An induction motor is an electrical device that converts electrical energy into mechanical energy. It is most widely used for industrial applications due to its self-starting attribute. Slip ring induction motor is one of the types of 3-phase induction motor and is a wound rotor motor type. Because of various advantages like low initial current, high starting torque, and improved power factor, it is used in applications that require high torque, cranes, and elevators. The rotor windings consist of more number of windings, higher induced voltage, and less current compared to the squirrel-cage rotor. The windings are connected to external resistance through slip rings, which helps to control the torque/speed of a motor.
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Definition: A slip ring induction motor is referred to as an asynchronous motor as the speed at which it operates is not equal to the synchronous speed of a rotor. The rotor of this type of motor is wound type. It comprises of a cylindrical laminated steel core and a semi-closed groove at the outer boundary to accommodate a 3-phase insulated winding circuit.
As seen in the figure above, the rotor is wound to match the number of poles on the stator. The three terminals of a rotor and three start terminals connecting through slip rings are connected to a shaft. The aim of the shaft is to transmit mechanical power.
Before we discuss the working principle of slip ring Induction motor, knowing the slip ring induction motor construction is important. So let’s begin with the construction which includes two parts: Stator and Rotor.
The stator of this motor comprises of various slots that are arranged to support the construction of a 3-phase winding circuit connecting to a 3-phase AC source.
The rotor of this motor consists of a cylindrical core with steel laminations. Besides this, the rotor has parallel slots to accommodate 3-phase windings. The windings in these slots are arranged at 120 degrees to each other. This arrangement can reduce noise and avoid irregular pausing of a motor.
This motor runs on the principle of Faraday’s law of electromagnetic induction. When a stator winding is excited with an AC supply, the stator winding produces magnetic flux. Based on faraday’s law of electromagnetic induction, the rotor winding gets induced and generates a current of magnetic flux. This induced EMF develops torque that enables the rotor to rotate.
However, the phase difference between the voltage and current do not meet the requirements to generate high starting torque as torque developed is not unidirectional. The external resistance of high value is connected with the circuit to improve the phase difference of a motor. As a result, inductive reactance and phase difference between I and V is reduced. Consequently, this reduction helps the motor to generate high stating torque. The slip ring induction motor diagram is shown below.
Slip is defined as the difference between the flux speed and the rotor speed. For an induction motor to produce torque, at least some difference should be there between stator field speed and rotor speed. This difference is called ‘slip’. The Slip Ring” is an electromechanical device that aids in transmitting power and electrical signals from stationary to a rotating component.
Slip rings are also known as rotary electrical interfaces, electric rotary joints, swivels, or collector rings. Sometimes, based on the application, the slip ring requires higher bandwidth to transmit data. Slip rings improve the efficiency and performance of a motor by improving system operation and eliminating wires that are dangling from motor joints.
The peak torque occurs if
r = Smax. X —— (I)
Where, Smax = Slip at pull-out torque
X = Inductance of a rotor
r = resistance of rotor winding
Adding external resistance R to equation (I),
r+R= (Smax)’. X —— (ii)
From equation (i) and (ii),
R = r(S’ max/Smax – 1) —— (iii)
By definition of Smax, we get Smax = 1 – (Nmax/Ns) —— (iv)
Putting S’max =1 in equation (iii), we get
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R = r. (1/Smax-1) —— (v)
Let’s say, Ns = synchronous speed of 1000rpm and pull-out torque happens at 900 rpm, equation (iv) reduces to Smax = 0.1 (i.e., 10% slip)
Substitute in equation (v),
R = r. (1/0.1 – 1)
R = 9. r
‘r’ is measured using a multimeter. The resistance value of 9 times higher than of a slip ring rotor resistance is connected externally to experience maximum starting torque.
The speed control of this motor can be done using two methods which include the following.
Generally, the initiation of these motors occurs when it draws full line voltage which is 6 to 7 times higher than the full load current. This high current can be controlled by external resistance connected in series with the rotor circuit. The external resistance acts as a variable rheostat during the motor kick-off and tweaks automatically to high resistance to get required starting current.
The external resistance reduces high resistance as soon as the motor obtains normal speed and increases the starting torque of a motor. The tweaking of external resistance also aids in decreasing of rotor and stator current but improves the power factor of a motor.
Thyristor On/Off circuit is another way to control the speed of a motor. In this method, rotor AC current is connected to a 3-phase bridge rectifier and connected to external resistance through a filter. The thyristor is connected across external resistance and is switched on/off at high frequency. The ratio of on-time to off-time estimates the actual value of rotor circuit resistance that helps in controlling the speed of a motor by controlling speed-torque characteristics.
The difference between these two motors is discussed below.
Slip Ring Motor Squirrel Cage Motor It has a rotor of wound type Its rotor is of squirrel cage type Rotor has cylindrical core has parallel slots, in which each slot has a bar Slots are not parallel to each other Construction is complicated because of slip rings and brushes Construction is simple External resistance circuit is connected with a motor No external resistance circuit as bars of the rotor is completely slotted Starting torque is high Torque is low Efficiency is low Efficiency is highThe advantages are
The disadvantages are
Some of the applications of slip ring induction motor are
1). What is slip in an electric motor?
Slip is defined as the difference between synchronous speed and operating speed, at the same frequency.
2). Where are squirrel cage induction motors used?
They are used in centrifugal pumps, large blowers, and fans, to run conveyor belts, etc.
3). What is a slip ring induction motor?
A motor with a wound-type rotor is known as a slip ring induction motor. Also, rotor windings are connected through slip rings to external resistance.
4). Name one disadvantage of slip ring induction motor and squirrel cage induction motor
The disadvantages are high copper losses and the low torque
5). What is the use of external resistance in slip ring induction motors?
The external resistance acts as a variable rheostat during the motor kick-off and tweaks automatically to high resistance to get required starting current.
Thus, this article discusses an overview of slip ring induction motor, the difference between slip ring induction motor and squirrel cage induction motor, applications, advantages, and disadvantages. Here is a question for you, what is the function of a slip ring induction motor?
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