The starting torque can be improved by increasing the rotor resistance but in case of squirrel cage induction motor external resistance can’t be added in the rotor circuit during starting since its rotor is permanently short circuited.
If the rotor circuit is made of high resistance to obtain high starting torque, then the motor will have poor efficiency under normal running condition because of more rotor copper losses.
However, the starting torque of this motor can be improved by employing another cage on the rotor, and the motor is called double squirrel cage induction motor.
Construction of Double Squirrel Cage Induction Motor
The upper slot conductors from the outer cage and the lower slot conductors form the inner cage. The outer cage consists of bars of high resistance, generally bars of brass alloy of a smaller area of cross-section, are employed.
The inner cage consists of bars of low resistance generally bars of copper metal of a larger area of cross section, are employed. Since the inner cage is embedded deep in the iron, it has relatively more leakage flux and has high reactance.
Thus the outer cage has high resistance and low reactance whereas the inner cage has low resistance and high reactance.
A slit is provided between upper and lower slot so that it offers high reluctance for the stator field. Thus the stator field instead of linking with only outer cage, links both the cages simultaneously.
Double Squirrel Cage Induction Motor Working
The working of double squirrel cage induction motor is as under:
- Since the maximum torque produced by each cage is independent of their resistance but inversely proportional to their standstill reactances. Therefore, the value of the maximum torque produced by the inner cage is more than the outer cage.
- At the start, the rotor current frequency is equal to the supply frequency; therefore, the reactances (2πfL) of the two cages are high as compared to their resistances.
The division of the rotor current between the cages is determined by their relative impedance (reactance).
Therefore, at the start, comparatively only small current flows through inner cage bars while large current flows through the outer cage in spite of its high resistance.
Thus, at the start the torque is mainly produced by the outer cage.
- As the motor picks up speed, the rotor current frequency decreases and is very low at the rated speed. Under this condition, the reactance of the two cages is very, low as compared to their resistances.
Therefore, the division of the rotor current between the cages is determined by their relative resistance and most of the current flows through the outer cage.
Thus, under normal running condition, torque is mainly produced by the inner cage.
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