This is a summary of the Electrical Apparatus October 2011 featured technical article, by Richard L. Nailen, P.E.

Although seemingly a simple concept, power factor in an a-c circuit is the subject of many misunderstandings. It expresses three relationships. First, power factor is the ratio of power in watts to the product of volts and amperes. Second, it is also the ratio of current in phase with applied voltage to the total current. Third, it is the ratio of circuit resistance to total impedance. All three ratios are identical.

A phasor triangle conveniently depicts these relationships. Usually the voltage is the reference, with the total current shown as either lagging the voltage (its waveform trailing behind in time) in an inductive circuit, or leading in a capacitive circuit.

The power factor angle separating current and voltage will be between 0 and 90 degrees. Therefore the cosine of that angle, which is called the power factor, must always be between 0 and 1. It can never exceed unity or have a negative value.

Adding capacitance in parallel to an inductive circuit increases power factor by decreasing the power factor angle. A similar effect results from connecting the capacitance in series, but that requires the capacitance to carry the entire load current and results primarily in decreasing the inductive voltage drop in the circuit conductors. Utilities find this useful in maintaining load voltage on distribution feeders.

Induction motor power factor and efficiency are not directly related. Hence, a higher motor efficiency does not necessarily mean higher power factor, or vice versa. Also, the various devices claiming to save large amounts of energy by raising motor circuit power factor are seldom useful except for motors that are lightly loaded. Reducing line current does reduce energy loss in supply conductors, but this is normally insignificant compared to losses within the motor.

Some utilities penalize their customers for power factors below some stated limit. However, such a penalty structure varies with the utility; adding enough capacitance to raise the power factor over the limit is not always cost-effective, and it does not apply to residential loads.