I'm hoping someone could explain something to me. My understanding is that in a 3-phase delta configuration, the voltage between any 2 of the 3 hot conductors would be 240 volts, but my question is: why wouldn't it be 208 volts (as it would be in the 3-phase wye configuration) since in both cases the current in those conductors are 120 degrees out of phase with each other. It seems to me that it would be 240 volts only if they were 180 degrees out of phase. Thanks for any explanations.
3 Phase Delta configuration voltage
- Thread starter Neilvet
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I'll give you my non-engineer, electrician's answer. With the Delta for example A-B you're measuring the voltage across one 240 volt coil of the transformer. With the Wye A-B is measured across two coils that share a common point so you have to factor in the phase angle between A and B which gives you 120*1.73 = 208.
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Welcome to the Forum.
kwired
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You could wire three 208 volt coils in delta but if you mid point grounded one phase you would have 104 volts from two corners to the ground and a high leg to ground voltage of about 180.
Or you could create a wye system with 240 between each ungrounded conductor but that would leave you with about 139 volts to neutral from each.
Vector representations of this don't lie. Draw an equilateral triangle with dimensions representing the volts mentioned and your neutral and high leg results will correspond to what I said above. Draw three lines 120 degrees apart and converging at a center point and let the dimensions represent the voltage and the voltage between other points mentioned above will be that corresponding distance in the drawing.
Or you could create a wye system with 240 between each ungrounded conductor but that would leave you with about 139 volts to neutral from each.
Vector representations of this don't lie. Draw an equilateral triangle with dimensions representing the volts mentioned and your neutral and high leg results will correspond to what I said above. Draw three lines 120 degrees apart and converging at a center point and let the dimensions represent the voltage and the voltage between other points mentioned above will be that corresponding distance in the drawing.
ggunn
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There are 208V, 240V, and 480V phase to phase three phase services. The 240V is almost always delta or high leg but the 208V and 480V can be either delta or wye. What you are describing is likely a high leg configuration where A and B are 240V to neutral and C to neutral is 208V. In a wye configuration all three phase to neutral voltages are the same and are the phase to phase voltage divided by sqrt(3). In a delta configuration there is no neutral.
But sometimes the 240V high leg is called 240 delta with a high leg even though it has a neutral
Thanks for the info....... the vector diagrams of Wye vs Delta are useful for visualizing the 208 vs 240 V difference, respectively, in the two configurations when looking at the voltages across the phases (not the high leg to neutral which I realize is 208 in the Delta config).
kwired
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If it doesn't have a neutral then it doesn't have a high leg. That leaves it being either ungrounded or a corner grounded system.
jaggedben
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Probably worth pointing out that the NEC definition of a neutral in high leg systems is inconsistent with the other systems. In split phase and wye systems, the vector sum of the voltages of all phase conductors to the neutral is zero. In a high-leg, that's only true if you leave the high-leg out of it: the vector sum of the non-high-leg conductors to neutral is zero, but the vector sum to all three phase conductors is not. So an unspoken part of the meaning of neutral is 'ignore high-legs'.
kwired
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NEC never used to define it the way they do now. Can't remember when they changed it, maybe 2002 or 2005. Don't think it was any later than that.
It was always a "grounded conductor" and still is. But isn't a "neutral" to the entire system. It is "neutral" to the "split phase" that is on one side of the delta. True neutral of a delta system is a point in the center of the triangle that has no physical connection to it.
jaggedben
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AFAIK the only reason they define it that way is to specify which conductor be grounded.
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It allows for a simpler definition of when and which conductor is required to be grounded when it exists (prohibiting ungrounded operaton.).
All of the messy details are pushed back to the definition of "neutral".
kwired
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Which from what I occasionally see posts about leads to confusion that I don't believe was intended.
It allows for a simpler definition of when and which conductor is required to be grounded when it exists (prohibiting ungrounded operaton.).
All of the messy details are pushed back to the definition of "neutral".
Just because a transformer has a center tap or even arranged to have dual voltage capability by connecting two separate windings in series vs parallel doesn't mean you must connect that mid point of your voltage source to a system conductor. But if you do connect it then it would be required to be the conductor that gets grounded.
An example is if you created a delta secondary using three separate single phase 120/240 transformers. You have three "center taps" but if you are creating a high leg delta system you only use one of them as a system conductor. You could also use none of them and just leave each mid termination "floating" which leaves no actual system conductor to derive either a corner ground system or an ungrounded system.
I also think you can apply this to wye connected sources. Most the time you probably will connect the neutral point of a wye source.
An example of where you possibly would not is say you had a delta system as the normal supply and it does not supply any "line to neutral" loads. You want a standby system for this and come up with a wye source. You can't really ground the neutral of the back up source if your normal source is a corner grounded system, but if you leave the neutral floating in the back up source and ground one of the phase conductors it will work just fine.
Transformer turns ratio.
kwired
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Turns ratio would be a comparison of primary to secondary voltages, at least with basic step up/step down transformer types.
Number of turns will dictate voltage across an individual coil, but when you go to multi-phase setups, that voltage across an individual along with the vectors of how they are arranged is what dictates voltage between any two points that don't have a simple coil running 180 degrees between them.
Not exactly sure what you are trying to say here.
My answer is correct for the OP
No need to make it harder than it really is.
ggunn
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But it's what we do.
kwired
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Three 120 volt coils in a wye configuration gives you 208 volts between each outer point of the wye.
Same three 120 volt coils connected in delta gives you 120 volts between each corner of the delta.
Number of turns is part of the result, but for the commonly used systems we have that delta configuration has a different number of turns in the windings used to get to the nominal 240.
At the same time 120 volts is a common desirable voltage to have within either system so that leaves us settling for 208 for line to line voltage for wye connected systems simply because of the vectors involved in how it is designed.
With a delta system you can get 120 volts on two of the lines and 240 line to line on all lines, but the vectors there make it impossible to get 120 on all three lines. If you made the delta so it is 208 line to line then you only get 104 to the neutral on two corners and high leg there would be about 180.
Your making this harder than it really is.
the OP said “3-phase delta configuration, the voltage between any 2 of the 3 hot conductors would be 240 volts, but my question is: why wouldn't it be 208 volts?”
my response; transformer turn ratio. Thats all there is to it.
7200 primary to 240 delta, ttr?
7200 primary to 208 delta, ttr?
winnie
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In the wye configuration, the L-L voltage goes through _two_ phase coils, and the 120° phase angle is relevant to calculating the voltage. Both the transformer turns ratio (which determines the individual coil voltage) and the phase angle between the _two_ phase coils are relevant to the final voltage.
In the delta configuration, the L-L voltage goes though a _single_ phase coil, and all that matters in the turns ratio which determines the voltage of that single coil.
If you had a transformer setup with 120V coils, in the wye configuration you would get 208V L-L. In the delta configuration you would get 120V L-L.
If you had a transformer setup with 240V coils, in the wye configuration you would get 416V L-L. In the delta configuration you would get 240V L-L.
-Jon
kwired
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