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Wound Rotor vs Squirrel Cage
14
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(Electrical)
(OP)
14 Dec 03 18:48Hi All,
I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
Thanks!
(Electrical)
15 Dec 03 05:06Hello OhioAviatorA wound rotor motor, with an appropriate secondary reistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not.The negatives, are that both the motor and the starter will require a lot more maintenance than a standard cage motor, and the purchased price is higher.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
(OP)
15 Dec 03 07:51Hi Mark,
Thank you for your reply to my question, I certainly appreciate it.
That leads me to my second question...
Can I achieve the benefits of a wound rotor motor (high starting torque w/ lower starting current) along with the added benefits of reduced maintenance by using a squirrel cage motor and an electronic soft starter?
Again, thanks for your help.
--John Ruble
(Electrical)
15 Dec 03 08:29Suggestion to the previous posting: The soft starter and induction motor will approximately accomplish the similar output to the wound rotor motor. However, the soft starter may be more expensive and more demanding on the service. Also, MTBF may be lower for the soft starter.
(Electrical)
(OP)
15 Dec 03 08:37Thank you jbartos,
I'm not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the 1000 HP+ range, either.
Back to wound rotor motors...
Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?
Again, thanks all for your help!
--John Ruble
(Electrical)
15 Dec 03 08:39Suggestion to the original posting marked ///\\\
I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
///The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor Nema Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor. Normally, starting and operating conditions of this size of motors are simulated by using software, e.g. EMTP.\\\
2
electricpete(Electrical)
15 Dec 03 10:23I don’t have knowledge for comprehensive evaluation of your options compared to your application.
Considering only the motors themselves (not the other parts of the starting/control): I agree with jbartos the wound rotor motors require more maintenance. Also, in my experience we have much higher failure rate on wound rotor motors than on our squirrel cage motors (although the applications are not comparable… wound rotor motors are probably used in the more demanding applications).
(Electrical)
15 Dec 03 13:13Hello OhioAviatorAlthough you can certainly reduce the starting current by using a soft starter, you will not get the same starting effectiveness (Start torque/start current) using this method.If you begin with a standard cage motor, particularly at this size, your starting torque will be low compared to that achievable with a wound rotor motor. The initial start torque may be in the region of 100 - 120% at 600 - 800% start current as opposed to to say 250% current for 200% torque with the would rotor.When we apply a soft starter to a standard cage motor, we reduce the start current and also reduce the start torque by the current reduction squared.For equal start current, the wound rotor motor and secondary resistance starter will produce many times the start torque of the soft starter and cage motor.If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations that I have been involved in are still operating correctly and without problems after twenty years.I suspect, that for this application, you will require a high start torque, hence the suggestion of the wound rotor machine. This is based on my experience, but would depend on your actual requirements and parameters.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
(OP)
15 Dec 03 15:12Hello Marke,
Thank you very much for your explanation of starting squirrel-cage motors vs wound-rotor motors. What you say makes perfect engineering sense, substantiated by my own personal observations in the field.
What is your opinion of solid-state wound-rotor motor starters? Are they more reliable than liquid rheostat starters? What about maintenance requirements? I've been looking at a couple of solid state wound-rotor motor starters from Benshaw, Inc. Any experience with these types of starters?
Again, many thanks!
--John Ruble
2
DougMSOE(Electrical)
15 Dec 03 15:20I have personally been to installations that used soft starters, and they were very sorry that they had them.
Like the previous posts, a wound rotor induction motor (WRIM) will have faster acceleration and an adjustable speed torque curve, which a squirrel cage motor (SCIM) does not have.
One important point that many SCIM manufactures do not understand is the KWH demand billing. What this means is that as the motor is crushing cars, there is a demand limit that you do not want to go over otherwise you will be paying substantially higher energy bills to compensate for that (for example) 5 second period that you went over that value.
Yes, you will get full load torque at standstill with about 70% of your full load (rated) starting current, provided that the liquid rheostat is operating correctly AND the brine solution is correct.
PS The best way to determine if the brine solution is of the correct concentration is to use a hydrometer (density test of the solution). Please take note that the concentration needs to be temperature compensated.
(Mechanical)
15 Dec 03 19:07Auto shredder motors see far more than shock loading, the main area which I did not see covered is the extended period of time where the load on the motor reduces the speed of the driven equipment. This is where the SCIM fail, they do not work well when punished at less than full speed.
I have seen many shredder motors and they are far beyond the capabilities of any other motor. The service factor is typically 2. Schorch make one of the best motors I have seen for this application. They are of course the wound rotor type. P&H may have a shredder motor also.
On SCIM motors the rotor will usally break up as a result of the rotor running slower than design.
(Electrical)
(OP)
16 Dec 03 08:14Thank you, all, and especially to DougMSOE and to Mendit. Both of you obviously have specific knowledge of the scrap metal processing industry (far more than I have, that's for sure!) and car shredders in particular. I originally come from the mining industry and while that industry has large crushers, I've never seen mining crushers (and motors) that take the punishment that car shredders do.
I think I'm convinced now that a WRIM really is the better choice for our applications in this industry.
One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters. Is this because you can continuously vary the resistance of a liquid rheostat while in operation? And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit? Does rotor resistance help limit KW Demand?
Again, thank you everyone!
--John R.
(Electrical)
16 Dec 03 09:04Suggestion to the previous posting marked ///\\\
One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters.
///The cost and reasonable reliability including simplicity may be the main reasons.\\\
Is this because you can continuously vary the resistance of a liquid rheostat while in operation?
///Yes, in comparison with wound resistors; no in comparison with soft starters.\\\
And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit?
///The resistors require less maintenance, can have better location, can be remotely controlled, etc.\\\
Does rotor resistance help limit KW Demand?
///In which context or comparison?\\\
///Please, notice that the above postings did not differentiate among squirrel-cage induction motor Nema Design letters. Nema design letter D has noticeably different characteristics from other Nema design letters. The Nema design letter D motor has substantially higher starting torque than other ones.
Another aspect that has not been addressed yet would be adding a flywheel. However, this would impact the operation of the crusher since it would have to start empty to minimize the starting torque unless the flywheel is disengageable. \\\
(Electrical)
4 Jan 04 19:41A Variable Speed Drive (VSD) will provide the most amount of torque per amp of any motor starting method available. If you choose a reputable supplier (of both motor and VSD), reliability will be much better than the WRM option.
An added bonus is that you will get full speed control.
(Electrical)
4 Jan 04 23:30Hello OhioAviator,
Although I fully agree with and support much of what has been said here, I believe you need to consider each application on a case-by-case basis, particularly if you are aiming achieve best performance with the lowest possible capital outlay.
Wound Rotor Motors are capable of producing high torque when controlled via a ‘properly engineered’ liquid resistance starter. Their torque/speed characteristics are also well suited to applications that present transient over-load conditions and this can be further enhanced in shredding applications through the addition of a flywheel to the mechanical system. On the down side they do require more maintenance than a Squirrel Cage Motors and this needs to be factored into the decision making process.
Medium (and High) Voltage Squirrel Cage Motors are notorious for their poor start performance, often exhibiting levels of Locked Rotor Torque less than rated or Full Load Torque. As has been advised by Marke the addition of a soft starter will act to reduce starting torque further and therefore it is likely problems will be experienced here. This might also explain comments by DougMSOE! On a more positive note some motor manufacturers will design and manufacture Medium Voltage Cage Motors to your specifications, thus allowing you (possibly) to utilize Full Voltage Starting techniques.
As suggested by TheDOG, a Cage Motor controlled via a Variable Speed Drive will provide the best overall performance......dollars permitting of-course!
If your clients are anything like mine (unlimited dollars are not available to them), an analysis of motor and load curves should assist to determine best starting method for the intended application.
Regards,
GGOSS
2
gsimson(Electrical)
5 Jan 04 02:45I will just add only one point.
With VFD with SQ. Cage motor performing satisfactorily there is a possibility of DC motors and Slip ring Motors becoming "Show case items" and limited to academics especially in large industries operating in dusty and humid atmosphere.
As on today there is no application which can not be satisfactorily performed by SC motor with VFD. Considering the maintenance, this option will workout cheaper in the long run
(Electrical)
(OP)
5 Jan 04 08:26"Thank You" to TheDog, GGoss, and GSimon for your replies to this thread. While I agree that a SCIM/VFD combination might produce starting torques comparable to a WRIM, the up-front capital costs tend to be prohibitively higher. (Remember, our world is dominated by bean counters who, by and large, look only to the end of the current fiscal quarter.)
What I don't have a good handle on, though, is how it is possible to continuously manage (control) KW Demand with a SCIM/VFD combination like I can a WRIM/Liquid Rheostat. Is it possible?
Thanks,
J Ruble
(Electrical)
5 Jan 04 09:32Total input power = motor output mechanical power plus motor/drive system losses.
Output mechanical power can be roughly controlled by adjusting speed for both the wound rotor and SCIM+VFD.
Losses would have to be considered for each system. I think the wound rotor will have increasing external (rheostat) losses as you attempt to adjust speed lower.
(Electrical)
5 Jan 04 12:08Ohio Aviator.
You should consider both, starting and full speed operation of the motor.
My opinion is that WRIM with a proper matched resistance will provide the higher starting torque with the lower KVA inrush.
At full speed operation, your load application could develop sudden high peak Torque overloads. A flywheel could provide the extra required torque combined with some resistance in the rotor circuit to allow the rotor to slip under a shock load and then gradually accelerate at operating speed again.
The kilowatt demand is due to load plus losses and you have to provide it since it is pure energy ( the flywheel, if you have one, will provide momentary kinetic energy but then the motor will return that energy while accelerating back to full speed ) A variable frequency driver will develop a Volt/Hertz electric input to the motor but the voltage drops following the frequency to avoid magnetic saturation, then your torque could be constant but not larger than under 60 Hz operation.
A Olalde.
(Electrical)
5 Jan 04 13:04The only advantage the wound rotor motor offers is during start up of high torque applications. External resisters are used in the rotor circuit to limit starting amps in the rotor. Once the wound rotor motor comes up to speed all external resistance is shorted out and the motor performs like a squirrel cage motor. Your application may not require high starting torque as I would assume the shredder is started unloaded, i.e. without a car in the shredder. Therefore the wound rotor motor may not be benificial. The Design class D squirrel motor are specifically design for surge amplications.
(Electrical)
5 Jan 04 14:11Having installed the 3 different types of motors in scrap yards WRIM,SCIM and Synch (and a few DC) the shock loading can take a motor from operation RPM to 1/4 of the at RPM in less than 2 seconds! Be advised, what is constant with scrap-- NOTHING! While in the theory world 'white goods' are only 'white goods', you may also have the "engine block" mixed in with them. By far a WRIM is totally the only motor that can handle 4* rated current for 30m seconds while the solid state world is in smoke. Been there and saw that. Reliabiliy is best when you can fix it youeself.
(Electrical)
(OP)
5 Jan 04 14:38Thank you, DougMSOE.
I'm finding myself pretty much in complete agreement with you, especially as time goes on and I gain more and more operating experience with these shredders. You are so right that 'white goods' ain't always 'white goods'. And I doubly agree with your statement that "Reliability is best when you can fix it yourself"! Been there, done that... too many times. I've spent way too many long nights and weekends learning how to fix the supposedly superior 'latest and greatest' electronic gadget, wishing I had a simple molded case circuit breaker and an across-the-line starter, instead.
Gentlemen (and ladies, too) thanks for your input on this thread. I think I have enough info for now. If a new posting comes up I'll check it out but I can't promise I'll respond to further postings.
Again, many thanks!
John R.
(Electrical)
8 Jan 04 03:16I know you've said you do not want any more emails, so please do not feel under obligation to replay.I was on holidays and missed the whole discussionApologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?Anyway, here is my contribution:Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .In summary major reasons for selecting WR motors are:1. The load can be started at the max torque2. Large starting torque can be obtained in comparison with low starting current3. In case when starting frequency is too high for the thermal resistivity of the an ordinary SC motor4. In case GD2 of the load is too large for the thermal resistivity of an ordinary SC motor5. In case load requires cushion startingToday, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.Note – in very large applications 8000 + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.
(Electrical)
8 Jan 04 08:50Suggestion/question: It is not clear from the original posting whether or not the cost is a factor. Please, would you address that point? Any better solution will probably be much costlier; especially, considering modern power electronics options aligned with the squirrel-cage induction motors and wound rotor induction motor.
(Electrical)
(OP)
8 Jan 04 09:38Hello Aquarius and Mr. Bartos...
Thanks for your postings to this thread.
Aquarius - The primary parameters I'm looking for are current limited starting, kW Demand control, high slip capability to accommodate very high short term shock loads, and reliability with simplicity. Variable speed control really isn't a goal, just high slip capability.
JBartos - Cost isn't the primary factor, but it probably ranks second. Reliability in this particular application (automobile shredders) is paramount. Simplicity is also important, as we typically don't have highly skilled electrical technicians on the payroll.
As I eluded to previously, I think I'm pretty well convinced that the WRIM is the best option, primarily due to the severity of the duty required for our application. Automobile shredders are "crushers" taken to the extreme. As DougMSOE stated above, a typical automobile shredder is shredding light metal (car bodies) together with heavy chunks of metal (engine blocks). The impact shock loads are tremendous.
Again, thanks!
2
jraef(Electrical)
8 Jan 04 14:26OhioAviator,I am coming into this late due to the holidays as well, but I would like to add my opinion. I am a big proponent of MV solid state controls, but I, like GGOSS, consider each application on its own merits.I have applied SS starters to SCIM motors in many shredder applications successfully and they work just fine. However they were waste shredders not AUTO shredders. IMHO, in your application the WR motor is probably better for the shock load capability as mentioned by others. Many of the above posts refer to starting torque issues but may not realize that a shredder NEVER starts with the load already in it (the exception being hydraulic powered shredders, but thats a different discussion). You are looking for a reduction in starting torque since the shredder is always unloaded at start. The slip recovery capability of the WR motor can be "adjusted" by altering the resistance on the secondary, a very useful feature for your application. Look at it this way, a WR motor has all of the capabilities of Design A, B, C and D SCIM motors by simply changing the rotor resistance. With a SS starter on a SCIM, once it is at full output the starter has NO control of torque capability. Your motor is on its own with just the torque and slip recovery capabilities inherent to its design.One other possibility is what was mentioned by rlpuck on 1/5/04, the use of a Design D squirrel cage motor. It has the slip recovery capability equal to the maximum available in the WR. If you go that route, the RVSS starter would be useful to keep the starting torque to a minimum. If you don't already have a motor you may want to consider this. If you already have a WR motor it would not be worth swapping it out.By the way, don't be fooled by the Benshaw marketing of their "solid state wound rotor" control. It is just a SS starter for the stator, combined with a fixed resistor in the rotor, which is shorted out at full speed. If you are going to go with WR, have a competent control manufacturer build a starter and resistor control package that matches your needs. SS is kind of a waste on WR motors. Also IMHO liquid rheostat WR controllers are good for things like flow control etc., but the maintenance costs and care required to keep them running would make them poor candidates for your application. Search this forum for the term "Liquid Rheostat" and you will see several discussions pertaining to LR maintenance issues. Shredder operators are not known for being mindful of routine maintenance!
Quando Omni Flunkus Moritati
(Electrical)
(OP)
8 Jan 04 14:41Hello jraef,
Thanks for your post. And thanks for confirming my suspicions that at WRIM is probably the best choice for our application. And you're right... shredder operators are DEFINITELY not known for routine maintenance.
BTW... thanks for making me look on the internet for the translation to your latin phrase (curiosity, you know)!
Cheers
(Electrical)
11 Jan 04 01:55Suggestion: Visitfor:Squirrel-Cage Induction Motors - The most simple and reliable of all electric motors. Essentially a constant speed machine, which is adaptable for users under all but the most severe starting conditions. Requires little attention as there are no commutator or slip rings, yet operates with good efficiency.Wound-Rotor (Slip Ring) Induction motor - Used for constant speed-service requiring a heavier starting torque than is obtainable with squirrel cage type. Because of its lower starting current, this type is frequently used instead of the squirrel-cage type in larger sizes. These motors are also used for varying-speed-service. Speed varies with this load, so that they should not be used where constant speed at each adjustment is required, as for machine tools.Comment marked ///\\\Reviewing the above postings and agreeing with the above link, the squirrel-cage induction motor will be the best solution. The flywheel for smoother ride-throughs should not be ruled out.
(Electrical)
11 Jan 04 16:48With all due respect jbartos.
Squirrel cage motors have been used in the auto shredder applications, BUT the tons/KWH is less for a SCIM than for a WRIM, all things the same.
Unless you have actually been to and worked in an auto/industrial scrap yard where the cars/busses, etc are sherdded from their present form to 'half-dollar size' in less than 10 seconds the true appreciation of the WRIM will never be appreciated. I have worked in the heavy power industry for over 30+ years and watching a shredder do its job is amazing to say the least. A 20,000 HP motor driving a BFP, ID or FD fan is no comparison to what a 7,000HP motor goes through in 10 min of its job in a scrap yard.
Yes, a SCIM is good but will NEVER come close to taking the shock loadings that a WRIM does. Further, the WRIM can be made to operate very closly to the SCIM in this application.
PS I have installed about 10 shredder motors and have worked on over 50 of then.
DougMSOE
(Electrical)
5 Feb 04 19:59The metal type resistive controller for WRIM is more reliable than liquid rheostat starters. The controller consists of only three durable parts: stainless steel resistors, vacuum contactors and PLC. It’s reliable and maintenance free. PLC controls contactor close/open to switch resistor in/out to control the torque and speed. The controller allows the motor to deliver up to its breakdown torque to the driven load during starting or running.The controller is pre-programmed to meet your specific load characteristic
(Electrical)
5 Feb 04 20:19Metal type resistors tend to have a positive temperature constant causing the value of the resistors to increase with heat. Liquid type resisters have a negative temperature coefficient resulting in reducing resistance with heat.A wound rotor starter requires reducing resistance as the motor accelerates so there is an advantage in using the liquid type resistor.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
5 Feb 04 21:06Suggestion: The Squirrel-Cage Induction Motor (SCIM) has some room for customization based on its intended application. SCIMs with NEMA Design Letter D can handle very demanding applications, e.g. openings of the rusty valve, punch presses, cranes, hoists, press brakes, shears, oil-well pumps, centrifugals, etc.Reference:Donald D. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, Inc., 1993,Section: Characteristics of Polyphase Induction Motors on page 20-33 objectively compares various motors characteristics.Visitetc. for more info
(Electrical)
6 Feb 04 10:08The resistor grid system has been used in several WRIM scrap yards and they do work reasonably well. The problem is that they become very hot and with all of the dust and fluff from the car seats you will have a fire. Been there and have installed the liquid rheostats that do not have the problem.
Further, the square footage req'd for the liquid Vs the grid system is much less about 60%. And with the grid system you need large contactors to control the speed torque curve such that the max torque can be placed at the resultant RPM from the load on the shredder.
Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job.
I do not care for any book that you might be able to cite that a SCIM is better than a WRIM in this application. There is NO COMPARISON in this application. A NEMA D does not even get close to what a WRIM can do in this application.
(Electrical)
8 Feb 04 06:01Question to the previous posting: Please, would you be more specific in engineering and design terms in your statement:
""Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job."" specifically, if the WRIM flexibility is clarified?
(Electrical)
9 Feb 04 10:24jbartos, To your question, I hope that this is what you are asking.
"Please, would you be more specific in engineering and design terms in your statement:
I would ask that you might take a look as to what the difference is between a NEMA design A,B,C,D,and F of a 3ph SCIM. What actually is the difference?
Given all other parameters the same the one major difference is the metallurgy of the rotor bars, and they are not Cu, but a Cu alloy!!! The resistivity of the bars AND the shorting rings are what make the difference.
Now thak a look at the equivalent circuit for an induction motor, NOT the equivalent circuit cited to simplify the circuit such that the well known circle diagram is made. You will notice that the maximun power transfer theorm applies, not just by load but also by RPM!
This is why the WRIM does this job better than any other motor or for that matter a diesel engine, which has also been used.
I the WRIM the secondary resistance may be changed at will to a range of resistances 20 times that of a SCIM even mor if required.
(Electrical)
9 Feb 04 15:27The WRIM could work with variable external resistance values connected to the rotor circuit.
That feature makes it feasible to be adjusted to almost any desired Torque-Line current- slip characteristic.
Put a high external resistance and the inrush current will drop close or even under full load current and at the same time the resultant Torque becomes more current effective.
Adjusting the external resistance properly,you will get any desired “Design Performance” from a NEMA design A to a D and much more.
Adjust the external rotor resistance when the motor is running with load and the speed will be reduced with an increase in the rotor slip.
An ISCM has a fix rotor construction and after it is constructed it will have a fixed rotor resistance and performance.
The heat generated in the rotor winding is dissipated into the rotor of an ISCM. For a WRIM most of the rotor heat is dissipated in the external resistors. A NEMA design A (ISCM) has high efficiency at full load and low slip but very high inrush current and only 150% LRT. A NEMA design D (ISCM) has 275% LRT and 400% inrush but generates too much heat and low efficiency under load.
That makes a (WRIM) Wound Rotor Induction Motor performance very superior if compared to an (ISCM) Induction Short Circuited-rotor Motor (squirrel cage).
Costs of Maintenance and Initial investment are certainly much lower for an ISCM, that is why it is the first selection as far as it could be tailored properly to the load.
(Electrical)
9 Feb 04 18:55You've got it aolalde!!!
The only addition to your post is that the starting current of the WRIM with a liquid rheostat is usually set at 70 to 75 % of the full load running current (this is where the maximum torque is usually developed). Typically the liquid rheostat uses a solution of soda ash and temperature (more on the soda ash) to adjuse the Istart via the secondary resistance.
Further, the WRIN is made top operate with the resistance in the rotor circuit.
Have a Great Day!!!
Doug
(Electrical)
9 Feb 04 23:47WRIM with resistor grid / Liquid resistor is a "Load dependent" drive. The torque developed at various speeds is dependendent on load hooked. SCIM with VFD is load independent and developes PULL OUT TORQUE at ALL speeds.
The current requirement is based on the load and the power "wasted" in the resistor. With SCIM /VFD there is no wastage of power . The motor draws power only to the extent required. "Solid state going into smoke" is only hypothetical and certainly not required for real applications.
(Electrical)
10 Feb 04 15:23Since the secondary resistance can be increased or decreased as required to match, as you put it, the PULL OUT TORQUE, the load WRIM is load dependent. Certainly, as you describe the SCIM with a VFD is, within the limits of the machine, is load independent. And yoe the heat developed by the resistor bank is some what wasted unless like a few scrap yards have done is to provide heating and cooling as required. Un fortunately I HAVE SEEN FIRST HAND secondary energy pump back systems going into smoke as well as the front end drives. What is required to understand in this application of WRIM in scrap yards is,
1.)easy to fix with only simple parts and simple equipment.
2.)TOUGH BEYOND BELIEF i.e. shock, vibration are standard. Again PLCs have had boards shaken out of their cages.
3.)First cost a big factor
4.)Easy to understand
Ultimate reliability is achieved when you can fix what is broken with a Simpson 260, a screw driver and an adjustable wrench.
(Electrical)
10 Feb 04 16:34Don't leave out the duct tape!
"Venditori de oleum-vipera non vigere excordis populi"
(Electrical)
(OP)
11 Feb 04 08:54The screwdriver (if you don't mind the broken handle or broken shaft), and the adjustable wrench (if you don't mind the finishing nail sticking out of where the knurled shift adjuster lockscrew is supposed to be), and the duct tape (after you warm it up and peel it off the old furnaces and ductwork) are all three easily obtained from the shredder feed pile. The Simpson 260 will probably need to be purchased new (I probably wouldn't trust a meter that I dug out of the shredder pile; at least not more than once!). As DougMSOE says, we are VERY cost conscious!
In all serious folks, I had no idea that this thread would be so popular. Thank you for all the great and valuable input. I learned a great deal from this thread.
Best Regards,
John Ruble
BTW... I'm thoroughly convinced that a WRIM is all-round the best way to go for a scrap yard shredder application.
(Electrical)
11 Feb 04 22:04Spoken like a man forced to improvise on the job trying to get something operating before he is allowed to go home...
Been there, done that, got the T-shirt and the hat.
(Electrical)
12 Feb 04 21:05Questions: How much time/downtime does it take to replace worn out slip rings of WRIM, how soon do they get worn out, and how much does it cost?
(Electrical)
13 Feb 04 09:45jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
Some designs can be changed out inplace in about 4 hours, others inplace, 2 to 4 days.
(Electrical)
13 Feb 04 22:54One of the more informative and entertaining threads I've seen. Thanks to all.
Smaller scale similar problem is the chipper in a sawmill, high shock loads when a chunk of oak or hard maple goes through. We used a slightly oversize SCIM with a Benshaw SS. The chipper itself includes a large inertia mass which helps. I guess my rambling point is that while a WRIM is probably best, look to increase inertia if possible.
(Electrical)
13 Feb 04 23:17FACB25:
I am glad that you mentioned INERTIA, which is a resource that has been forgotten lately. It is interesting the way it works on heavy peak load applications.
(Electrical)
15 Feb 04 02:49Comment on DougMSOE (Electrical) Feb 13, 2004 marked ///\\\
jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
///This 20+ years life-expectancy is probably what the automobile shredders WRIM need.\\\
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
///The frequent current transients due to variable and impulsing automobile shredder loads reduces the slip ring life expectancy.\\\
Some designs can be changed out inplace in about 4 hours,
///4 hours for the slip ring replacement seems to be optimistically short time; especially, if the rotor balancing is included.\\\
others inplace, 2 to 4 days.
///This is more like it, if the good unionized workmanship is in place.\\\
(Electrical)
16 Feb 04 04:36Some of the requirements which are required to be met as indicated are better met by SqIM along with VFD.
Easy to fix- No rotor resistance, Rotor cables , Rotor contactors . Only a Sq. cage motor with 3 leads connected to VFD.
Simple parts & simple equipment - What is nmore simpler than SQIM?
Tough beyond belief ( Shock , Vibration) - Sliprings & rotor resistances are certainly not tough.
Cost - A slipring motor with rotor resistance / rotor panel is certainly costlier than a Sq. Cage motor & VFD.
Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?
ALL repeat ALL the applications can be met by SQIM & VFD at a cheaper cost both capital & running, with min. maintenace and much better speed /Torque control. Both Dc motor and WRIM will become museum pieces & for academic interests as stated earlier.
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
(Electrical)
16 Feb 04 12:54gsimson - I appreciate all your expert advice and opinions.I have to take exception with one thought process:"Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?"I wouldn't buy from anyone that told me I don't need to understand it because it's never gonna break
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
(Electrical)
17 Feb 04 22:02Suggestion to gsimson (Electrical) Feb 16, 2004 marked ///\\\
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
///ABB rep mentioned sometimes ago that the cost MV VFD for large HP application is coming down so that the LV VFD is not that much better for such applications.\\\
(Electrical)
18 Feb 04 16:14gsimon,You made some good points. I am a big promoter of VFDs, but I beg to differ with you here. As mentioned earlier in this thread, the APPLICATION is not appropriate for a MV VFD. I have been on startups and retrofit projects for plenty of MV VFDs; AB, ABB, Robicon and Ross Hill (now owned by Robicon), and I can report that my experiences lead me to believe that conditions need to be near prefect for them to operate reliably. By that I mean clean, air conditioned spaces, clean, reliable power, clean, knowledgeable operators and clean. If any of those conditions are less than perfect, the equipment tends to cause moe headaches that it is worth. Did I mention it needs to be clean?An Auto Shredder operation meets NONE of these requirements, and in fact fails them in with extreme prejudice. I have put low voltage VFDs in rock crusher applications for yerars, and only after 10+ years of exposure to them are they finally gaining acceptance by users. That said, I just had an "electrician" at a quarry connect 120VAC to the 4-20ma input on one of mine yesterday, blew up a 250HP VFD. He didn't know exactly what "speed reference signal" meant, he thought it was the Start button! Auto shredder operations are the same if not worse.
"Venditori de oleum-vipera non vigere excordis populi"
(Electrical)
18 Feb 04 16:27FACB25Your point about inertia is correct, except that most auto shredders were not designed around a flywheel effect as were chippers. You can't add significant inertia just by increasing the motor size, you need to add mass to the machinery. A wood chipper is inherently a high mass machine. You may have needed to increase HP a bit because in the old days, chippers handled occasional loads of scrap material. When a load came into them, inertia allowed them to chew it up, and the motor simply re-accelerated it after it was gone. Now chippers are generating revenue and are fed a continuous stream of material. The extra HP is sometimes needed on older machines to maintain speed of the mass (intertia) under that more constant loading.
"Venditori de oleum-vipera non vigere excordis populi"
(Electrical)
19 Feb 04 04:48Thank you every one for your valuable comments.
Certainly VFDs are understood by engineers. When we are in the age of "Prevent Maintenance" VFDs are preferred as their reliability is very high with minimum or rather no maintenance. As on date the electricians may not understand and considering the reliabilty their understanding is not a must.
MV VFD is a general comment and not relevent to this thread.
As I mentioned 'all' applications, the cost factor was posted for information.
For isolated drives located far away with 1 electrician I agree that the SRIM with resistor is better option.
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This article provides an in-depth look at the often-encountered but rarely-discussed squirrel cage motor. We’ll explore what it is, how it works, its applications, and design types, and answer common questions revolving around its cost-effectiveness, durability, speed control, and performance.
Introduction: What is Squirrel Cage Motor?
At its simplest, a squirrel cage motor, or squirrel cage induction motor, is an electric motor that harnesses the principle of electromagnetic induction to generate motion. It’s a type of asynchronous motor, which means that the electric current in the rotor (the rotating part of the motor) needed to produce torque – or rotating force – is obtained by electromagnetic induction from the magnetic field of the stator winding (the stationary part of the motor). In short, it’s a workhorse enabling modern-day conveniences and industries through the conversion of electrical energy into mechanical energy, driving everything from ceiling fans to industrial machinery.
The moniker “squirrel cage” isn’t due to the motor’s affinity for our acorn-loving friends. Instead, it’s borrowed from the rotor’s construction that uncannily resembles a hamster wheel or, indeed, a squirrel cage. The rotor consists of conductive bars, aluminum or copper, arranged in a laminated cylinder, similar to how a small animal’s cage is constructed. Shorted (electrically connected) at both ends by conductive rings, the overall structure imitates a cage. The embedded bars in the rotor’s core are skewed to reduce magnetic hum and unnecessary induction heating, thus enhancing the motor’s efficiency.
Squirrel cage motors are ubiquitous, forming an integral part of various applications in a diverse expanse of industries. Often going unnoticed in day-to-day life, they power household appliances such as washing machines, air conditioners, refrigerators, and water pumps. From a broader perspective, industrial applications are even more prolific – ventilation systems, conveyor belts, industrial pumps, and compressors, to name just a few. This prevalence stems from their boundless advantages, such as their robustness, low-cost production, superior performance rate, and importantly, their minimal requirement for maintenance, owing to the absence of brushes, slip rings, or commutators. Furthermore, their considerable torque enables the swift start-up of appliances and machinery.
Given these compelling characteristics, it should come as no surprise that the squirrel cage motor has been an unwavering cornerstone in motor-driven systems since its inception, continuously evolving with technology’s stride. Now, having unraveled what it is and why it’s paramount, the succeeding segments will delve further into how this motor variety works and the essential aspects that characterize its performance.
What Is Squirrel Cage Motor: Basic Working Principle
The working principle of squirrel cage motors, fascinating in its simplicity yet profound in its application, hinges on the foundational laws of physics and electromagnetism. At their core, these motors employ the ingenious concept of electromagnetic induction to convert electrical energy into mechanical energy, facilitating movement in a myriad of devices and machinery that form the backbone of both industrial and domestic settings.
To understand how squirrel cage motors work, one must grasp the roles played by its two primary components: the stator and the rotor. The stator, an unmoving part of the motor, comprises a series of electromagnets arranged in a circle around the rotor. This arrangement is not merely structural but serves the pivotal function of generating a rotating magnetic field within the motor. When alternating current (AC) power is supplied to the stator, a magnetic field is created around each coil of wire within the stator. Due to the configuration and the phase difference in the AC supply to these coils, a rotating magnetic field is produced.
The rotor, placed inside the magnetic influence of the stator and being the moving part of the motor, is what gives the squirrel cage motor its name. Structured remarkably like the familiar rodent’s wheel, the rotor consists of aluminum or copper bars connected at both ends by rings of the same material, forming a ‘cage’. This construction is crucial because it allows the rotor to interact with the stator’s magnetic field through electromagnetic induction.
Electromagnetic induction, a principle discovered in the early 19th century by Michael Faraday, is the process by which a conductor moving through a magnetic field has an electric current induced within it. In the context of the squirrel cage motor, as the rotating magnetic field produced by the stator sweeps past the rotor bars, a current is induced in these bars due to electromagnetic induction. According to Lenz’s Law, the direction of this current is such that its magnetic effect opposes the cause, which is the moving magnetic field. Thus, the induced current generates its magnetic field in the rotor bars, which interacts with the stator’s rotating magnetic field. It’s this interaction – essentially a push-and-pull dynamic – that causes the rotor to move, in an attempt to catch up with the rotating magnetic field of the stator. However, an inherent characteristic of the induction motor is that the rotor can never actually ‘catch up’ to the stator field speed; this difference in speed is essential for continuous motor operation.
As straightforward as it appears, the electromagnetic induction process within a squirrel cage motor is a delicate balance of physical laws enabling efficient energy conversion. The elegant dance between the stator’s magnetic field and the induced currents in the rotor, mediated by the principles of electromagnetism, underpins the operation of these motors, making them a remarkably efficient means to drive mechanical loads. This understanding of the basic working principle sets the stage for exploring deeper aspects of the squirrel cage motor’s design, types, and applications in ensuing discussions.
What Is Squirrel Cage Motor: Construction and Design
The robustness and efficiency of squirrel cage motors, which have solidified their status as the workhorses of the electric motor family, are largely attributable to their construction and design. These motors are engineered to be simple yet effective, utilizing materials and a structure that lends to their durability and versatility.
Materials Used in Squirrel Cage Motor ConstructionThe choice of materials in the construction of squirrel cage motors is pivotal to their performance. Typically, the stator consists of a steel frame that houses a core made from stacked laminations of silicon steel, a material known for its excellent magnetic properties. This lamination helps in reducing eddy current losses, a form of energy dissipation that occurs due to the induction of currents in the core material itself.
For the rotor, aluminum or copper is used to make the conductive bars and end rings that define the ‘cage’. Aluminum, being lighter and cheaper, is commonly used, especially in standard motors, due to its adequate conductivity and lower rotor inertia, which is beneficial in applications requiring frequent starts and stops. Copper, offering superior conductivity, makes for a more efficient motor with lower energy losses, albeit at a higher cost and greater weight. The choice between these materials often hinges on the specific requirements of the application, including considerations of efficiency, cost, and operational dynamics.
The Distinctive Structure of the Squirrel Cage RotorCage RotorThe rotor’s unique structure is a defining characteristic of the squirrel cage motor. Resembling a cylindrical cage, it consists of conductive bars laid parallel to the rotor’s shaft, connected at both ends by conductive rings. This configuration ensures a uniform response to the rotating magnetic field generated by the stator, allowing the rotor to spin smoothly and efficiently. The bars are often slightly skewed relative to the shaft’s axis, a setup designed to reduce magnetic humming for quieter operation and to minimize locking tendencies between the rotor and the stator’s magnetic fields.
Design Differences Between Squirrel Cage Motors and Other Types of Induction MotorsWhile the squirrel cage motor is an induction motor at heart, it’s distinct from its counterpart, the wound rotor motor, in several key aspects. The immediate difference lies in the rotor design; unlike the open-loop, wire-wound rotor of the latter, the squirrel cage rotor employs a closed-loop design. This fundamental distinction leads to variations in performance and application suitability. For instance, while squirrel cage motors are prized for their simplicity, ruggedness, and lower costs, wound rotor motors offer the advantage of the controllable speed and torque, making them preferable in applications demanding such functionality.
Moreover, the simplistic design of squirrel cage motors translates into lower maintenance requirements and enhanced reliability over wound rotor motors, which feature brushes and slip rings that can wear down over time. However, this same simplicity limits the squirrel cage motor’s speed control capabilities compared to the wound rotor motors, which can be adjusted through external resistance.
Through their construction and design, squirrel cage motors embody a balance of efficiency, durability, and cost-effectiveness that has cemented their place in myriad applications. The specific choice of materials, the unique design of the rotor, and the differences in construction compared to other induction motors collectively underpin the capabilities that make squirrel cage motors a ubiquitous choice in the realms of industrial and domestic machinery.
What Is Squirrel Cage Motor: Types and Variants
Though the foundational principle of squirrel cage motors is the same across various designs, their differences lie in size, power ratings, rotor bar architecture, and specific characteristics catering to diverse types of applications. Classifications of squirrel cage motors can be articulated based on these parameters.
Different Types of Squirrel Cage Motors Based on Power Ratings and SizesMuch like any other machine, squirrel cage motors come in a plethora of power ratings and sizes designed to meet varying application requirements. From tiny motors that power miniature fans within personal computers to gigantic ones moving heavy machinery in industries, their power ratings widely differ. Commonly, these motors are available with power ratings starting from a fraction of a watt to many megawatts.
The size of a squirrel cage motor goes hand-in-hand with its power rating. Larger motors possess more substantial amounts of conductive and magnetic materials, designed to handle higher current and magnetic field strengths, and thus can deliver more power. Conversely, smaller motors are suitable for applications where space is at a premium, and the power requirement is modest.
The Significance of Different Rotor Bar Designs (Deep Bar, Double Cage, etc.)The rotor bar design is a strategic factor in the function and efficiency of a squirrel cage motor, which can be altered to meet specific application needs. A generic squirrel cage rotor contains bars of uniform cross-section, but innovative design improvements, including deep bar and double cage rotors, are utilized in certain circumstances.
Deep bar rotors contain bars with a larger cross-sectional area at the bottom than at the top. The added depth gives these bars a higher resistance near the surface, reducing current during startup and allowing the motor to produce high starting torque without an excessive inrush of current.
Double cage rotors consist of two “cages” or sets of rotor bars, placed one inside the other, with differing electrical properties. The outer cage has a high resistance but low reactance, making it responsive during startup for high torque, while the inner cage possesses low resistance but high reactance, providing a steady state for high efficiency during regular operation.
Specialty Squirrel Cage Motors and Their ApplicationsCertain applications may demand specific motor characteristics not encapsulated by standard motors. Here, specialty squirrel cage motors come into the picture. Among these are explosion-proof squirrel cage motors, used in hazardous environments where flammable gases or dust can pose a risk of ignition. These motors are designed with robust enclosures that contain and isolate any internal electrical faults or sparks that could potentially ignite the surrounding explosive atmosphere.
On the other hand, braking motors are built with an inherent braking system that assists in quick stopping, making them suitable for applications requiring frequent stops/start-ups or precise positioning, such as elevators or hoisting equipment.
The versatility of the basic squirrel cage motor designs, and the ability to tweak them to meet specific requirements, highlights the sheer adaptability of this humble piece of equipment. As technology continues to evolve, so will the variants and innovations around squirrel cage motors, expanding their utility in multiple spheres.
What Is Squirrel Cage Motor: Common Applications
Squirrel cage motors are remarkably versatile, finding utility across a broad spectrum of applications, from the mundane to the highly specialized. Their inherent simplicity, reliability, and efficiency make them the go-to choice for numerous demands. This omnipresence can be seen in industrial settings, household appliances, and in operations amid hazardous environments.
Industrial Uses, Including Pumps, Fans, and Conveyor SystemsIn the industrial realm, squirrel cage motors are indispensable. They are the driving force behind a wide array of machinery and systems critical to production processes and facility management. For instance, these motors power pumps that are essential for the movement of fluids in sectors such as water management, chemical processing, and oil and gas. The choice of squirrel cage motors for pumps stems from their robustness and uninterrupted operation capability, crucial in environments where continuous flow is non-negotiable.
Fans and blowers, tasked with ventilation, cooling, or even air supply for combustion, heavily rely on squirrel cage motors for their operation. The simplicity and reliability of these motors make them suitable for long-term, continuous use—a staple requirement in HVAC systems and industrial cooling. Similarly, conveyor systems, which form the backbone of material handling in warehouses, mining, and manufacturing plants, also depend on these motors. The ability to produce a consistent torque reliably over time allows for the smooth functioning of conveyor belts, thus ensuring uninterrupted industrial operations.
The Role of Squirrel Cage Motors in Household AppliancesWithin the domestic sphere, squirrel cage motors quietly facilitate comfort and convenience, embedded within numerous household appliances. Their presence is felt in air conditioners and refrigerators, where their task revolves around compressing refrigerants or driving fans for air circulation. Washing machines and dishwashers also utilize these motors for agitating or rotating drums, providing the mechanical action necessary for cleaning. The prevalence of squirrel cage motors in these applications is largely due to their compact form, efficiency, and minimal maintenance they require—attributes highly valued in consumer appliances.
Their Use in Specialized Settings, Such as Hazardous EnvironmentsFurther extending their applicability, squirrel cage motors are specifically designed to operate in hazardous environments, highlighting their versatility. In settings where explosive gases, dust, or volatile chemicals pose significant risks, explosion-proof variants of these motors ensure safety alongside functionality. These specialized motors are constructed to prevent any internal spark or high temperature from igniting the external atmosphere, crucial in petrochemical plants, mines, and grain silos. The reliability and robustness of squirrel cage motors augment their suitability for such environments, providing a secure, efficient solution where standard motors would pose a significant hazard.
In essence, the widespread use of squirrel cage motors, from powering essential industrial machinery to seamlessly integrating within household appliances and ensuring safety in hazardous settings, underscores their fundamental role in modern technology and everyday life. Their adaptability across different applications is a testament to the ingeniously simple yet effective design that has made squirrel cage motors ubiquitous in both public and personal spheres.
What Is Squirrel Cage Motor: User Concerns and Questions
As ubiquitous as squirrel cage motors are in various applications, potential users or operators often have pertinent questions and concerns about their performance, operational characteristics, and cost implications. Understanding these elements can significantly influence decision-making processes regarding the selection and deployment of these motors.
Performance EfficiencyWhen discussing performance efficiency, squirrel cage motors are renowned for their high levels of operational efficiency, which often reach up to 90% under optimal loading conditions. This is largely due to their simple construction and the absence of brushes or slip rings, which minimize energy loss through friction and electrical resistance. Compared to other motor types, such as wound rotor motors or motors with mechanical commutators, squirrel cage motors offer an advantageous blend of efficiency and reliability. However, it’s worth noting that their efficiency can vary based on size, with larger motors generally being more efficient. While squirrel cage motors are not inherently the most efficient in every scenario, for most standard industrial and domestic applications, they provide an excellent balance between cost, efficiency, and durability.
Speed ControlSpeed control within squirrel cage motors is a topic of considerable interest, especially given the motor’s inherent design which naturally lends itself to a constant speed operation. Traditionally, squirrel cage motors are not as flexible in speed control compared to DC motors or wound rotor induction motors. However, with advancements in technology, speed control of these motors has become possible and increasingly sophisticated through the use of Variable Frequency Drives (VFDs). VFDs adjust the motor’s speed by varying the frequency of the electrical power supplied to the motor, allowing for flexible control over motor speed without significant losses in efficiency or power.
Maintenance and DurabilitySquirrel cage motors are esteemed for their durability and longevity. The simplicity of their design — lacking brushes and commutators — not only contributes to their efficiency but also reduces the potential points of failure, thus lowering maintenance requirements. General maintenance might include routine inspections, bearing lubrication, and keeping the motor clean from dust and debris. With proper maintenance, these motors can operate reliably for many years, making them a stalwart choice for applications where minimizing downtime is critical.
Cost-effectivenessFrom a financial standpoint, squirrel cage motors tend to be more cost-effective both in initial purchase and operational costs compared to motors of comparable power but different designs. The straightforward manufacturing process of squirrel cage motors, coupled with their widespread availability, keeps their initial purchase price competitive. Additionally, their high efficiency and low maintenance requirements contribute to lower operational costs over the motor’s lifespan. When considering the total cost of ownership — including energy consumption, maintenance, and potential downtime — squirrel cage motors often emerge as a financially prudent option.
Compatibility with VFDs (Variable Frequency Drives)The compatibility of squirrel cage motors with VFDs is a crucial consideration for applications requiring versatile speed control. Fortunately, most squirrel cage motors can be used in conjunction with VFDs to achieve adjustable speed and torque control. This compatibility extends the motor’s application possibilities immensely. However, it’s important to ensure that the motor is rated for VFD use, as the high-frequency signals from a VFD can potentially cause additional stress on the motor insulation and bearings without proper design considerations.
Addressing these common concerns and questions provides a clearer picture of the capabilities, limitations, and practicalities of using squirrel cage motors in various contexts. With their proven efficiency, durability, and now adaptable speed control via VFDs, squirrel cage motors continue to be a fundamentally sound choice across a myriad of applications.
What Is Squirrel Cage Motor: Advantages and Disadvantages
The wide usage of squirrel cage motors across numerous sectors speaks volumes about their utility and effectiveness. However, like any technology, they come with their own set of advantages and drawbacks. Understanding these can help users and engineers alike in choosing the right motor for their specific needs, balancing between the benefits and limitations.
Pros of Using Squirrel Cage Motors, Including Robustness and SimplicityThe advantages of squirrel cage motors are rooted in their design and operational characteristics, which contribute to their popularity. One of the most significant benefits is their robustness. These motors are built to withstand harsh conditions, including dust, moisture, and fluctuating temperatures, without significant degradation in performance. This durability stems from their simple construction, which lacks brushes, slip rings, or other contact points prone to wear and tear, reducing the likelihood of failure and extending the motor’s lifespan.
Simplicity is another hallmark of the squirrel cage motor, contributing not just to its robustness but also to ease of use and maintenance. Their construction is straightforward, with fewer moving parts compared to other motor types. This simplicity translates to lower initial costs, minimal maintenance requirements, and ease of installation and operation. The lack of brushes means there’s no need for regular replacements or upkeep related to commutation components, further reducing operational costs.
Furthermore, squirrel cage motors are known for their high efficiency and self-starting capability. They can operate at a constant speed under varying loads, making them suitable for a wide range of applications without the need for complex control systems. Their efficiency, especially in larger-sized motors, ensures that energy consumption is kept to a minimum, contributing to lower running costs and a reduced environmental footprint.
Cons, Such as the Limited Ability to Control the SpeedDespite the numerous advantages, squirrel cage motors do have their limitations. The primary drawback stems from their inherent design, which traditionally allowed for limited speed control. Unlike DC motors or other AC motor types, the speed of a squirrel cage motor is determined by the frequency of the supply voltage and the motor’s construction. This characteristic made it challenging to use these motors in applications requiring precise speed adjustments or variable speed control.
While the introduction of variable frequency drives (VFDs) has mitigated this issue by allowing the speed of squirrel cage motors to be controlled more precisely, it does add complexity and cost to the motor system. VFDs require additional installation space, have their own maintenance needs, and can introduce electrical noise that may affect other equipment. Additionally, not all squirrel cage motors are designed to be VFD-compatible, especially older models, which means their speed still cannot be adjusted without potentially damaging the motor.
Another potential drawback is the starting current of squirrel cage motors, which can be significantly higher than their running current. This high inrush current can cause a voltage drop that might affect other equipment. While this issue can be addressed through the use of soft starters or VFDs, it does require consideration during the design phase of systems incorporating squirrel cage motors.
In summary, while squirrel cage motors offer considerable advantages in terms of robustness, simplicity, and efficiency, considerations around speed control and starting current highlight the need for careful planning and potential additional components like VFDs to fully realize their benefits in specific applications.
What Is Squirrel Cage Motor: Troubleshooting and Maintenance
The operational supremacy of squirrel cage motors in an array of settings is well-documented; however, as with any mechanical device, issues can arise over time. A robust troubleshooting strategy paired with diligent maintenance practices is vital in thwarting potential failures and prolonging the motor’s service life. The following expands on practices to pinpoint and rectify common issues, as well as routines for keeping squirrel cage motors in their prime operating condition.
Tips for Common Troubleshooting IssuesTroubleshooting squirrel cage motors involves keeping an eye out for symptoms that may indicate underlying issues. Here are several common problems and tips on how to address them:
Implementing a meticulous maintenance routine is crucial in avoiding breakdowns, ensuring efficiency, and prolonging the life of a squirrel cage motor. Here are recommended practices:
Regular Cleaning
: Keep the motor and its surroundings clean from dust, dirt, and debris that might obstruct the cooling airflow or accumulate on windings, potentially causing insulation failure or overheating.Lubrication
: Bearings require periodic lubrication to reduce friction and wear. Follow the manufacturer’s recommendations for lubrication type and schedule, and avoid over-lubrication which can cause overheating.By following these troubleshooting tips and adhering to a disciplined maintenance schedule, you can help ensure that your squirrel cage motor operates effectively, retains its inherent efficiency, and continues serving reliably for the duration of its designed life span. These proactive steps are an investment in performance stability, energy economy, and overall savings in terms of reducing the likelihood of costly unscheduled downtime and repairs.
What Is Squirrel Cage Motor: Recent Developments and Future Trends
The domain of squirrel cage motor technology has not remained static but has continuously evolved to incorporate new materials, designs, and control methodologies, enhancing performance and efficiency. Ongoing research and development efforts aim to push the boundaries of what these motors can achieve. This section delves into the recent advancements that have been pivotal in shaping the present capabilities of squirrel cage motors and speculate on future trends that might further transform their application spectrum.
Advancements in Squirrel Cage Motor TechnologyRecent years have witnessed significant technological advancements in squirrel cage motors that have contributed to their improved efficiency, reliability, and application versatility. A notable development is the integration of advanced computational tools in the design phase, enabling better optimization of motor parameters for specific applications. This computational approach allows for more precise control over the magnetic flux distribution within the motor, reducing energy losses and improving overall efficiency.
Material science has also played a crucial role in the evolution of squirrel cage motors. The adoption of high-grade electrical steels in the construction of stators and rotors has led to a reduction in core losses, a significant factor in motor efficiency. Additionally, the development and use of improved insulation materials have enhanced the thermal endurance of these motors, allowing them to operate at higher temperatures without the risk of insulation failure.
Another noteworthy advancement is the enhancement of cooling techniques. Improved cooling systems, including the use of external fans, heat exchangers, and advanced internal ventilation designs, have enabled squirrel cage motors to dissipate heat more effectively. This not only protects the motor components from overheating but also allows the motors to function efficiently under higher loads.
Moreover, the proliferation of Variable Frequency Drives (VFDs) has revolutionized how squirrel cage motors are controlled, particularly in terms of speed and torque. VFDs have enabled precise, energy-efficient control, making squirrel cage motors adaptable to a broader range of applications than ever before.
Predictions about How These Motors Might Evolve with New Materials or DesignsLooking ahead, the trajectory of squirrel cage motor technology suggests a continued focus on materials and design innovations that promise even greater efficiency and optimization for specific uses. One of the most exciting prospects is the potential use of superconducting materials in motor windings. Such materials could drastically reduce or even eliminate electrical resistance, dramatically enhancing efficiency and reducing energy losses.
Nanotechnology also holds promising predictions for the future of squirrel cage motors. Nano-engineered materials could be used to create lighter, stronger, and more thermally conductive components, which would increase the motor’s lifespan and allow for more compact motor designs.
Additionally, the integration of advanced sensors and IoT (Internet of Things) technologies into squirrel cage motors is envisioned to become more widespread. This integration would facilitate real-time monitoring and predictive maintenance, significantly reducing downtime and extending the motor’s useful life by preempting malfunctions before they lead to failure.
Another anticipated design evolution is the further optimization of the motor’s electromagnetic design, using sophisticated algorithms and machine learning to tailor motor characteristics precisely to specific operational requirements. This could lead to motors with better performance, customized for niche applications with unique demands.
In conclusion, the continuous advancements in squirrel cage motor technology highlight an industry that is dynamically evolving, driven by the pursuit of efficiency, reliability, and adaptability. As materials science and computational capabilities progress, squirrel cage motors are expected to become even more integral to industries reliant on electric motor solutions, embodying cutting-edge innovations that redefine performance standards.
Conclusion
Squirrel cage motors are undoubted workhorses of the modern world. Their unrivaled benefits and versatility in application underscore their importance in our daily lives and their potential to adapt to the dynamic technological landscape.
FAQs about What Is Squirrel Cage Motor
Q: Can squirrel cage motors operate at variable speeds?
A: Traditionally, squirrel cage motors were known for their constant speed operation, which is inherently linked to the supply frequency. However, with the advent and integration of Variable Frequency Drives (VFDs), it is now possible to control the speed of these motors accurately. By adjusting the frequency of the electrical supply, VFDs allow squirrel cage motors to operate over a wide range of speeds, thus expanding their application in processes requiring variable speed control.
Q: How do squirrel cage motors compare to slip ring motors in terms of efficiency?
A: Squirrel cage motors generally offer higher efficiency than slip ring motors due to their simpler and robust construction. The absence of brushes and slip rings reduces energy losses associated with friction and contact resistance. Additionally, squirrel cage motors typically have lower maintenance requirements, further enhancing their overall operational efficiency over time.
Q: What makes squirrel cage motors highly reliable?
A: The reliability of squirrel cage motors stems from their simple yet rugged construction. With fewer moving parts (notably the absence of brushes or slip rings), there’s less wear and tear, leading to a lower likelihood of failure. Additionally, their capability to operate in various environmental conditions without significant performance degradation contributes to their reputation for reliability.
Q: Are squirrel cage motors suitable for high-starting torque applications?
A: Squirrel cage motors typically have a lower starting torque compared to some other types of motors, such as slip ring motors. However, certain designs and configurations, such as double squirrel cage motors, are designed to improve starting torque while maintaining high efficiency during normal operation. For applications that require very high starting torque, careful selection of the motor or the use of external starting aids may be necessary.
Q: Can squirrel cage motors be used in explosive atmospheres?
A: Yes, squirrel cage motors can be designed and certified for use in explosive atmospheres. These motors are constructed with specific materials and protective measures to prevent the ignition of explosive gases or dust. Motors intended for such applications are designated with appropriate explosion-proof ratings, ensuring compliance with safety standards for hazardous environments.
Q: How long do squirrel cage motors typically last?
A: With proper installation, use, and maintenance, squirrel cage motors can have an extensive operational life, often spanning several decades. The exact lifespan depends on various factors, including the application, operating conditions, and adherence to recommended maintenance practices. Routine inspections and maintenance, such as bearing lubrication and checking for wear and tear, can significantly extend a motor’s service life.
Q: Can the efficiency of a squirrel cage motor be improved?
A: The efficiency of a squirrel cage motor can be optimized through several means. Choosing the correct motor size for the application to avoid underloading or overloading, using VFDs for precise speed control, ensuring proper maintenance, and retrofitting older motors with energy-efficient models are all strategies that can enhance motor efficiency. Additionally, ensuring that the motor operates under optimal load conditions can significantly impact its efficiency and energy consumption.
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Wound Rotor vs Squirrel Cage
14
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(Electrical)
(OP)
14 Dec 03 18:48Hi All,
I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
Thanks!
(Electrical)
15 Dec 03 05:06Hello OhioAviatorA wound rotor motor, with an appropriate secondary reistance starter is able to produce a high starting torque from zero speed through to full speed. This will result in a higher acceleration rate than you will achieve with a squirrel cage motor. The starting current will be lower and the motor will be able to start in loaded situations where a standard cage motor will not.The negatives, are that both the motor and the starter will require a lot more maintenance than a standard cage motor, and the purchased price is higher.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
(OP)
15 Dec 03 07:51Hi Mark,
Thank you for your reply to my question, I certainly appreciate it.
That leads me to my second question...
Can I achieve the benefits of a wound rotor motor (high starting torque w/ lower starting current) along with the added benefits of reduced maintenance by using a squirrel cage motor and an electronic soft starter?
Again, thanks for your help.
--John Ruble
(Electrical)
15 Dec 03 08:29Suggestion to the previous posting: The soft starter and induction motor will approximately accomplish the similar output to the wound rotor motor. However, the soft starter may be more expensive and more demanding on the service. Also, MTBF may be lower for the soft starter.
(Electrical)
(OP)
15 Dec 03 08:37Thank you jbartos,
I'm not sure about the MTBF being lower for a soft starter; they seem to be getting more and more reliable these days. But I do not have experience with medium voltage starters in the 1000 HP+ range, either.
Back to wound rotor motors...
Are liquid rheostat starters still the current, most reliable technology? If not, are there other types of wound rotor starters out there that are more reliable and less maintenance? If so, what manufacturers?
Again, thanks all for your help!
--John Ruble
(Electrical)
15 Dec 03 08:39Suggestion to the original posting marked ///\\\
I have a basic question regarding wound rotor motors vs squirrel cage motors in high horsepower (4,000 HP+) automobile shredder applications. Automobile shredders, like any large rock crusher, experience very high shock loading. Which type of motor is better suited for this application, and why?
///The mechanical load profile torque-speed needs to be known to be able to match the motor torque-speed characteristics. Assuming that high starting torque is required, then the squirrel-cage induction motor Nema Design D may be required. This could be the better solution than the wound-rotor induction motor since the motor may be DOL started, if the power distribution allows it, and it will be simpler to maintain than the wound-rotor induction motor. Normally, starting and operating conditions of this size of motors are simulated by using software, e.g. EMTP.\\\
2
electricpete(Electrical)
15 Dec 03 10:23I don’t have knowledge for comprehensive evaluation of your options compared to your application.
Considering only the motors themselves (not the other parts of the starting/control): I agree with jbartos the wound rotor motors require more maintenance. Also, in my experience we have much higher failure rate on wound rotor motors than on our squirrel cage motors (although the applications are not comparable… wound rotor motors are probably used in the more demanding applications).
(Electrical)
15 Dec 03 13:13Hello OhioAviatorAlthough you can certainly reduce the starting current by using a soft starter, you will not get the same starting effectiveness (Start torque/start current) using this method.If you begin with a standard cage motor, particularly at this size, your starting torque will be low compared to that achievable with a wound rotor motor. The initial start torque may be in the region of 100 - 120% at 600 - 800% start current as opposed to to say 250% current for 200% torque with the would rotor.When we apply a soft starter to a standard cage motor, we reduce the start current and also reduce the start torque by the current reduction squared.For equal start current, the wound rotor motor and secondary resistance starter will produce many times the start torque of the soft starter and cage motor.If you do not require a high start torque, then the soft starter and cage motor are definitely a very viable option. The reliability of correctly engineered soft start applications is very high. Some installations that I have been involved in are still operating correctly and without problems after twenty years.I suspect, that for this application, you will require a high start torque, hence the suggestion of the wound rotor machine. This is based on my experience, but would depend on your actual requirements and parameters.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
(OP)
15 Dec 03 15:12Hello Marke,
Thank you very much for your explanation of starting squirrel-cage motors vs wound-rotor motors. What you say makes perfect engineering sense, substantiated by my own personal observations in the field.
What is your opinion of solid-state wound-rotor motor starters? Are they more reliable than liquid rheostat starters? What about maintenance requirements? I've been looking at a couple of solid state wound-rotor motor starters from Benshaw, Inc. Any experience with these types of starters?
Again, many thanks!
--John Ruble
2
DougMSOE(Electrical)
15 Dec 03 15:20I have personally been to installations that used soft starters, and they were very sorry that they had them.
Like the previous posts, a wound rotor induction motor (WRIM) will have faster acceleration and an adjustable speed torque curve, which a squirrel cage motor (SCIM) does not have.
One important point that many SCIM manufactures do not understand is the KWH demand billing. What this means is that as the motor is crushing cars, there is a demand limit that you do not want to go over otherwise you will be paying substantially higher energy bills to compensate for that (for example) 5 second period that you went over that value.
Yes, you will get full load torque at standstill with about 70% of your full load (rated) starting current, provided that the liquid rheostat is operating correctly AND the brine solution is correct.
PS The best way to determine if the brine solution is of the correct concentration is to use a hydrometer (density test of the solution). Please take note that the concentration needs to be temperature compensated.
(Mechanical)
15 Dec 03 19:07Auto shredder motors see far more than shock loading, the main area which I did not see covered is the extended period of time where the load on the motor reduces the speed of the driven equipment. This is where the SCIM fail, they do not work well when punished at less than full speed.
I have seen many shredder motors and they are far beyond the capabilities of any other motor. The service factor is typically 2. Schorch make one of the best motors I have seen for this application. They are of course the wound rotor type. P&H may have a shredder motor also.
On SCIM motors the rotor will usally break up as a result of the rotor running slower than design.
(Electrical)
(OP)
16 Dec 03 08:14Thank you, all, and especially to DougMSOE and to Mendit. Both of you obviously have specific knowledge of the scrap metal processing industry (far more than I have, that's for sure!) and car shredders in particular. I originally come from the mining industry and while that industry has large crushers, I've never seen mining crushers (and motors) that take the punishment that car shredders do.
I think I'm convinced now that a WRIM really is the better choice for our applications in this industry.
One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters. Is this because you can continuously vary the resistance of a liquid rheostat while in operation? And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit? Does rotor resistance help limit KW Demand?
Again, thank you everyone!
--John R.
(Electrical)
16 Dec 03 09:04Suggestion to the previous posting marked ///\\\
One thing that still puzzles me though, is the fact that we still seem to purchase and use liquid rheostats instead of electronic starters.
///The cost and reasonable reliability including simplicity may be the main reasons.\\\
Is this because you can continuously vary the resistance of a liquid rheostat while in operation?
///Yes, in comparison with wound resistors; no in comparison with soft starters.\\\
And if so, what benefits are to be gained from running a WRIM with resistance in the rotor circuit?
///The resistors require less maintenance, can have better location, can be remotely controlled, etc.\\\
Does rotor resistance help limit KW Demand?
///In which context or comparison?\\\
///Please, notice that the above postings did not differentiate among squirrel-cage induction motor Nema Design letters. Nema design letter D has noticeably different characteristics from other Nema design letters. The Nema design letter D motor has substantially higher starting torque than other ones.
Another aspect that has not been addressed yet would be adding a flywheel. However, this would impact the operation of the crusher since it would have to start empty to minimize the starting torque unless the flywheel is disengageable. \\\
(Electrical)
4 Jan 04 19:41A Variable Speed Drive (VSD) will provide the most amount of torque per amp of any motor starting method available. If you choose a reputable supplier (of both motor and VSD), reliability will be much better than the WRM option.
An added bonus is that you will get full speed control.
(Electrical)
4 Jan 04 23:30Hello OhioAviator,
Although I fully agree with and support much of what has been said here, I believe you need to consider each application on a case-by-case basis, particularly if you are aiming achieve best performance with the lowest possible capital outlay.
Wound Rotor Motors are capable of producing high torque when controlled via a ‘properly engineered’ liquid resistance starter. Their torque/speed characteristics are also well suited to applications that present transient over-load conditions and this can be further enhanced in shredding applications through the addition of a flywheel to the mechanical system. On the down side they do require more maintenance than a Squirrel Cage Motors and this needs to be factored into the decision making process.
Medium (and High) Voltage Squirrel Cage Motors are notorious for their poor start performance, often exhibiting levels of Locked Rotor Torque less than rated or Full Load Torque. As has been advised by Marke the addition of a soft starter will act to reduce starting torque further and therefore it is likely problems will be experienced here. This might also explain comments by DougMSOE! On a more positive note some motor manufacturers will design and manufacture Medium Voltage Cage Motors to your specifications, thus allowing you (possibly) to utilize Full Voltage Starting techniques.
As suggested by TheDOG, a Cage Motor controlled via a Variable Speed Drive will provide the best overall performance......dollars permitting of-course!
If your clients are anything like mine (unlimited dollars are not available to them), an analysis of motor and load curves should assist to determine best starting method for the intended application.
Regards,
GGOSS
2
gsimson(Electrical)
5 Jan 04 02:45I will just add only one point.
With VFD with SQ. Cage motor performing satisfactorily there is a possibility of DC motors and Slip ring Motors becoming "Show case items" and limited to academics especially in large industries operating in dusty and humid atmosphere.
As on today there is no application which can not be satisfactorily performed by SC motor with VFD. Considering the maintenance, this option will workout cheaper in the long run
(Electrical)
(OP)
5 Jan 04 08:26"Thank You" to TheDog, GGoss, and GSimon for your replies to this thread. While I agree that a SCIM/VFD combination might produce starting torques comparable to a WRIM, the up-front capital costs tend to be prohibitively higher. (Remember, our world is dominated by bean counters who, by and large, look only to the end of the current fiscal quarter.)
What I don't have a good handle on, though, is how it is possible to continuously manage (control) KW Demand with a SCIM/VFD combination like I can a WRIM/Liquid Rheostat. Is it possible?
Thanks,
J Ruble
(Electrical)
5 Jan 04 09:32Total input power = motor output mechanical power plus motor/drive system losses.
Output mechanical power can be roughly controlled by adjusting speed for both the wound rotor and SCIM+VFD.
Losses would have to be considered for each system. I think the wound rotor will have increasing external (rheostat) losses as you attempt to adjust speed lower.
(Electrical)
5 Jan 04 12:08Ohio Aviator.
You should consider both, starting and full speed operation of the motor.
My opinion is that WRIM with a proper matched resistance will provide the higher starting torque with the lower KVA inrush.
At full speed operation, your load application could develop sudden high peak Torque overloads. A flywheel could provide the extra required torque combined with some resistance in the rotor circuit to allow the rotor to slip under a shock load and then gradually accelerate at operating speed again.
The kilowatt demand is due to load plus losses and you have to provide it since it is pure energy ( the flywheel, if you have one, will provide momentary kinetic energy but then the motor will return that energy while accelerating back to full speed ) A variable frequency driver will develop a Volt/Hertz electric input to the motor but the voltage drops following the frequency to avoid magnetic saturation, then your torque could be constant but not larger than under 60 Hz operation.
A Olalde.
(Electrical)
5 Jan 04 13:04The only advantage the wound rotor motor offers is during start up of high torque applications. External resisters are used in the rotor circuit to limit starting amps in the rotor. Once the wound rotor motor comes up to speed all external resistance is shorted out and the motor performs like a squirrel cage motor. Your application may not require high starting torque as I would assume the shredder is started unloaded, i.e. without a car in the shredder. Therefore the wound rotor motor may not be benificial. The Design class D squirrel motor are specifically design for surge amplications.
(Electrical)
5 Jan 04 14:11Having installed the 3 different types of motors in scrap yards WRIM,SCIM and Synch (and a few DC) the shock loading can take a motor from operation RPM to 1/4 of the at RPM in less than 2 seconds! Be advised, what is constant with scrap-- NOTHING! While in the theory world 'white goods' are only 'white goods', you may also have the "engine block" mixed in with them. By far a WRIM is totally the only motor that can handle 4* rated current for 30m seconds while the solid state world is in smoke. Been there and saw that. Reliabiliy is best when you can fix it youeself.
(Electrical)
(OP)
5 Jan 04 14:38Thank you, DougMSOE.
I'm finding myself pretty much in complete agreement with you, especially as time goes on and I gain more and more operating experience with these shredders. You are so right that 'white goods' ain't always 'white goods'. And I doubly agree with your statement that "Reliability is best when you can fix it yourself"! Been there, done that... too many times. I've spent way too many long nights and weekends learning how to fix the supposedly superior 'latest and greatest' electronic gadget, wishing I had a simple molded case circuit breaker and an across-the-line starter, instead.
Gentlemen (and ladies, too) thanks for your input on this thread. I think I have enough info for now. If a new posting comes up I'll check it out but I can't promise I'll respond to further postings.
Again, many thanks!
John R.
(Electrical)
8 Jan 04 03:16I know you've said you do not want any more emails, so please do not feel under obligation to replay.I was on holidays and missed the whole discussionApologies to jbartos, electricpete,dougmsoe, gsimson if I repeate what you have already said; you guys provided v.good replays.OhioAviator- you did not state at the beginning what is the application you are looking for, speed control or soft start ?Anyway, here is my contribution:Solid State Soft Start Systems reduce the available full voltage start torque when applied to SQ or WR induction motors. Hence applications requiring a maximum starting torque cannot be used with a soft starter. However, where the WR motor has been used to reduce and control the starting torque application of soft start control can be readily adopted.The characteristic of increasing torque with decreasing current frequently determines the selection of the WR motor when starting current limitations are severe. I.e if a power company regulations limits LRC to 1.5 FLC , WR motor would still produce a LRT of 1.5 FLT.Each addition of R in the rotor circuit reduces motor speed . Speed reduction is practical only to 50 % of synchronous speed. Beyond that speed becomes unstable because high slip characteristics are produced in the a rotor that operates with a high resistance in its circuit. For this reason application where speed is to be reduced <50% are used only where a constant loads are involved( bridge, trolley, crane)Normally, a WR motor is designed to operate with small slip and high efficiency at full load. So, when speed is reduced , slip increases and efficiency decreases.When operating at anything other then max speed the resistance control of WR motor consumes power that would otherwise be used to move the load. This inherent inefficiency of WR design has lead to its obsolescence in variable speed application.Consequently, applications requiring variable speed 3 ph motors now use a variable frequency drive (VFD) to adjust the speed of a standard SQ motor. The VFD electronically changes the frequency of the AC current supply , thus changing the synchronous speed of the motor. This has a huge benefits in power consumption over the resistance control.The other problem with application of WR motor in variable speed application is that if the secondary resistance value of each phase become unbalance, the vibratory torque is generated.WR motor can be applied on either constant speed or adjustable-speed drives. They are particularly suitable for smooth acceleration of loads in application which require high starting torque with low starting current or impact load (elevators, ventilating fans, printing press, pumps, compressors, conveyors, pulverizers, stokers, positive pressure blowers, crushers, shredders .In summary major reasons for selecting WR motors are:1. The load can be started at the max torque2. Large starting torque can be obtained in comparison with low starting current3. In case when starting frequency is too high for the thermal resistivity of the an ordinary SC motor4. In case GD2 of the load is too large for the thermal resistivity of an ordinary SC motor5. In case load requires cushion startingToday, owning to their complicated construction and hence high maintenance and cost, WR motors are used mainly in applications when relatively high starting torque is required, but in which the starting current may not exceed the FLC much-application 2 above.Note – in very large applications 8000 + kW a solid state slip energy recovery (SER) system in conjunction with the conventional slipring resistors is used to perform variable speed control.
(Electrical)
8 Jan 04 08:50Suggestion/question: It is not clear from the original posting whether or not the cost is a factor. Please, would you address that point? Any better solution will probably be much costlier; especially, considering modern power electronics options aligned with the squirrel-cage induction motors and wound rotor induction motor.
(Electrical)
(OP)
8 Jan 04 09:38Hello Aquarius and Mr. Bartos...
Thanks for your postings to this thread.
Aquarius - The primary parameters I'm looking for are current limited starting, kW Demand control, high slip capability to accommodate very high short term shock loads, and reliability with simplicity. Variable speed control really isn't a goal, just high slip capability.
JBartos - Cost isn't the primary factor, but it probably ranks second. Reliability in this particular application (automobile shredders) is paramount. Simplicity is also important, as we typically don't have highly skilled electrical technicians on the payroll.
As I eluded to previously, I think I'm pretty well convinced that the WRIM is the best option, primarily due to the severity of the duty required for our application. Automobile shredders are "crushers" taken to the extreme. As DougMSOE stated above, a typical automobile shredder is shredding light metal (car bodies) together with heavy chunks of metal (engine blocks). The impact shock loads are tremendous.
Again, thanks!
2
jraef(Electrical)
8 Jan 04 14:26OhioAviator,I am coming into this late due to the holidays as well, but I would like to add my opinion. I am a big proponent of MV solid state controls, but I, like GGOSS, consider each application on its own merits.I have applied SS starters to SCIM motors in many shredder applications successfully and they work just fine. However they were waste shredders not AUTO shredders. IMHO, in your application the WR motor is probably better for the shock load capability as mentioned by others. Many of the above posts refer to starting torque issues but may not realize that a shredder NEVER starts with the load already in it (the exception being hydraulic powered shredders, but thats a different discussion). You are looking for a reduction in starting torque since the shredder is always unloaded at start. The slip recovery capability of the WR motor can be "adjusted" by altering the resistance on the secondary, a very useful feature for your application. Look at it this way, a WR motor has all of the capabilities of Design A, B, C and D SCIM motors by simply changing the rotor resistance. With a SS starter on a SCIM, once it is at full output the starter has NO control of torque capability. Your motor is on its own with just the torque and slip recovery capabilities inherent to its design.One other possibility is what was mentioned by rlpuck on 1/5/04, the use of a Design D squirrel cage motor. It has the slip recovery capability equal to the maximum available in the WR. If you go that route, the RVSS starter would be useful to keep the starting torque to a minimum. If you don't already have a motor you may want to consider this. If you already have a WR motor it would not be worth swapping it out.By the way, don't be fooled by the Benshaw marketing of their "solid state wound rotor" control. It is just a SS starter for the stator, combined with a fixed resistor in the rotor, which is shorted out at full speed. If you are going to go with WR, have a competent control manufacturer build a starter and resistor control package that matches your needs. SS is kind of a waste on WR motors. Also IMHO liquid rheostat WR controllers are good for things like flow control etc., but the maintenance costs and care required to keep them running would make them poor candidates for your application. Search this forum for the term "Liquid Rheostat" and you will see several discussions pertaining to LR maintenance issues. Shredder operators are not known for being mindful of routine maintenance!
Quando Omni Flunkus Moritati
(Electrical)
(OP)
8 Jan 04 14:41Hello jraef,
Thanks for your post. And thanks for confirming my suspicions that at WRIM is probably the best choice for our application. And you're right... shredder operators are DEFINITELY not known for routine maintenance.
BTW... thanks for making me look on the internet for the translation to your latin phrase (curiosity, you know)!
Cheers
(Electrical)
11 Jan 04 01:55Suggestion: Visitfor:Squirrel-Cage Induction Motors - The most simple and reliable of all electric motors. Essentially a constant speed machine, which is adaptable for users under all but the most severe starting conditions. Requires little attention as there are no commutator or slip rings, yet operates with good efficiency.Wound-Rotor (Slip Ring) Induction motor - Used for constant speed-service requiring a heavier starting torque than is obtainable with squirrel cage type. Because of its lower starting current, this type is frequently used instead of the squirrel-cage type in larger sizes. These motors are also used for varying-speed-service. Speed varies with this load, so that they should not be used where constant speed at each adjustment is required, as for machine tools.Comment marked ///\\\Reviewing the above postings and agreeing with the above link, the squirrel-cage induction motor will be the best solution. The flywheel for smoother ride-throughs should not be ruled out.
(Electrical)
11 Jan 04 16:48With all due respect jbartos.
Squirrel cage motors have been used in the auto shredder applications, BUT the tons/KWH is less for a SCIM than for a WRIM, all things the same.
Unless you have actually been to and worked in an auto/industrial scrap yard where the cars/busses, etc are sherdded from their present form to 'half-dollar size' in less than 10 seconds the true appreciation of the WRIM will never be appreciated. I have worked in the heavy power industry for over 30+ years and watching a shredder do its job is amazing to say the least. A 20,000 HP motor driving a BFP, ID or FD fan is no comparison to what a 7,000HP motor goes through in 10 min of its job in a scrap yard.
Yes, a SCIM is good but will NEVER come close to taking the shock loadings that a WRIM does. Further, the WRIM can be made to operate very closly to the SCIM in this application.
PS I have installed about 10 shredder motors and have worked on over 50 of then.
DougMSOE
(Electrical)
5 Feb 04 19:59The metal type resistive controller for WRIM is more reliable than liquid rheostat starters. The controller consists of only three durable parts: stainless steel resistors, vacuum contactors and PLC. It’s reliable and maintenance free. PLC controls contactor close/open to switch resistor in/out to control the torque and speed. The controller allows the motor to deliver up to its breakdown torque to the driven load during starting or running.The controller is pre-programmed to meet your specific load characteristic
(Electrical)
5 Feb 04 20:19Metal type resistors tend to have a positive temperature constant causing the value of the resistors to increase with heat. Liquid type resisters have a negative temperature coefficient resulting in reducing resistance with heat.A wound rotor starter requires reducing resistance as the motor accelerates so there is an advantage in using the liquid type resistor.Best regards,
Mark Empson
http://www.lmphotonics.com
(Electrical)
5 Feb 04 21:06Suggestion: The Squirrel-Cage Induction Motor (SCIM) has some room for customization based on its intended application. SCIMs with NEMA Design Letter D can handle very demanding applications, e.g. openings of the rusty valve, punch presses, cranes, hoists, press brakes, shears, oil-well pumps, centrifugals, etc.Reference:Donald D. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, Inc., 1993,Section: Characteristics of Polyphase Induction Motors on page 20-33 objectively compares various motors characteristics.Visitetc. for more info
(Electrical)
6 Feb 04 10:08The resistor grid system has been used in several WRIM scrap yards and they do work reasonably well. The problem is that they become very hot and with all of the dust and fluff from the car seats you will have a fire. Been there and have installed the liquid rheostats that do not have the problem.
Further, the square footage req'd for the liquid Vs the grid system is much less about 60%. And with the grid system you need large contactors to control the speed torque curve such that the max torque can be placed at the resultant RPM from the load on the shredder.
Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job.
I do not care for any book that you might be able to cite that a SCIM is better than a WRIM in this application. There is NO COMPARISON in this application. A NEMA D does not even get close to what a WRIM can do in this application.
(Electrical)
8 Feb 04 06:01Question to the previous posting: Please, would you be more specific in engineering and design terms in your statement:
""Yes the SCIM does have some room for customization but does not have the flexibility that the WRIM needs to have to do its job."" specifically, if the WRIM flexibility is clarified?
(Electrical)
9 Feb 04 10:24jbartos, To your question, I hope that this is what you are asking.
"Please, would you be more specific in engineering and design terms in your statement:
I would ask that you might take a look as to what the difference is between a NEMA design A,B,C,D,and F of a 3ph SCIM. What actually is the difference?
Given all other parameters the same the one major difference is the metallurgy of the rotor bars, and they are not Cu, but a Cu alloy!!! The resistivity of the bars AND the shorting rings are what make the difference.
Now thak a look at the equivalent circuit for an induction motor, NOT the equivalent circuit cited to simplify the circuit such that the well known circle diagram is made. You will notice that the maximun power transfer theorm applies, not just by load but also by RPM!
This is why the WRIM does this job better than any other motor or for that matter a diesel engine, which has also been used.
I the WRIM the secondary resistance may be changed at will to a range of resistances 20 times that of a SCIM even mor if required.
(Electrical)
9 Feb 04 15:27The WRIM could work with variable external resistance values connected to the rotor circuit.
That feature makes it feasible to be adjusted to almost any desired Torque-Line current- slip characteristic.
Put a high external resistance and the inrush current will drop close or even under full load current and at the same time the resultant Torque becomes more current effective.
Adjusting the external resistance properly,you will get any desired “Design Performance” from a NEMA design A to a D and much more.
Adjust the external rotor resistance when the motor is running with load and the speed will be reduced with an increase in the rotor slip.
An ISCM has a fix rotor construction and after it is constructed it will have a fixed rotor resistance and performance.
The heat generated in the rotor winding is dissipated into the rotor of an ISCM. For a WRIM most of the rotor heat is dissipated in the external resistors. A NEMA design A (ISCM) has high efficiency at full load and low slip but very high inrush current and only 150% LRT. A NEMA design D (ISCM) has 275% LRT and 400% inrush but generates too much heat and low efficiency under load.
That makes a (WRIM) Wound Rotor Induction Motor performance very superior if compared to an (ISCM) Induction Short Circuited-rotor Motor (squirrel cage).
Costs of Maintenance and Initial investment are certainly much lower for an ISCM, that is why it is the first selection as far as it could be tailored properly to the load.
(Electrical)
9 Feb 04 18:55You've got it aolalde!!!
The only addition to your post is that the starting current of the WRIM with a liquid rheostat is usually set at 70 to 75 % of the full load running current (this is where the maximum torque is usually developed). Typically the liquid rheostat uses a solution of soda ash and temperature (more on the soda ash) to adjuse the Istart via the secondary resistance.
Further, the WRIN is made top operate with the resistance in the rotor circuit.
Have a Great Day!!!
Doug
(Electrical)
9 Feb 04 23:47WRIM with resistor grid / Liquid resistor is a "Load dependent" drive. The torque developed at various speeds is dependendent on load hooked. SCIM with VFD is load independent and developes PULL OUT TORQUE at ALL speeds.
The current requirement is based on the load and the power "wasted" in the resistor. With SCIM /VFD there is no wastage of power . The motor draws power only to the extent required. "Solid state going into smoke" is only hypothetical and certainly not required for real applications.
(Electrical)
10 Feb 04 15:23Since the secondary resistance can be increased or decreased as required to match, as you put it, the PULL OUT TORQUE, the load WRIM is load dependent. Certainly, as you describe the SCIM with a VFD is, within the limits of the machine, is load independent. And yoe the heat developed by the resistor bank is some what wasted unless like a few scrap yards have done is to provide heating and cooling as required. Un fortunately I HAVE SEEN FIRST HAND secondary energy pump back systems going into smoke as well as the front end drives. What is required to understand in this application of WRIM in scrap yards is,
1.)easy to fix with only simple parts and simple equipment.
2.)TOUGH BEYOND BELIEF i.e. shock, vibration are standard. Again PLCs have had boards shaken out of their cages.
3.)First cost a big factor
4.)Easy to understand
Ultimate reliability is achieved when you can fix what is broken with a Simpson 260, a screw driver and an adjustable wrench.
(Electrical)
10 Feb 04 16:34Don't leave out the duct tape!
"Venditori de oleum-vipera non vigere excordis populi"
(Electrical)
(OP)
11 Feb 04 08:54The screwdriver (if you don't mind the broken handle or broken shaft), and the adjustable wrench (if you don't mind the finishing nail sticking out of where the knurled shift adjuster lockscrew is supposed to be), and the duct tape (after you warm it up and peel it off the old furnaces and ductwork) are all three easily obtained from the shredder feed pile. The Simpson 260 will probably need to be purchased new (I probably wouldn't trust a meter that I dug out of the shredder pile; at least not more than once!). As DougMSOE says, we are VERY cost conscious!
In all serious folks, I had no idea that this thread would be so popular. Thank you for all the great and valuable input. I learned a great deal from this thread.
Best Regards,
John Ruble
BTW... I'm thoroughly convinced that a WRIM is all-round the best way to go for a scrap yard shredder application.
(Electrical)
11 Feb 04 22:04Spoken like a man forced to improvise on the job trying to get something operating before he is allowed to go home...
Been there, done that, got the T-shirt and the hat.
(Electrical)
12 Feb 04 21:05Questions: How much time/downtime does it take to replace worn out slip rings of WRIM, how soon do they get worn out, and how much does it cost?
(Electrical)
13 Feb 04 09:45jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
Some designs can be changed out inplace in about 4 hours, others inplace, 2 to 4 days.
(Electrical)
13 Feb 04 22:54One of the more informative and entertaining threads I've seen. Thanks to all.
Smaller scale similar problem is the chipper in a sawmill, high shock loads when a chunk of oak or hard maple goes through. We used a slightly oversize SCIM with a Benshaw SS. The chipper itself includes a large inertia mass which helps. I guess my rambling point is that while a WRIM is probably best, look to increase inertia if possible.
(Electrical)
13 Feb 04 23:17FACB25:
I am glad that you mentioned INERTIA, which is a resource that has been forgotten lately. It is interesting the way it works on heavy peak load applications.
(Electrical)
15 Feb 04 02:49Comment on DougMSOE (Electrical) Feb 13, 2004 marked ///\\\
jbartos,
That depends on;
The design of the slip ring assembly.
I have seen these motors in this duty last 20+ years with only cleaning and a air dry insulating varnish and no ring change outs.
///This 20+ years life-expectancy is probably what the automobile shredders WRIM need.\\\
Other machines with a poorly designed slip ring assembly and poor maintenance 5 years.
///The frequent current transients due to variable and impulsing automobile shredder loads reduces the slip ring life expectancy.\\\
Some designs can be changed out inplace in about 4 hours,
///4 hours for the slip ring replacement seems to be optimistically short time; especially, if the rotor balancing is included.\\\
others inplace, 2 to 4 days.
///This is more like it, if the good unionized workmanship is in place.\\\
(Electrical)
16 Feb 04 04:36Some of the requirements which are required to be met as indicated are better met by SqIM along with VFD.
Easy to fix- No rotor resistance, Rotor cables , Rotor contactors . Only a Sq. cage motor with 3 leads connected to VFD.
Simple parts & simple equipment - What is nmore simpler than SQIM?
Tough beyond belief ( Shock , Vibration) - Sliprings & rotor resistances are certainly not tough.
Cost - A slipring motor with rotor resistance / rotor panel is certainly costlier than a Sq. Cage motor & VFD.
Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?
ALL repeat ALL the applications can be met by SQIM & VFD at a cheaper cost both capital & running, with min. maintenace and much better speed /Torque control. Both Dc motor and WRIM will become museum pieces & for academic interests as stated earlier.
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
(Electrical)
16 Feb 04 12:54gsimson - I appreciate all your expert advice and opinions.I have to take exception with one thought process:"Easy to understand - The requirement is only for maintenance. When SqIM & VFD do not need any maintenance why every one need to understanmd ?"I wouldn't buy from anyone that told me I don't need to understand it because it's never gonna break
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
(Electrical)
17 Feb 04 22:02Suggestion to gsimson (Electrical) Feb 16, 2004 marked ///\\\
Only the cost of VFD is abnormally high for MV applications. Over a period of time this also will come down.
///ABB rep mentioned sometimes ago that the cost MV VFD for large HP application is coming down so that the LV VFD is not that much better for such applications.\\\
(Electrical)
18 Feb 04 16:14gsimon,You made some good points. I am a big promoter of VFDs, but I beg to differ with you here. As mentioned earlier in this thread, the APPLICATION is not appropriate for a MV VFD. I have been on startups and retrofit projects for plenty of MV VFDs; AB, ABB, Robicon and Ross Hill (now owned by Robicon), and I can report that my experiences lead me to believe that conditions need to be near prefect for them to operate reliably. By that I mean clean, air conditioned spaces, clean, reliable power, clean, knowledgeable operators and clean. If any of those conditions are less than perfect, the equipment tends to cause moe headaches that it is worth. Did I mention it needs to be clean?An Auto Shredder operation meets NONE of these requirements, and in fact fails them in with extreme prejudice. I have put low voltage VFDs in rock crusher applications for yerars, and only after 10+ years of exposure to them are they finally gaining acceptance by users. That said, I just had an "electrician" at a quarry connect 120VAC to the 4-20ma input on one of mine yesterday, blew up a 250HP VFD. He didn't know exactly what "speed reference signal" meant, he thought it was the Start button! Auto shredder operations are the same if not worse.
"Venditori de oleum-vipera non vigere excordis populi"
(Electrical)
18 Feb 04 16:27FACB25Your point about inertia is correct, except that most auto shredders were not designed around a flywheel effect as were chippers. You can't add significant inertia just by increasing the motor size, you need to add mass to the machinery. A wood chipper is inherently a high mass machine. You may have needed to increase HP a bit because in the old days, chippers handled occasional loads of scrap material. When a load came into them, inertia allowed them to chew it up, and the motor simply re-accelerated it after it was gone. Now chippers are generating revenue and are fed a continuous stream of material. The extra HP is sometimes needed on older machines to maintain speed of the mass (intertia) under that more constant loading.
"Venditori de oleum-vipera non vigere excordis populi"
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(Electrical)
19 Feb 04 04:48Thank you every one for your valuable comments.
Certainly VFDs are understood by engineers. When we are in the age of "Prevent Maintenance" VFDs are preferred as their reliability is very high with minimum or rather no maintenance. As on date the electricians may not understand and considering the reliabilty their understanding is not a must.
MV VFD is a general comment and not relevent to this thread.
As I mentioned 'all' applications, the cost factor was posted for information.
For isolated drives located far away with 1 electrician I agree that the SRIM with resistor is better option.
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This article provides an in-depth look at the often-encountered but rarely-discussed squirrel cage motor. We’ll explore what it is, how it works, its applications, and design types, and answer common questions revolving around its cost-effectiveness, durability, speed control, and performance.
Introduction: What is Squirrel Cage Motor?
At its simplest, a squirrel cage motor, or squirrel cage induction motor, is an electric motor that harnesses the principle of electromagnetic induction to generate motion. It’s a type of asynchronous motor, which means that the electric current in the rotor (the rotating part of the motor) needed to produce torque – or rotating force – is obtained by electromagnetic induction from the magnetic field of the stator winding (the stationary part of the motor). In short, it’s a workhorse enabling modern-day conveniences and industries through the conversion of electrical energy into mechanical energy, driving everything from ceiling fans to industrial machinery.
The moniker “squirrel cage” isn’t due to the motor’s affinity for our acorn-loving friends. Instead, it’s borrowed from the rotor’s construction that uncannily resembles a hamster wheel or, indeed, a squirrel cage. The rotor consists of conductive bars, aluminum or copper, arranged in a laminated cylinder, similar to how a small animal’s cage is constructed. Shorted (electrically connected) at both ends by conductive rings, the overall structure imitates a cage. The embedded bars in the rotor’s core are skewed to reduce magnetic hum and unnecessary induction heating, thus enhancing the motor’s efficiency.
Squirrel cage motors are ubiquitous, forming an integral part of various applications in a diverse expanse of industries. Often going unnoticed in day-to-day life, they power household appliances such as washing machines, air conditioners, refrigerators, and water pumps. From a broader perspective, industrial applications are even more prolific – ventilation systems, conveyor belts, industrial pumps, and compressors, to name just a few. This prevalence stems from their boundless advantages, such as their robustness, low-cost production, superior performance rate, and importantly, their minimal requirement for maintenance, owing to the absence of brushes, slip rings, or commutators. Furthermore, their considerable torque enables the swift start-up of appliances and machinery.
Given these compelling characteristics, it should come as no surprise that the squirrel cage motor has been an unwavering cornerstone in motor-driven systems since its inception, continuously evolving with technology’s stride. Now, having unraveled what it is and why it’s paramount, the succeeding segments will delve further into how this motor variety works and the essential aspects that characterize its performance.
What Is Squirrel Cage Motor: Basic Working Principle
The working principle of squirrel cage motors, fascinating in its simplicity yet profound in its application, hinges on the foundational laws of physics and electromagnetism. At their core, these motors employ the ingenious concept of electromagnetic induction to convert electrical energy into mechanical energy, facilitating movement in a myriad of devices and machinery that form the backbone of both industrial and domestic settings.
To understand how squirrel cage motors work, one must grasp the roles played by its two primary components: the stator and the rotor. The stator, an unmoving part of the motor, comprises a series of electromagnets arranged in a circle around the rotor. This arrangement is not merely structural but serves the pivotal function of generating a rotating magnetic field within the motor. When alternating current (AC) power is supplied to the stator, a magnetic field is created around each coil of wire within the stator. Due to the configuration and the phase difference in the AC supply to these coils, a rotating magnetic field is produced.
The rotor, placed inside the magnetic influence of the stator and being the moving part of the motor, is what gives the squirrel cage motor its name. Structured remarkably like the familiar rodent’s wheel, the rotor consists of aluminum or copper bars connected at both ends by rings of the same material, forming a ‘cage’. This construction is crucial because it allows the rotor to interact with the stator’s magnetic field through electromagnetic induction.
Electromagnetic induction, a principle discovered in the early 19th century by Michael Faraday, is the process by which a conductor moving through a magnetic field has an electric current induced within it. In the context of the squirrel cage motor, as the rotating magnetic field produced by the stator sweeps past the rotor bars, a current is induced in these bars due to electromagnetic induction. According to Lenz’s Law, the direction of this current is such that its magnetic effect opposes the cause, which is the moving magnetic field. Thus, the induced current generates its magnetic field in the rotor bars, which interacts with the stator’s rotating magnetic field. It’s this interaction – essentially a push-and-pull dynamic – that causes the rotor to move, in an attempt to catch up with the rotating magnetic field of the stator. However, an inherent characteristic of the induction motor is that the rotor can never actually ‘catch up’ to the stator field speed; this difference in speed is essential for continuous motor operation.
As straightforward as it appears, the electromagnetic induction process within a squirrel cage motor is a delicate balance of physical laws enabling efficient energy conversion. The elegant dance between the stator’s magnetic field and the induced currents in the rotor, mediated by the principles of electromagnetism, underpins the operation of these motors, making them a remarkably efficient means to drive mechanical loads. This understanding of the basic working principle sets the stage for exploring deeper aspects of the squirrel cage motor’s design, types, and applications in ensuing discussions.
What Is Squirrel Cage Motor: Construction and Design
The robustness and efficiency of squirrel cage motors, which have solidified their status as the workhorses of the electric motor family, are largely attributable to their construction and design. These motors are engineered to be simple yet effective, utilizing materials and a structure that lends to their durability and versatility.
Materials Used in Squirrel Cage Motor ConstructionThe choice of materials in the construction of squirrel cage motors is pivotal to their performance. Typically, the stator consists of a steel frame that houses a core made from stacked laminations of silicon steel, a material known for its excellent magnetic properties. This lamination helps in reducing eddy current losses, a form of energy dissipation that occurs due to the induction of currents in the core material itself.
For the rotor, aluminum or copper is used to make the conductive bars and end rings that define the ‘cage’. Aluminum, being lighter and cheaper, is commonly used, especially in standard motors, due to its adequate conductivity and lower rotor inertia, which is beneficial in applications requiring frequent starts and stops. Copper, offering superior conductivity, makes for a more efficient motor with lower energy losses, albeit at a higher cost and greater weight. The choice between these materials often hinges on the specific requirements of the application, including considerations of efficiency, cost, and operational dynamics.
The Distinctive Structure of the Squirrel Cage RotorThe rotor’s unique structure is a defining characteristic of the squirrel cage motor. Resembling a cylindrical cage, it consists of conductive bars laid parallel to the rotor’s shaft, connected at both ends by conductive rings. This configuration ensures a uniform response to the rotating magnetic field generated by the stator, allowing the rotor to spin smoothly and efficiently. The bars are often slightly skewed relative to the shaft’s axis, a setup designed to reduce magnetic humming for quieter operation and to minimize locking tendencies between the rotor and the stator’s magnetic fields.
Design Differences Between Squirrel Cage Motors and Other Types of Induction MotorsWhile the squirrel cage motor is an induction motor at heart, it’s distinct from its counterpart, the wound rotor motor, in several key aspects. The immediate difference lies in the rotor design; unlike the open-loop, wire-wound rotor of the latter, the squirrel cage rotor employs a closed-loop design. This fundamental distinction leads to variations in performance and application suitability. For instance, while squirrel cage motors are prized for their simplicity, ruggedness, and lower costs, wound rotor motors offer the advantage of the controllable speed and torque, making them preferable in applications demanding such functionality.
Moreover, the simplistic design of squirrel cage motors translates into lower maintenance requirements and enhanced reliability over wound rotor motors, which feature brushes and slip rings that can wear down over time. However, this same simplicity limits the squirrel cage motor’s speed control capabilities compared to the wound rotor motors, which can be adjusted through external resistance.
Through their construction and design, squirrel cage motors embody a balance of efficiency, durability, and cost-effectiveness that has cemented their place in myriad applications. The specific choice of materials, the unique design of the rotor, and the differences in construction compared to other induction motors collectively underpin the capabilities that make squirrel cage motors a ubiquitous choice in the realms of industrial and domestic machinery.
What Is Squirrel Cage Motor: Types and Variants
Though the foundational principle of squirrel cage motors is the same across various designs, their differences lie in size, power ratings, rotor bar architecture, and specific characteristics catering to diverse types of applications. Classifications of squirrel cage motors can be articulated based on these parameters.
Different Types of Squirrel Cage Motors Based on Power Ratings and SizesMuch like any other machine, squirrel cage motors come in a plethora of power ratings and sizes designed to meet varying application requirements. From tiny motors that power miniature fans within personal computers to gigantic ones moving heavy machinery in industries, their power ratings widely differ. Commonly, these motors are available with power ratings starting from a fraction of a watt to many megawatts.
The size of a squirrel cage motor goes hand-in-hand with its power rating. Larger motors possess more substantial amounts of conductive and magnetic materials, designed to handle higher current and magnetic field strengths, and thus can deliver more power. Conversely, smaller motors are suitable for applications where space is at a premium, and the power requirement is modest.
The Significance of Different Rotor Bar Designs (Deep Bar, Double Cage, etc.)The rotor bar design is a strategic factor in the function and efficiency of a squirrel cage motor, which can be altered to meet specific application needs. A generic squirrel cage rotor contains bars of uniform cross-section, but innovative design improvements, including deep bar and double cage rotors, are utilized in certain circumstances.
Deep bar rotors contain bars with a larger cross-sectional area at the bottom than at the top. The added depth gives these bars a higher resistance near the surface, reducing current during startup and allowing the motor to produce high starting torque without an excessive inrush of current.
Double cage rotors consist of two “cages” or sets of rotor bars, placed one inside the other, with differing electrical properties. The outer cage has a high resistance but low reactance, making it responsive during startup for high torque, while the inner cage possesses low resistance but high reactance, providing a steady state for high efficiency during regular operation.
Specialty Squirrel Cage Motors and Their ApplicationsCertain applications may demand specific motor characteristics not encapsulated by standard motors. Here, specialty squirrel cage motors come into the picture. Among these are explosion-proof squirrel cage motors, used in hazardous environments where flammable gases or dust can pose a risk of ignition. These motors are designed with robust enclosures that contain and isolate any internal electrical faults or sparks that could potentially ignite the surrounding explosive atmosphere.
On the other hand, braking motors are built with an inherent braking system that assists in quick stopping, making them suitable for applications requiring frequent stops/start-ups or precise positioning, such as elevators or hoisting equipment.
The versatility of the basic squirrel cage motor designs, and the ability to tweak them to meet specific requirements, highlights the sheer adaptability of this humble piece of equipment. As technology continues to evolve, so will the variants and innovations around squirrel cage motors, expanding their utility in multiple spheres.
What Is Squirrel Cage Motor: Common Applications
Squirrel cage motors are remarkably versatile, finding utility across a broad spectrum of applications, from the mundane to the highly specialized. Their inherent simplicity, reliability, and efficiency make them the go-to choice for numerous demands. This omnipresence can be seen in industrial settings, household appliances, and in operations amid hazardous environments.
Industrial Uses, Including Pumps, Fans, and Conveyor SystemsIn the industrial realm, squirrel cage motors are indispensable. They are the driving force behind a wide array of machinery and systems critical to production processes and facility management. For instance, these motors power pumps that are essential for the movement of fluids in sectors such as water management, chemical processing, and oil and gas. The choice of squirrel cage motors for pumps stems from their robustness and uninterrupted operation capability, crucial in environments where continuous flow is non-negotiable.
Fans and blowers, tasked with ventilation, cooling, or even air supply for combustion, heavily rely on squirrel cage motors for their operation. The simplicity and reliability of these motors make them suitable for long-term, continuous use—a staple requirement in HVAC systems and industrial cooling. Similarly, conveyor systems, which form the backbone of material handling in warehouses, mining, and manufacturing plants, also depend on these motors. The ability to produce a consistent torque reliably over time allows for the smooth functioning of conveyor belts, thus ensuring uninterrupted industrial operations.
The Role of Squirrel Cage Motors in Household AppliancesWithin the domestic sphere, squirrel cage motors quietly facilitate comfort and convenience, embedded within numerous household appliances. Their presence is felt in air conditioners and refrigerators, where their task revolves around compressing refrigerants or driving fans for air circulation. Washing machines and dishwashers also utilize these motors for agitating or rotating drums, providing the mechanical action necessary for cleaning. The prevalence of squirrel cage motors in these applications is largely due to their compact form, efficiency, and minimal maintenance they require—attributes highly valued in consumer appliances.
Their Use in Specialized Settings, Such as Hazardous EnvironmentsFurther extending their applicability, squirrel cage motors are specifically designed to operate in hazardous environments, highlighting their versatility. In settings where explosive gases, dust, or volatile chemicals pose significant risks, explosion-proof variants of these motors ensure safety alongside functionality. These specialized motors are constructed to prevent any internal spark or high temperature from igniting the external atmosphere, crucial in petrochemical plants, mines, and grain silos. The reliability and robustness of squirrel cage motors augment their suitability for such environments, providing a secure, efficient solution where standard motors would pose a significant hazard.
In essence, the widespread use of squirrel cage motors, from powering essential industrial machinery to seamlessly integrating within household appliances and ensuring safety in hazardous settings, underscores their fundamental role in modern technology and everyday life. Their adaptability across different applications is a testament to the ingeniously simple yet effective design that has made squirrel cage motors ubiquitous in both public and personal spheres.
What Is Squirrel Cage Motor: User Concerns and Questions
As ubiquitous as squirrel cage motors are in various applications, potential users or operators often have pertinent questions and concerns about their performance, operational characteristics, and cost implications. Understanding these elements can significantly influence decision-making processes regarding the selection and deployment of these motors.
Performance EfficiencyWhen discussing performance efficiency, squirrel cage motors are renowned for their high levels of operational efficiency, which often reach up to 90% under optimal loading conditions. This is largely due to their simple construction and the absence of brushes or slip rings, which minimize energy loss through friction and electrical resistance. Compared to other motor types, such as wound rotor motors or motors with mechanical commutators, squirrel cage motors offer an advantageous blend of efficiency and reliability. However, it’s worth noting that their efficiency can vary based on size, with larger motors generally being more efficient. While squirrel cage motors are not inherently the most efficient in every scenario, for most standard industrial and domestic applications, they provide an excellent balance between cost, efficiency, and durability.
Speed ControlSpeed control within squirrel cage motors is a topic of considerable interest, especially given the motor’s inherent design which naturally lends itself to a constant speed operation. Traditionally, squirrel cage motors are not as flexible in speed control compared to DC motors or wound rotor induction motors. However, with advancements in technology, speed control of these motors has become possible and increasingly sophisticated through the use of Variable Frequency Drives (VFDs). VFDs adjust the motor’s speed by varying the frequency of the electrical power supplied to the motor, allowing for flexible control over motor speed without significant losses in efficiency or power.
Maintenance and DurabilitySquirrel cage motors are esteemed for their durability and longevity. The simplicity of their design — lacking brushes and commutators — not only contributes to their efficiency but also reduces the potential points of failure, thus lowering maintenance requirements. General maintenance might include routine inspections, bearing lubrication, and keeping the motor clean from dust and debris. With proper maintenance, these motors can operate reliably for many years, making them a stalwart choice for applications where minimizing downtime is critical.
Cost-effectivenessFrom a financial standpoint, squirrel cage motors tend to be more cost-effective both in initial purchase and operational costs compared to motors of comparable power but different designs. The straightforward manufacturing process of squirrel cage motors, coupled with their widespread availability, keeps their initial purchase price competitive. Additionally, their high efficiency and low maintenance requirements contribute to lower operational costs over the motor’s lifespan. When considering the total cost of ownership — including energy consumption, maintenance, and potential downtime — squirrel cage motors often emerge as a financially prudent option.
Compatibility with VFDs (Variable Frequency Drives)The compatibility of squirrel cage motors with VFDs is a crucial consideration for applications requiring versatile speed control. Fortunately, most squirrel cage motors can be used in conjunction with VFDs to achieve adjustable speed and torque control. This compatibility extends the motor’s application possibilities immensely. However, it’s important to ensure that the motor is rated for VFD use, as the high-frequency signals from a VFD can potentially cause additional stress on the motor insulation and bearings without proper design considerations.
Addressing these common concerns and questions provides a clearer picture of the capabilities, limitations, and practicalities of using squirrel cage motors in various contexts. With their proven efficiency, durability, and now adaptable speed control via VFDs, squirrel cage motors continue to be a fundamentally sound choice across a myriad of applications.
What Is Squirrel Cage Motor: Advantages and Disadvantages
The wide usage of squirrel cage motors across numerous sectors speaks volumes about their utility and effectiveness. However, like any technology, they come with their own set of advantages and drawbacks. Understanding these can help users and engineers alike in choosing the right motor for their specific needs, balancing between the benefits and limitations.
Pros of Using Squirrel Cage Motors, Including Robustness and SimplicityThe advantages of squirrel cage motors are rooted in their design and operational characteristics, which contribute to their popularity. One of the most significant benefits is their robustness. These motors are built to withstand harsh conditions, including dust, moisture, and fluctuating temperatures, without significant degradation in performance. This durability stems from their simple construction, which lacks brushes, slip rings, or other contact points prone to wear and tear, reducing the likelihood of failure and extending the motor’s lifespan.
Simplicity is another hallmark of the squirrel cage motor, contributing not just to its robustness but also to ease of use and maintenance. Their construction is straightforward, with fewer moving parts compared to other motor types. This simplicity translates to lower initial costs, minimal maintenance requirements, and ease of installation and operation. The lack of brushes means there’s no need for regular replacements or upkeep related to commutation components, further reducing operational costs.
Furthermore, squirrel cage motors are known for their high efficiency and self-starting capability. They can operate at a constant speed under varying loads, making them suitable for a wide range of applications without the need for complex control systems. Their efficiency, especially in larger-sized motors, ensures that energy consumption is kept to a minimum, contributing to lower running costs and a reduced environmental footprint.
Cons, Such as the Limited Ability to Control the SpeedDespite the numerous advantages, squirrel cage motors do have their limitations. The primary drawback stems from their inherent design, which traditionally allowed for limited speed control. Unlike DC motors or other AC motor types, the speed of a squirrel cage motor is determined by the frequency of the supply voltage and the motor’s construction. This characteristic made it challenging to use these motors in applications requiring precise speed adjustments or variable speed control.
While the introduction of variable frequency drives (VFDs) has mitigated this issue by allowing the speed of squirrel cage motors to be controlled more precisely, it does add complexity and cost to the motor system. VFDs require additional installation space, have their own maintenance needs, and can introduce electrical noise that may affect other equipment. Additionally, not all squirrel cage motors are designed to be VFD-compatible, especially older models, which means their speed still cannot be adjusted without potentially damaging the motor.
Another potential drawback is the starting current of squirrel cage motors, which can be significantly higher than their running current. This high inrush current can cause a voltage drop that might affect other equipment. While this issue can be addressed through the use of soft starters or VFDs, it does require consideration during the design phase of systems incorporating squirrel cage motors.
In summary, while squirrel cage motors offer considerable advantages in terms of robustness, simplicity, and efficiency, considerations around speed control and starting current highlight the need for careful planning and potential additional components like VFDs to fully realize their benefits in specific applications.
What Is Squirrel Cage Motor: Troubleshooting and Maintenance
The operational supremacy of squirrel cage motors in an array of settings is well-documented; however, as with any mechanical device, issues can arise over time. A robust troubleshooting strategy paired with diligent maintenance practices is vital in thwarting potential failures and prolonging the motor’s service life. The following expands on practices to pinpoint and rectify common issues, as well as routines for keeping squirrel cage motors in their prime operating condition.
Tips for Common Troubleshooting IssuesTroubleshooting squirrel cage motors involves keeping an eye out for symptoms that may indicate underlying issues. Here are several common problems and tips on how to address them:
Implementing a meticulous maintenance routine is crucial in avoiding breakdowns, ensuring efficiency, and prolonging the life of a squirrel cage motor. Here are recommended practices:
Regular Cleaning
: Keep the motor and its surroundings clean from dust, dirt, and debris that might obstruct the cooling airflow or accumulate on windings, potentially causing insulation failure or overheating.Lubrication
: Bearings require periodic lubrication to reduce friction and wear. Follow the manufacturer’s recommendations for lubrication type and schedule, and avoid over-lubrication which can cause overheating.By following these troubleshooting tips and adhering to a disciplined maintenance schedule, you can help ensure that your squirrel cage motor operates effectively, retains its inherent efficiency, and continues serving reliably for the duration of its designed life span. These proactive steps are an investment in performance stability, energy economy, and overall savings in terms of reducing the likelihood of costly unscheduled downtime and repairs.
What Is Squirrel Cage Motor: Recent Developments and Future Trends
The domain of squirrel cage motor technology has not remained static but has continuously evolved to incorporate new materials, designs, and control methodologies, enhancing performance and efficiency. Ongoing research and development efforts aim to push the boundaries of what these motors can achieve. This section delves into the recent advancements that have been pivotal in shaping the present capabilities of squirrel cage motors and speculate on future trends that might further transform their application spectrum.
Advancements in Squirrel Cage Motor TechnologyRecent years have witnessed significant technological advancements in squirrel cage motors that have contributed to their improved efficiency, reliability, and application versatility. A notable development is the integration of advanced computational tools in the design phase, enabling better optimization of motor parameters for specific applications. This computational approach allows for more precise control over the magnetic flux distribution within the motor, reducing energy losses and improving overall efficiency.
Material science has also played a crucial role in the evolution of squirrel cage motors. The adoption of high-grade electrical steels in the construction of stators and rotors has led to a reduction in core losses, a significant factor in motor efficiency. Additionally, the development and use of improved insulation materials have enhanced the thermal endurance of these motors, allowing them to operate at higher temperatures without the risk of insulation failure.
Another noteworthy advancement is the enhancement of cooling techniques. Improved cooling systems, including the use of external fans, heat exchangers, and advanced internal ventilation designs, have enabled squirrel cage motors to dissipate heat more effectively. This not only protects the motor components from overheating but also allows the motors to function efficiently under higher loads.
Moreover, the proliferation of Variable Frequency Drives (VFDs) has revolutionized how squirrel cage motors are controlled, particularly in terms of speed and torque. VFDs have enabled precise, energy-efficient control, making squirrel cage motors adaptable to a broader range of applications than ever before.
Predictions about How These Motors Might Evolve with New Materials or DesignsLooking ahead, the trajectory of squirrel cage motor technology suggests a continued focus on materials and design innovations that promise even greater efficiency and optimization for specific uses. One of the most exciting prospects is the potential use of superconducting materials in motor windings. Such materials could drastically reduce or even eliminate electrical resistance, dramatically enhancing efficiency and reducing energy losses.
Nanotechnology also holds promising predictions for the future of squirrel cage motors. Nano-engineered materials could be used to create lighter, stronger, and more thermally conductive components, which would increase the motor’s lifespan and allow for more compact motor designs.
Additionally, the integration of advanced sensors and IoT (Internet of Things) technologies into squirrel cage motors is envisioned to become more widespread. This integration would facilitate real-time monitoring and predictive maintenance, significantly reducing downtime and extending the motor’s useful life by preempting malfunctions before they lead to failure.
Another anticipated design evolution is the further optimization of the motor’s electromagnetic design, using sophisticated algorithms and machine learning to tailor motor characteristics precisely to specific operational requirements. This could lead to motors with better performance, customized for niche applications with unique demands.
In conclusion, the continuous advancements in squirrel cage motor technology highlight an industry that is dynamically evolving, driven by the pursuit of efficiency, reliability, and adaptability. As materials science and computational capabilities progress, squirrel cage motors are expected to become even more integral to industries reliant on electric motor solutions, embodying cutting-edge innovations that redefine performance standards.
Conclusion
Squirrel cage motors are undoubted workhorses of the modern world. Their unrivaled benefits and versatility in application underscore their importance in our daily lives and their potential to adapt to the dynamic technological landscape.
FAQs about What Is Squirrel Cage Motor
Q: Can squirrel cage motors operate at variable speeds?
A: Traditionally, squirrel cage motors were known for their constant speed operation, which is inherently linked to the supply frequency. However, with the advent and integration of Variable Frequency Drives (VFDs), it is now possible to control the speed of these motors accurately. By adjusting the frequency of the electrical supply, VFDs allow squirrel cage motors to operate over a wide range of speeds, thus expanding their application in processes requiring variable speed control.
Q: How do squirrel cage motors compare to slip ring motors in terms of efficiency?
A: Squirrel cage motors generally offer higher efficiency than slip ring motors due to their simpler and robust construction. The absence of brushes and slip rings reduces energy losses associated with friction and contact resistance. Additionally, squirrel cage motors typically have lower maintenance requirements, further enhancing their overall operational efficiency over time.
Q: What makes squirrel cage motors highly reliable?
A: The reliability of squirrel cage motors stems from their simple yet rugged construction. With fewer moving parts (notably the absence of brushes or slip rings), there’s less wear and tear, leading to a lower likelihood of failure. Additionally, their capability to operate in various environmental conditions without significant performance degradation contributes to their reputation for reliability.
Q: Are squirrel cage motors suitable for high-starting torque applications?
A: Squirrel cage motors typically have a lower starting torque compared to some other types of motors, such as slip ring motors. However, certain designs and configurations, such as double squirrel cage motors, are designed to improve starting torque while maintaining high efficiency during normal operation. For applications that require very high starting torque, careful selection of the motor or the use of external starting aids may be necessary.
Q: Can squirrel cage motors be used in explosive atmospheres?
A: Yes, squirrel cage motors can be designed and certified for use in explosive atmospheres. These motors are constructed with specific materials and protective measures to prevent the ignition of explosive gases or dust. Motors intended for such applications are designated with appropriate explosion-proof ratings, ensuring compliance with safety standards for hazardous environments.
Q: How long do squirrel cage motors typically last?
A: With proper installation, use, and maintenance, squirrel cage motors can have an extensive operational life, often spanning several decades. The exact lifespan depends on various factors, including the application, operating conditions, and adherence to recommended maintenance practices. Routine inspections and maintenance, such as bearing lubrication and checking for wear and tear, can significantly extend a motor’s service life.
Q: Can the efficiency of a squirrel cage motor be improved?
A: The efficiency of a squirrel cage motor can be optimized through several means. Choosing the correct motor size for the application to avoid underloading or overloading, using VFDs for precise speed control, ensuring proper maintenance, and retrofitting older motors with energy-efficient models are all strategies that can enhance motor efficiency. Additionally, ensuring that the motor operates under optimal load conditions can significantly impact its efficiency and energy consumption.
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