Question

Explain the shape of both the grid connected motor armature current vs. field current and armature current vs. reactive power plots 7. Grid-connected Motor (larm, PF vs. Ifield) Grid-connected Motor (Q vs. larm) 0.250.50.75 1.25 15 1.75 2000 1000 1000 2000

SYNCHRONOUS MACHINES

Answer 1

The armature current of the motor changes with the change in field current.Let us assume that the motor is operating at no load.If field current is increased from this value,the armature current decreases until the armature current becomes minimum.At this armature current motor is operating at unity power factor. Up to this point the motor was operating at lagging a power factor.If the field current is increased further,the armature current increases again and the motor starts to operate at leading power factor.

Since the shapes of these curves resembles the letter "V" these curves are known as V curves of a synchronous motor.The point at which unity power factor occurs is the point where armature current is minimum.

The V curves are useful in adjusting the field current.Increasing the field current beyond the level for minimum armature current results in leading power factor.Similarly, decreasing the field current below that for minimum armature current results in lagging power factor.Therefore, by controlling the field current of a synchronous motor, the reactive power supplied to or consumed from the power system can be controlled.