Answer Posted / ann
For DC machine
when when we energize rotor of the machine .flux is produce this flux is static flux ... when the rotor of the dc machine rotors the flux cuts the stator winding according to Faraday's law a produced emf is produced which cancelled the effect of original emf .the emf is called back emf which oppose the effect of original emf
For ac machines,
both parts of machine are rotating .. when we energize the rotor of synchronous machine with dc supply a rotating emf is induce which cuts the winding of stator in the result back emf induce
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