The advantage of a double-squirrel cage motor is that it provides high starting torque and low starting current.
How does a double-squirrel cage induction motor operate (i) at starting (ii) under running conditions ?
A double-squirrel cage motor has two rotor windings, one inside the other. impedance of the outer winding is less than that of the inner winding, resulting in a large proportion of current flowing in the outer high-resistance winding. This provides good starting torque.
As the motor accelerates, the rotor frequency decreases, thereby lowering the reactance of inner winding, allowing it to carry a largy proportion of the total current. At the normal operating speed, the rotor frequency is so low that nearly all the rotor current flows in the low-resistance inner cage, resulting in high efficiency and good speed regulation.