POWER FACTOR

Power factor is a quantity which has important implications when sizing a UPS system and power distribution equipment. Power is a measure of the delivery rate of energy and in DC (direct current) electrical circuits is expressed as the mathematical product of Volts and Amps (Power = Volts x Amps). However, in AC (alternating current) power system, a complication is introduced; namely that some AC current (Amps) may flow into and back out of the load without delivering energy. This current, called reactive or harmonic current, gives rise to an “apparent” power (Volt x Amps) which is larger than the actual power consumed. This difference between the apparent power and the actual power gives rise to the power factor. The power factor is equal to the ratio of the actual power to the apparent power. The apparent power is expressed as the Volt-Amp or VA rating. Therefore, the actual power in any AC system is the VA rating multiplied by the power factor.

For many types of electrical equipment the difference between apparent power (VA) and actual power (Watts) is very slight and can be ignored, but for some computers the difference is very large and important. Many desktop personal computers present a nonlinear load to the AC supply. This is because they have a

power supply design known as a "capacitor input switch mode power supply". In a study done by PC Magazine, it was found that typical personal computer systems exhibit a power factor of .65 which means that the apparent power (VA) was 50% larger than the actual power (Watts)! Information Technology equipment including servers, routers, hubs, and storage systems almost universally use a different power supply design known as "Power Factor Corrected". These devices present a very linear load to the AC supply and do not generate harmonic currents. In fact they are one of the cleanest loads on the power grid and generate less harmonic current than many other devices such as fluorescent

lighting or variable speed motors. Ten years ago, these devices were nonlinear loads like Personal Computers, but today all of these loads are subject to international regulation IEC 1000-3-2 which require them to be made with the "Power Factor Corrected" design.