Answer / farmesh vasava

TO IMPROVE STARTING TORQUE

Answer / raul

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

Answer / pramod attri

increasing the starting torque

A 15 kw squirrel cage motor having a maximum efficiency of 90% on a continuous full load has temperature rise of 42 degrees celcius after 30 min and 52 degrees celcius after 1 hr under the above operation conditions. a) calculate its final steady state temperature rise on continuous load and the heating time constatnt. b) if the motor is operated on a short load for 9 min estimate its temperature rise. c) calculate the maximum overload that can be applied on a short, 9 min rating so that the temperature rise is the same as the final steady state temperature rise on continuous load.

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