The power factor of an AC electric power system is defined as the ratio of the active (true or real) power to the apparent power
Reactive power is required for the magnetization of a motor but doesn't perform any action. The reactive power required by inductive loads increases the amounts of apparent power - measured in kilovolt amps (kVA) - in the distribution system. Increasing of the reactive and apparent power will cause the power factor - PF - to decrease.
It is common to define the Power Factor - PF - as the cosine of the phase angle between voltage and current - or the "cosφ".
The power factor defined by IEEE and IEC is the ratio between the applied active (true) power - and the apparent power, and can in general be expressed as:
PF = P / S (1)
PF = power factor
P = active (true or real) power (Watts)
S = apparent power (VA, volts amps)
A low power factor is the result of inductive loads such as transformers and electric motors. Unlike resistive loads creating heat by consuming kilowatts, inductive loads require a current flow to create magnetic fields to produce the desired work.
Power factor is an important measurement in electrical AC systems because
International standards such as IEC 61000-3-2 have been established to control current waveform distortion by introducing limits for the amplitude of current harmonics.
A industrial plant draws 200 A at 400 V and the supply transformer and backup UPS is rated 200 A × 400 V = 80 kVA.
If the power factor - PF - of the loads is only 0.7 - only
80 kVA × 0.7
= 56 kW
of real power is consumed by the system. If the power factor is close to 1 (purely resistive circuit) the supply system with transformers, cables, switchgear and UPS could be made considerably smaller.
Any power factor less than 1 means that the circuit's wiring has to carry more current than what would be necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load.
A low power factor is expensive and inefficient and some utility companies may charge additional fees when the power factor is less than 0.95. A low power factor will reduce the electrical system's distribution capacity by increasing the current flow and causing voltage drops.
Power factors are usually stated as "leading" or "lagging" to show the sign of the phase angle.
Inductive and capacitive loads stores energy in magnetic or electric fields in the devices during parts of the AC cycles. The energy is returned back to the power source during the rest of the cycles.
The total power required by an inductive device as a motor or similar consists of
The power factor for a three-phase electric motor can be expressed as:
PF = P / [(3)1/2 U I] (2)
PF = power factor
P = power applied (W, watts)
U = voltage (V)
I = current (A, amps)
|Power (hp)||Speed (rpm)||Power Factor|
|1/2 load||3/4 load||full load|
|0 - 5||1800||0.72||0.82||0.84|
|5 - 20||1800||0.74||0.84||0.86|
|20 - 100||1800||0.79||0.86||0.89|
|100 - 300||1800||0.81||0.88||0.91|