Power Factor Quiz Questions

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Power Factor Basic Theory:

In a dc supply system, energy storing elements such as capacitors and inductors doesn’t consume energy and hence entire dc circuit acts as resistive circuit. There will be only resistive drop in dc circuits. Hence, voltage and currents are in phase with each other.

In case of ac supply system, Inductor (L) stores the energy in the form of magnetic field and Capacitor stores the energy in the form of electrostatic field. Also, there exist a phase shift (current lags voltage by 90o) between current and voltage waveform when inductor present in the circuit and similarly current waveform leads the voltage waveform by 90o in case of capacitive circuits. Therefore, in a circuit consists of resistance, inductor and capacitors there will be phase shift between the voltage waveform and current waveform. The cosine of the phase difference between voltage and current waveform is termed as Power Factor.

Electrical power supplied to ac circuits is either absorbed in inductor or capacitor (stored energy) and do no useful work or the power which do useful electrical work (like rotating the machine, providing heat to the furnace, illuminating electrical bulb). This energy which do useful work is called real power and the power which stores in energy storing devices is termed as reactive power.

The vector product of both real power (kW) and reactive power (vAR) is called Apparent Power.

The product of rms values of current and voltage, V X I is called the apparent power and is measured in volt-amperes or kilo volt amperes (kVA).

The True Power in an ac circuit is obtained by multiplying the apparent power by the power by the power factor and is expressed in watts or kilo watts (kW).

Power Factor may be defined as:


The ratio of the resistance to the impendance

The ratio of true power to the apparent power

Power factor can never be greater than unity. Power factor is usually expressed in terms of fraction or percentage value.

Q-Factor of Coil:

Q-Factor is also called as Quality Factor. It is the reciprocal value of the power factor.

Apparent Power = V x I volt-amp or \dpi{100} \bg_white \frac{V X I}{1000} kVA