Answer Posted / thuranga
construction of the synchronous motor is as follows:
stator: 3-phase construction so as to form rotating magnetic
field.
rotor: concentric winding designed so as to form fixed poles
through the 2 slip rings provided at the end of the
shaft,and also the copper bars are placed in the rotor
core,and short circuited so as to form the virtual squirrel
cage rotor which is used while starting.
(note: damper windings are also provided for the starting
purpose of the squirrel cage)
for the motor, the 3-phase supply is given first to the
stator. then the rotor will start rotating (squirrel cage
action),now if the rotor is reached approximately the
synchronous speed(i.e 95-96 % of synchronous speed) of the
stator,(most probably it takes 56 secs to reach that
speed),then through some arrangement D.C supply is given to
the rotor, so that fixed poles form and gets magnetically
locked with the rmf,i.e North to south attract, and then
rotate with the synchronous speed.
That is the reason for giving D.C supply to the rotor,so
that we get constant speed,which is required for some
particular industrial applications.
This is also a disadvantage because we cannot use this one
where variable speed is required,so we go for squirrel care
and slip ring motors.
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