Spots
Re types of power:
Someone sent me the following APC material. I don't know if it adds anything to what's already been said.
Crest factor
In addition to a lower factor, computer loads are also unusual in that they exhibit a very high crest factor. Crest factor is the ratio between the instantaneous peak current required by the load and the RMS current (RMS stands for Root Mean Square, which is a type of average). Most common electrical appliances exhibit a crest factor of 1.4 (1.4 is the ratio of the peak value of a sine wave to its RMS value). When a load exhibits a crest factor of more than 1.4, the source (UPS) must supply the peak current desired by the load. If the source does not supply the current, then the source voltage will become deformed (distorted) by the excess peak current. Therefore, if a UPS is not sized to supply the crest factor desired by the load, the output voltage waveform of the UPS will be distorted.
The crest factor requirement of a computer load will vary depending on the source which it is supplied from. The crest factor may even vary when the computer load is moved from one AC receptacle to another in the same room. It is widely believed that the crest factor is an inherent characteristic of a computer load, when in fact crest factor results from an interaction between the load and the AC source. The crest factor required by a computer load depends on the AC source waveform. For a sine wave source, a computer will typically exhibit a crest factor of 2 to 3. For a source waveform which is a stepped approximation to a sine wave, a computer will exhibit a crest factor of 1.4 to 1.9.
It is widely but mistakenly believed that it is desirable to operate a computer at as high a crest factor as possible. In fact, computer manufactures go to great lengths to reduce the crest factor of the computer because high crest factor causes overheating of power supply components.
The reduction in crest factor which occurs when a computer load is operated from a UPS, surge suppressor, or power conditioner is a positive side benefit, except if the reduction is accompanied by excessive distortion of the input voltage waveform to the computer load. Such distortion may result in a significantly reduced peak voltage being supplied to the load, which is equivalent to a brownout condition. The UPS or line conditioner must be designed to maintain the proper peak voltage.
American Power Conversion sine wave UPS systems have a very high crest factor capability of approximately 3 when operated at full load, 4 when operated at 1/2 load, and 8 when operated at 1/4 load. The smaller stepped wave models have a crest factor capability of 1.6 at full load and 2 at 1/2 load. This is much better performance than a typical power conditioner or UPS. APC UPS systems are designed to maintain the proper peak voltage into the computer load for computers with any input crest factor specification.
In all computers tested, it was found that operating the computer from an American Power Conversion UPS caused the efficiency of the computer power supply to increase by a small but measurable amount, resulting in a decreased operating temperature of the computer.
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Spots or anyone: I highlighted the following (in the first paragraph above) because I wasn't sure it was right.
>>If the source does not supply the current, then the source voltage will become deformed (distorted) by the excess peak current.<<
Instead of "excess peak current" should it be excess crest factor?
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FWIW - I believe APC is misusing UPS (uninterruptable power supply) based on my understanding of what UPS means. I understand UPS to mean the power supply is always outputting it's power not what's coming in through the wall. Whereas an SPS (standby power supply) allows the "wall" power to pass through until it detects an out-of-bounds condition. In a split second after the out-of-bounds condition has been detected the SPS begins outputting "conditioned" power. It continues doing so until the out-of-bounds condition no longer exists. Then it reverts to outputting "wall" power.. So in my mind everywhere APC has used UPS it should be replaced with SPS.
Emory |
