How do you calculate actual power factor?

Answer 1

The power factor of a circuit is defined as the cosine of the phase angle -which is the angle by which the supply current lags or leads the supply voltage in AC circuits.

Power factor is always expressed as either a 'lagging power factor' or as a 'leading power factor'.

The terms 'lagging' or 'leading' describe the relationship of the supply current to the supply voltage. Since current lags voltage in an inductive circuit, 'lagging power factors' describe inductive circuits; since current leads voltage in capacitive circuits, 'leading power factors' describe capacitive circuits. In practice, lagging power factors are more common than leading power factors, because most practical loads are inductive (e.g. motors, etc.).

Power factors are normally expressed as a decimal (e.g. '0.8 lagging') although, in the past they were often expressed as a percentage (e.g. '80% lagging'). 'High' power factors tend towards unity, whereas 'low' power factors tend towards zero.

In terms of power, the cosine of a circuit's phase angle and, therefore, its power factor is the ratio of that circuit's true power (expressed in watts) and its apparent power (expressed in volt amperes).

Power factor has no effect whatsoever upon the energy consumed by a load, but it does effect the amount of current drawn from the supply. 'Low' power factors result in unnecessarily-large load currents for any given load, which mean that the supply utilities need to use larger than necessary conductor sizes (expensive!). For industrial or commercial (but not residential) loads, therefore, it is often desirable to 'improve' the load's power factor towards unity, which acts to reduce the load current. This is most-usually done by installing capacitors close to the load, and is termed 'power-factor correction' or 'power-factor improvement'. Capacitors used in this way are rated in reactive volt amperes, rather than in microfarads.

Answer 2

There are a number of different ways of determining the power factor of a load, depending on what information you have access to. For example:

• power factor = cosine of phase angle
• power factor = true power / apparent power
• power factor = resistance / impedance
• power factor = votage across resistive component / supply voltage
• etc.