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.
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