What is Power Factor in Electric Motors?

Real power (measured in kW) does the actual work — producing torque, heat or light. Reactive power (kVAR) does no net work; it flows back and forth to build and collapse the magnetic fields that inductive devices like motors need. Apparent power (kVA) is the combination of the two, and it is what the supply must actually deliver.

Power factor is the ratio of real power to apparent power. A power factor of 1.0 means every unit of current delivered does useful work; a lower value means some of the current is circulating without doing work.

Motors are inductive loads: they must draw reactive current to establish the magnetic fields in their windings and cores. This magnetising current is why induction motors typically show a power factor between about 0.7 and 0.9 at rated load — and lower at partial load.

The reactive component adds nothing to the mechanical output, but it still flows through cables, transformers and switchgear, occupying capacity and causing losses along the way.

A low power factor means more current is needed to deliver the same real power. Higher current means larger cable and switchgear sizing, greater resistive losses in the distribution system, and heavier loading on transformers.

Many utilities also apply penalty charges to industrial customers whose power factor falls below a threshold, so poor power factor has a direct billing impact on top of the infrastructure cost.

Permanent magnet motors generally achieve a higher power factor than induction motors of similar rating, because the rotor's magnetic field is supplied by the magnets rather than by reactive magnetising current drawn from the supply.

That can reduce apparent power demand and ease pressure on the electrical infrastructure, an efficiency benefit beyond the headline IE class.

Power factor correction — typically capacitor banks — supplies reactive power locally so that less has to travel from the supply. Fixed banks suit steady loads; automatic systems switch capacitance in and out to track varying loads.

When drives are involved, harmonic considerations come into play and may call for filters. To discuss motor selection with power quality in mind, get in touch.