Answer MET Question 37

Question: With reference to A.C Distribution system; Define power factor and explain the effects of low power factor.

Answer: In a purely resistive AC circuit, the voltage and current waveforms are in phase, i.e., changing polarity at the same instant in each cycle. Where reactive loads are present, such as with capacitors or inductors, the energy stored in the loads results in a time difference between the current and voltage waveforms, as the stored energy is not available to do work at the load it is termed apparent power. This is known as a lagging power factor (which is less than 1.0).
P.F. is the ratio of true power or real power (watts (W) = amps x volts) to the apparent power (VA = amps x volts) flowing to the load in an alternating current (AC) system.  Watts and VA are more commonly quoted in thousands as kW and kVA. kW and kVA in an AC system are only the same when P.F. has a value of one (unity).  Commonly the PF is found to be equal to 0.8.

Let;

Apparent Power = S

Reactive Power = Q. and

Real Power = P

Thus by definition

$\displaystyle \small \mathrm{p.f\ (Cos\phi )= \frac{P}{S}=\frac{watt}{VI}}$

Effects of low power factor.
1. A Low P.F. draws a higher internal current and the excessive heat generated will damage and/or shorten equipment life.
2. Increased reactive loads can reduce output voltage and damage equipment sensitive to reduced voltage.
3. Low P.F. requires equipment to be constructed heavier to absorb internal energy requirements
4. Low P.F. will result in a more expensive system with equipment able to absorb internal loads and larger load requirements
5. Operation cost will be higher with a low P.F.
6. Higher current is required by the equipment, gives rise to high copper losses in the system.
7. The efficiency of the system is reduced.
8. Higher current produced a large voltage drop in the apparatus. This results in the poor voltage regulation.