Power Factor Correction

By Sanjay Chawla On Dec 29, 2010 Site: Electronics Design Engineering Lounge (Public) Type: Blog - # of views: 1482

In electrical power systems the common way to reduce harmonic distortion and increase efficiency is to increase the power factor (PF). This can be achieved by power factor correction (PFC). The power factor correction reduces the cost of generating power and thus saves money for both utilities and customers.

Since a pure resistive circuit produces no phase shift between voltage and current, power is maximized. This is known as true power and it is the power that performs work. True power is measured in watts (W). Conversely, a pure capacitive or inductive circuit will draw current that does no work. Power that performs no work is known as reactive power and is measured in volts amp reactive (VAr). Reactive power is undesirable and in turn produces energy loss in terms of heat on the conductors. Unlike true power, it does not perform work on load. This forces the source to output more power in order to compensate for the losses. As a result, it is important to know how much true power vs. reactive power exists in the system. Taking these two values (True Power, Reactive Power) into account, the overall output is known as apparent power and it is measured in volt amps (VA).

To determine the efficiency of a circuit, the input power is compared to the output power. This tells us how much total power needs to be provided in order to get the necessary output at the load. This value is known as Power Factor.

PF = P/S Where

PF = Power Factor

P = True Power (W)

S = Apparent Power in Volt-Amps

Multiplying this by 100% gives the efficiency of the system.

For example if motor operates at 1200W but requires 1600W of power, this means there are 400W of power loss in the system. So the Power Factor is:

PF = 1200W/1600W = 0.75

ie Efficiency = 75%

So 25% of the input power is lost due to the reactive components in the system.

An electrical system with low power factor is less efficient because it requires more current. A system that is 100% efficient has a unity power factor (PF = 1).

There are two main methods of power factor correction: passive power factor correction and active power factor correction. Passive PFC is the simpler. It is generally done by introducing an inductance to a system to counteract the reactive current due to capacitance. As a result, the power factor increases and the system become more efficient because reactive losses have been minimized.

Active PFC involves using complex circuitry to counteract the effects of the reactive components. Most power sources and systems manufactured presently utilize active power factor correction. The reason is because once a source or system reaches a certain size (over about 200-250VA), the inductor necessary for passive PFC becomes too large for a feasible design.