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01-23-2009, 09:23 PM
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#1
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Senior Member
Join Date: Dec 2007
Location: Hamburg, PA
Posts: 263
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120/240 Volt 3-Phase Delta High Leg
Ok I have a question about a high leg delta transformer. I know that phases A and C to neutral you get 120 and that phase B to neutral you get 208. I also know that between any two phases you get 240. My question is why don't you get 328 volts from phase B to either phase A or C? I believe it has to do with the was the sine waves are off from each other but am not totally sure. Does any one have a good explaination? Or maybe a diagram of the sine waves?
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01-23-2009, 10:41 PM
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#2
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el abogado del diablo
Join Date: Mar 2007
Location: belly of the beast
Posts: 1,347
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08-05-2009, 11:15 AM
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#3
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Junior Member
Join Date: Aug 2009
Location: California
Posts: 1
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208 / three phase
I am an electronic technician by trade, not an electrician, but have been forced into performing electrician type work from time to time. I am always reluctant to more than the most basic work - I know the difference, and my limitations.
Anyway, 208V has always been a bit of a mystery to me, but your diagram using the triangle and pythagorean theory finally made it very clear to me.
My only question now, is why do we use 208V in the first place?
Thanks,
Goober Pat
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08-05-2009, 11:27 AM
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#4
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Senior Member
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,035
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Quote:
Originally Posted by Goober Pat
I am an electronic technician by trade, not an electrician, but have been forced into performing electrician type work from time to time. I am always reluctant to more than the most basic work - I know the difference, and my limitations.
Anyway, 208V has always been a bit of a mystery to me, but your diagram using the triangle and pythagorean theory finally made it very clear to me.
My only question now, is why do we use 208V in the first place?
Thanks,
Goober Pat
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The 208 V in the delta setup is just an artifact. We don't use it. In a three phase wye connected system where we want single phase 120 V, the phase to phase voltage turns out to be 208, and we use this just like we would 240 V in a single phase system, or all three phases for large loads.
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08-05-2009, 05:26 PM
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#5
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Senior Member
Join Date: Apr 2009
Location: Canada
Posts: 783
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I must say in my 30+ years in the trade I have never seen a transformer like that.
Why would one use this?
__________________
 Don't fight .. Play nice!
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08-05-2009, 05:47 PM
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#6
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Senior Member
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,035
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Quote:
Originally Posted by Toronto Sparky
I must say in my 30+ years in the trade I have never seen a transformer like that.
Why would one use this?
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It was used here alot for heavy commercial and industrial buildings with lots of motors. There is less stress on a delta connected transformer from heavy motor starts than on a wye. And, of course, there is alot of single phase 240 V equipment as well as 240 V three phase stuff. And you need 120 V for general power and lighting, so the 120/240 V delta is pretty good for that.
Of course, the 208 V high leg isn't used for anything normally.
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08-05-2009, 09:02 PM
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#7
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Member
Join Date: May 2008
Location: Texas
Posts: 91
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Or when the POCO wanted to save money by supplying a three phase service with two pots.
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08-05-2009, 09:28 PM
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#8
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Senior Member
Join Date: Sep 2007
Location: Delmarva, USA
Posts: 607
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In which case, that is called an "open delta" service.
__________________
-KB
Life is uncertain -- eat dessert first!
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08-05-2009, 09:35 PM
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#9
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Senior Member
Join Date: Sep 2007
Location: Phoenix, AZ
Posts: 1,160
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Quote:
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why don't you get 328 volts from phase B to either phase A or C?
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Because, that's the way it is.
In my line of work, I don't need to know everything. My brain isn't big enough so I limit my knowledge to "need to know"
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08-05-2009, 09:41 PM
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#10
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Member
Join Date: May 2008
Location: Texas
Posts: 91
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Of which has 57% capacity of a three transformer bank if the transformers are of the same KVA rating. Most 240 delta banks I've seen have a large pot with a smaller " kicker" pot.
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08-06-2009, 12:10 AM
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#11
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Licensed Pro
Join Date: Jan 2007
Location: Rahway, NJ
Posts: 2,772
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Power companies are slowly doing away with these systems.
__________________
There's only one way to succeed in anything, and that is to give it everything.
Vince Lombardi
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08-06-2009, 08:07 PM
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#12
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Bilge Rat
Join Date: Aug 2007
Location: Fernley, Nevada (near Reno)
Posts: 651
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Too bad that POCOs are not connecting deltas as much anymore.
A closed delta (3 transformers) will tend to balance voltage. This is why 480 to 120/208 transformers are almost always delta on the 480 side. Also why most larger motors are delta connected.
If there were 3 transformers on a pole connected wye on the primary, and delta on the secondary, and you opened one of the cut-outs, you'd still have 3 phase power available on the secondary. If you completely remove one of the transformers, you'd still have 3 phase at the secondary.
Voltage balance is very critical to 3 phase motors, and POCOs must maintain it within certain tolerances. Since they are getting away from delta secondaries, most of them are getting away from wye-wye padmounts in favor of delta-wye.
Rob
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