Power, Crest & Surge Factors
Power Factor (PF)
Power factor is a quantity which has important implications when sizing a UPS system. Power is a measure of the delivery rate of energy and in DC (direct current) electrical circuits is expressed as the mathematical product of Volts and Amps (Power = Volts x Amps). However, in an AC (alternating current) power system, a complication is introduced; namely that some AC current (Amps) may flow into and back out of the load without delivering energy. This current, called reactive or harmonic current, gives rise to an apparent power (Volt x Amps) which is larger than the actual power. This difference between the apparent power and the actual power gives rise to the power factor. The power factor is equal to the ratio of the actual power to the apparent power. The apparent power is expressed as the Volt-Amp or VA rating. Therefore, the actual power in any AC system is the VA rating multiplied by the power factor.
For many types of electrical equipment the difference between apparent power (VA) and actual power (Watts) is very slight and can be ignored, but for virtually all computers the difference is very large and important. In a recent study done by PC Magazine, it was found that typical computer systems exhibit a power factor of .65 which means that the apparent power (VA) was 50% larger that the actual power (Watts)!
To size a UPS and insure that the UPS output capacity is sufficient, the VA rating of the UPS should be larger than the VA requirement of the load. The power or Watt rating of the load is lower and should not be used because this rating does not include the extra reactive or harmonic current that the UPS will actually be required to supply during service.
Many UPS manufacturers do not include both Watt and VA specifications for their UPS products. When the VA rating of a UPS is not furnished, it can be very difficult to determine if the UPS is capable of supplying a specific load. In fact, many UPS systems which are specified for operation at a specific Watt rating will not actually run rated power if the load is a typical computer system with a .65 power factor! When the Watt rating is the only rating specified for a UPS, we have found that a general rule is to assume that a UPS Watt rating is equal to the VA rating.
In addition to a low power 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 manufacturers 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.
Most properly designed sine wave UPS systems have a very high crest factor capability of approximately 3 when operated at full load, 4 when operated at ½ load, and 8 when operated at ¼ load. The smaller stepped wave models have a crest factor capability of 1.6 at full load and 2 at ½ load. This is much better performance than a typical power conditioner or UPS. Most quality UPS systems are designed to maintain the proper peak voltage into the computer load for computers with any input crest factor specification.
Surge Factor (In-rush Current, Start-up Current)
This term is often mistakenly related to the surge suppression characteristic of a UPS or surge suppressor, which is a different and unrelated characteristic. Surge factor relates to the momentary overload capacity of the UPS and is a measure of the ability of the UPS to start-up loads which temporarily require extra power when they start-up. Examples of loads which have high surge factor requirement include motors and disk drives.
For a typical computer system with 5.25″ hard disk drives, the surge factor required is approximately 1.15 times the steady-state power consumption. For large systems with 8”,10” or 15” hard disks, the surge factor required is approximately 1.5 times the steady-state power consumption.
All properly designed UPS systems have enough surge factor capability to start typical hard disk loads even when the UPS is operated at the full power rating. For systems with multiple large form factor hard drives over sizing the UPS is not necessary to prevent the UPS from reaching an overload situation. All quality online UPS systems with the rating of 1kVA and higher incorporate a static bypass switch for severe or abnormal overloads.