An induction machine is connected to a 60Hz supply The noloa

An induction machine is connected to a 60-Hz supply. The no-load rotor speed is 1200rpm. The machine is driven mechanically to a speed of 1250rpm. Under this condition, determine the frequency of the stator and rotor currents in the stationary, rotor, and synchronous reference frames. Determine the speed and direction of the stator and rotor MMF when viewed from an observer positioned on (1) the stator and (2) the rotor.

Solution

As the rotor is accelerated mechanically to a speed greater than no load speed of machine ,the induction motor now works as a generator

Stationary reference Frame

To calculate frequencies assume no load speed as synchronous speed

slip s=(1200-1250)/1200=-0.0417; & slip is negative as it is generator now

Frequency of rotor currents:Fr=sFs=2.5 Hz

Frequency of stator currents=Fs=60Hz

Rotor reference Frame:

Frequency of rotor currents:Fr= 0 Hz

Frequency of stator currents:Fs=sFs=0.0417*60= 2.5 Hz

Synchronous reference Frame:

Frequency of stator currents:0 Hz

Frequency of rotor currents:Fr=sFs=0.0417*60= 2.5 Hz

from stator frame

Speed and Direction of stator MMF= 1200 Rpm ,assume clock wise

Speed and Direction of Rotor MMF= 1200 Rpm clock wise

from Rotor

Speed and Direction of stator MMF= 50 Rpm , clock wise

Speed and Direction of Rotor MMF= 50 Rpm anti clock wise