Advantages and Disadvantages of Slip Ring Induction Motor

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The advantages are Slip Ring in Motors

1-Heavy beginning torque with great support for heavy inertia loads .

2-Due to external resistance, it has a low beginning current.

3-Able to handle currents that are 6&#;7 times greater at full load .

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The disadvantages are Slip Ring in Motors

1-includes more expensive maintenance since brushes and slide rings are used instead of squirrel cage motors.
2-Intricate construction

3-High copper loss

4-pricey compared to a three-phase squirrel cage induction motor

A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.[1]

Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator. As a result, the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.

The speed and torque characteristics of a wound-rotor motor can be adjusted by changing the external resistance, unlike a squirrel cage motor which has a fixed characteristic. This is useful for speed control of the motor.[1]

A wound-rotor motor can be used in several forms of adjustable-speed drive. Common applications include hoists and conveyor systems. Certain types of variable-speed drives recover slip-frequency power from the rotor circuit and feed it back to the supply, allowing wide speed range with high energy efficiency. Doubly-fed electric machines use the slip rings to supply external power to the rotor circuit, allowing wide-range speed control. Today speed control by use of slip ring motor is mostly superseded by induction motors with variable-frequency drives.

References

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1. a b

   Harold J. Herbein Rotating Machinery, Rinehart Press, , SBN 03--7, pages 215-218

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