What you should know about UPS Inverter waveform, to eliminate possibility of computer dropping, when using Standby or Line Interactive UPS systems.

Uninterruptible Power Supply (UPS) systems provide output voltage either from the utility mains (line) or from backup batteries by means of an internal DC to AC inverter that converts the direct 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 also a sinusoidal waveform in most Online UPS systems, but mostly a non sinusoidal, pulsed type waveform in Off Line ( Stand By 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, which works fine with all common pulsed voltage waveform types. Therefore, most Line Interactive and Standby UPS systems, based on economical considerations provide non sinusoidal, pulsed output waveform.

These systems however, are prone to drop load when powering new computer servers build to comply with recent green oriented legislative requirements.

A 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.

New computers are designed to work with sinusoidal input voltage, and as evidenced, drop in certain cases the load if the UPS Inverter provides a non sinusoidal waveform. 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 non sinusoidal Inverter and a particular computer or load size cannot be predicted.

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

Things You Should Know about UPS Inverter for Computer Protection:

1. New computers build to meet Green Oriented legislation use Active Power Factor Correction designed to work with sinusoidal waveform. Such computers can turn off, trip the input breaker, or shut down the UPS due to overload, when powered from non sinusoidal waveform.

2. The problem will be evident mainly with Standby UPS systems and Line Interactive UPS systems upon power outage, when the load is transferred from utility to the non-sinusoidal Inverter fed from the backup battery.

3. Modified Sine wave, Semi sinusoidal waveform, Stepped waveform, Square waveform are all pulsed type Non – Sinusoidal waveforms.

4. Almost 100% of Standby UPS systems and 70% of Line Interactive UPS systems from most brands have non sinusoidal waveform Inverters.

5. A UPS that works fine with a particular Green Labeled computer may not work properly with another green labeled computer.

6. A UPS which works fine with existing green label computers may cause problems if the load on the UPS is increased or when additional computers are added.

7. Type of waveform, if not sinusoidal is generally not stated on UPS trading websites, sometimes even not in UPS technical specifications.

Currently most personal computers as well as existing servers use conservative power supplies with simple rectifier circuits, which work fine with pulsed type UPS Inverters. You should however prefer sinusoidal UPS Inverter systems, when buying a UPS intended to protect future computers or new servers which might have an APFC type power supply.

Author Bio: Meir Portnoy is VP of UPSonNet, UPS information and guidance provider. UPS waveform is only one aspect to consider when buying a computer backup. For more information, relevant survey results and how to select and find a proper UPS read “UPS Inverter & New Computers” on http://www.upsonnet.com/.

Category: Computers and Technology
Keywords: UPS Inverter, Modified sinewave, Stepped waveform, UPS backup, Line Interactive UPS, Stnadby UPS