Power Factor Explained for Industrial Motors

Power factor is one of those electrical concepts that sounds abstract but has real financial and practical consequences in industrial facilities. Electric motors are a major influence on it, and understanding power factor helps explain electricity costs and the behaviour of the supply system.

What power factor describes

In alternating current systems, not all the current a motor draws does useful work. Some of it flows back and forth establishing the magnetic fields the motor needs, without being consumed. Power factor describes the relationship between the current that does useful work and the total current drawn. A power factor near one means most of the current is doing useful work; a lower power factor means a larger share is this non-working, circulating current.

Why it matters financially

Although the circulating current does no useful work, it still flows through the supply system, and the utility must provide for it. For this reason, many industrial electricity tariffs penalise a poor power factor, charging more when a facility draws a large amount of non-working current. Improving power factor can therefore reduce electricity costs directly, quite apart from any energy saved.

Motors and power factor

Induction motors, which dominate industry, inherently draw this magnetising current and so operate at less than unity power factor. Crucially, their power factor is worse at light load, because the magnetising current stays roughly constant while the useful current shrinks. This gives another reason to avoid running motors lightly loaded and to size them appropriately.

Improving power factor

Power factor can be improved by adding equipment that supplies the magnetising current locally, reducing the amount the facility must draw from the supply. This is a well-established practice in industrial facilities with many motors. Done correctly, it lowers the total current drawn, can reduce electricity charges, and eases the load on the supply infrastructure, sometimes freeing capacity for expansion.

Power factor, though invisible on the factory floor, has a tangible effect on electricity costs and supply capacity. Understanding that motors draw both working and non-working current, that a poor power factor is penalised and worsens at light load, and that it can be improved, allows a facility to manage this often-overlooked aspect of its electrical system and reduce unnecessary cost.