Engineering Tutorial

Difference Between Squirrel Cage and Slip Ring Motors

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
Starting Current	High	Low
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
Cost	Low	High