Power in AC Circuits

Froth

True Pint

Apparent Pint

True, reactive and apparent power


The power dissipated by a load is the true power and is measured in Watts (W).


Reactive loads (capacitive and inductive) dissipate no real power. But because reactive loads drop voltage and draw current it gives the illusion that they are dissipating power. The power absorbed and returned to the supply is called the reactive power and is measured in Volt-Amps-Reactive (VAR).


The sum of the power dissipated (true power) and the reactive power is called the apparent power and is measured in Volt-Amps (VA). The circuit shows the equivalent circuit of an induction motor in terms of inductive and resistive components.

A Boozy Analogy

You can think of this in terms of a visit to the pub: When you go to the bar you are given an apparent pint of bitter. You actually get less bitter because of the froth on the top, what you actually get to drink is the true pint. The froth just sits on the top costing you money and makes no contribution to you getting drunk.

Reactive Power (VAR)

True Power (Watts)

Apparent Power (VA)

Apparent Power (VA)

True Power (W)

Power Triangle

You can also determine the apparent power by drawing a triangle to scale and measuring the length of the hypotenuse.

Reactive Power (VAr)