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THE POWER FACTOR

by George Novacek

Start ý Nonlinear Loads ý Harmonic Distortion ý Power Factor Correction ý Sources and PDF

For over a century now, alternating current (AC) electric power distribution has been used around the world. Some countries use 50 Hz, others 60 Hz, each swearing by the significant advantages their respective systems deliver. A third, uncommon frequency is 400 Hz, which is mainly used by military and aerospace vehicles. It is also interesting to note that the traditional, 400-Hz ± 5% generators are being gradually replaced in many circumstances by 400-Hz VF (variable frequency generators), which delivers power at frequencies between 320 and 780 Hz.

The reason for the variable frequency is efficiency, weight, and economics. To maintain constant frequency, a constant speed of the alternator must be maintained. This is no small task when the generator is on the shaft of a vehicleýs engine. It costs money, weight, and reduces efficiency. Because few systems today need to depend on a stable power supply frequency, the complications caused by generating it are hardly justified. To power the electronics and brushless DC actuators, you only need to rectify the distributed AC. And herein lies the problem.

Resistive loads are easy for power distribution systems. The immediate load current is always in proportion to the immediate voltage as defined by Ohmýs Law. The power delivered to the load is expressed in watts and is strictly a product of the voltage (V) and current (I), P = V ý I. Both the voltage and current remain sinusoidal; no distortion is caused to either wave and, therefore, no harmonic frequencies are created.

In real life, things are not this straightforward. The wiring alone adds inductance to the system. Many loads, such as induction motors, are reactive, so the true loads are always complex. With the voltage and current remaining sinusoidal and with no harmonic distortion created, the current is rarely in phase with the voltage. The power delivered to the load is no longer the product of the voltage and current, but is reduced by a power factor. The power factor is the ratio of the real power and apparent power. In cases of reactive loads, the power factor is the cosine of the phase angle between the voltage and current. Cosine equals 1 at 0ý phase shift and reaches 0 at 90ý phase shift. The apparent power (P) expressed in volt-amperes (VA), will be:

Predictably, there are always people looking for a free lunch, so over the years there have been many schemes to take advantage of the cosine hitting zero at 90ý phase and cheating power companies of their revenues. How successful these conspiracies have been you can judge for yourselves.

The real problem arises when you use nonlinear loads, where there is no constant ratio between the load voltage and current. Such loads are typically fluorescent lamps, phase angle controlled light dimmers, or DC power supplies. In the past, this was not a problem. But with the proliferation of small electronic appliances, TVs, plug-in power adapters, battery chargers, and untold millions of computers, power companies had to take notice.