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Article Date: 19th July 2013

Power Capacitors - Power Factor Correction

Power Factor Correction - Power Factor - Power Quality Systems

Power Pylon

Power Factor Correction is the common name for what may more accurately be described as Reactive Power Compensation.

Most three phase electrical loads draw not only active power (kW, the part that does useful work) from the supply, but also reactive power (kVAr, essential but ‘Wattless’ power). Examples of these loads would be induction motors, motor drives (both AC and DC), switch-mode power supplies, and most forms of lighting.

To make a very crude example of an induction motor, the active power is what comes out of the shaft and does work, whilst the reactive power is what is used by the coils of the stator to create the magnetic field. This magnetic field is essential for the operation of the motor, but does no actual work, only permitting the transfer of power from the stator to the rotor.

The total demand on the supply for a load of this type is called the apparent power, and is the vector sum of both the active and reactive components of power.

The ‘power factor’ of a load is the ratio of active power to apparent power, kW to kVA, sometimes referred to as cos.

When a load draws reactive power from the supply, its power factor is said to be lagging, when the reactive power is exported to the supply, its power factor is said to be leading. This is a reference to the phase of the load current with respect to the supply voltage.

This is the ‘Displacement Power Factor’ or DPF, more commonly known as just the ‘Power Factor’ and only takes into consideration the 50Hz components of voltage and current.

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