O-Curves and V -Curves
The circular current loci for constant mechanical power of a synchronous motor is called the O-curves. The curves of armature current vs field current are called the V-curves.
O-Curves and V –Curves:
Fig 1: Load diagram-O-curves
Fig 2: Load diagram-V-curves
- The current loci in Fig. 1 are continued below the base line for generator operation. The horizontal lines of constant mechanical power are now constant input (from the prime mover) and a departure from unity-power-factor working, giving increased currents, increases the I2R loss and lowers the available electrical output.
- The whole system of lines depends, of course, on constant bus-bar voltage. The circular current loci are called the O - curves for constant mechanical power. Any point P on the diagram, ﬁxed by the percentage excitation and load, gives by the line OP the current to scale in magnitude and phase. Directly from the O-curves, Fig. 1, the V -curves, relating armature current and excitation for various constant mechanical loads can be derived. These are shown in Fig. 2