Power factor is the measure of evaluating how effectively the incoming electrical power is used in an electrical system. It is defined as the ratio of Active Power (kW) to Apparent Power (kVA).
In
this article we are going to discuss what power factor is and why it is
important? What is the physical significance of power factor in Electrical
system?
But
before that, we have to understand the basic concepts of Electrical power and
brief analysis of Active, Reactive and Apparent Power, so that it will be easy
to understand the concepts of Power Factor. Because the concepts of power
factor are derived from Electrical power concepts. We have already discussed the concepts
of Electrical power (Active, Reactive and Apparent Power) in our previous
Article. So please follow my previous Article about Electrical Power titled as:
Electrical Power- Active, Reactive and Apparent Power.
Well,
now coming to our main topic which has mainly two parts. First, what is power
factor? And other is why it is important?
So let’s discuss it one by one in detail so that
it will be easy to understand.
What is Power Factor..?
Power factor is the measure
of evaluating how effectively the incoming electrical power is used in an electrical
system. If the power factor is high, then we can say that more effectively the
electric power is being used in an electrical system. A load with power factor of 1 results most efficient loading of the system. But if the power factor is
poor (say 0.8), then the effectiveness of usage of electrical power reduces which results in higher losses in the supply system and a higher bill for consumers. PF 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 in an inductor or electrostatic energy in the capacitor.
A high PF benefits both
consumers and Power Company. Whereas Low PF indicates poor utilization of
Electrical Power. In Electrical Engineering the concept of power factor is only
discussed in AC circuits. Whereas there are no power factor concepts in case of
DC Circuit due to Zero frequency. Its value becomes 1 (unity) for DC Circuit.
But in the case of AC circuit, the value of Power Factor always lies between
the ranges 0 to 1 (0 < Cosθ
< 1).
Generally a high and leading
PF is preferred in the Electrical system. Ideally the minimum and maximum value for
PF becomes 0 and 1 respectively. But in Practical case, it is very difficult to
achieve PF unity (1). A low power factor is generally the result of inductive loads such as Induction motors, Power transformers, ballast in the luminaire, a welding set or an induction furnace. PF value near to 0.9 is considered as satisfactory.
It is calculated with the help of Power Triangle.
Power Factor Definition
There are three ways to
define the concepts of power factor in electrical engineering.
- Power factor is defined as the ratio of Active Power (kW) to Apparent Power (kVA).
P.F = Active Power(kW) / Apparent Power (kVA)
- Also Power factor is the cosine of the phase angle difference between voltage and current pharos.
P.F = CosθWhere θ is the angle between V and I.
- One more way of defining the Power Factor is, It is the ratio between resistance (R) and Total Impedance (Z) of AC circuit.
P.F = Resistance / Impedance = R / Z
Why Power Factor is Important?
It
is a very common question comes in our mind that “why Power Factor is important in Electrical System?” What is its physical significance?
To
understand this let’s consider an example. An Electrical machine runs at 100 kW
(Working Power) and the Apparent Power supplied by Power utilities is 125 kVA. Then
the first question arises in our mind is why the working power of machines and
power supplied by the Power company is different?
Well,
there is a concept of Power Factor behind that. So we have to understand it
clearly. Here if we find the PF, we divide 100 kW by 125 kVA (according to the definition of Power Factor), then we got a PF of 0.8 (80%). It means that only
80% of the incoming current does useful work in the circuit and 20% is used by
reactive elements in the circuit. In other words, we can say that 80% of the power
is useful power which is also known as real or True Power, and the rest 20% of
power is Reactive Power used by reactive element in the circuit.
Have a Look: Discovery of Electricity
Because
the Electric Company must supply total apparent power (kVA) to the consumer and
in return consumer pay them only for real power (kW). Due to poor PF, the consumer
pays fewer amounts to Power company. It will be considered a loss for power
company as well as no benefit for the consumer. So power company penalizes the
consumer for maintaining the poor PF. So higher PF is beneficial for both power
utilities as well as consumers.
Problem with Low Power Factor
It can be observed that an increase in reactive power causes
a corresponding decrease in Active Power as well as power factor. It means the
power distribution system is operating less efficiently because not all the current
is performing useful work in the circuit. For example, a 50 kW load with a power factor of unity (Reactive power = 0 kVAR)
could be supplied by a transformer rated for 50 kVA. However, if the power factor is reduced to 0.7 (70 %), then the transformer must
also supply additional power for the reactive load. So In this example, a
larger transformer which is capable of supplying 71.43 kVA (50/0.7) would be required. In addition to that, the size
of the conductors would have to be increased to supply an increased amount of
current due to low PF. So due to poor PF the cost of equipment also increased.
There
are three types of power factor in electrical science. These are leading power factor, Lagging power factor and Unity power factor. Let’s discuss all
these types of power factor one by one in detail.
1.
Leading
Power Factor: A leading power factor signify that the load in
the circuit is capacitive. As the load supplies reactive power to the circuit,
therefore in this case the reactive component (Q) of Electrical power will be negative. If in the electrical circuit
there is more capacitive reactance than inductive reactance, then PF of the circuit will be leading power factor. In
this case, the operating current will lead the voltage phasor by an angle θ. whereas in
pure capacitive circuit current leads the voltage by θ = 90 degrees.
2.
Lagging
Power Factor: A lagging power factor signify that the load in
the circuit is inductive. As the load will consume reactive power, therefore in
this case the reactive component (Q)
of Electrical power will be positive. If in the electrical circuit there are more
inductive reactance than capacitive reactance, then PF of the circuit will be lagging power factor. In this case, the
operating current will lag the voltage phasor by an angle θ. Whereas in
pure inductive circuit current lags voltage by θ = 90 degrees.
3. Unity Power Factor :
A unity power factor signifies that the load in the circuit is purely resistive.
As the load will consume active power, therefore, in this case, the reactive
component (Q) of Electrical power
will be zero. Hence in case of a unity power
factor, total apparent power will be utilized in the circuit. In the electrical circuit, if inductive reactance and capacitive reactance balance each
other, the circuit will be in resonance condition and considered as purely
resistive hence PF of the circuit will be unity
power factor. In this case, the operating current will in phase with voltage
phasor. In this case θ will be 0 degrees.
Must Read: Causes and Disadvantages of Low Power Factor
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