V Curve of a Synchronous Motor

V Curve of a Synchronous Motor

In this article we will study about the V Curves of Synchronous Motor.
Graph plotted between “Armature Current flowing through Stator” and “field current” at a constant load is known as V curve of synchronous motor.

V Curves of Synchronous Motor for no load, half load and full load is shown in the figure below.

Fig-1: Family of V curves of synchronous motor


From the V curve, we can observe that armature current is changed due to change in field current. When field current is increased from smaller value (keeping load constant), then armature current decreases and at a point it becomes minimum. At this minimum value of armature current, synchronous motor is operating at unity power factor and before this point power factor was lagging.
When field is further increased, then armature current starts to rise and motor operates at leading power factor.

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Lowest Curve in the figure is plotted at no load and if similarly graph is plotted for different values of load, a family of curve is obtained as shown in figure above.

Since the shape of curve is similar to letter “V”, therefore these curves are called V curves. Curve connecting the lowest point of all V curves for different values of load is called Locus of Unity Power Factor or Unity Power Factor Compounding Curve (as shown in figure by dotted lines). In the same manner, compounding curve for other values of power factor can also be drawn. Compounding curve shows the manner in which field current should be varied in order to maintain constant power factor.


Conclusively, we can say that points to the right of unity power factor corresponds to the over excitation and leading power factor. Points to the left of unity power factor corresponds to under excitation and lagging power factor.
V curves are useful for adjusting the field current. By controlling the field current, reactive power supplied to the motor can be controlled.

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A family of curves can be obtained by plotting the power factor versus field current at a constant load of synchronous motor. These curves are called Inverted V Curves.

Fig-2: Inverted Curve

Highest Point on each curve denotes unity power factor. Note that field current for unity power factor at full load is greater than field current for unity factor at no load. Now suppose if motor is operating at full load on unity power factor and whole shaft load is removed, then motor will operate at leading power factor and this property of synchronous motor is used for Power Factor Improvement and then synchronous motor is called as Synchronous Condenser.

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