Basic AC Power POWER TRIANGLETruePowerTrue 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)whereP = true power (watts)I = RMS current (A)E = RMS voltage (V)R = resistance (W)q = angle between E and I sine wavesReactivePowerReactive power (Q) is the power consumed in an AC circuit because of the expansion andcollapse of magnetic (inductive) and electrostatic (capacitive) fields. Reactive power is expressedin volt-amperes-reactive (VAR). Equation (9-3) is a mathematical representation for reactivepower.Q= I2X = EI sinq(9-3)whereQ = reactive power (VAR)I = RMS current (A)X = net reactance (W)E = RMS voltage (V)q = angle between the E and I sine wavesUnlike 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 anattempt to keep current constant, and by capacitors, because they charge and discharge in anattempt to keep voltage constant. Circuit inductance and capacitance consume and give backreactive power. Reactive power is a function of a system’s amperage. The power delivered tothe inductance is stored in the magnetic field when the field is expanding and returned to thesource when the field collapses. The power delivered to the capacitance is stored in theelectrostatic field when the capacitor is charging and returned to the source when the capacitordischarges. None of the power delivered to the circuit by the source is consumed. It is allreturned to the source. The true power, which is the power consumed, is thus zero. We knowthat alternating current constantly changes; thus, the cycle of expansion and collapse of themagnetic and electrostatic fields constantly occurs.Rev. 0 Page 3 ES-09
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