How does 3 phase power work?


How does 3 phase power work?

In: Physics

You have four wires (typically). Three carry power, one is neutral/ground and doesn’t (normally) do anything. Each of the power wires is carrying the same AC frequency and voltage *but* out of phase…the “peaks” of power on the wires are 120 degrees out of phase. So over one “rotation”, one wavelength, of 360 degrees, each wire goes through it’s peak voltage in turn.

Electricity transmitted as alternating current travels in waves, here in the usa the waves take 1/60th of a second to pass. The phase is the timing of when the waves pass, so if you divide a second into 120 chunks a on even numbered chunks you would be at the peak of a wave and at odd numbered chunks you would be at the bottom of the valley of a wave. To change the phase of the wave you would change it to be a peak on odd numbered chunks and the bottom of a valley on even numbered chunks. If you combine those two phases at every 1/60th of a second you would be at the peak of one phase and the valley of another phase. 3 phase power works the same way where at every 1/60th of a second you would be at the top, bottom and middle of a wave. There are some applications where more than 3 phases of power are used but they tend to only have very specific uses and are pretty rare.

Any time there is a difference in voltage potential, power can flow. This is how static shocks work.

Now your house just has 120V (technically 240, but it’s center-tapped to divide it in half unless it’s a big appliance), with the ground being 0 Volts which allows electricity to flow. But you just get a single sine wave of electricity from this, which means there are instantaneous points during which power going through the system is 0.

3-phase involves having 3 different power wires all connected to the same thing. As they’re out of phase with each other, power can flow between them as while say one is at 480, another phase is at say 240 (as an imaginary example) But since you have 3 phases, there will never be a point where the net power is 0 volts to the circuit. It dips down and goes up but is never zero, nor “negative.” This can be quite helpful with things like electric motors — they can put out more work, and to make them run backwards you just flip any 2 phase connections.

Think of it this way (ELI5 vast-oversimplification) power comes in cycles called waves where it goes from high to low to high to low. The high part is what we need to make electricity do something useful, the rest of the cycle doesn’t help us (I know, I know, keeping it simple here). Let’s say a single phase of power cycles once a second,-

Single Phase: 0 seconds it’s high, 0.25 its middle, 0.5 it’s low, 0.75 its middle again, at 1 sec it’s high. That’s a lot of time where we can’t use the electricity.

A 2 phase system is has a second wave half a step out of sequence with the first. so at 0 seconds one phase is High and the other is low. at 0.5 seconds the first phase is low but the second is now high. This gives us 2x the Highs per second, so this system is 2x more useful than a single phase.

A 3 phase is the same logic, now we have 3x the highs per second making the system 3x more powerful.

The implication is that we can build systems 3x more powerful on a 3 phase BUT WE COULD ALSO use a machine with the same output as the single phase, but 3x smaller using 3 phase. Not the just the machine, we can use smaller wires, smaller *other complicated electrical stuff*, making a much cheaper device in 3 phase than the same output in 1 phase.

Normal power is like pedalling a bike with two pedals; it’s very hard at the instant when one of the pedals is all the way down and the other is all the way up. If you could have a bike with three evenly-spaced pedals (and three legs!) then that problem is solved and the power flows more efficiently and smoothly.

Single phase has 2 wires carrying one unit of power. 3 phase has 3 wires carrying 3 units of power. One more wire, two more units of power.

Each phase uses the other two phases as the return wires. You can supply power with nothing but hot wires.