Power Supplies - Upgrading and Repairing PCs - page 1131

• Inductive —Sustains an electromagnetic field, such as in a transformer or motor
A resistive load is often called working power and is measured in kilowatts (KW). An in-
ductiveload,ontheotherhand,isoftencalled reactive power andismeasuredinkilovolt-
amperes-reactive(KVAR).Workingpowerandreactivepowertogethermakeup apparent
power , which is measured in kilovolt-amperes (KVA). The power factor is measured as
the ratio of working power to apparent power, or working power/apparent power (KW/
KVA). The ideal power factor is 1, where the working power and apparent power are the
same.
The concept of a resistive load or working power is fairly easy to understand. For ex-
ample,alightbulbthatconsumes100Wofpowergenerates100Wworthofheatandlight.
This is a pure resistive load. An inductive load, on the other hand, is a little harder to
understand. Think about a transformer, which has coil windings to generate an electro-
magnetic field and then induce current in another set of windings. A certain amount of
power is required to saturate the windings and generate the magnetic field, even though
no work is being done. A power transformer that is not connected to anything is a perfect
example of a pure inductive load. An apparent power draw exists to generate the fields,
but no working power exists because no actual work is being done.
When the transformer is connected to a load, it uses both working power and reactive
power. In other words, power is consumed to do work (for example, if the transformer is
poweringalightbulb),andapparentpowerisusedtomaintaintheelectromagneticfieldin
the transformer windings. In an AC circuit, these loads can become out of sync or phase,
meaning they don't peak at the same time, which can generate harmonic distortions back
down the power line. I've seen examples in which electric motors have caused distortions
in television sets plugged into the same power circuit.
PFC usually involves adding capacitance to the circuit to maintain the inductive load
without drawing additional power from the line. This makes the working power and ap-
parent power the same, which results in a power factor of 1. It usually isn't just as simple
as adding some capacitors to a circuit, although that can be done and is called passive
power factor correction. Active power factor correction involves a more intelligent circuit
designedtomatchtheresistiveandinductiveloadssotheelectricaloutletseesthemasthe
same.
A power supply with active power factor correction draws low distortion current from the
AC source and has a power factor rating of 0.9 or greater. A nonpower factor-corrected
supply draws highly distorted current and is sometimes referred to as a nonlinear load.
The power factor of a noncorrected supply is typically 0.6-0.8. Therefore, only 60% of
the apparent power consumed is actually doing real work!