Question { TVS, 50025 }

Synchrounous motor is three phase motor,but its not self
starting .why?

Answer

In order to understand the principle of operation of a
synchronous motor, let us examine
what happens if we connect the armature winding (laid out in
the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source
and the field winding to a D.C
source of appropriate voltage. The current flowing through
the field coils will set up sta-
tionary magnetic poles of alternate North and South.On the
other hand, the 3-phase currents flowing in
the armature winding produce a rotating magnetic field
rotating at synchronous speed. In
other words there will be moving North and South poles
established in the stator due to the
3-phase currents i.e at any location in the stator there
will be a North pole at some instant of
time and it will become a South pole after a time period
corresponding to half a cycle. (after
a time = 1/2f
where f = frequency of the supply). Let us assume that the
stationary South
pole in the rotor is aligned with the North pole in the
stator moving in clockwise direction
at a particular instant of time.These two poles get
attracted and try to maintain this alignment ( as per lenz’s
law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the
production of torque in the clockwise
direction. However the rotor cannot move instantaneously due
to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator
pole would quickly (a time
duration corresponding to half a cycle) change its polarity
and becomes a South pole. So
the force of attraction will no longer be present and
instead the like poles experience a force of repulsion.In
other words, the conditions are now suitable for the
production of torque in the anticlockwise direction. Even
this condition will not last longer
as the stator pole would again change to North pole after a
time of 1/2f. Thus the rotor will
experience an alternating force which tries to move it
clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding
to 1/2f seconds. As this duration is
quite small compared to the mechanical time constant of the
rotor, the rotor cannot respond
and move in any direction. The rotor continues to be
stationary only. so need some prime mover