Power factor
Power factor is the fraction of the current that is actually doing useful work — the ratio of real power to the total apparent power, somewhere between 0 and 1. When it is 1, every amp is earning its keep. When it is lower, some of the current is moving energy back and forth without doing anything useful, and it still has to be carried.
Why current can flow without doing work
A motor, a transformer, a ballast — anything with a coil in it — stores energy in a magnetic field on part of the cycle and gives it back on the rest. That exchange draws current, but over a full cycle it nets out to no useful work; it just fills and empties the field. Because it makes the current lag the voltage, the trade calls the load inductive, and the lagging current is the part power factor is describing. It is real current in the conductors, warming them and taking up capacity, even though it turns nothing.
That is why the numbers matter in the field. Real power in kilowatts is the work being done; apparent power in kVA is the total the system has to supply; power factor is the first divided by the second. A poor power factor means you haul a lot of kVA to deliver a little kW, so conductors and transformers run fuller for the same useful output — which is exactly why utilities and engineers push to correct it.
Questions people ask
What is power factor in plain terms?
The fraction of the current that is actually doing useful work. It runs from 0 to 1. A power factor of 1 means every amp is pulling its weight; lower than that means some of the current is sloshing back and forth without doing work, yet still has to be carried.
Why do motors have a low power factor?
Because they are inductive. An inductive load makes the current lag behind the voltage, and that out-of-step portion moves energy into and out of the motor's magnetic field without turning it into useful output. That lagging current still flows in the conductors, so it still counts.
How does power factor relate to kVA and kW?
kW is the real power doing work; kVA is the total apparent power the system has to supply. Power factor is kW divided by kVA. A poor power factor means you move a lot of kVA to get a little kW, which is why utilities care about it.
Related
- Wye and delta
- Phase formulas sheet — where pf appears in the power equations.
- Back to the glossary
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