How to Select a UPS or Inverter for Standby Computer Power Protection
Conservatively, type of UPS Inverter waveform was a minor consideration when selecting
a UPS; therefore most UPS systems, based on economical considerations provide non sinusoidal output waveform. These systems however, are prone to drop load when powering new computer servers build to 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 either a sinusoidal waveform, or a pulsed waveform.
The type of Inverter's waveform depends on the type of the UPS. An online UPS which provides output continuously from the Inverter has generally a sinusoidal voltage, like the mains. A simple Standby UPS type, based on cost consideration has almost always a non sinusoidal inverter. A Line Interactive UPS may provide either a sinusoidal or pulsed waveform from the inverter.
Non Sinusoidal Inverter waveforms include 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. These harmonics amount to 45% of fundamental waveform in a square type Inverter waveform, and about 20% in pulsed type waveforms called semi, modified or stepped sine waves.
These non sinusoidal waveforms may cause problems with numerous loads but they work fine with conservative computers and servers which generally include a simple capacitive bridge rectifier circuit in the computer power supply.
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, server manufacturers have to redesign the power supply input by introducing an Active Power Factor Correction (APFC) circuit instead of the passive simple rectifying bridge. New servers systems are already designed for Energy Star labeling. We can expect future standard computer to include APFC circuits as manufacturers follow suit.
The new computers are designed to work with sinusoidal input voltage. They function properly if the Inverter supplies a sinusoidal waveform, but, as reported, may drop load if the Inverter provides a non sinusoidal waveforms. 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. 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 when buying a UPS or selecting an Inverter for new computers.
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.
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.
Modified Sine wave, Semi sinusoidal waveform, Stepped waveform, Square waveform are all pulsed type Non Sinusoidal waveforms.
Almost 100% of Standby UPS systems and 70% of Line Interactive UPS systems from most brands have non sinusoidal waveform Inverters.
A UPS that works fine with a particular Green Labeled computer may not work properly with another green labeled computer.
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.
Type of waveform, if not sinusoidal is generally not stated on UPS trading websites, sometimes even not in UPS technical specifications.
Hence, make sure that UPS Inverter supplies a sinusoidal waveform, to ascertain that it will be able to protect your current computer and any future computer you might need.
How to Select a UPS or Inverter for Standby Computer Power Protection
By: Meir portnoy
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