EPE301C Course 1 Practice Test 2026 - Free Practice Questions and Study Guide

Reactive power describes the part of electrical power that doesn’t do net work because energy is being temporarily stored and returned by reactive elements, causing voltage and current to be out of step. In a three-phase system, we can think of power as having two components: real (active) power, which actually delivers energy, and reactive power, which oscillates energy between the source and the load. The relationship is P = VI cos φ and Q = VI sin φ, where φ is the phase angle between voltage and current. When φ is 90 degrees, cos φ is zero and all the power is reactive—there’s a 90-degree phase difference and no net energy transfer per cycle. The total apparent power combines these as S = √(P^2 + Q^2). So reactive power is the portion of apparent power that does not perform real work due to the 90-degree phase difference between voltage and current. The average energy consumed per cycle corresponds to real power, not reactive power.