Answer / prasanth

"A synchronous motor is not a self starting motor because
torque is developed when running at synchronous
speed,therefore,the motor needs some type of device to
bring the rotor to synchronous speed."

EXPLAINATION:
As per operating principle of synchronous
motor,due to continuous and rapid rotation of stator
poles,the rotor is subjected to a torque which tends to
move it first in one direction and then in the opposite
direction.Owing to its large inertia,the rotor cannot
instantaneously respond to such quickly reversing
torque,with the result that it remains stationary or in
other words it is not self starting.

Answer / koti1965

Seen 20 answers, none of them are correct. since the rotor is not closed, no induced current and hence no torque(the basic principle of rotation ac motor). So give supply to rotor(runs as SM) or short it(runs as IM).

Answer / lloyd kaunde

Its simply because each rotor pole of the motor is subjected
to both attraction and repulsion forces

Answer / leo thomas

Since the rotor is supplied from a DC source the rotor have
a permanent polarity . But applying 3 phase supply to
stator, a 3 phase magnetic field is produced , but polarity
changing according to the supply frequency . Actually at
some regular time intrvals the stator poles and rotor poles
comes into exat 120 degree phase shift (this will exist
only for some micro seconds)the motor try to rotate a
clockwise direction . Another time intervels motor getting
torque to counter clockwise direction .
Thus the rotor poles oscillating to clockwise and
anticlockwise direction, due to the inertia of the moving
system it cannot move into any direction

Answer / vikash kumar gaurav

similar pole are alike each other it is a three phase motor
bt very low starting torque and the average exerted torque
in operation of synchronous motor is zero and we have to
supply both the rotor and stator this is another cause it
is nt aself starting machine.

Answer / tulsi_lotus@yahoo.com

sync motor has no self starting torque this is bcaz its
high inertia prevents it from rotating in either of the
direction,so it is not self starting

Answer / rahul

becoz in synchronous machine uni-directional torque not
produce.

Answer / vidhya

because of the rotor inertia it is unable to rotate in the
anticlockwise direction as the energy supplied is
insufficient to make it to spin because it is very
heavy....so a prime mover is required to produce a
unidirectional torque.

Answer / sangeetha

Synchronous motor is not self starting. Because to develop
torque motor has to run at synchronous speed. the motor
needs some type of device to bring rotor to synchronous
speed.

Answer / vinod

low starting tarque

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