what is power factor in electricity?
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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.
Each power cycle, you give a big push on a rod of power to the grid. When we have perfect power factor, the grid simply takes it all perfectly – you have a constant resistance on the ride you’re pushing.
When power factor is off, you give a push, but at first the resistance is very low, so the rod goes forward really far and takes a lot of energy from you. But later in the cycle, the grid actually starts pushing it back.
Essentially it’s a short-term load of energy you give to the grid each cycle. The grid ultimately pays it back, but the transmission lines always take a commission on each transaction.
Imagine voltage and current flowing down a wire. In a purely resistive load the Voltage is in the vertical axis and the current is in the horizontal axis, such that if your where to draw it, it would resemble a string of beads (AC system). When Voltage is zero, current is zero. When Voltage is maximum, current is maximum.
The total volume inside of each bead is the power dissipated in the resistive load.
In a reactive load such as a motor, the winding inductance causes the current to rotate off the 90 degree and the string of beads is now kinda oval or squashed. The volume inside the oval is reduced. Power dissipated in the load is reduced.
The Power Factor (PF) represents the amount of squashy relative to a perfect bead. It’s a ratio so it has no units.
It is important to consider PF because to dissipate 600W into a reactive load vs dissipating 600W into a resistive load requires more power from the source.
Another problem is the current can rotate so much that it become in-phase with the voltage. The area of the bead drops to zero and no power is imparted into the load. This is used by inductors in blocking high frequency signals such as in speaker crossovers. But the voltage and current sum in phase and create a peak and this can exceed breakdown voltages in components or wire sheathing.
Finally something with a large Power Factor will also mean a large inrush current before the reluctance fields reach equilibrium, and this can exceed systems’ specs at startup.
Really what it is saying is how much energy is your system using efficiently. I really describe it as this. Imagine you are pushing a heavy object to slide it straight across the room . The most efficient way to do that is you try to push it from directly behind it. In this way your pushing (voltage) and it moving across the floor( current) is completely in phase. So PF =1.
Now the above is great in a perfect world. But now let’s say let’s say the heavy object(maybe think fridge or stove), and you can’t get straight behind it so along with pushing it back you also push it towards the left. Now your pushing in two directions but really only care about pushing it straight back. You are still expending energy to push it to the left though. In this case you are spending desired energy to push it back but also in effect using energy to push it to the left. PF is the measure of how efficient you are pushing that object.
Don’t think I can do this unless the 5yo understands basic trign functions.
You know voltage is measured in volts, and current is measured in amps. Well, power is measured in watts.
A watt is a volt multiplied by an amp.
So, to get the power drawn by a device, you multiply the voltage by the current it draws. This gives you the power it is drawing.
Except that it doesn’t really, particularly with alternating current.
The simplistic amps x voltage works if the load behaves more or less like a simple resistor, but mant loads aren’t
If we’re talking about an AC motor, it’s coils will act like an inductor, which will cause the current to be out of phase with the voltage. Thus a simple multiply becomes a complicated trign expression with a ‘phase difference’ between two sine waves.
If we’re talking about a modern switch-mode power supply, we get the opposite problem because the load now looks like a big capacitor.
So power factor is a measure of the difference between the current and voltage phases. Some energy companies bill differently according to power factor, as it affects where the waste heat energy is dumped and how hard the generators have to work.
Power factor is the ratio between Real Power (the power used to actually power things) and Apparent Power, the total power that is the sum of the Real and Reactive Power.
Think of it in terms of a glass of beer. Reactive power is the froth, while the beer is the Real Power. The whole glass is the Apparent Power. The ideal ratio is minimum amount of froth (in this case 0%) and maximum amount of beer (100%, since you want the most beer out of your buck).
This kind of situation is called unity, when the power factor is 1 (or 100%). But in real life, there’s always some froth to your beer, as there is Reactive Power to your Real Power. Hope this helps!
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