reactive

In AC circuits, reactive power is the portion of electrical power that oscillates between the source and the load without being dissipated as heat or mechanical work. It arises from reactive components: inductors and capacitors. Where a resistor consumes power linearly, reactive elements store energy in one half-cycle and return it in the next, creating a phase shift between voltage and current. This returned power does real damage to power distribution systems, forcing larger cables and transformers than the actual work being performed would require.

Inductive reactance (XL) comes from coils, transformer windings, and motor windings. It opposes changes in current flow and causes current to lag behind voltage by up to 90 degrees. Capacitive reactance (XC) comes from capacitor plates and stray capacitance in cables; it opposes changes in voltage and causes current to lead. Both are measured in ohms, just like resistance, but they do not dissipate power. The reactance of an inductor increases with frequency; a capacitor's reactance decreases. At a given frequency, XL equals capacitive reactance only at resonance, where they cancel and the circuit behaves as purely resistive.

Power Factor and Practical Consequences

Power factor is the cosine of the phase angle between voltage and current. A purely reactive circuit has power factor of zero; purely resistive has power factor of one (unity). Most industrial loads, especially motors and welders, are inductive and operate at power factors between 0.7 and 0.95. A facility drawing 100 kW of real power at 0.8 power factor must supply 125 kVA to the load. The extra 75 kVAR reactive power floods the grid, heating distribution lines and wasting money through higher peak demand charges. Many utilities penalize customers whose average power factor falls below 0.95.

Power factor correction uses capacitor banks installed near inductive loads to cancel out excess reactive power. A 30-horsepower induction motor at 0.85 power factor might have a 15 to 20 kVAR capacitor bank installed across its terminals to bring combined power factor above 0.95. Fixed capacitors work at rated load; switched banks with reactors protect against over-correction and transients. Harmonic filtering must be considered in plants with variable frequency drives or rectifiers, which can excite capacitor resonance.

The reactive impedance (Z) of a circuit combines resistance (R) and reactance (X) according to Z = sqrt(R^2 + X^2). This is why a 10-ohm resistor in series with a 10-ohm inductive reactance does not give 20 ohms total impedance, but rather 14.14 ohms. Reactive components also determine bandwidth and stability in filters, oscillators, and feedback circuits. In distribution engineering, reactive power is tracked separately on power bills as kVAR (kilovolt-amperes reactive) or KVAR, distinct from real power in kilowatts.