The ‘power factor’ of an electrical supply is the ratio of the ‘real power’ to the ‘apparent power’ of the supply. Put more simply, it is the useful power used by the site (‘real’ power, in Watts) divided by the total power that is actually drawn. The latter includes power that is unusable, so clearly it is desirable to have a power factor close to 1, meaning that no unusable power is being returned to the grid. As low power factor means that the electricity supplier would effectively have to supply more energy than the consumer’s bill would indicate, suppliers are entitled to penalise customers with poor power factors. Typically, Power Factor Correction (PFC) equipment consists of a bank of capacitors which acts to correct the phase shift introduced by inductive site loads such as motors.
The name given to the ‘unusable power’ above is reactive power. Reactive power does not do “work” in the traditional sense of electrical or mechanical power. This energy is instead used to charge a capacitor or produce a magnetic field around the field of an inductor. Current in an AC circuit will rise and fall many times a second. As the current rises, energy is stored as a magnetic field in an inductor or as an electrical field in a capacitor. As the current falls, the energy is returned to the system as the magnetic field collapses or the capacitor is discharged. Reactive power needs to be generated and distributed through a circuit to provide sufficient true power (kW) to enable processes to run.
Electricity suppliers discourage customers from using large amounts of reactive power and will penalise them according to their Power Factor. A perfect Power Factor would be 1; this would mean all power drawn is consumed (reactive power = 0).
Power Factor = True power (kW)
Apparent power (kVA)
Where apparent power = √(kW2+kVAr2 )
Reactive power generally increases significantly with increasing voltage, as the ‘reactance’ of equipment increases. Correcting this will therefore lead to a reduction in reactive power, and an improvement in power factor.
