Double Squirrel Cage Induction Motor
When motor using squirrel cage type of rotor is known as the Squirrel cage induction motor. Single cage rotor has high current and low starting torque. By using doubles squirrel cage rotor we can get High starting torque. Double squirrel cage induction motor has high starting torque and low current.
Squirrel cage induction motor has high starting torque because double cage are used.hence, rotor has more resistance compare to single cage rotor. So current rating is low.
Therefore, the double squirrel cage induction motor us design to provide high starting torque with the low starting current.Rotor is designed as that under normal running condition it has low resistance and at starting operated with high resistance.
The double squirrel cage induction motor is consists in two main parts.
- Double cage rotor
Rotor is consists of two cages. Which are known as outer cage and inner cage. outer cage has high resistance and low reactance ,inner cage has low resistance and high reactance.And stator is same as the single cage IM.
Outer cage is kept at nearer to the air gape. It is made of high resistance materials like aluminum or brass with the bars of small cross sectional area.
The inner cage is kept inner to the outer cage and which are separated by the narrow slit. inner cage is made from low resistance material like copper with larger cross sectional area.
At starting, rotor current frequency is same as the supply frequency. Conductor is wound on the both cages. Leakage flux of the inner cage is very more because of it is in deep inside of rotor.
At staring of motor leakage reactance of upper cage is very high. But inductive reactance is directly proportional to the current. Hence, at the starting current rating of lower cage is very low. but upper cage has high resistance. Hence, the current rating at starting is very less.At starting, upper cage is active and lower cage is inactive. Therefore, upper cage is known as the starting cage.Conductor
Wounded on upper cage has high resistance so current of upper cage is low. Which is improve power factor and gives the high starting torque.Under normal running condition, when rotor speed is increase ,rotor frequency will decrease as a result leakage reactance is decrease. Hence, at normal running condition lower cage produced more torque. During normal running condition the most torque is produced by the lower cage.
At running, upper cage is inactive and lower cage is active.
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