True, Reactive, and Apparent power
We know that reactive loads such as
inductors and capacitors dissipate zero power, yet the fact that they
drop voltage and draw current gives the deceptive impression that they
actually do dissipate power. This "phantom power" is called
reactive power, and it is measured in a unit called
Volt-Amps-Reactive (VAR), rather than watts. The mathematical symbol
for reactive power is (unfortunately) the capital letter Q. The actual
amount of power being used, or dissipated, in a circuit is called
true power, and it is measured in watts (symbolized by the capital
letter P, as always). The combination of reactive power and true power
is called apparent power, and it is the product of a circuit's
voltage and current, without reference to phase angle. Apparent power is
measured in the unit of Volt-Amps (VA) and is symbolized by the
capital letter S.
As a rule, true power is a function of a
circuit's dissipative elements, usually resistances (R). Reactive power
is a function of a circuit's reactance (X). Apparent power is a function
of a circuit's total impedance (Z). Since we're dealing with scalar
quantities for power calculation, any complex starting quantities such
as voltage, current, and impedance must be represented by their polar
magnitudes, not by real or imaginary rectangular components. For
instance, if I'm calculating true power from current and resistance, I
must use the polar magnitude for current, and not merely the "real" or
"imaginary" portion of the current. If I'm calculating apparent power
from voltage and impedance, both of these formerly complex quantities
must be reduced to their polar magnitudes for the scalar arithmetic.
There are several power equations
relating the three types of power to resistance, reactance, and
impedance (all using scalar quantities):
Please note that there are two equations
each for the calculation of true and reactive power. There are three
equations available for the calculation of apparent power, P=IE being
useful only for that purpose. Examine the following circuits and
see how these three types of power interrelate:
Resistive load only:
Reactive load only:
Resistive/reactive load:
These three types of power -- true,
reactive, and apparent -- relate to one another in trigonometric form.
We call this the power triangle:
Using the laws of trigonometry, we can
solve for the length of any side (amount of any type of power), given
the lengths of the other two sides, or the length of one side and an
angle.
- REVIEW:
- Power dissipated by a load is referred
to as true power. True power is symbolized by the letter P and
is measured in the unit of Watts (W).
- Power merely absorbed and returned in
load due to its reactive properties is referred to as reactive
power. Reactive power is symbolized by the letter Q and is
measured in the unit of Volt-Amps-Reactive (VAR).
- Total power in an AC circuit, both
dissipated and absorbed/returned is referred to as apparent power.
Apparent power is symbolized by the letter S and is measured in the
unit of Volt-Amps (VA).
- These three types of power are
trigonometrically related to one another. In a right triangle, P =
adjacent length, Q = opposite length, and S = hypotenuse length. The
opposite angle is equal to the circuit's impedance (Z) phase angle.
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