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POWER FACTOR CORRECTION (PFC)
Introduction to Power Factor Correction
What is power factor correction, what does it do and how do I work it out what rating I need for my
design.
With rising electrical costs and more power being used every day power factor correction is an
important part of many designs.
What is Power Factor Correction?
Power factor is the ratio between the actual load (kW) and the apparent load (kVA) drawn by an
electrical load. Simply, it is a measure of how efficiently the load current is being converted into
useful output and more particularly is a good indicator of the effect of the load current on the
efficiency of the supply system.
In an electric power system, a load with a low power factor draws more current than a load with a
high power factor for the same amount of useful power transferred. The higher currents increase
the energy lost in the distribution system, and require larger wires and other equipment. Because
of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher
cost to industrial or commercial customers where there is a low power factor.
What does it do to an electricity bill?
In a 3 phase power supply, kW consumed is 3 phase VOLTS x AMPS x 1.73 x Power Factor.
The Electricity Company supply you VOLTS x AMPS and they have to supply extra to make up for
the loss caused by poor Power Factor.
When the power factor falls below a set figure, the electricity supply companies charge a premium
on the kW being consumed, or, charge for the whole supply as kVA by adding reactive power
charges (kVar) to the bill.
How does PFC work?
Capacitive PFC is applied to electric circuits as a means of minimising the inductive component of
the current and thereby reducing the losses in the supply. The introduction of PFC capacitors is a
widely recognised method of reducing an electrical load, thus minimising wasted energy and
hence improving the efficiency of a plant and reducing the electricity bill. It is not usually necessary
to reach unity, ie a power factor (PF) of 1, since most supply companies are happy with a PF of
0.95 to 0.98 by installing suitably sized and controlled switched banks of capacitors into the circuit,
the PF is improved and the value becomes nearer to unity thus minimising wasted energy and
improving the efficiency of a plant. The purchase cost of the installation is usually repaid in less
than 1 year of electricity savings.
How much PFC do I need and how do I work it out?
You can calculate the PFC required by using the following equations:
Single phase circuit calculation
Power factor calculation:
PF = |cos φ| = 1000 × P(kW) / (V(V) × I(A))
Apparent power calculation:
|S(kVA)| = V(V) × I(A) / 1000
Reactive power calculation:
Q(kVAR) = √(|S(kVA)|2 - P(kW)2)
Power factor correction capacitor's capacitance calculation:
C(F) = 1000 × Q(kVAR) / (2πf(Hz)×V(V)2)
Three phase circuit calculation
For three phase with balanced loads:
Calculation with line to line voltage
Power factor calculation:
PF = |cos φ| = 1000 × P(kW) / (√3 × VL-L(V) × I(A))
Apparent power calculation:
|S(kVA)| = √3 × VL-L(V) × I(A) / 1000
Reactive power calculation:
Q(kVAR) = √(|S(kVA)|2 - P(kW)2)
Power factor correction capacitor's capacitance calculation:
C(F) = 1000 × Q(kVAR) / (2πf(Hz)×VL-L(V)2)
Calculation with line to neutral voltage
Power factor calculation:
PF = |cos φ| = 1000 × P(kW) / (3 × VL-N(V) × I(A))
Apparent power calculation:
|S(kVA)| = 3 × VL-N(V) × I(A) / 1000
Reactive power calculation:
Q(kVAR) = √(|S(kVA)|2 - P(kW)2)
Power factor correction capacitor's capacitance calculation:
C(F) = 1000 × Q(kVAR) / (3×2πf(Hz)×VL-N(V)2)