Double Cage Induction Motor - Construction & Working

Need For Double Cage Induction Motor :

     The starting torque of a induction motor is given by

     Where, R2 and X2 are the  rotor resistance and inductive reactance at starting respectively, E2 is the rotor induced EMF.

     Here in this equation starting torque of the induction motor Tsh is proportional to the rotor resistance  R2.

     The starting torque in a squirrel cage motor is small because of its low rotor resistance and there is no way to insert external resistance as the winding is permanently short circuited. The starting torques of the squirrel cage motors are improved by using a double cage rotor i.e., a method of obtaining a comparatively high rotor resistance at starting (to give good starting torque) and low rotor resistance when running ( to give good efficiency ) in cage motors.

Construction of Double Cage Induction Motor :

     The double cage rotor consists of two separate cages on the rotor as shown in figure.   

Double Cage Induction Motor

     The rotor punchings on a double cage rotor is shown in above figure. The bars on the cage A is made up with low resistance by increasing its cross section area and high inductive reactance. It is deeply embedded in the rotor. The bars on the surface of the rotor shown as cage B are with small cross section i.e., with high resistance and low reactance values.

Working of Double Cage Induction Motor :

Double Cage Induction Motor

     At starting, the frequency of the rotor induced emf or current is high ( same as stator supply frequency ). So the reactance of the inner cage ( = 2πfL ) and therefore its impedance are both high. Hence, very little current flows in it. Most of the rotor current will flow through the high resistance outer cage and high starting torque is obtained. 

     The frequency of the rotor decreases as the motor gains its speed. Under normal running conditions, the reactance and hence the impedance of inner cage decreases. Most of the rotor current then flows through the inner low resistance cage, thus the IR loss in the rotor circuit will not be high and therefore giving a high efficiency under running condition.

Characteristic of Double Cage Induction Motor :

Double Cage Induction Motor

     When s = 1 ( i.e., at starting ) a greater portion of torque is contributed by outer cage. During running, the outer cage contributes only a small portion of the total torque.

Advantages of Double Cage Induction Motor :

1. It permits high starting torque and good efficiency during running conditions.

2. Useful for frequent starting under heavy load conditions.

3. No need of maintaining slip rings and brushes when compared to wound rotor.

Comparison Between Single Cage and Double Cage :

1. A double cage induction motor has high starting torque and takes low currents during starting. Which is well suitable for direct-on-line starting and in some cases can be started without a starter.

2. The reactance of a double cage induction motor is higher due to presents of inner cage. Which interns reduces full-load power factor of the motor.

3. Due to higher resistance of outer cage the effective resistance of the motor increases. Therefore it increases the full-load copper losses and decreases the overall efficiency.

4. We can obtain a wide range of torque-slip characteristics with double-cage motor by changing resistances and reactances values of the inner and outer cage to suitable values, which is not possible with single-cage motor.

5. Cost of double cage motor is more than the single cage motor of the same rating ( 20% to 30% higher ).

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