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Friday, May 6, 2011

Power Factor Correction

What is Power Factor?

Power Factor is a characteristic of alternating current, and can be defined as the ratio of working power to total power.
 
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 unitless Power Factor is obtained from
Power Factor =    Real Power     = kW
Available Power kVA

Power Factor is generally represented as a percentage or a decimal. Perfect power factor, often referred to as unity power factor would be 100% or 1.0.
 

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 results 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. These loses have to be paid for, and result in higher utility bills. It is possible to modify the supply and distribution system to bring the power factor closer to unity. This is called power factor correction.
 

Correcting Power Factors

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.
 

Any Other Benefits?

Besides savings in electricity bills, power factor correction offers improved voltage regulation due to reduced lower voltage drop. 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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