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Power Factor | Calculation and Power Factor Improvement

Published on 24/5/2013 & updated on 19/8/2018
HOME / TRANSMISSION / TRANSMISSION EFFECT
In general power is the capacity to do work. In electrical domain, electrical power is the amount of electrical energy that can be transferred to some other form (heat, light etc) per unit time. Mathematically it is the product of voltage drop across the element and current flowing through it. Considering first the DC circuits, having only DC voltage sources, the inductors and capacitors behave as short circuit and open circuit respectively in steady state. Hence the entire circuit behaves as resistive circuit and the entire electrical power is dissipated in the form of heat. Here the voltage and current are in same phase and the total electrical power is given by

Now coming to AC circuit, here both inductor and capacitor offer a certain amount of impedance given by, The inductor stores electrical energy in the form of magnetic energy and capacitor stores electrical energy in the form of electrostatic energy. Neither of them dissipates it. Further, there is a phase shift between voltage and current. Hence when we consider the entire circuit consisting of resistor, inductor and capacitor, there exists some phase difference between the source voltage and current. The cosine of this phase difference is called electrical power factor. This factor (-1 < cosφ < 1 ) represents the fraction of the total power that is used to do the useful work. The other fraction of electrical power is stored in the form of magnetic energy or electrostatic energy in the inductor and capacitor respectively.

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Power Factor | Calculation and Power Factor Improvement

The total power in this case is, This is called apparent power and its unit is VA (Volt Amp) and denoted by ‘S’. A fraction of this total electrical power which does our useful work is called as active power. We denote it as ‘P’.
P = Active power = Total electrical power.cosφ and its unit is watt.

The other fraction of power is called reactive power. Reactive power does no useful work, but it is required for the active work to be done. We denote it with ‘Q’ and mathematically is given by,
Q = Reactive power = Total electrical power.sinφ and its unit is VAR (Volt Amp Reactive). This reactive power oscillates between source and load. To help understand this better all these power are represented in the form of triangle. power factor triangle Mathematically, S2 = P2 + Q2 and electrical power factor is active power / apparent power.

Power Factor Improvement

The term power factor comes into the picture in AC circuits only. Mathematically it is the cosine of the phase difference between the source voltage and current. It refers to the fraction of total power (apparent power) which is utilized to do the useful work called active power. Need for Power Factor Improvement

Methods of Power Factor Improvement

Power Factor Calculation

In power factor calculation, we measure the source voltage and current drawn using a voltmeter and ammeter respectively. A wattmeter is used to get the active power. Now, we know P = VIcosφ watt Hence, we can get the electrical power factor. Now we can calculate the reactive power Q = VIsinφ VAR This reactive power can now be supplied from the capacitor installed in parallel with load in local. Value of capacitor is calculated as per following formula: IMPORTANT: In power factor improvement, the reactive power requirement by the load does not change. It is just supplied by some device in local, thus reducing the burden on source to provide the required reactive power.




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