Steady-state operation of motor:
Effect of field current changes
A plot of armature current vs. field current is
called a synchronous motor V curve. V
curves for different levels of real power
have their minimum at unity PF, when only
real power is supplied to the motor. For field
currents less than the one giving the
minimum IA, the armature current is lagging
and the motor consumes reactive power.
For field currents greater than the one
giving the minimum IA, the armature current
is leading and the motor supplies reactive
power to the system.
Therefore, by controlling the field current of a synchronous
motor, the reactive power consumed or supplied to the power
system can be controlled.