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Answer Posted / satya
a synchronous motor is not self starting.
when three phase current displaced in time by 120 degree is
applied to the three phase stator windings displaced in
space by 120 degree then a rotaing magnetic field is
produced in the air gap. the rotor tends to allign with this
rotating field. the rotor windings are supplied by dc . so
consatnt poles are produced on the rotor.
now suppose the instant when the rotating stator field has
its north pole in line with the south pole of the rotor. the
rotor will be attracted and tend to move with the stator.
but due to its inertia before it starts to move with the
stator the south pole of the stator comes in line with the
rotor south pole. so now the rotor gets repelling force and
it tends to move in the opposite direction as that of the
stator. but again before it starts to move the stator north
pole comes in line with the rotor south pole. thus overall
the efect is that the syn. motor is not self starting. once
it is brought to the syn speed by an external primemover it
rotates at syn. speed irrespective of the load.
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