AC FUNDAMENTALS - PART – 10 - SIGNIFICANCE OF POWER FACTOR

TRUE POWER - One watt of true power means that electric energy is converted into heat energy at a rate of one J/sec.

APPARENT POWER - One volt-ampere (VA) of apparent power converts less than one J/sec.

How much less depends on how far the voltage and current are out of phase.

Alternators and Transformers and Circuit Breakers are rated in kVA because the power factor of the load is not known when the machinery is manufactured in the factory.

CAUSES OF LOW POWER FACTOR

The low power factor is mainly due to the fact that most of the power loads are inductive and, therefore, take lagging currents.

1. Single phase and three phase induction motors which have low lagging p.f. (Inductive load).

2. The motors work at a power factor which is extremely small on light load (0.2 to 0.3) and rises to 0.8 to 0.9 at full load.

3. Arc lamps, electric discharge lamps and electric furnaces and welding equipments operate at low lagging power factor.

4. The load on the power system is varying; being high in the morning and evening and low at other times.

5. Low load period, supply voltage is increased which increases the magnetization current which leads to poor power factor.

DISADVANTAGES OF LOW POWER FACTOR

1. Large kVA rating of equipment

kVA = [Active power / Power factor] = kW / cosθ 

The rating is inversely proportional to power factor. If the power factor is low the kVA rating of the machine is high, hence the making the equipment larger and expensive.

2. Large copper losses

The larger current at low power factor leads to more I^2 losses in all the elements of the supply system. When losses are more results in poor efficiency.

3. Large conductor size

To carry higher current the size if the conductor will have more cross section which increases the cost and size of the cable.

4. Poor voltage regulation

The large current at low lagging power factor causes greater voltage drop in alternators, transformers, transmission lines and distributors. The decreased voltage available at the supply end, thus affecting the performance of utilization devices. 

5. KVA rating of the equipment increases

Power factor and KVA rating are inversely proportional and hence if power factor increases KVA rating decreases resulting in the reduction of capital cost.