3- to 1-phase?

Would it be practical to drive a single phase motor, by tapping into 2 of the wires in a 3-phase system?
Jordan

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Jordan wrote:

Sure. Any pair of the 3 wires in a 240V 3phase system will have 240V single phase power between them. It won't be perfectly balanced, but it will work fine electrically.
GWE
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Yep----that's all single phase is, assuming you want 240 or 208 volts. If not, all you need is one leg to neutral, but it has to be the proper leg if you have delta service. The high leg to neutral in that case would be 208 Volts, and is usually the center leg, or B phase, on the panel. Be sure to run a ground. If none of this makes sense, get someone to help that better understands electricity.
Harold
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Harold and Susan Vordos wrote:

^^^^^^^^^^^^^

Okay , I have to disagree with you . Delta configuration , all three legs should be at specified voltage in relation TO EACH OTHER . Whether or not any leg shows a potential to ground depends on how the service is grounded - the service at the cabinet shop I work at appears to have one leg at ground potential . I have no idea what code says . Wye connected , same in relation leg/ leg , but quite often the center connection is grounded , giving half of rated voltage between any leg and ground . Of course I could be full of shit , it's been a long time and some of the details are a bit fuzzy .
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wrote:

No. In a center-grounded Y, line-to-line is sqrt(3) or 1.732 x line to neutral. 208 line-to-line 3phase is 120 line to neutral. 480 line to line 3phase is 277 line to neutral, used in some lighting circuits.
In a delta with one leg grounded, line to line and line to neutral will be the same for the other two legs.
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Don Foreman wrote:

And that's exactly where the details were fuzzy . Thank you for correcting my mistake .

I though I was right on that part ... my training was over 30 years ago , not that that matters , but I've used it only casually thru the years , and have lost a lot of details .
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I was wondering how it worked out to sqrt(3) seeing it should be based on trig. But it does. The answer using trig is 2 * cos(30deg) which is the same as the sqrt(3). With trig, you just form a triangle representing the voltage vectors. So you have two legs for 120 V at 120 deg out of phase and the vector from tip to tip is the 208 leg. It's a triangle with one 120 deg angle and two 30 degree angles. The lengths of the sides are 120, 120, and 207.846....

Isn't there also a configuration where they put a center tap on one of the delta windings and use that as the ground? I think I saw that once on some book or web site where they had 220 V 3 phase and the center tap gave you 110 V to two of the 3 phases. Ground to the other phase would be some other strange value (190 I think). I've never actually worked with 3 phase circuits so I don't know if this is actually used or not.
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snipped-for-privacy@kcwc.com (Curt Welch) wrote:

Ah I see now, 240 V 3 phase delta with one winding center tapped for the ground gives you 120 V to two of the phases and 208 to the third phase. The 190 number is what you get if you do the math using 110V/220V instead of 120V/240V for the numbers.
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The key word here is *delta* (or Wye wiring).
A phase and C phase to neutral are 120 volts. B phase to neutral is 208 volts. I'm not disputing phase to phase voltage, which, in the case of delta, should be 240 volts. I suggested that if the motor required 120 volts that one must then select the proper phase for achieving the desired voltage. If I'm wrong, then the wiring in my shop, and past shops I've owned with three phase delta service, have a serious problem, because that's the voltage, measured to ground with a respectable meter. It's also why there used to be blank spaces in the panel in my previous installations. The current shop has more than one panel, so all single phase service comes from a single phase panel.
Harold
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Harold and Susan Vordos wrote:

I've obviously forgotten more than I realized . Next time I'll study up before I speak up ...
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I like to post with what I think I know and then learn when people correct me. :) If you don't post, you don't get the chance to learn.
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Curt Welch wrote:

I don't usually mind looking ignorant , as you say I learn lots when better minds step in . My downfall is the math . I useta know a couple of formulae for specific items , but never could "get" algebra . I still know a few formulae for things , just not these things ... And a day I didn't learn something is a day wasted . I learned a *bunch* today ! Thanks for the education , I appreciate it .
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On 26 Feb 2007 00:58:36 GMT, with neither quill nor qualm, snipped-for-privacy@kcwc.com (Curt Welch) quickly quoth:

Read any good books lately, Curt? <bseg>
=========================================================CAUTION: Do NOT look directly into laser with remaining eyeball! =========================================================
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I read the new Miller 2007 catalog this week. That's a good book. :) I recently read most of "Welding Technology Fundamentals" by Bowditch Bowditch and Bowditch for the two welding classes I'm taking. That's also a good book.
I'm currently reading "Consciousness Explained" by Daniel Dennett. It's good, but a bit slow - I haven't picked it up in about a month now but it's on the top of the stuff I'm reading. I found Darwin's Dangerous Idea by Dennett to be a far more interesting read. I find his views refreshing. I'm also trying to get though "Spikes, Exploring the neural code". It's mostly math which makes it fairly slow going. Not too long ago I read Dawkins' "The Selfish Gene". That's a classic. I've got Minsky's new book "The Emotion Machine" sitting in the "to be read" queue. I'm not sure if I'm going to find that very interesting or not. His SOM is a classic I read not too long ago. I've got about 20 books and a large assortment of technical papers on AI related stuff sitting to be read. Not to mention a nice backlog of Popular Science, and New Scientist, Robotics, and Scientific American, and other such magazines waiting to be read.
But sadly, nothing on political issues like global warming in the stack. :)
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On Sun, 25 Feb 2007 21:11:17 GMT, "Harold and Susan Vordos"

Interesting! Took me a minute to figure this one out though it's probably quite common as you say. (I'm not an electrician)
Start with a Y-connected xfmr producing 240 line-to-line 3phase. The center of the Y is merely that--center. We haven't defined a neutral yet.
Now connect a voltage (xfmr winding) of 69 volts to the center that is in phase with B. Make the other end of that neutral. Now lines A and C are 120V WRT neutral and are opposite in phase, while B to neutral is 208 volts, in quadrature with A and C -- leading one and lagging the other. Result: A and C are 120 line to neutral and 240 line to line (like single phase power) while AB, AC and CA comprise 240V threephase. I can sure see how that would make a lot of sense in a shop. The center of Y-connected threephase loads will be 69 volts above neutral, which doesn't matter a bit as long as they're not connected to anything.
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Or do what they probably really do and just use a delta transformer and center tap one of the delta windings for the ground. That gives you your 69 volts to what would be the center of the Y if it existed.
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On 26 Feb 2007 01:08:38 GMT, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

That does make sense and it sounds practical as well.
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How does a 3 phase panel work? I've never seen one. It will have 4 wires coming into the box right, and if it's 3 phase delta, then you end up with 240/240/240 across each phase and 120/120/208 from phase to neutral right (for 240 delta service)?
Is it like a home panel with the phases alternating so that every 3rd breaker is the same phase? So to hook up a 3 phase feed, you need a breaker with 3 gangs feeding it? And for 120 circuits for lights etc you just use breakers in the A and C phase slots? And for 240 you can use any two side by side breakers (assuming it's a pure 240 circuit you are creating and not a 240/120?) Or do you just always use A-C phase pairs for 220 V circuits so the ground is always 120V to both?
Is this type of 240 V 3 phase service common in commercial settings because it gives you the standard 120V circuits at the same time?
I guess 208 Wye gives you 120 on all phases to ground. Is that a common service as well?
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5 if you count the ground, unless the metallic conduit serves as a ground.
and if it's 3 phase delta, then you end up with

Three phase breakers connect to three stabs, or bolt-ons screw on to three buss bars, just like a double pole breaker, only three poles.
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snip---

I've always specified and used Square D products. They are amongst a few that are still using copper buss bars, and are of reasonably high quality. Well respected in the industry, although not cheap. What I'll describe is proper for Square D. I have no experience with other panels.
As you allude, the panel accommodates a breaker with three poles. There are three buss bars that run vertically in the panel, with the center bar the high leg when wired delta. That is the second position on the breaker panel, so if you use the three phase panel for single phase, that position may not get used. For sure it won't for 120 volt service, but there's nothing preventing you from using it for 240 volt single phase service.
The panel is wired with the three phases, a neutral, and a ground. Five wires total. Not all three phase service has the neutral. If there are no 120 volt loads, it's possible that a panel wouldn't be able to provide such service considering it's no longer legal to use the ground as a conductor, although it would work fine.

Correct
Correct.
Or do you just always use A-C phase pairs for

You need not use them in pairs. Each slot has its own breaker, single pole. You'd have a blank space in the panel where the high leg would be if it wasn't used for either 240 volts or three phase service.

All depends on the load needs and panel size. Because 1/3 of a three phase delta panel is wasted when the panel is used for 120 V service, you may find that a second panel, strictly single phase, is needed. That's what I've done with my shop. Everything is fed from the same transformer source, but it separates at the pole and goes through two different meters. That, in my case, was a good thing, because I'm on a demand meter that changes the price of power when I exceed 50 kw demand. 50 kw is the size of my induction furnace, so I'm damned glad to not have everything running through the demand meter.

Indeed, and is far more common than delta. There are some issues with delta that present a fire hazard, according to PUD (our provider). They really push the star (wye) service, which is 208 volts, but that's not in the best interest of almost anyone that runs motors. While they'll usually run on 208, they draw more amperage and run somewhat hotter. In some instances, devices don't perform as intended when they're designed around 240 volts. A friend had this problem when he moved his saw sharpening service from a delta wired shop to a wye wired shop.
Harold
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