We have seen the construction of the induction motor. The starting torque of the motor depends on rotor resistance. In the case of three-phase squirrel cage induction motor, we cannot add external resistance. So, here we cannot increase starting torque. If we increase the rotor resistance by using a high resistance rotor winding, the rotor CU losses will increase and it will decrease the efficiency of the motor.
Now, the question is how to increase the starting torque?
The double cage induction motor uses to overcome the above difficulties. It gives good starting torque with good efficiency.
How It works?? How this motor is different compare to the simple three-phase squirrel cage induction motor?
As the name suggests, the double cage induction motor has two cages. Upper cage is near to the surface of the rotor. This cage is known as the starting cage. The conductor in starting cage is made of high resistance brass or iron material. The lower cage is located deeply. This cage is known as the running cage and this cage is made of low resistance copper.
In the starting condition, the impedance of the lower cage is more compared to the upper cage and hence the low cage accepts small current and more part passes through high resistance outer cage. The starting torque is increase about 200 to 250% of the full load torque and starting current is 4 to 5 times the full load current.
But under the running condition, when the motor reaches near to the rated speed, the frequency of the rotor current becomes very small. So that the impedance of the lower cage is very less compared to the upper cage. Therefore, in this condition, the lower cage copper conductors take a large current and give good running torque.
In a this type of induction motor, the torque slip characteristic due to the inner cage is different compare to the torque slip characteristic due to the outer cage. In the below figure, both characteristics are given. The resultant characteristic is an overall characteristic of the double cage induction motor.
Equivalent Circuit of Double cage Induction Motor
- Rolling Mills
- Reciprocating pumps
Important Points for Double cage Induction Motor
- Higher starting torque
- Outer cage increases CU loss due to high resistance
- Inner cage reduces full load power factor
- Efficiency reduces
- Cost is higher
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