What is Power Factor in Electric Motors?

In an alternating-current circuit, not all the power drawn from the supply is converted into useful work. Real power, measured in kilowatts (kW), is the portion that actually performs work, such as turning a shaft or producing heat. Reactive power, measured in kilovolt-amperes reactive (kVAR), flows back and forth between the source and the magnetic and electric fields of the load without doing net work, yet it is essential for establishing those fields.

The combination of the two, as seen by the supply, is the apparent power, measured in kilovolt-amperes (kVA). The three quantities are related by the so-called power triangle: apparent power is the vector sum of real and reactive power. Power factor is simply the ratio of real power to apparent power, a dimensionless number between zero and one.

A power factor close to one means almost all the supplied power is doing useful work. A lower value means a larger share is circulating as reactive power, so the supply must deliver more current to achieve the same useful output. The table below sets out the three powers side by side.

Electric motors are inductive loads: they need reactive power to magnetise their iron cores and establish the rotating magnetic field that produces torque. This magnetising requirement means an induction motor always draws some reactive power, so its power factor is inherently less than one even when it is perfectly healthy and correctly applied.

The magnetising component is relatively fixed, while the real-power component rises and falls with mechanical load. As a result, power factor is strongly load-dependent. At rated load an induction motor commonly exhibits a power factor in the region of 0.7 to 0.9, but at light load the fixed magnetising current dominates and the power factor falls markedly.

This behaviour explains why an oversized motor running well below its rated load is doubly penalised: it is both less efficient and operating at a poorer power factor than it would near its design point. Right-sizing a motor to its duty is therefore one of the simplest ways to keep power factor healthy.

Poor power factor increases the current a system must carry for a given amount of useful work. Higher current means greater resistive losses in cables, transformers and switchgear, and it consumes capacity that could otherwise serve additional load. In effect, a low power factor makes the whole electrical distribution system work harder for the same output.

For larger consumers, network operators frequently levy charges based on apparent power or impose penalties for poor power factor, because the utility must size its infrastructure for the current actually drawn rather than the useful power delivered. Improving power factor can therefore reduce demand charges as well as internal losses.

Power factor should not be confused with efficiency, though the two are related in effect. Efficiency describes how much of the real input power becomes useful output; power factor describes how much of the apparent power is real. A motor can be efficient yet still burden the supply with reactive current, which is why both metrics deserve attention.

Different motor technologies present different power-factor characteristics. Conventional induction motors always draw magnetising current from the supply, which sets a ceiling on their achievable power factor. Line-fed synchronous machines can be arranged to run at, or even above, unity power factor, and some can supply reactive power to the network.

When a motor is fed through a variable speed drive, the picture changes again. The drive decouples the motor from the mains, and the input stage of the drive largely determines the power factor seen by the supply. Modern drives with appropriate front-end designs can present a high power factor to the network regardless of the motor's own magnetising behaviour, though they introduce other considerations such as harmonic content.

Because technology choice affects power factor, it is worth considering alongside efficiency class and materials when specifying a drive system. The best combination depends on the application, the duty cycle and how the installation connects to the wider network.

The classic remedy for poor power factor is power-factor correction, most commonly by connecting capacitors that supply reactive power locally so that it need not be drawn all the way from the source. Correction can be applied at an individual motor, at a distribution board, or centrally for a whole installation, each with its own trade-offs in cost and flexibility.

Other measures work by reducing the reactive demand in the first place. Right-sizing motors to their load keeps them near their design point where power factor is best, while switching off or unloading idle machines avoids the light-load penalty. Where variable speed operation is needed, a drive with a high-power-factor front end can address the issue at source.

Any correction scheme should be designed with care, because over-correction, resonance with system harmonics or switching transients can cause problems of their own. Measurement and, where appropriate, expert design ensure that correction delivers the intended benefit without unintended side effects.