Power System Fundamentals 49
measured with a wattmeter. This actual power is referred to as true power.
Ordinarily, it is desirable to know the ratio of true power converted in a
circuit into apparent power. This ratio is called the power factor and is ex-
pressed as:
P
pf = ——
VA
or
P
%pf = —— × 100
VA
where:
pf is the power factor of the circuit,
P is the true power in watts, and
VA is the apparent power in volt-amperes.Sample Problem:
Given: a 240 volt, 60 hertz, 30 ampere electric motor is rated at 6000
Watts.
Find: power factor at which the motor operates.
Solution:
P
%pf = —— × 100
VA6000 W
————— = 100
240V × 30A% pf = 83.3%The maximum value of the power factor is 1.0, or 100%, which would
be obtained in a purely resistive circuit. This is referred to as the unity pow-
er factor.
The phase angle between voltage and current in an AC circuit deter-
mines the power factor. If a purely inductive or capacitive circuit existed,
the 90° phase angle would cause a power factor of zero to result. In practi-
cal circuits, the power factor varies according to the relative values of re-
sistance and reactance.
The power relationships we have discussed may be simplified by