V Curves and Inverted V Curves of Synchronous Motor


     The performance characteristics of a synchronous motors is obtained by v-curves and inverted v-curves. Synchronous machines have parabolic type characteristics ( the graph drawn is in the shape of parabolic ). If excitation is varied from low ( under excitation ) to high ( over excitation ) value, then current Ia also changes i.e., becomes minimum at unity p.f. 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.

V Curves and Inverted V Curves of Synchronous Motor

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

V Curves and Inverted V Curves of Synchronous Motor

     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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