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Tool Talk Discussion Forum

Re: Different 3-phase question

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04-22-2013 10:46:29

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I don't know about that John T. I know you have a lot of real world experience, but I also trust my formal education. What I was trying to say is one leg is high, but still the same voltage. Our professor said two will be 208 and one will be 218 (or something like that). He said you should then tap off the 208 legs if you need single phase power.

Assuming the red leg is L2 and 208 volts to ground and L1 and L3 read 110 to ground. What would you read from L1 to L2?
This post was edited by sflem849 at 10:51:04 04/22/13 2 times.

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04-22-2013 12:25:54

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 Re: Different 3-phase question in reply to sflem849, 04-22-2013 10:46:29  
"Assuming the red leg is L2 and 208 volts to ground and L1 and L3 read 110 to ground. What would you read from L1 to L2?"

It is a fixed math calculation. A matter of summing the vectors (phasors), sines, cosines. Your set of numbers do not fit the math. IF you actually had 110 for L1 to N and L3 to Neutral, then the math would yield 190.5 for high-leg, not 208. And if high leg were 208, then L1/L3 to neutral would be 120.

In either case, L1 to L2 to L3 & L3 to L1 would be equal at 220, or 240.

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John T

04-22-2013 14:24:18

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 Re: Different 3-phase question in reply to JMOR, 04-22-2013 12:25:54  
These guys are just having trouble with the MATH as you correctly point out. The figures I keep using are typical and nominal (based on math such as the 208 derives from 120 x square root of 3 etc) and science, but that dont mean any given voltmeter is gonna yield those exact results. The laws of physics and math must apply and the Y and Delta configurations I keep quoting them are correct based on vector sums and pretty basic trig etc. Also a non electrical person has a tough time with Y versus Delta when they start throwing out figures they heard about somewhere down at the pool hall.

But hey God Bless them all, they have some good questions, and I still try my best to help and will keep doing so and thanks to you also

John T

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John T

04-22-2013 11:51:11

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 Re: Different 3-phase question in reply to sflem849, 04-22-2013 10:46:29  
Yep I have 40 + years of real world experience. I was an AC Power Distribution Design Engineer and have a BSEE as part of my "formal education" as you call it from good old Purdue University a top notch Engineering School, so not to brag but YEP I DO KNOW SOMETHING ABOUT THAT LOL

To answer your question:

In a 120/240 Volt Three Phase Four Wire center Tapped DELTA, IF L2 is the Red/High Leg then:

Its 240 Volts L1 to L2 Its 240 Volts L2 to L3 Its 240 Volts L3 to L1 Its 120 Volts L1 to Neutral Its 120 volts L3 to Neutral Its 208 Volts L2 to Neutral (Red/High Leg)

Its the transformer between L1 & L3 thats center tapped with its center grounded and that point is the Neutral

In a 208 Y 120 Volt Three Phase Four Wire System

Its 120 L1 to Neutral Its 120 L2 to Neutral Its 120 L3 to Neutral Its 208 L1 to L2 Its 208 L2 to L3 Its 208 L3 to L1

So thats about as plain as I can describe it to a lay person sure hope this helps. If some "Professor" says different then Hes wrong and Im right is all I can tell you. NOTE sure there can be some slight different voltages (204, 206, 208, 210, 212 etc etc) depending on resistances and loads and the tap settings etc. etc. Im unsure that "Professor" understands the difference in a Delta versus Y configuration???

NOTE do NOTTTTTTTT feel bad if you cant understand this and sorry I can NOT put in a paragraph here what takes volumes of books to explain and years of engineering practice and experience to fully comprehend. I try my best to help and educate the fine gents here but I cant make them understand volumes of theory and years of expreience here in a few sentences BUT BY GOLLY I DO TRY MY BEST.

God bless you for your good thoughtful questions and sparky chat and I sincerely hope I helped you understand all this, sorry if I couldnt.......


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