What is power factor correction?

bg triangle top left

Read on for a simple explanation of power factor correction, but first: what is power factor?

What is power factor?

Power factor is essentially a measure of the proportion of power an electrical load can actually put to use. The power consumed by a load (i.e. if you measure the electrical power flowing to the load) is known as ‘apparent power’. This apparent power can be split into two parts:

  • ‘Real power’ (sometimes call ‘true power’), which is the part that a motor, for example, can actually convert into rotating kinetic energy
  • ‘Reactive power’ – power that is unusable by the load

The ‘power factor’ is the ratio of real power to apparent power.

For example, a 4kW motor (i.e. a motor that can produce 4kW of power on its rotating shaft) may actually consume closer to 6kW of electrical power. This is partly due to inefficiencies of the motor in turning electrical energy into kinetic energy; but it is also due to the power factor – a typical motor will have power factor of around 0.85, meaning it can only convert 85% of the power it draws into useful rotating power.

Loads such as lights or heaters have a power factor close to 1 – known as ‘unity’. This means (in theory) they can convert all of the power they draw from the supply into useful light or heat. By far the most common low power factor loads are motors.

Why do I need to know about power factor?

Low power factor means that more power needs to flow to get work done. This extra current flow means higher loading and heating of transformers, generators, cables etc. As a result, electricity distributors set limits on the power factor of consumers loads – typically they insist on a minimum power factor of 0.8. Also, bringing the power factor of your load closer to 1 or unity means you will be using less power, which in some applications can translate to substantial savings on your electricity bill.

What can be done about it? (Power factor correction)

If the power factor of the total load of a building is less than 0.8, power factor correction usually needs to be installed to comply with the distributors requirements. This may in the form of one or more capacitor banks that can be automatically connected and disconnected as required; or an active electronic power filter that can ‘inject’ currents to counteract the reactive power. Capacitor banks are often cheaper, however less flexible and less accurate meaning both over and under correction; active filters can much more accurately provide the exact amount of compensation required, however are typically more expensive.

Your switchboard builder will be able to recommend a suitable power factor correction solution for your application.