Power Factor

Power Factor :

It is defined in several ways

(i) In alternating-current power transmission and distribution, the cosine of the phase angle between the voltage and current "cosφ".

(ii) In AC networks power factor is the ratio of resistane to impedance of the circuit ( R/Z ).

(iii) The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power ( P/S ), and is a number between 0 to 1 inclusive .

Power factors other than unity have deleterious effects on power transmission systems, including excessive transmission losses and reduced system capacity.

When the load is inductive, e.g., an induction motor, the current lags the applied voltage, and the power factor is said to be a lagging power factor. When the load is capacitive, e.g., a synchronous motor or a capacitive network, the current leads the applied voltage, and the power factor is said to be a leading power  factor.Power factor equals unity (1) when the voltage and current are in phase, and is zero when the current leads or lags the voltage by 90 degrees.

Capacitive circuits cause reactive power with the current waveform leading the voltage wave by 90 degrees, while inductive circuits cause reactive power with the current waveform lagging the voltage waveform by 90 degrees. The result of this is that capacitive and inductive circuit elements tend to cancel each other out. By convention, capacitors are said to generate reactive power while inductors are said to consume it (this probably comes from the fact that most real-life loads are inductive and so reactive power has to be supplied to them from power factor correction capacitors).

In power transmission and distribution, significant effort is made to control the reactive power flow. This is typically done automatically by switching inductors or capacitor banks in and out, by adjusting generator excitation, and by other means. Electricity retailers may use electricity meters which measure reactive power to financially penalise customers with low power factor loads. This is particularly relevant to customers operating highly inductive loads such as motors at water pumping stations. 

reference : 

http://en.wikipedia.org/wiki/Power_factor