Power Factor is a product of both resistive and reactive loads. A

reactive load may be either an inductor or a capacitor. A resistive

load contains only a resistive component. An reactive load contains

both a resistive and inductive or capacitive component.

If you connect AC across a purely resistive load, current will flow

and be in phase with the voltage. Power in this case is measured in

watts (real power) and is the product of voltage (in volts RMS) times

current (in amperes). For a resistive load with one volt across it

and drawing one ampere of current, there is one watt of real power

being used. An example of a resistive load is an incandescent light

bulb.

If you connect AC across a purely inductive load, the current will

either lead or lag the voltage by 90 degrees. At a particular instant

in time, the voltage will be negative while the current is positive or

the voltage is positive while the current is negative. This means

that during any instant in time, the energy is actually being

transferred back from the inductor into the voltage source. An

inductor stores energy. For a purely inductive load with one volt

across it and one ampere of current flowing, the power is 1 VAR

(reactive power). Inductive power is not measured in watts, but in

VAR or Volt-Amp-Reactive. A pure inductor produces no heat, unlike a

resistive load. An example of an inductive load is a transformer.

In the real world, an inductive load is made up of both a resistive

and inductive component. The resistive component is drawing current

in phase with the voltage, while the inductive component is drawing

current 90 degrees out of phase with the voltage. The resultant Power

Factor, or PF, is the instantaneous summation of these two currents

and is somewhere above zero, but less than 90 degrees. It separate

measurements are made of the voltage and current, the product is not

AC power as is the case with a purely resistive load or DC. Power is

calculated taking into consideration the phase of the voltage to the

current and is also measured in VA (apparent power). Apparent power

is the vector sum of real power and reactive power. PF is equal to

the real power divided by apparent power.

AC Watts = Voltage x Current x PF (cos theta) PF = Watts / VA

An inductor such as a motor winding affects PF as much as a capacitor

can. The difference is that a motor winding produces lagging PF since

the current lags the voltage, while a capacitor produces leading PF

since the current leads the voltage. Since most of the worlds power

load is inductive, electric utilities need a form of power factor

correction. This lagging PF is corrected by placing capacitors on the

power lines. This is done in substations or by capacitor banks

mounted on utility poles.

A TRIAC is affected by this PF. Since the voltage and current are

out of phase on an inductive load, it will not turn off completely.

Also, high voltage inductive spikes can damage unprotected

semiconductor devices such as a TRIAC.

The difference between a rheostat and a dimmer is how they control

the power. A dimmer simply switches out the load. As more of the AC

cycle is switched out, less and less power is delivered to the load.

A rheostat does not switch off the load, but reduces both the current

and voltage going to the load. However, it is very inefficient for it

must absorb whatever power is held back and dissipate it in the form

of heat. For either device, the power ratings must be taken into

consideration.

I hope this helps.

Charles Heick

Cincinnati, Ohio