V Curves and Inverted V Curves of Synchronous Motor
1. V Curves: 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 due to the shape similar to English letter V
The VCurve of a synchronous motor shows how the Ia varies with excitation for same input. The VCurves for no. load, 1/2 FL. and FL. are shown in Fig. 1.20 (a) From V-Curves it is observed that, the armature current has large values both for load and high values of excitation (through it is lagging for low excitation and leading higher excitation). In between, it has minimum value corresponding to unity power factor (normal excitation).
2. Inverted V-Curves: If the power factor is plotted against excitation for various load conditions, we obtain a set of curves known as Inverted V-Curves' as shown in Fig. 1.20 (b).
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.
