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In a synchronous motor armature current flows due to
revolving flux which in turn rotates the load,but in no
load condition armature current is zero hence thr is no
revolving flux then how motor rotates?
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Answer Posted / rakesh
This is funny , how on earth could u assume that the no
load condition of a motor means open circuit condition ,
now listen the 3 phase supply given to stator produces a
rotating magnetic flux which reacts wit the initially at
rest field conductors to set up an induced emf lagging the
flux by 90* , but the field mmf set up leads the stator mmf
and rotates in the same direction as the rotating field (
note that i m already considering that the motor has
starting mechanism i.e. damper winding and later on dc
excitation ) thus the two rotating mmfs are stationary wrt
to each other but rotating at synchronous speed wrt
stator , since the rotor mmf i.e field mmf has the tendency
to follow the resultant air gap mmf so electromagnetic
torque acts that sets the rotor rotating . it must be noted
that the field mmf is proportional to field current while
the armature mmf is prprtnl to armtr crrnt , hence at no
load the armature mmf is maxm, because the arm current is
maxm, while with increase in load the current decreases and
hence the arm mmf , however the reasultant air gap mmf
always remain constant till voltage supply and the
frequemncy are maintained constant, it is only that the
field mmfs and the armature mmfs change in orientation and
magnitude.
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