‘V curves’ and ‘inverted V curves’ for Synchronous Motor:
V curve:
V curve is a plot of the stator current versus field current for different constant loads. The graph is plotted between the armature current Ia and field current If at no load. This curve is known as V curve because the shape of this curve is similar to the letter “V”. For higher values of field current the power factor is leading whereas for lower values of field current the power factor is lagging. Similar curves are plotted for various constant loading conditions. When such number of V-curves are plotted, the unity power factor line is locus of minimum armature current. The family of curves for different loads are as shown below:
Inverted V Curve: When the power factor is plotted against field current for any constant load, the shape of the graph looks like an inverted V. Such curves obtained by plotting p.f. against field current at various constant load conditions, are called Inverted Vcurves of synchronous motor. The highest point on each of these curves indicates unity power factor.