Difference between Single-Cage and Double-Cage Induction Motors

Let us first take a look at the construction diagrams of single-cage and double-cage induction motors −

Following are the key differences between a single-cage induction motor and a double-cage induction motor of the same rating −

• A double-cage induction motor has high starting torque and low starting current as compared to a single-cage induction motor. Therefore, the double-cage motors are more suitable for direct on-line starting.
• A double-cage induction motor has high rotor resistance. Consequently, a large amount of heat is generated in the rotor circuit of a double-cage motor at the time of starting as compared to a single-cage motor.
• Due to the high resistance of the outer cage of the rotor in a double-cage motor, the effective resistance of the double-cage motor is increased and hence, the full-load copper losses. Therefore, a double-cage motor has low efficiency than a single-cage motor.
• A double-cage induction motor high leakage reactance due to additional reactance of the inner rotor cage. Hence, the full-load power factor of a double-cage motor is less than that of a single-cage motor.
• A wide range of torque-slip characteristics can be obtained with double-cage motors by a proper choice of resistances and reactances of the outer and inner rotor cages. This is not possible with a single-cage rotor.
• The breakdown or pull-out torque of a double-cage induction motor is smaller than that of a single-cage motor. Because in a double-cage motor, the two cages produce the maximum torque at different speeds.
• The double-cage motors are more expensive than the single-cage motors, i.e., the cost of a double-cage induction motor is about 20 to 30 % higher than that of a single-cage motor of the same rating.