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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

NONLINEAR LOADS

Letýs consider a simple, hypothetical situation. Say you need a power supply to deliver 12 VDC to a 100-ohm load, that is a 120-mA current with a reasonably low (<1%) ripple. Nothing could be simpler, right? The schematic diagram of the rectifier is shown in Figure 1. And, the voltage across the load and its ripple are plotted in Figure 2. Thatýs exactly what you need, so whereýs the problem?

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Figure 1ýThis simple, full-wave rectifier delivers 12 V at 120 mA to a 100-ohm load.

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Figure 2ýThe DC ripple satisfies your requirement. The ripple is less than 1%.

Take a look at Figure 3, which is the time domain plot of the input AC voltage and current. The voltage (shown in blue trace) is a nice 60-Hz sine wave. The current (shown by the red trace) does not resemble a sinusoidal wave even with a great deal of imagination. It is distorted beyond recognition, and one of the results is immediately obivious when you look at the current axis. With the AC input voltage approximately 9-V rms, the input current should be roughly 150-mA rms, or 210-mA peak. Instead, the current reaches almost 900-mA peak, which is more than four times as much.

(Click to enlarge)

Figure 3ýThe rectifier is a nonlinear load. With the input voltage perfectly sinusoidal, the current resembles a train of pulses.

This may not be a big deal when building a home project, but if youýre working for a power company supplying power to zillions of such devices, with all the current peaks synchronized to the same sine wave, it is a serious problem. Imagine that you are responsible for the design of a vehicle power distribution system where the power is generated (as usual) by an AC alternator. The vehicle has a number of servo systems with DC motor actuators, each equipped with appropriate rectifiers. All of these actuators may be functioning at the same time. And now, because of the nonlinear distortion that the rectifiers cause, your generator and wiring may have to be rated for several times the power the system actually needs. You add cost and weight and the efficiency will be terrible, thereby wasting fuel.

By observing the waveforms on the oscilloscope you can easily tell that they are distorted, but how do you quantify them? For that you must consider the waveform in the frequency domain as opposed to the time domain (as seen on a scope) by performing a Fourier analysis.

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