Power factor is defined as which ratio?

• A. The ratio of apparent power to real power.
• B. The ratio of real power to apparent power.
• C. The ratio of real power to reactive power.
• D. The sum of real power and apparent power.

Answer: (B)

Power factor tells you how effectively the input power is used to do useful work. It is defined as the ratio of real (active) power to apparent power. Real power P is the actual energy converted to useful work each second, measured in watts. Apparent power S is the product of voltage and current (VI) and represents the total power flow in the circuit, including both useful and reactive components, measured in volt-amperes.

In sinusoidal AC systems, P equals VI cos(phi) and S equals VI, so power factor PF = P / S = cos(phi), where phi is the phase angle between voltage and current. A PF of 1 means all the power does useful work with no reactive power; a PF less than 1 means some power is tied up in reactive components (inductors or capacitors). Leading or lagging indicates whether the load is predominantly capacitive or inductive. To improve PF, you can add capacitors for correction to reduce the reactive current and losses.