A question for real sparks (Not DIY)

[TheGaffersSon]

Hi all, i'm an apprentice due to start in college in a couple of weeks. I have been using this site as a useful resource for helping me understand more abput electrics. I intend to stick around, mainly to learn from the sparks on here, and maybe give out help as I go along.

Anyway, I was wondering if anyone on here could explain to me three-phase electricity. Why do we use it?

I have a vague idea, but when I look to back up what I already know with facts I am presented with maths equations which are far to advanced for me yet. I hope you can help me understand how 3-phase works.

Many thanks in advance.

 

[Steve]

someone asked this a few weeks ago. someone will give you a link. ended up spanning about 15 pages in 2 days

 

[Nige F]

because the power stations have 3 sets of windings on the generators ducks for cover back to the plumbing forum

 

[Steve]

because the power stations have 3 sets of windings on the generators ducks for cover back to the plumbing forum

there you have it. the reason we have 3 phase. from a plumber.

 

[Spark123]

Basically its the more efficient way of distributing power, also 3 phase motors are a lot more efficient than 1 phase.

 

[plugwash]

Anyway, I was wondering if anyone on here could explain to me three-phase electricity. Why do we use it?

ok first a bit of background:

DC as you probablly know is a PITA to voltage convert

single phase AC suffers from 2 main problem
Firstly the power varies from zero to twice the average, this variation inevitablly gets transmitted to the mechanical side of motors and generators making them run in a manner thats not very smooth.
Secondly motor design for single phase AC is quite awkward.

So various other systems were tried out, eventually 3 phase AC was settled on, in this system there are 3 live conductors 120 degrees out of phase with each other, power flow is constant, its easy to voltage change, all wires carry the same current and it only needs one more wire.

low voltage (<1KV) 3 phase AC systems nearly always add a fourth wire known as the neutral, this is mainly done to allow 240V single phase equipment while making the phase-phase voltage and hence the efficiancy higher than in a 240V phase-phase system.

 

[Steve]

So where you have a 3 phase distribution board, the incoming is 5 wires - 3 lives, neutral and earth. There are 3 busbars that the lives connect to, and the MCB ways alternate between L1, L2 and L3. This allows you to fit a 3-phase MCB or single phase MCBs, depending on what sort of circuits you are connecting.

All the neutrals connect together on a single busbar. Therefore a perfectly balanced board will have zero current on the neutral, as the 3 phases will balance out on the neutral (the sum of the 3 sine waves superimposed on each other is zero). In reality, most 3 phase DBs have a load of single phase circuits, which are usually intermittent, such as ring mains, fridges, lights etc, so it is hard to balance it perfectly. Pure motor DBs are possible to balance perfectly though.

 

[RF Lighting]

someone asked this a few weeks ago. someone will give you a link. ended up spanning about 15 pages in 2 days

Didn't that thread end up getting deleted when the OP got all offensive?

 

[Spark123]

Wondered why I couldn't see it.

Pure motor DBs are possible to balance perfectly though.

To expand on this, is not uncommon on some 3 phase motor installations to omit the neutral alltogether and just have the 3 phase wires going to the motor.

 

[keyplayer]

I have a vague idea, but when I look to back up what I already know with facts I am presented with maths equations which are far to advanced for me yet. I hope you can help me understand how 3-phase works.

Many thanks in advance.

Yes, GCSE mathematics is far too advanced for most diyers. Perhaps the Gaffer could help?

 

[Softus]

Firstly the power varies from zero to twice the average...

Twice? An AC voltage varies from -Vpeak to +Vpeak, so the average voltage is zero.

Surely you mean that the peak is (2¨²)*Vrms ?

 

[JohnD]

power

voltage

 

[Softus]

Indeed so, but is the power waveform not also sinusoidal?

 

[TheGaffersSon]

I have a vague idea, but when I look to back up what I already know with facts I am presented with maths equations which are far to advanced for me yet. I hope you can help me understand how 3-phase works.

Many thanks in advance.

Yes, GCSE mathematics is far too advanced for most diyers. Perhaps the Gaffer could help?

Don't worry, my old man will be getting a grilling too.

Just wanted to have as much info as possible. I know it's not the real purpose of the forum, but i'm sure the mod's and posters won't mind people passing on their knowledge to a keen learner.

 

[TheGaffersSon]

Many thanks to all that have posted so far.

Can you tell me, why are 3-phases used to connect to a metre. I fitted one the other day, and didn't like not knowing why I did so.

(Obviously not in Japanese though)

Another question. I may be wrong but is 3-phase used, because the magnetic field produced can make objects rotate by all three at 120 degrees pulling it towards them?

I was told if you fit a 3 phase item incorrectly (ie the wrong way round) it can work in reverse. I assumed this was because the magnetic fields would be different.