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Voltage between phases

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LAN_403

Download

Wed Mar 25 2009, 01:25AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

So I'm building a 3 phase 240v input, single phase 240v output power box for a LAN party and I wasn't sure how the thing should be wired up

Should the outputs be between the phases or between the phase and neutral to get 240v out?

Picture example for my poor explanation:

Also, what would the maximum load be on each single phase if the 3 phase is 32A per/phase?

hotcrazyfruit

Wed Mar 25 2009, 01:33AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Posts: 147

phase and neutral. if you went between phases, you would get a really weird looking wave (picturing it in my mind it would almost look like DC). so yea, phase and neutral is what you want. remember , 3 phase is just 3, 60Hz lines, just offset by 1/3 phase from the rest (or 120 degrees as many call it)

tesla500

Wed Mar 25 2009, 02:40AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

Check how your 3 phase voltage is measured, it can be measured either be phase to phase (delta), or phase to neutral (wye).

The waveshape you'd see when measuring between phases is a sinewave, just like phase to neutral, but phase to phase is sqrt(3) higher voltage than phase to neutral.

David

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Wed Mar 25 2009, 03:37AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

tesla500 wrote ...

Check how your 3 phase voltage is measured, it can be measured either be phase to phase (delta), or phase to neutral (wye).

The waveshape you'd see when measuring between phases is a sinewave, just like phase to neutral, but phase to phase is sqrt(3) higher voltage than phase to neutral.

David

I'm guessing it's measured phase to neutral, because most houses single phase connections round here use one phase of the 3 phase connection

Can someone answer my question regarding how much current I can put on each phase? Because putting 96amps on the neutral (32amps per phase) doesn't sound good

hotcrazyfruit

Wed Mar 25 2009, 04:05AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
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Download wrote ...

Can someone answer my question regarding how much current I can put on each phase? Because putting 96amps on the neutral (32amps per phase) doesn't sound good

what is your power source? if you have a 96 amp combined power source, then you could draw any combination of current on each phase until one phase goes over 32 amps i believe. but give some more details with your post.

Download

Wed Mar 25 2009, 04:11AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

hotcrazyfruit wrote ...

Download wrote ...

Can someone answer my question regarding how much current I can put on each phase? Because putting 96amps on the neutral (32amps per phase) doesn't sound good

I put a load of 32amps on each phase, each phase is connected to neutral meaning there is a load of 96 amps on neutral. The cable will probably melt, won't it? There's no fuse on neutral so it can't trip

tesla500

Wed Mar 25 2009, 04:16AM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

If your source is 32amps 3 phase, you can draw 32amps on each of the three phase to neutral pairs, and the neutral will carry zero current, assuming the load is linear and balanced across phases. If you're hosting a LAN party, you likely are going to be running all SMPSs that have very poor power factor and are nonlinear loads, so there will be significant 3rd harmonic currents flowing down the neutral. Older buildings sometimes have a reduced size neutral, from the days before SMPSs, and there have been reports of the neutral wires and connections overheating when lots of computers were installed.

If you're in a modern building you should be fine loading it up all the way to 32A. Although if you're drawing that much current, you're talking about dissipating 14,000+W, and cooling the room may be a problem.

David

...

Wed Mar 25 2009, 04:24AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

If you had exactly 32a on each of the phases, (each with perfect power factor) there would actually be no current on the neutral. Imagine the normal center tapped 240v service (2x 120v 'phases' that most houses have) if you put a resistor from each of the hots to the nuetral and imagined the resistor as a 2:1 voltage divider (ie the voltage at the middle of the resistor is the average voltage between the phases) there will always be 0v at the middle. The same idea also applies to the 3 phase systems, just they phases are split by 120 degrees each instead of 180.

As a consequence of this it is pretty hard to actually draw more than 32a off the neutral, hard enough in fact that we don't even bother putting in an extra neutral breaker.

edit looks like tesla beat me to it but you get the idea

Download

Wed Mar 25 2009, 04:33AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

I've just been looking at a picture of the 3 phase waveform on Wikipedia and I understand what you mean. So if I have an unbalanced load I'll need PF caps won't I? If so are there any formulas for PF caps?.

A lot of you seem to be talking about 120v, I'm in Australia so I've got 240v 50Hz

aonomus

Wed Mar 25 2009, 04:44AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Depending on the venue you are at, you may be required to hire a professional electrician to do your service wiring due to insurance policy requirements. It could be a barn, shed, or professionally run venue, however insurance policies vary... check with the owner, and your own insurance company.

Also, you should read up and understand star or Y (wye?) 3-phase vs delta 3-phase. Essentially delta is the L1-L2, L2-L3, L3-L1 wiring in your second diagram with no neutral, and iirc is used more for 3-phase motors, whereas the L1-N, L2-N, L3-N configuration is used for separate 1-phase services.

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