Uninterruptible Power Supply:: Facts You Should Know About Ups Inverter Output Waveform

What you should know about UPS Inverter waveform

, to eliminate the possibility of computer dropping when using Standby or Line Interactive UPS systems.

Conservatively, Computer Power Supplies use a simple rectifier bridge to convert the incoming alternating line voltage to the direct voltage required to power computer circuits. Such rectifier works fine with all common voltage waveform types, therefore, most Line Interactive and Standby UPS systems, based on economical considerations provide non sinusoidal output waveform. These systems however, are prone to drop load when used to protect new computer servers built to comply with recent green oriented legislative requirements.

Uninterruptible Power Supply (UPS) systems provide output voltage either from the utility mains (line) or from internal batteries by means of an internal DC to AC inverter which converts the DC battery voltage to Alternating AC waveform. Utility line voltage has a sinusoidal waveform with a grid frequency of 50Hz or 60 Hz. The inverter voltage has a sinusoidal waveform in most Online UPS systems, and mostly a non sinusoidal, pulsed type waveform in Off Line ( Stand By or Line Interactive) UPS systems.

Pulsed waveform includes the basic sinusoidal waveform with the fundamental frequency of the mains, and additional high frequency parasitic harmonics which do not contribute to the UPS output power. Recent green oriented legislations, such as Energy Star require increasing computer server's efficiency and its power factor by eliminating input current harmonics. In order to comply, new server power supplies use an Active Power Factor Correction (APFC) circuit instead of the passive simple rectifying bridge. We can expect most future standard computers to include APFC circuits as manufacturers follow suit.

The new computers designed to work with sinusoidal input voltage, as reported, if the UPS Inverter provides a non sinusoidal waveform, may turn off the computer or the the UPS, by activating the computer power supply or UPS protecting devices. Contrary to the conservative passive rectifier circuits, which all contain similar input circuits; the specific design of the APFC circuit depends on the manufacturer. Thus, the exact behavior of a particular UPS with a particular computer cannot be predicted.

Both Standby and Line Interactive UPS system are normally powered from the utility line, which provides sinusoidal waveform as required by the computer's power supply circuit. Only when the line power fails (or deviates beyond an allowed voltage window) the protected load is transferred to the internal Inverter.

The main threat with Line Interactive and Standby UPS systems with non sinusoidal inverters lies in the fact that its inability to backup new computers is revealed at the very moment when the utility power fails and the Inverter kicks in. It is like finding out that the brakes in your car aren't working when you are on a steep downhill.

Hence, when buying a UPS prefer UPS Inverter which supplies a sinusoidal waveform. These way you will ascertain that it will be able to protect both your current computer and any future computer you might need.

by: Meirp

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