Uninterruptible Power Supplies (UPS's) supply uninterrupted, clean power to computers of all sizes. What many Internet Service Providers (ISP's) who use UPS's do not realize is that they create and cause distortion, harmonics, and power factor problems on the supply side of the UPS's. UPS's are designed to disconnect the load from utility power if it senses low or no voltage, harmonic wave distortions of over 10%, or low frequency. Significant induction loads, caused by the transformers used in UPS's, decrease power factor. Distributed UPS (many small ones) may also cause poor power factor. Ideal power factor should be a 1 or what is known as unity. Poor power factors are .7 and below. The lower the power factor, the more this type of load distorts the AC waveform. A generator sized to the multiple distributed or a large UPS’s may not be adequate.

When planning to use a generator to power a load (which includes a UPS) during utility outages, requires careful planning. The following 3 points should be considered.

1. A generator should be sized 4 times larger than the total UPS kVA load. Of course the sizing must take into account all other inductive (motors), and resistive (incandescent lighting) loads.

2. A generator used in this application should have less than 10% inherent waveform distortion and should have automatic electronic voltage regulation to within ±1/2%.

3. An electronic (isochronous) engine speed governor should be used to maintain frequency regulation to within ±1/2% also.

In summary, most UPS's produce much noise and distortion back into the supply side of the UPS. A generator should be sized much larger than the total UPS load. All Gen set sizing must still be done on a case by case basis and by competent generator specialist.

Alan Ferrin