power factor [PF]


(n.) The ratio of the total power in watts under a resistive load to the total apparent power in voltamperes (VA) under a reactive load. The difference between watts and VA is due to reactive load impedance. Apparent power equals watts only for a purely resistive load with zero degrees phase shift between the applied voltage and the resultant current. Power may be considered the multiplier (ranging between 0 and 1) that is used to obtain the real power from the apparent power. To obtain the apparent power, measure the rms voltage and current of a circuit and multiply them. To obtain the real power, multiply this value by the power factor. If the load is purely resistive, then the phase difference between the voltage and current will be zero, the power factor will be one, and the apparent power will equal the true power. For any load with inductive or capacitive reactance, there will be a phase difference between the voltage and the current due to phase delay introduced by the reactive elements. Maximum voltage and current do not occur at the same instant in time, so the amount of power developed is less than the measured rms voltage and current multiplied together. Power factor is a ratio that can be expressed in several ways. It is the ratio of watts to voltamperes, of resistance to reactance, and if the phase shift in degrees is known (phase angle), it is the cosine of that angle. If the angle is zero, the PF is 1, and if the angle is 90 degrees, the PF is 0.