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CUI “Power Factor and Power Factor Correction” application note

12th August 2013

CUI Inc

Nat Bowers

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Whitepaper

Long gone are the days when only engineers that worked with large electric motors and high power electric loads need worry about power factor. The introduction of switching power supplies into electronic systems has led to increasing international legislation, moving power factor up the list as a key concern for most engineers developing systems that run on ac mains power. This paper will help you understand power factor and its implications in today's power systems.

Defining power factor

Power factor (pf) is the ratio of real power (P) flowing to the load, to the apparent power in the circuit (S). It is a sinusoidal waveform and therefore is expressed as a dimensionless number between -1 and 1:

Pf = P/S

For a purely resistive load, the two figures are identical; for a reactive load the arithmetic for the apparent power produces the same figure, that is, the product of the RMS values of voltage and current. However, to find the actual (real) power delivered to the load, the instantaneous product of voltage and current must be integrated over the complete sine-wave cycle.

Adding reactive power into the equation

Real (or active) power, measured in watts, is defined as the circuit’s capacity to perform work at a given time. Apparent power, measured in volt amperes, is the product of the current and voltage of the circuit.

When current is leading or lagging voltage, the value of that integral will always be less than the value for the in-phase case over the same interval. This reflects the attribute of an inductor or a capacitor to act as energy stores; at various points through the ac cycle the reactive component (Q) is either storing energy, or returning it to the system.

Download the full application note below.