Question

In 3 - phase induction motor some times copper bars are placed deep in the rotor to

Answer 1

This design is known as Deep bar rotor, which is basically 
a modified rotor design in Squirrel cage induction motor. 
During starting, the rotor frequency (i.e. Fr = slip * 
supply frequency) is very high and the rotor reactance 
value is very high, due to which the rotor current is 
restricted to the near slot rotor copper and the area of 
cross section of the conducting rotor conductors reduces 
and the net resistance is increased due to which the 
starting torque and the slip at which the max. torque takes 
place also increases. And as the rotor picks up speed, the 
rotor frequency and hence rotor reactance starts decreasing 
and the complete deep bars carries uniform current and 
torque reached steady state torque.

Another design with the almost the same above principle is 
the DOUBLE CAGE ROTOR


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