A wound-rotor motor (Slip Ring Motor) is a type of induction motor where the rotor windings are connected through slip rings to external resistances. 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.
Squirrel Cage Motors have a heavy winding made up of solid bars, usually aluminum or copper, joined by rings at the ends of the rotor. When one considers only the bars and rings as a whole, they are much like an animal’s rotating exercise cage, hence the name.
|PROPERTY||SQUIRREL CAGE MOTOR||SLIP RING MOTOR|
|Rotor Construction||Bars are used in rotor. Squirrel cage motor is very simple, rugged and long lasting. No slip rings and brushes||Winding wire is to be used. Wound rotor required attention. Slip ring and brushes are needed also need frequent maintenance.|
|Starting||Can be started by D.O.L., star-delta, auto transformer starters||Rotor resistance starter is required.|
|Starting torque||Low||Very high|
|Speed variation||Not easy, but could be varied in large steps by pole changing or through smaller incremental steps through thyristors or by frequency variation.||Easy to vary speed. Speed change is possible by inserting rotor resistance using thyristors or by using frequency variation injecting emf in the rotor circuit cascading.|
|Maintenance||Almost ZERO maintenance||Requires frequent maintenance|