Synchronous Motor

Electrical Machines II

6
6.1

Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao

Synchronous motor
Principle of operation

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 stationary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). 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 = 21 , where f = frequency of the supply). Let us assume that the stationary South
f
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and

Direction of rotation of stator poles

S

N

N
T

S

N

S

Stationary
rotor poles

Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
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...