What’s the difference between single and three phase?


Please explain the difference between single phase and three phase. I’ve tried to figure it out but I can’t get a grasp of it.

In: Physics

Are you talking about electricity, specifically alternating current (AC) ?

Single phase current is what you have in your house–the electricity comes in on a single wire, and it alternates (switches direction). If you were to actually look at the current you’d see it doesn’t just switch abruptly from one direction to the other, though, it follows a sine wave.

Three phase current is basically the same, but instead of having just a single wire, you have three of them, and the sine waves travelling through each are offset from each other so they come to their peaks at different times. It’s actually how power is generated in the first place, and it’s also used in most AC electric motors because it’s really easy to create a rotating magnetic field using it.

Lets say you have a spinning magnet, and put a coil next to it, this generates 1 phase power, and uses 2 wires. Now lets add 2 more coils, evenly spaced around the spinning magnet, and wire them up so that the current going out on one wire comes back on the other 2. You get 3x the power with 1.5x the wires, making it more efficient. It’s also convenient having 3 phases for motors, as you can control which direction the motor rotates very easily, just swap two of the wires if you want to reverse the direction. With single phase, if it’s just a coil and a magnet, it’s essentially random which direction the motor will spin when you turn it on, so you have to do more complicated stuff like using a capacitor or shaded pole to make it start spinning a specific direction.

if speaking about electricity, single phase uses only two wires, a load-bearing phase wire and a return wire and usually operates around 240V. It’s cheaper and more efficient for things that require less power. A three phase uses 3 wires with alternating current and a fourth that remains neutral / depending on the configuration you may need the ground wire. Three phase costs much more to repair but that’s about all I know

I’m assuming you are referring to AC power transmission, so I’ll explain that.

For single phase power, there is a ground wire, and a live wire that alternates from positive to negative in a sine wave, using the ground wire as its zero point. It’s convenient for short runs and lower voltages, as Single phase is easy to get into DC power.

Three phase power’s quite a bit more complicated, but enables using thinner wires (think three smaller wires instead of two large ones). It has three wires. Each one has a similar sine wave to the live wire of the single phase, except they’re all out of phase. That means that when Wire 1 is at it’s zero point, wire 2’s a third of the way through it’s wave, and wire 3’s two thirds through its cycle.

For a graphical explanation that may or may not help, imagine a normal X-Y graph (I know, this isn’t 5 year old math.), with a circle centered at (0,0). There are three spokes 120 degrees apart, and where those spokes meet the circle, there are points. The Y value of points 1, 2, and 3 represent the voltage of wires 1, 2, and 3. So as time goes on, the circle rotates, each point following a third of a rotation behind the previous one.