ELI5 - Electricians, Single Phase & 3 Phase Power
AC stands for alternating current. The electricity comes through (current) in waves up and down (alternating) 230v plus to 230v minus.
In a single phase supply, there is just one wave. In three phase there are three waves.
Picture this as kayakers.
In single phase there is one kayaker, putting the left hand paddle in (230v +) and then the right (230v -) which pushes the kayak along. Albeit in an uneven, jolting way.
In three phase, there is one kayak (supply), but three kayakers. They don’t paddle at the same time but instead sequentially one after the other but still equally spaced apart. This means that the paddles are still alternating, but the thrust created is more evenly distributed - three left hand paddles are hitting per second instead of one per second with the single kayaker and vice versa for the right hand paddle resulting in a smoother ride. Under normal load they are power is delivered more evenly and when more thrust (load) is required they have more capacity to smoothly accellerate.
Single phase is (was) fine for standard households as lighting and general household appliances were well within a single kayaker’s ability. Picture this as them towing something small along - slight extra effort to give it momentum and then you can maintain it, but it might be quite jerky with one kayaker doing the paddling - the jerk getting worse the heavier the load.
This is particularly important for electrical motors - where a circular magnetic field is created/required - as the three kayakers have less down time between paddles (waves) so they have the ability to provide a smooth energy flow. This is why industrial installs are three phase as standard.
As we have got more used high draw items domestically like air conditioners and heat pumps, a single kayaker can still handle the workload but a ‘pushstart’ might be necessary in the form of capacitors or similar to give an initial boost particularly as there is a noticeable gap between the times the paddles are in the water. However this is more a work around than an ideal way forward so new installs tend to have three phase installed.
The first thing to understand is that AC power is a sine wave. The voltage goes up and down over time. With single-phase power, there is only a single sine wave. With three-phase power, there are three sine waves, 120 degrees out of phase.
The important difference here is that with single-phase power the load is connected between the phase and the neutral wire, while with three-phase power the load is connected between phases. Some three-phase-only equipment will directly connect between two phase wires in a delta (Δ) configuration, but most loads are placed in a wye (Y) configuration. When properly balanced the neutral wire of a Y configuration will have nearly zero load, as the power going “in” through the load on one phase will go “out” through the loads on the other two phases. The neutral wire only needs to carry the remaining power due to load differences between phases. This means three-phase systems need far less wire to transmit the same amount of power: a three-phase system can transmit twice as much power for only 1.5x as much wire - or even less!
In practice the entire power distribution system is set up as a three-phase system. If your house has a single-phase connection, it is simply connected to only one of the three phases. Your neighbor will be connected to a different phase, so on average your entire street will present itself as a roughly balanced three-phase load.
If your house has a three-phase connection, most of your equipment will be connected to a single phase and neutral. Your breakers will alternate between the different phases, just like the houses will be with single-phase connections. Although the individual breakers aren’t balanced, the total load is - and any slight imbalance is compensated by the rest of the neighborhood being imbalanced on other phases.
Some high-power equipment is designed for three-phase power. In a residential home this will be stuff like a car charger, heat pump, or a heavy-duty electric stove top. They will be connected to a special three-phase breaker.
North America is a little bit different, because they have something called a “split phase”. In essence, they take a single phase of a three-phase system and split it in two by creating a new neutral point in the middle. This means you have two “phases” at your home which are 180 degrees out of phase, so if one is at +120V the other one is at -120V. This gives you access to either a regular 120V, or 240V across the two phases for high-power equipment. Neat idea, but the downside is that in the same setup the voltage difference between two of the three original phases is 208V - so you now have two different voltages for high-power equipment.
See Also
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- - ELI5: Human Progress
- - ELI5: Insects and sunlight