The idea in one picture
Imagine pouring a glass of beer. You want beer, but you also get foam on top. The beer is the useful part, and the glass has to be big enough to hold beer plus foam. Power factor describes how much of the glass is beer.
In an alternating current system, some current does useful work, such as turning a shaft or producing heat. Some current only moves back and forth to build the magnetic fields that motors and transformers need. The second kind does no net work, but it still flows through wires, transformers and breakers.
Three kinds of power
- Real power (kW) is the power that does useful work.
- Reactive power (kVAr) is the power that sustains magnetic or electric fields. It moves back and forth rather than being consumed.
- Apparent power (kVA) is the combination of the two, and it is what the supply equipment must be able to deliver.
They form a right triangle, with real and reactive power as the two shorter sides and apparent power as the longest side. Power factor is the ratio of real power to apparent power. A value of 1.0 means all of the current is doing useful work. Lower values mean more of the current is reactive.
A hypothetical example
Suppose a plant draws 80 kW of real power and 100 kVA of apparent power. These are made-up numbers for illustration.
- Power factor = 80 / 100 = 0.8
- Reactive power = the square root of (100 squared minus 80 squared) = the square root of (10,000 minus 6,400) = the square root of 3,600 = 60 kVAr
If power factor were improved to 1.0 for the same 80 kW, the apparent power would drop to 80 kVA. That is 20 percent less than the 100 kVA, which means the same wiring and transformers would carry noticeably less current for the same useful work. In real plants, the improvement is rarely complete, but the principle holds.
Why low power factor is a problem
A low power factor means more current for the same useful output. That has several effects:
- Cables, transformers and switchgear carry extra current, producing more heat and using up capacity that could serve other loads.
- Voltage drop along feeders increases.
- Losses in the plant’s distribution system rise.
- Some utilities add a charge or penalty when a customer’s power factor falls below a set level, or bill based on apparent power or demand. Whether and how this happens varies by utility and by country, so tariffs should be checked rather than assumed.
What causes it
Induction motors are the classic cause. They need magnetizing current, and when they run lightly loaded the power factor can be poor, because the magnetizing current is a bigger share of the total. Transformers, lighting ballasts, induction furnaces and welding equipment also draw reactive current. Oversized motors that run well below their rating are a typical contributor.
A second, separate effect comes from distortion. Nonlinear loads such as variable speed drives can lower the true power factor through harmonics even when the basic displacement is good. This ties power factor to the wider subject of power quality.
Common ways to improve it
Capacitors. Because capacitors supply reactive power, they can offset the reactive demand of motors. They can be installed at individual motors, at distribution points or as a central bank at the main service. Automatic banks switch sections in and out as the load changes.
Right-sizing motors. Avoiding large motors running at light load keeps power factor higher.
Synchronous motors and drives. In some applications, synchronous machines can be set up to supply reactive power, and some drives present a better power factor to the supply than direct-on-line starting.
Active correction. Electronic equipment can correct both power factor and harmonics, though at higher complexity.
Cautions
Capacitors are not simply bolted on, and the equipment around them, such as electrical panels, must be rated for them. Too much capacitance can overcorrect and raise voltage, and capacitors can interact with harmonics, creating resonance that damages equipment. They also store energy after being switched off and need discharge time before anyone works on them. Selection, installation and maintenance should be done by qualified people following manufacturer instructions, lockout tagout procedures and local rules.
Where it fits in energy planning
Power factor is one item among several in a plant’s energy picture. It often appears next to energy management systems, motor sizing and motor energy efficiency discussions, because oversized or lightly loaded motors affect all three. Plants with large banks of three phase motors usually see the biggest effect.
Is it worth fixing?
Whether correction pays off depends on the tariff, the equipment already installed and the cost of capacity upgrades. Some plants find it valuable, others find little benefit. A sensible first step is to look at utility bills and to measure the plant’s actual power factor and load profile before deciding.