have seen previously that when load changes, for constant excitation,
current drawn by the motor increases. But if excitation i.e. field
current is changed keeping load constant, the synchronous motor reacts
by by changing its power factor of operation. This is most interesting
feature of synchronous motor. Let us see the details of such operation.
V i.e. induced e.m.f. is equal to applied voltage. Such an excitation
is called Normal Excitation of the motor. Motor is drawing certain
current from the supply and power input to the motor is say Pin. The power factor of the motor is lagging in nature as shown in the Fig. 1(a).
when excitation is changed, changes but there is hardly any change in
the losses of the motor. So the power input also remains same for
constant load demanding same power output.
So far this entire operation of variable excitation it is necessary to
remember that the cosine component of armature current, Ia cos? remains constant.
? remains constant when excitation of the motor is changed keeping load
constant. This is the reason why synchronous motor reacts by changing
its power factor to variable excitation conditions.
? component constant. This is shown in the Fig. 1(b). So in under
excited condition, current drawn by the motor increases. The p.f. cos ?
decreases and becomes more and more lagging in nature.
in over excited condition. So power factor of the motor becomes
leading in nature. So overexcited synchronous motor works on leading
power factor. So power factor decreases as over excitation increases but
it becomes more and more leading in nature.
When the excitation is changed, the power factor changes. The
excitation for which the power factor of the motor is unity (cos ? =
1) is called critical excitation. Then Iaph is in phase with Vph. Now Ia cos
? must be constant, cos ? = 1 is at its maximum hence motor has to
draw minimum current from supply for unity power factor condition.
So for critical excitation, cos ? = 1 and current drawn by the motor
is minimum compared to current drawn by the motor for various
excitation conditions. This is shown in the Fig. 1(d).
|Fig. 1 Constant load variable excitation operation|