Eli5: What’s the difference between a single phase electric current versus a 3 phased one?
In: 18
I always think of old movies I’ve seen of men driving railroad spikes with sledge hammers. One man swinging a hammer goes *clang……clang……clang……clang……* as he drives the spike and swings the hammer again. But sometimes a team of three men will all drive one railroad spike and it goes *clang clang clang clang clang clang* as they swing their hammers in perfect timing and the spike is being hit continuously.
That’s what electric power is doing. Three magnets are “pushing” the power in sequence instead of one magnet “pushing” and then a pause as it draws back to “push” again.
If a house has both single and 3 phase power to it (due to renovation and extension of an old house) and there is a power blackout in the area, it is possible to lose lights in some rooms but not power points and lose power points but not lights in others?
Odd question, I know, but the rare occasion that power goes off in my street I get some things working and others not but my neighbours are all powerless. The builder I used told me it was due to different phase power coming into the building…..
Does this make sense?
For practical purposes: A three fase system can carry three times the energy using only 1.5 as much wiring, so it saves cost on infrastructure.
In a 50 hz system, a one fase system completes a full sinus-curve 50 times a second, while a three fase system completes three times as many. So the power is «cleaner».
There are also many other advantages to a three fase system, but it is difficult to explain in simple terms.
Imagine 3 points on a circle equally spaced apart, and another point in the centre of the circle.
When we talk about 3 phase, we are talking about the voltage between the 3 points on the outside of the circle. When we talk about single phase, we are talking about only one (any one) of the outside points and the point in the centre.
The distance between the outside points are further away from each other, which is kind of the same as a higher voltage between each of the 3 phases, compared to the distance from one of the points to the centre.
The peak voltage between one outside point and the others changes in a kind of rotational “dance”. Big heavy motors are able to use this as it is like having a little extra help to get started.
There are two types of power. One is like a steady pull and the other shakes the object to get it moving. 3 phase is the shaking kind.
By shaking instead of a steady pull, you can put much more energy into your wire (power loss issues) but with the trade off that your object is shaking violently.
Now, you aren’t REALLY shaking the object, that’s just a metaphor. What 3 phase does is get three people “shaking” objects, at the same speed when it was one person, but at different times so that they move like they are doing the wave.
In this example, the “output” only cares when an object is pushed forward, and as soon as another object is farther forward than the other two, will look at that one.
So your “power supply” goes from
– ^ v / ^ v / ^ v
To
– ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
Which makes for a smoother operation while still getting the larger power output than a steady push.
All of these smart people here are having a problem explaining this at a conceptual level:
There are two forms of power, DC and AC, and you can imagine them as a rope, and the power as friction in your hands. DC (direct current) is like having a rope tied in a circle and constantly spinning it. The rope goes one direction and causes friction (power) on your hands. However, this requires all power to be looped to the power source– not practical on a large scale.
So the power that goes to your house is AC (alternating current) , which you can think of as a single rope. Instead of going in a loop, the rope is tugged back and forth, also creating friction (power) in your hands.
The problem with AC is that when the rope switches direction, the power being produced hits zero at some point– sort of pulse-like. This is not good for some types of equipment. To mitigate this, they take 3 different ropes that are all in different “phases” of being pulled, so there is never a time where the power equals zero, and the power is more or less consistent.
Here’s a diagram:
https://www.otterbine.com/media/1504/cablewave.png?width=400&height=299
Edit: I’m not an engineer or electrician. My analogy is purely eli5, and I recommend you read other comments below that expand or correct it.
Single phase is poorly named. It’s actually two phases, but electricians have a weird sense of humour.
Electricity is generated as three phase power at the power plant by turning a motor also known as a generator with a few extra parts. Ultimately, you will receive two of these phases into your home. This is known as a single phase system although you are getting two phases.
Simply put (without explaining how alternating current waves work) it is important to remember that nominal voltages are the average of the voltages sign wave. I believe typical max volts is around 169v for 120v system, but my math may be wrong on that as I have been out of school for quite some time.
Three phase is 3 electrical supplies that are 120 degrees apart from each other (in relation to their sign wave pattern) that results in an efficient system that allows 3 “hot” or supply conductors to feed circuits while only using 1 neutral. With the phases being 120 degrees out of phase with each other, it does not result in a voltage that doubles similar to single phase. An example is 1 conductor + neutral = 120v and 2 conductors = 208v
Single phase has 2 electrical supplies that are not out of degree with each other, which results in doubling the voltage when using two conductors. An example, 1 supply conductor + neutral = 120v and 2 supply conductors = 240v this example applies to North America.
These comments suck to the core. Single phase AC means the current fluctuates between +240V to -240V then back to 240V about 50 times a second (for outside USA). Because a single phase AC crosses 0V, it can make things that needs massive constant power to sag when current passing thru 0V. So what do we do to solve this? We add more AC lines but each AC line is slightly more delayed; hence, a different phase. Three phase AC basically means three AC lines that are timed preciswly that the AC of combined lines never cross 0V.
If your workshop has a massive AC electric motor, chance is that it is designed to use three phase lines natively, so that the motor runs smoother and can sustain massive power over multiple AC lines. Three phases have 4 to 5 lines.
Here is the real kicker. All hydro turbines generate three phase AC natively. If your house runs on a single phase, it means your house is connected to one of the three phases. Other houses connect to the other phases.
Latest Answers