Equivalent Circuit of Double Squirrel-Cage Motor
Equivalent Circuit of Double Squirrel-Cage Motor:
Fig. (1) shows a section of the double squirrel cage motor. Here Ro and Ri are the per phase resistances of the outer cage winding and inner cage winding whereas Xo and Xi are the corresponding per phase standstill reactances. For the outer cage, the resistance is made intentionally high, giving a high starting torque.
For the inner cage winding, the resistance is low and the leakage reactance is high, giving a low starting torque but high efficiency on load. Note that in a double squirrel cage motor, the outer winding produces the high starting and accelerating torque while the inner winding provides the running torque at good efficiency.
Fig. (2 (i)) shows the equivalent circuit for one phase of double cage motor referred to stator. The two cage impedances are effectively in parallel. The resistances and reactances of the outer and inner rotors are referred to the stator. The exciting circuit is accounted for as in a single cage motor.
If the magnetizing current (I0) is neglected, then the circuit is simplified to that shown in Fig. (2 (ii)). From the equivalent circuit, the performance of the motor can be predicted. Total impedance as referred to stator is