In AC circuits, current and voltage are
normally out of phase and, as a result, not all the powerproduced by the
generator can be used to accomplish work. By the same token, power
cannotbe calculated in AC circuits in the same manner as in DC circuits.
The
power triangle, equates AC
power to DC power by showing the
relationship between generatoroutput (apparent power - S) in
volt-amperes (VA), usable power (true power - P) in watts, andwasted or stored
power (reactive power - Q) in volt-amperes-reactive (VAR). The
phase angle(q)
represents the inefficiency of the AC circuit and corresponds to the total
reactive impedance(Z) to the current flow in the circuit.
The power triangle represents comparable
values that can be used directly to find the efficiencylevel
of generated power to usable power,
which is expressed as the power
factor (discussedlater).
Apparent power, reactive power, and
true power can be calculated by using
the DC equivalent (RMS
value) of the AC voltage and current components along with the power factor.
APPARENT POWER
Apparent Power (S) is
the power delivered to an electrical
circuit. Equation is a mathematical representation of apparent power. The
measurement of apparent power is in volt-amperes (VA).S = I2Z = ITwhereS =
apparent power (VA)I = RMS current
(A)E = RMS voltage (V)Z = impedance (W)
TRUE POWER
True power (P) is the power consumed by the
resistive loads in an electrical circuit. The measurement of true power
is in watts.P = I2R = EI cosq, whereP = true power (watts)I
= RMS current (A)E = RMS voltage (V)R =
resistance (W) q= angle between E and I sine waves.
REACTIVE POWER
Reactive power (Q)
is the power consumed in an AC
circuit because of the expansion and collapse
of magnetic (inductive) and electrostatic (capacitive) fields. Reactive
power is expressed in volt-amperes-reactive
(VAR).
POWER FACTOR
Power factor (pf)
is the ratio between true power and apparent power. True power is the power
consumed by an AC circuit, and reactive
power is the power that is stored
in an AC circuit. Cos theta is called
the power factor (pf) of an AC circuit. It is the ratio of true
power to apparent power, where q is
the phase angle between the applied voltage and current sine waves and also between
P and S on a power triangle shown on the figure above.
By: Kirk Macaraeg