Answer / 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.

Answer / kuttivenu

Single-phase or polyphase synchronous motors can't start
without being driven, or having their rotor connected in
the form of a self-starting circuit. Since the field is
rotating at synchronous speed, the motor must be
accelerated before it can pull into synchronism.
Accelerating from zero speed requires slip until
synchronism is reached. Therefore, separate starting means
must be employed.

Answer / don

buy p s bimbra..u wl get answer.

Answer / marjo

because the relative velocity b/w the stator and rotor mmfs is zero.

Answer / gaurav srivastava

synchronous motor have 3 phase winding which is having
altarnating rotating nature and the rotor of motor have dc
wwindig which is connected to a dc source due to this the
rotar become magnetished thus there are two poles form in
rotar. that is South and North pole...since the stator
winding are 3phase so that the stator become magnetised but
it is change alternatively.....but there is main reason
that to zero starting is the alternating magnetisation is
very fast (in approx 1/20 sec)so that there is no pole
coupling in rotating stator field and rotor..so that is is
not self starting...

for this there is a prime mover is used to rotate it ..

by this there is magnet copling is formed so that it can
run...

there is main function of synchronous motor it always run
on synchronous speed..

MUST REED THIS/////
=>if motor start to run and there is a phase cut (one of
three)....it is not affect on the motor speed


GAURAV SRIVASTAVA (UNITED COLLEGE OF ENGINEERING AND
RESEARCH)ALLAHBAD

Answer / manik

interlocking of stator & rotor poles therefore at starting
rotor cant rotate automatically. synch motor is not self
starting (slip=0)

Answer / iamdon

The 3 phase balanced supply given to a 3 phase stator
balanced winding produce synchronously rotating magnetic
field. This field cuts the stationary conductor and the emf
induced is sinosoidal.The average of torue induced is zero
over a complete cycle.Hence the starting torque is zero.

Author:arbelectbit@yahoo.com

Answer / sheik

staring torque is low

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