15.1 Power Factor What Is It and Why Must It Be Corrected?

The term power factor is borrowed from elementary AC circuit theory. When a sinusoidal AC power source feeds either an inductive or a capacitive load, the load current is also sinusoidal but lags or leads the input voltage by some angle x. For an rms input voltage V _i and rms input current I _i, the apparent power taken from the line is V _i I _i.

But the actual power delivered to the load is only V _i I _i cos x. It is only the component of input current which is in phase with the voltage across the load resistance ( I _i cos x) which contributes to the load power. The component of input current normal to the voltage across the load resistor ( I _i sin x) does not contribute to the actual load power. At one phase of the AC input waveform, it represents power drawn from the input source which is temporarily stored in the reactive component of the load device. In a later phase of the input voltage, this stored current or energy is returned to the input source. But this excessive current, which does not contribute to load power, wastes power in the winding...