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
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
I hope this helps.