Power Factor Correction - Compaq 226824-001 - ProLiant - ML750 Basics Manual

Power factor correction

Before computing and storage devices can use electrical power, the AC provided from the source
must be transformed to direct current (DC) by a power supply. The term power indicates the rate at
which the electricity does work, such as running a central processing unit (CPU) or turning a cooling
fan. The power that the electricity provides (apparent power) is simply the voltage times the current,
measured in volt-amperes (VA).
There is a difference between the power supplied to a device and the power actually used by the
device because the capacitive and inductive nature of AC circuits will change the phase relationship
of current and voltage as shown in Figure 12. The true power, measured in watts rather than VA, can
only be delivered when the current and voltage overlap.
Figure 12. Aligned current and voltage for power delivery
The power factor (PF) of a device is a number between zero and one that represents the ratio
between the real power in watts and the apparent power in VA. A power supply that has a PF of
1.0 indicates that the voltage and current peak together (the voltage and current sine waves are
always the same polarity), which means that the VA and watt values are the same. A device with a
Power Factor of 0.5 would have a watt value that is half the VA value; for example, a 400VA device
with a Power Factor of 0.5 would be a 200W device.
A common misconception is that the power factor and the power supply efficiency are related, but
this is not the true. Power supply efficiency is the ratio of output power in watts to input power in watts
at peak efficiency. For example, a typical white box power supply with a peak efficiency of
75 percent would waste at least 25 percent of the incoming energy by converting it to heat that must
then be dissipated. HP ProLiant server power supplies all have peak efficiencies of 85 percent or
greater, which increases the amount of power that performs useful work.
Devices with a low power factor, on the other hand, do not waste energy. Unused energy is simply
returned to the utility and is not paid for by the customer. Utilities charge for true power used as
measured in kWhours, not in VA. The main costs associated with a low power factor are for higher
amperage circuits to deliver the same amount of true power as a device with a power factor closer to
one.
Power supplies for servers usually contain circuitry to correct the power factor (that is, to bring input
current and voltage into phase). Power-factor correction allows the input current to continuously flow,
reduces the peak input current, and reduces energy loss in the power supply, thus improving its
operational efficiency. Power-factor-corrected (PFC) power supplies have a power factor near
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