Discussion
In three phase induction motor the double cage rotors have :
Answer: A
In a double cage rotor the outer cage has high resistance and the inner cage has low resistance.
The outer cage, near the air gap, is made of small bars of high-resistance material and has low leakage reactance. The inner cage, buried deep in the iron, has low-resistance bars and high leakage reactance. At starting the rotor frequency is high, so the inner cage's reactance keeps its current small and most current flows in the outer, high-resistance cage, giving a high starting torque.
Near full speed the rotor frequency is low, reactance hardly matters, and the current flows mainly in the low-resistance inner cage, giving good efficiency. Reversing the arrangement, or making both cages alike, loses that combination.
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