It is seen that, when synchronous motor is on no load, the stator and rotor pole axes almost coincide with each other.
motor is loaded, the rotor axis falls back with respect to stator. The
angle by which rotor retards is called load angle or angle of
If the load connected to the motor is suddenly changed by a large
amount, then rotor tries to retard to take its new equilibrium position.
But due to inertia of the rotor, it can not achieve its final position
instantaneously. While achieving its new position due to inertia it
passes beyond its final position corresponding to new load. This will
produce more torque than what is demanded. This will try reduce the load
angle and rotor swings in other direction. So there is periodic
swinging of the rotor on both sides of the new equilibrium position,
corresponding to the load. Such a swing is shown in the Fig. 1.
|Fig. 1 Hunting in synchronous motor|
oscillations of the rotor about its new equilibrium position, due to
sudden application or removal of load is called swinging or hunting in
the current drawn by the motor also changes. Hence during hunting
there are changes in the current drawn by the motor which may cause
problem to the other appliances connected to the same line. The changes
in armature current due to hunting is shown in the Fig. 2.
|Fig. 2 Current variations during hunting|
oscillations continue for longer period, there are large fluctuations in
the current. If such variations synchronous with the natural period of
oscillation of the rotor, the amplitude of the swing may become so great
that motor may come out of synchronism. At this instant mechanical
stresses on the rotor are sever and current drawn by the motor is also
very large. So motor gets subjected to large mechanical and electrical
It is mentioned earlier that in the slots provided in the pole faces, a
short circuited winding is placed. This is called damper winding.
When rotor starts oscillating i.e. when hunting starts a relative
motion between damper winding and the rotating magnetic field is
created. Due to this relative motion, e.m.f. gets induced in the damper
winding. According to Lenz’s law, the direction of induced e.m.f. is
always so as to oppose the cause producing it. The cause is the hunting.
So such induced e.m.f. oppose the hunting. The induced e.m.f. tries to
damp the oscillations as quickly as possible. Thus hunting is minimised
due to damper winding.
The time required by the rotor to take its final equilibrium position
after hunting is called as setting time of the rotor. If the load angle
is plotted against time, the schematic representation of hunting can be
obtained as shown in the Fig. 3. It is shown in the diagram that due to
damper winding the setting time of the rotor reduces considreably.
|Fig. Effect of damper winding on hunting|