Can anyone define 3 phase electrics please ?

Question

...as in wiring differences etc and when/why it's used ?

Answer 1

Hi,
There is two and six phase juice, but for special applications.
Basically, most electrical generators drive an alternator with the three coils of copper, equally opposed. This in then distributed.
Most efficient. Look at an overhead power line - it will have three lines, called red at the top, blue in the middle, and yellow at the bottom, or the same looking left to right in the direction of transmission, an say, on a 66kV, 33kv local distribution line.
Drop that down to 415V at your local sub-station transformer, across phases, to a domestic area then takes one phase to neutral (earth) to give you root three of that in the UK, nominally 240V
However, if you run to a farm or factory, then a lot of kit will have 3-phase motors, so you bring all three in. It is more efficient to do so.
Your house may be on any of the three, as the load is then balanced between areas, a street away perhaps. This balances the distribution lines, and therefore the alternator is happy, the prime mover is happy and all is well.
I could draw diagrams, but those are the main issues.
Apart from some roadside sparks who seemingly often just hook up a new housing estate to red phase, without sprading the load out. It is annoying !
Same with street lighting.
I wish they would stick a bit more load on yellow now and then.
So basically, the rotor spins past three sets of coils when generating, at 120 degrees apart - thus three out of phase supplies.

Oh dear, that was long-winded !

Bob.

Answer 2

If you could see the waveform of electricity for the house, it's a single sinusoidal waveform. That is like an 'S' on it's side. now times it by 3. It means there's a more constant supply with 3 phases. No dips in supply. If the top of the 'S' is a peak you have will have more at their peak if it's 3 phase. Draw an S on it's side. Then from a different point draw another to fill in the gaps the n draw another to fil in those gaps. You should now see there's a more constant peak as the gaps of the 'S' are filled in. I wish I could include a picture. I find it hard to put into words. It's used for example in x-ray equipment so there is a constant supply so a more constant output of x-rays.

Answer 3

There are several kinds, but the easiest is where all 3 phases are balanced, and 120 degrees separating the sinusoids.

It is generated by turbines operating an alternator system.
Typically, after going through transmission (at high voltages, eg 132kV), the voltage between phases in the UK is 415V. But in the US and Japan it can be around 200V.

A normal domestic house would only need a small amount of power, and one phase would be sufficient. This is generated by a local substation (perhaps for a street or district) where a transformer reduced voltage to 240V. In the UK, the "neutral" line is earth bonded. (Never assume its 0v though!)

Answer 4

when you get to studying the waves that describe electricity, you will see that they are sine waves. so if you put together 2 waves (two of the three phases), then they add up to double the voltage (actually, 73% larger voltage, but that's details).

if you put together 3 waves, to get 3 phases, they're going to be 120 degrees out of phase. you can achieve higher voltages by combining them,and this helps in industrial applications, and for things like a/c and stove that take up a lot of electricity. read up here, they got good explanations

http://en.wikipedia.org/wiki/Three-phase_electric_power

Answer 5

3 phase is more used commercially to handle higher currents i believe.
a single phase system is a single signwave of alternating current usually from 0 to 240v both in the positive and negative scale.
three phase has three sign waves superimpossed on one another but "out of phase so when one wave is at zero one is at +240 and one at -240 i think so the overall average (rms) is higher than that of single phase.
im tryin to remember this from school 10 -12 years ago so sorry bit vague

Answer 6

so