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 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 ( 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'.

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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