what is power factor in electricity?


what is power factor in electricity?

In: 362

Power factor is the amount of effective energy coming out of a system, divided by the amount of energy that was put into the system.
Meaning the efficiency of the system, or in other words how much power is lost to the system.

Think about what happens when you push someone on a swing. You start pushing when they are swinging backwards, but they don’t start swinging forwards immediately. First they slow down, then accelerate in the other direction. The swinging is an oscillation, and the swing is out of phase with the pushing. This means that when the push is at a maximum (top of the swing) the speed is at a minimum (the person is momentarily stationary at the top of the swing).

When talking about power factor we are talking about AC electricity – the current and voltage are oscillating backwards and forwards like the person on a swing. The current is like the speed of the person, and the voltage is like the amount of pushing that you are doing.

The reason the person doesn’t change direction immediately when you start pushing is that they have momentum, because they have mass. An electric circuit can also have momentum. Electric motors in particular (a major load in industrial applications) have a property called inductance which resists any change in current (just like mass resists change in speed). Inductance is caused by magnetic fields generated by coils of wires carrying current.

If a circuit has lots of inductance then it will resist changes in current, so as the voltage oscillates the current will lag behind. The amount of lag is the power factor.

Alternatively a circuit can have lots of capacitance which is sort of the opposite of inductance. Electronics have capacitance typically. A capacitive circuit will have current actually ahead of voltage, which gives a negative power factor.

This all matters because it is only the in-phase part of the current and voltage that consume power (called real power). The out of phase part adds to the current oscillating backwards and forwards and so affects losses in transmission lines, but doesn’t consume power (this is called imaginary power). If you are familiar with work equalling force multiplied by distance then you can see that pushing someone on a swing requires no work at the very top of the swing when they are momentarily not moving, despite this being the moment that you are pushing hardest.

You can correct power factor; a factory with lots of motors may have a bank of capacitors to reduce the power factor and so the load on their distribution infrastructure (and in some jurisdictions I believe industrial users pay for the imaginary as well as real part of the power they receive).

Edit: thanks for all the kind words.

Analogies like this are really useful because they are not only helpful illustrations; the underlying mathematics of oscillating systems is identical regardless of whether they are mechanical or electrical. The way you adjust the position of a weight on a clock pendulum to change its frequency is exactly equivalent to the way you turn the dial of a traditional radio and adjust a variable inductor and so change the frequency of station it picks up.

In fact an old lecturer of mine invented a component used in formula 1 cars called an inerter by looking at suspension systems and realising that they were missing a component that was analogous to an inductor in electrical circuits.

But it very much is, when talking about the Power Factor of the insulation of a transformer. Putting X amount of power into either the primary or secondary side of a transformer, and measuring how much “leaks” out the other side. Hense, the “Power Factor” of the windings.

Think of 2 people jumping on a trampoline. If they land at exactly the same time, it’ll totally launch them. That would be a “perfect” power factor.

If one lands right after the other, then there isn’t as much “power” because they landed at different times (“out of phase”).

It isn’t a perfect analogy, but when voltage and current are out of phase, the power factor is reduced.

For the record: even most power systems specialized electrical engineers don’t have an intuitive understanding of what’s going on with power factor and reactive power, at least right out of college or in their early careers. They learn the math and know what they need to *do* with it, but most wouldn’t answer this question very well until after working with it for years in real applications.

I think the top post is as good an analogy as I’ve seen. But all analogies are lacking, because there’s no other mechanic for comparison where oscillation has to happen on two dimensions for power to transfer.