Power Factor Correction Formula
What is Electrical Power Factor?
Electrical Power Factor is a characteristic of alternating current, and can be explained simply as the ratio of working (true) power (kW) to total (apparent) power (kVA) consumed by an item of electrical equipment, e.g. an electric motor.
Power Factor Correction Calculations
Alternating current has the following components
| Real Power | - | Power which produces work (kW) |
| Available Power | - | Power delivered or total volt amps (kVA) |
| Reactive Power | - | Power needed to generate magnetic fields required for the operation of inductive electrical equipment. (kVAR) No useful work is performed with reactive power. |
Therefore the formula for power factor is:
| Power Factor = | Real Power | = | kW |
| Available Power | kVA |
Power Factor Correction Units
Power Factor does not have any units, and is generally represented as a percentage or a decimal. Therefore it is common to see power factors expressed as, for example 0.95, or 95%.
What is Ideal Power Factor?
Perfect power factor, often referred to as unity power factor, or ideal power factor is 100% or 1.0 meaning that all the energy supplied by the source is consumed by the load.
What is Power Factor Correction?
All flowing current causes losses in the supply and distribution system. A load with a power factor of 1.0 is the most efficient loading for the supply and a load with a power factor of 0.6 will have much higher losses in the supply system. In this case, only 60% of the incoming current does useful work and 40% is wasted through heating up the conductors. These loses have to be paid for, and result in higher utility bills. It is possible, using the above power factor formula, to determine what modifications are required to bring the supply and distribution system power factor closer to unity. This is called power factor correction.
How to Correct Power Factor
The simplest form of power factor correction, sometimes referred to as static correction, is by the addition of capacitors in parallel with the connected inductive load. The resulting capacitive current is a leading current and is used to cancel the lagging inductive current flowing from the supply. The capacitors can be applied at the starter, or the switchboard or at the distribution panel. Note that power factor correction should not be used when a motor is controlled by a variable speed drive.
Rather than correcting each individual load, the total current supplied to the distribution board can be monitored by a controller which switches capacitor banks to maintain the power factor at its predetermined setting. The controller switching in capacitors as new loads come on line, and switching out capacitors as loads go off line. This type of correction is sometimes referred to as bulk correction.
Common Inductive Loads
Commonly used electrical equipment that provide an inductive load include lighting circuits, heaters, arc welders, distribution transformers and electric motors. See our page on
motor calculation formula for more details on calculating motor power.
Benefits of Power Factor Correction
Performing a simple power factor calculation, and installing power factor correction equipment based on your results i.e. bringing the power correction factor closer to unity, can bring many benefits. Besides savings in electricity bills, the benefits of power factor correction include improved voltage regulation due to reduced voltage drop in the circuit. Also, by considering power factor correction at the design stage it is possible to reduce capital investment through reduction in the size of transformers, switchgear and cable diameters.
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