Steady-state operation of motor:
Effect of field current changes
Assuming that a synchronous motor operates
initially with a lagging PF.
If, for the constant load, the field current on the
motor increases, the magnitude of the internal
generated voltage EA increases.
Since changes in IA do not affect the shaft
speed and the motor load is constant, the
real power supplied by the motor is
unchanged. Therefore, the distances
proportional to power on the phasor
diagram (EAsinδ and IAcosθ) must be
constant.
Notice that as EA increases, the magnitude of the armature current IA first
decreases and then increases again. At low EA, the armature current is lagging and
the motor is an inductive load that consumes reactive power Q. As the field current
increases , IA eventually lines up with Vφ, and the motor is purely resistive. As the
field current further increases, IA becomes leading and the motor is a capacitive
load that supplies reactive power Q to the system (consumes –Q).