Total Power

Electrical Science Volume 3 of 4
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POWER TRIANGLE Basic AC Power Total Power The total power delivered by the source is the apparent power. Part of this apparent power, called true power, is dissipated by the circuit resistance in the form of heat. The rest of the apparent power is returned to the source by the circuit inductance and capacitance. 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. Cosq 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 (Figure1). Equation (9-4) is a mathematical representation of power factor. (9-4) cosq P S where cosq = power factor (pf) P = true power (watts) S = apparent power (VA) Power factor also determines what part of the Figure 2 Lagging Power Factor apparent power is real power. It can vary from 1, when the phase angle is 0°, to 0, when the phase angle is 90°. In an inductive circuit, the current lags the voltage and is said to have a lagging power factor, as shown in Figure 2. In a capacitive circuit, the current leads the voltage Figure 3 Leading Power Factor and is said to have a leading power factor, as shown in Figure 3. A mnemonic memory device, "ELI the ICE man," can be used to remember the voltage/current relationship in AC circuits. ELI refers to an inductive circuit (L) where current (I) lags voltage (E). ICE refers to a capacitive circuit (C) where current (I) leads voltage (E). ES-09 Page 4 Rev. 0

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