In AC circuits, power has three components. Real power (P) performs useful work and is measured in watts. Reactive power (Q) oscillates between source and reactive elements, measured in VAR. Apparent power (S) is the vector sum, measured in VA. Understanding these helps correctly size transformers, generators, and power factor correction equipment.
⚠Inductive loads (motors) have positive Q (lagging PF). Capacitive loads have negative Q (leading PF). Power factor correction adds capacitors to cancel inductive Q, reducing apparent power S and line current.
Understanding AC Power
AC power is more complex than DC because voltage and current can be out of phase. Real power does work, reactive power represents energy stored and returned each cycle, and apparent power is what the utility must supply. The power factor measures the efficiency of power delivery.
Real Power P
P = VI cos φ (W). Does useful work. Resistive loads only. Measured by wattmeter. Billed by utility.
Reactive Power Q
Q = VI sin φ (VAR). Stored in L/C fields. Inductive = +Q, capacitive = -Q. No work but causes line loss.
Apparent Power S
S = VI (VA). Total power drawn. S = √(P²+Q²). Transformers and generators rated in kVA. Sizing based on S, not P.
Power Triangle
P on x-axis, Q on y-axis, S as hypotenuse. φ = arctan(Q/P). PF = cos φ = P/S. Unity PF when Q=0.
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