# V Curves and Inverted V Curves of Synchronous Motor

The performance characteristics of a synchronous motor are obtained by v-curves and inverted v-curves. Synchronous machines have parabolic type characteristics (the graph drawn is in the shape of parabolic).

If the excitation is varied from low (under-excitation) to high (over-excitation) value, then the current Ia also changes i.e., becomes minimum at unity PF and then again increases.

But at starting lagging current becomes unity and then becomes leading in nature. V-curves and inverted V-curves of a synchronous motor are used to analyze efficiency on no-load and on-load conditions.

## V-Curves of Synchronous Motor :

If the armature current Ia is plotted against excitation or field current for various load conditions, we obtain a set of curves known as 'V-Curves' clue to their shape similar to english letter V.

In the below figure V-Curve of a synchronous motor shows how armature current Ia changes with excitation for the same input, at no-load, half full-load, and full-load.

From V-Curves it is observed that the armature current has large values both for low and high values of excitation (though it is lagging for low excitation and leading for higher excitation). In between, it has a minimum value corresponding to the unity power factor (normal excitation).

## Inverted V-Curves of Synchronous Motor :

If the power factor is plotted against excitation for various load conditions, we obtain a set of curves known as Inverted V-Curves.

The inverted V-Curves of synchronous motor shows how the power factor varies with excitation. From inverted V-curves, it is observed that the power factor is lagging when the motor is under excited and leading when it is over-excited. In between, the power factor is unity.

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