Answer Posted / vikas

Synchronous machine can run on both AC and DC
excitaion...The speed at which the machine will rotate will
depend on the relative frequency of stator and rotor
supply...i.e.if machine is connected to grid of 50 Hz
frequency and rotor is also provided with 50 Hz frequency
source then relative frequency will be 0 and machine will
not rotate because average torque produced will be 0 as
someone has already mentioned, but if rotor frequency is
say 20 Hz then the relative frequency will be 50-20=30 Hz.
so the machine speed will be 120f/p..substitute f= 30
Hz..when the rotor supply is DC then the relative frequency
is 50-0= 50 Hz. so the machine speed is given by
substituting f= 50 Hz in this case...

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