One of the main drawbacks of the squirrel cage induction motor is that it has very low starting torque. Hence, for applications requiring high starting torques, the slip ring motor is the only choice.
However, by providing a double squirrel cage rotor to the induction motor, the starting torque of the motor can be increased.
The double squirrel cage induction motor consists of a rotor which has two cages. The outer cage consists of rotor bars with high reactance and low resistance. The inner cage, on the other hand, consists of rotor bars with low reactance and high resistance.
When the motor is started, the slip is high. As a result, the frequency of the currents induced in the rotor is high. This causes the rotor currents to flow in the outer cage. Thus the torque of the motor increases due to the high initial resistance in the outer cage.
When the motor reaches the rated speed, the slip between the stator and the rotor decreases and the frequency of the rotor currents drop. This causes the reactance of the inner cage to drop. Thus the current flows through the inner cage.
During normal operation, current flows through both the inner and the outer cages as the reactance is very low.
Thus, the double cage induction motor provides excellent speed torque characteristics during start-up.
The double cage induction motor is used in applications such as mixers and crushers in the industry where high initial torque is necessary. It is cheaper than the slip ring motor and does not require the complicated and costly starter circuits like the slip ring motor.