LukeScotty

Hiya all, Could someone possbile explain 3 phase to me, or link us to a decent website which could tell me. I work for an industrial electricial company, but funnierly enough we havnt done much on 3 phase

Cheers Luke

If You Build It, They Will Come n If it aint broken don't fix it!

TonyM58

Ok, VERY briefly:-

The substation transformer supplying your house is three phase, exactly as if a factory was being supplied.

The three windings are star connected, cand draw it here but put a dot in the middle of a page. Draw a line vertically up from it . Go clockwise one third (120 degrees) and put another line from the central dot, then draw another at a further 120 degrees. you should now have three lines spaced at one thrid intervals around a circle. the first label L1 at the end, the second L2 and the thrid L3 going clockwise.

You now have a respresentaion of a three phase supply (the lines are the windings) From the central dot, draw line horizontally accross the page, this is the neutral. Also draw another line from the central dot and connect tit to earth. The central dot is called the 'star point'

ok, you know what a sine wave looks like? well each of three phases produces an identical sine wave, but they are 120 degress apart. So when the first sine wave peaks, the second is at a different point in the cycle, as is the third wave.

So, if you was to measure between any of the Lines (L1, L2 or L3) as shown on your diagram and Neutral you would have 230V, but iff you was to measure between any of the Lines (Like L1 to L2) you would measure 400V.

With your house supply they just run the first block of houses off L1, the second block off L2 etc.

Now think if you has a 100A supply coming n to your house, then so would the next block etc. Now think of the same supply coming in to a factory, you would now have three 100A supplies (so obviously you have more available power)

Now you can either use this from a three phase dis board as three seperate single phase supplies (shared out so they are loaded equally) or take it off as a three phase 400V supply (for industrail machinery, motors etc). In practice you would use both the three pases and individual single phases in an industral phases.

Main thing you need to know if you start working on three phase equipment is mortors will run in one direction using L1, L2 and L3 (or red yellow blue, or brown black grey using new colours), and if you want to run it in the other direction, you simply swap any two of the phases at the motor. (i would explain why but its another story, and you need good motor principles)

I can expand on any of this or can answer specific queries you have, no probs. i seem to remember you are doing 2391 soon? You will be testing on a three phase dis board, and three phase contactor with BS4343 outlet when you do it. Its all the same principles, but the PFC is twice the single phase value

Good luck!

Martin

Three phase power is the most efficient way to generate and distribute electricity. The basics are easy, but it gets more complicated when you start taking about 3ph phasor diagrams, phase balancing and looking at how three phase motors work.

Three phase motors are perfectly blanced loads, in that they draw the same current from each phase, so they don't need a neutural. Each live takes it in turns to act as the neutural return path for the other lives. This happens 50 times a second or 50Hz.

If you are drawing different currents from each live then you need a neutural to return the difference in current between all the phases.

Wikipedia has some more info on three phase here:
Three-phase electric power - Wikipedia, the free encyclopedia

and deals with some of the scary maths here:
Three-phase - Wikipedia, the free encyclopedia

Also HowStuffWorks has some info here:
Howstuffworks "How Power Grids Work"