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#5366 - 11/17/01 02:45 PM two phases
Mike Shn Offline

Registered: 06/13/01
Posts: 26
Loc: New York
I'm helper in electric company. We use 2 phases for boiler and blower but we don't use neutral for it. Why is that? How that works? For outlet we need phase and NEUTRAL.


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#5367 - 11/17/01 02:54 PM Re: two phases
sparky Offline

Registered: 10/18/00
Posts: 5545
Hi Mike,
Would I be correct to assume your Q eludes to how the 'juice' returns. We had quite a thread going on this a while back, some very indepth posts by Scott T, and others in the 'theory' section i believe. Bill, has this passed into the great cybervoid???

#5368 - 11/17/01 05:38 PM Re: two phases
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hello Mike,

Well, explaining 3-phase power in a concise way is quite a task.

Basically, you have three "hot" wires supplying power. Are you familiar with the sinewave pattern of AC? Each hot wire is 120 degrees out of phase with the others, which in simple terms means that the peaks and nulls of the sinewaves on each phase don't coincide. That means that as well as there being voltage between any phase and neutral, there is also a voltage between phases. Thus you can obtain power by connecting either phase-to-neutral or phase-to-phase.

You can always work out the relationship between the voltages by multiplying or dividing by the square root of 3.
So a common 3-phase system specified as 120/208 volts will have 120V from any phase to neutral, but if you measure between any two phases you will get 208V. So a heater, blower motor, or whatever connected between two phases must be designed for the higher voltage.

The same relationship holds true for other 3-phase systems, such as your 277/480V system in America or the 240/415V we use here in England.

Perhaps a little easier to understand is the residential 3-wire system which has a neutral and two "hot" lines.

Technically, this is single-phase, but the lines are 180 degrees out of phase with each other, so when one is at its positive peak the other is at its negative peak. Thus you can get 120V from either line to neutral for normal lights & outlets, but 240V by connecting between the two hot lines. Hence the 120/240V specification for residential services.

That's all pretty simplified, but I hope it helps.

[This message has been edited by pauluk (edited 11-17-2001).]

#5369 - 11/18/01 08:11 AM Re: two phases
Bill Addiss Offline

Registered: 10/07/00
Posts: 4196
Loc: NY, USA

Everything should still be there. When you go to that Forum check that the dropdown menu (Top Right) says "Show all Topics" rather than "last 100 days" or whatever.

If not, you can always do a search.

Personally, I think that a picture of a center-tapped coil showing 120v to center (Neutral) on each side and 240v between the 2 "ends" would be easier to get the point across.


#5370 - 11/18/01 01:04 PM Re: two phases
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England

For newcomers I've also found it helpful to start by explaining a 3-wire DC system, as this seems to be more easily grasped by many people. Once that concept is understood, it's then easier to explain a 3-W AC setup.


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