Answer Posted / arnab choudhury

the essential requirement for starting a synchronous machine
is that in the initial stages the rotor( which houses d.c
excited field windings) must be rotated at the synchronous
speed(determined using(120*f)/p where'f'is the supply
frequency and 'p' is the no. of poles) so that the rotor
poles may lock on to the stator rotating poles( created by
three phase balanced excitation given to the stator armature
windings) following which the rotor will rotate
synchronously with stator field at that speed. hence without
the use of an external prime mover in the beginning, the
syn. motor cannot self start....

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