The induction motors are widely used in industrial applications. Induction motor can be classified as squirrel cage induction motor, and slip ring or wound rotor induction motor and also linear induction motor.
Squirrel cage induction motor costs nearly one third of a DC motor of the same rating. It is extremely rugged and required practically no maintenance. Can be built for higher speeds, torques and power ratings.
A Slip Ring Induction Motor (SRIM), also known as a wound rotor induction motor, is a type of three-phase induction motor where the rotor is wound with insulated wire and connected to external resistance through slip rings and brushes. Unlike the squirrel cage rotor, which is permanently short-circuited, the SRIM allows external access to the rotor circuit. The stator of SRIM is similar to any three-phase induction motor, consisting of laminated steel core and three-phase windings. The rotor is also wound with three-phase windings, which are connected to slip rings mounted on the rotor shaft. Through these slip rings, external resistances can be inserted into the rotor circuit during operation.
The slip ring induction motor has a number of disadvantages compared to squirrel cage motor such as
The stator side control methods such as stator voltage control, V/f control and stator current control are applicable to both squirrel cage and slip ring induction motors. Additionally, speed control of slip ring induction motor can be achieved using rotor side control. Rotor side control method is very simplest and oldest method. The speed can be controlled by mechanically varying rotor circuit rheostat.
The main feature of this machine is that slip power becomes easily available from the slip rings, which can be electronically controlled to control speed of the motor. For limited range speed control applications, where the slip power is only a fraction of the total power rating of the machine, the converter cost should be reduced. The main applications of slip power recovery drives are,