Basic AC Power
POWER TRIANGLE
True Power
True power (P) is the power consumed by the resistive loads in an electrical circuit. Equation
(9-2) is a mathematical representation of true power. The measurement of true power is in watts.
P = I2R = EI cosq
(9-2)
where
P = 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). Equation (9-3) is a mathematical representation for reactive
power.
Q= I2X = EI sinq
(9-3)
where
Q = reactive power (VAR)
I = RMS current (A)
X = net reactance (W)
E = RMS voltage (V)
q = angle between the E and I sine waves
Unlike true power, reactive power is not useful power because it is stored in the circuit itself.
This power is stored by inductors, because they expand and collapse their magnetic fields in an
attempt to keep current constant, and by capacitors, because they charge and discharge in an
attempt to keep voltage constant. Circuit inductance and capacitance consume and give back
reactive power. Reactive power is a function of a system’s amperage. The power delivered to
the inductance is stored in the magnetic field when the field is expanding and returned to the
source when the field collapses. The power delivered to the capacitance is stored in the
electrostatic field when the capacitor is charging and returned to the source when the capacitor
discharges. None of the power delivered to the circuit by the source is consumed. It is all
returned to the source. The true power, which is the power consumed, is thus zero. We know
that alternating current constantly changes; thus, the cycle of expansion and collapse of the
magnetic and electrostatic fields constantly occurs.
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