3- to 1-phase?

- J
- Jordan
  
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posted
15 years ago

Sun, Feb 25, 2007 2:59 AM

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

- G
- Grant Erwin
  
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posted
15 years ago

Sun, Feb 25, 2007 3:18 AM

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

- H
- Harold and Susan Vordos
  
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posted
15 years ago

Sun, Feb 25, 2007 3:23 AM

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

- J
- JimL
  
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posted
15 years ago

Sun, Feb 25, 2007 3:28 AM

Yes,assuming the voltage and frequency matches the motor nameplate data. Measure the voltage between the two wires you have chosen to be sure.

- C
- clutch
  
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posted
15 years ago

Sun, Feb 25, 2007 4:07 AM

Happens all the time. But since you had to ask the question it indicates there might be a few more things you might need to know.
Tell us about your project.
Wes

- J
- Jordan
  
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posted
15 years ago

Sun, Feb 25, 2007 8:32 AM

Thanks Wes and all.
I don't have a current project - just something I wondered about and thought would be good to know for reference. I'd been aware that in theory this was possible, but thought there might be a "gotcha" in practice. As I thought, the answer was right here. Long live RCM.
Jordan

- D
- David Billington
  
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posted
15 years ago

Sun, Feb 25, 2007 1:44 PM

Jordan as you seem to be posting from Australia I wonder if your mains is the same as that in the UK. My understanding of UK mains is the single phase is one 3 phase leg and neutral giving 240V between phase and neutral but the voltage between any 2 3 phase legs is 415V due to the 120 degree phase difference. As was mentioned by another check the voltage before connecting. AFAIK the UK setup is the same across Europe but the US seems to have many 3 phase supply arrangements from what others have said at various times on RCM.

- S
- Snag
  
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posted
15 years ago

Sun, Feb 25, 2007 3:54 PM

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

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 .

- D
- Dixon
  
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posted
15 years ago

Sun, Feb 25, 2007 4:57 PM

Depends on if you are using a three phase generator or not. My 50kw three phase generator can do single phase by using two of the three legs but on heavy draw it creates an imbalance that shuts the generator off. Dixon

- D
- Don Foreman
  
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posted
15 years ago

Sun, Feb 25, 2007 5:47 PM

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.

- S
- Snag
  
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posted
15 years ago

Sun, Feb 25, 2007 7:31 PM

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 .

- H
- Harold and Susan Vordos
  
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posted
15 years ago

Sun, Feb 25, 2007 9:11 PM

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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- Snag
  
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posted
15 years ago

Sun, Feb 25, 2007 9:43 PM

I've obviously forgotten more than I realized . Next time I'll study up before I speak up ...

- D
- Don Foreman
  
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posted
15 years ago

Mon, Feb 26, 2007 12:37 AM

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.

- C
- Curt Welch
  
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posted
15 years ago

Mon, Feb 26, 2007 12:47 AM

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.

- C
- Curt Welch
  
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posted
15 years ago

Mon, Feb 26, 2007 12:56 AM

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.

- C
- Curt Welch
  
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posted
15 years ago

Mon, Feb 26, 2007 12:58 AM

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.

- C
- Curt Welch
  
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posted
15 years ago

Mon, Feb 26, 2007 1:08 AM

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.

- C
- Curt Welch
  
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posted
15 years ago

Mon, Feb 26, 2007 1:28 AM

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?

- S
- Snag
  
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posted
15 years ago

Mon, Feb 26, 2007 1:31 AM

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 .