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Q7. What is Power Factor and how do you correct a bad Power Factor?

A7.

Power Factor

Essentially, power factor (PF) is a simple notion. It is a measurement of the efficiency of an alternating current power system, that is, the proportion of its energy, which is available for useful work in a machine or electrical load.

It may be better understood by taking the following mechanical analogy. If a Barge is being towed along a channel by a short rope then the large angle between the tow line and the direction of travel means that much less of the applied force is available for forward travel - it has a poor "power factor". By using a longer towline, the angle will be less and the "power factor" will be improved.

The problem does not lie with the power supply itself; electrical utilities go to great lengths to supply "high quality electricity". This problem is caused by the effects, which certain types of load have on the power supply. The theory behind this effect show that most machinery introduce a phase difference between the current and voltage waveforms. This means that the electrical load cannot convert all the supplied electricity into useable mechanical energy. To make up for this, the load takes more current than it really needs. In theory this current just circulates and is not used by the load - so where's the problem?

Let's look at another analogy. The supply authority fills our bath with electrical power, we drain off that power to use, but the bath has a small leak - bad power factor. The supply authority, in the past, charged the consumer for the power drained off but now we are being charged for the power supplied to fill the bath. This means that the consumer has to bare the cost of the leakage and for many plant managers a poor power factor has long been seen as just one of the necessary evils of an engineer's life. Now he has to improve the bad power factor or pay the extra cost.

Power Factor Correction.

To understand power factor correction we need a quick refresher on the basics of a.c. theory. First we need to recall that in a purely resistive ( R ) circuit, carrying an alternating current:

V = I x R

where I and V are in phase. In such a circuit, the current and voltage are exactly in phase and all the power supplied by the current is delivered to the load - the resistance.

However, in a purely inductive ( L ) circuit, the current I lags the voltage V by 90 degrees, whereas in a purely capacitive ( C ) circuit, the current I leads the voltage V by the same amount.

But what happens when we mix R, C and L in a simple series circuit? The resultant phase angle between I and V will be a function of the total impedance ( Z ) in the circuit, and:

V = I x R x Pf

In the more common inductive circuit (resistance R plus inductance L), the inductance element causes a lagging effect (I toV) or inefficient phase angle - normally 0.7 to 0.8 PF. To counter this effect and improve the phase angle closer to unity (1.0), we introduce a capacitive C load element. This capacitance can be of a fixed value or it can be switched into circuit when the individual inductive load is switched.

Clearly, it makes sense to have the power factor as close to unity as possible to keep the cost of power used as close as possible to the charged cost of power supplied.

It boils down to this: the addition of capacitors to a network supplies the reactive energy component of the load. Upstream of the capacitors, therefore, reactive demand is reduced. The result is the reduction of total power and an improvement in power factor.

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