Application of Squirrel Cage And Slip Ring Induction Motor

There are two types of 3 phase induction motor. These are squirrel cage induction motor & slip ring induction motor. We shall discuss the application of squirrel cage induction motor & slip ring induction motor in this article. The principle of working of squirrel cage and slip ring induction motor is based on Faraday’s law of electromagnetic induction. The torque of the induction motor depends on the rotor resistance.

The rotor of a squirrel cage induction motor is short-circuited by the end rings and it is not possible to connect external resistance to the rotor winding. Whereas, it is possible in slip ring induction motor.

The Squirrel cage induction motor is simple, rugged and reliable. However, the motor can’t produce as much starting torque as the slip ring motor can produce. The squirrel cage induction motor is used for driving the loads that demand low starting torque.

The torque of the induction motor is,

T=slip*Es*Ir*cosΦ

cosΦ=Rr/√(Rr^2+s*Xr^2)

The power factor of the squirrel cage induction motor is very low at the start because the rotor is highly inductive. The power factor of the motor improves as the speed of the motor increase.

The starting torque of the squirrel cage motor is low, therefore the motor is not suitable for the applications that demand higher starting torque. The starting torque of the squirrel cage induction motor can be improved by use of double cage rotor.

Applications of Squirrel Cage Induction Motor

The squirrel cage induction motor is suitable for the application where;

The moderate starting torque is required
The constant speed is required.( The speed can be controlled through VFD). The induction motor is fairly a constant speed motor.

Applications of Slip ring Induction Motor

The large inertial loads like rolling mills, bucket elevators, cranes demand higher starting torque, and for driving such loads the slip ring motor is a perfect choice. A slip ring induction motor can produce 250–300 % starting torque.

The torque of the slip ring induction motor,

T=slip*Es*Ir*cosΦ

cosΦ=(Rr+R)/√(Rr+R)^2+s*Xr^2)

R- External resistance

The external resistance is added to the rotor conductor to improve the starting torque of the motor.

application of slip ring induction motor

The external resistance improves the power factor, reduce the starting current and as a result improves the starting torque.The maximum resistance is added to the rotor resistance to produce higher starting torque and, the resistance is decreased as the motor accelerates towards its base speed. At the base speed, the rotor conductor is short-circuited by the shorting contactor of Liquid rotor starter( LRS )/ Grid Rotor Resistance(GRR).

The slip ring induction motor is suitable for the application where,

The applications which demand higher starting torque needs a slip ring induction motor. Therefore, slip ring induction motor is suitable for driving high inertia loads.
The speed control is required ( the speed can be controlled through Grid resistance regulator. Also, there is substantial power saving if used for driving the centrifugal fans and pumps. The other option of power saving is the slip power recovery system. The rotor copper loss or the slip power is recovered and back feed to the mains. Thus, with the use of SPRS, the total recovery of the slip power is possible. The SPRS is more efficient than GRR.