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Old 09-07-2010, 09:04 PM   #1
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Default Wye to Wye or Delta to Wye

When you have 480/277 4wire Wye supply. You usally stepdown the voltage to 208/120 to run single phase loads. You use the 480 without the neutral for the primary. Why do they call that Delta to Wye? Isn't still a Wye system even if you don't use the Neutral for the transformers primary?

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Old 09-07-2010, 09:07 PM   #2
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Originally Posted by Skipp View Post
When you have 480/277 4wire Wye supply. You usally stepdown the voltage to 208/120 to run single phase loads. You use the 480 without the neutral for the primary. Why do they call that Delta to Wye? Isn't still a Wye system even if you don't use the Neutral for the transformers primary?

isnt it because the connection at the primary side of the transformer is delta.

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Old 09-07-2010, 09:19 PM   #3
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isnt it because the connection at the primary side of the transformer is delta.
So if I don't use the neutral in any 4 wire WYE system, it automatically becomes a Delta?
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Old 09-07-2010, 09:28 PM   #4
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So if I don't use the neutral in any 4 wire WYE system, it automatically becomes a Delta?

no the coils in the transformer are wired for a delta connection.
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Old 09-07-2010, 10:09 PM   #5
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When you have 480/277 4wire Wye supply. You usally stepdown the voltage to 208/120 to run single phase loads. You use the 480 without the neutral for the primary. Why do they call that Delta to Wye? Isn't still a Wye system even if you don't use the Neutral for the transformers primary?
First of all, how would you have a 4 wire supply? Wye simply means the addition of the 4th Xo neutral.

Delta is 3 wire, 4 wire is always Wye
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Old 09-07-2010, 10:25 PM   #6
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There are plenty of 4 wire deltas. They all have a high leg; 208 to neutral/ground for a 240 system, and 416 for a 480 system.

It's called delta/wye only because the transformer is delta primary and wye secondary. A delta connection can be fed by 3 hots from a 3 or 4 wire delta, or 3 hots of a 4 wire wye.

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Old 09-07-2010, 10:45 PM   #7
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It's not a matter of simply not using the neutral...they are two different ways of connecting the same thing. in a wye setup the neutral is created where the one end of each of the three coils meets in the middle. If you delta connect, there's no "middle" as only two coils ever meet at any given connection point.

If you brought balanced 3 power to a transformer with a delta primary and for some reason hypothetically brought a neutral too, aside from the lack of a place to connect it to, the currents between the phases would cancel out and leave no unbalanced current for the neutral to carry back to its source anyway(making it pointless). If you wye-connect the secondary you get a brand new neutral where there wasn't one before and you also get a higher phase to phase voltage than the voltage across a single coil. The secondary could be delta connected too, but the phase to phase voltage would be the same as each coil on its own, and there would be no neutral created*.

*Except if it was a high leg delta, but unlike with a wye the neutral in one of those systems is not located an equal "distance" from all the phases, so the "far away" phase has a higher voltage to neutral than the other two.




I've only seen one wye to wye transformer and that was inside of a 10 kw 3 phase solar power inverter. I guess in that case you could/might have to bring a neutral, but I really don't know any more about transformers than that.

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Old 09-07-2010, 11:42 PM   #8
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Quote:
Originally Posted by Skipp View Post
When you have 480/277 4wire Wye supply. You usally stepdown the voltage to 208/120 to run single phase loads. You use the 480 without the neutral for the primary. Why do they call that Delta to Wye? Isn't still a Wye system even if you don't use the Neutral for the transformers primary?
A 277 volt load is still a single phase load. So is a double pole 480 volt load considered a single phase load. Sure it's derived from a three-phase system but so is a 120 volt load.

All single and double pole loads are single-phase loads regardless of their voltage. It's only a three-phase load if it uses all three poles from it's source.
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Old 09-08-2010, 01:09 AM   #9
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There is a slight difference between the power derived from a delta system vs. a wye. A closed (3 transformer) delta system will tend to balance voltages of all 3 phases, where a wye doesn't.

If you open up one corner of a closed delta, and all 3 voltages are balanced, the voltage across the open corner will be zero. If the voltages are not balanced, a voltage will appear across the open corner.

This is the basis of a phase failure relay, as well as a grounding transformer. If a voltage relay is placed across the open corner (along with a resistor, to cancel out transients), excessive phase voltage imbalance will cause a breaker to trip or a power relay to open, thus preventing damage to motors and such.

If the transformers are used for ground fault detection, if no faults are present, the voltage across the open corner will be zero. When a ground fault occurs, full voltage will appear. This can be used to trigger an alarm or trip a breaker.

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Old 09-08-2010, 04:12 AM   #10
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A 277 volt load is still a single phase load. So is a double pole 480 volt load considered a single phase load. Sure it's derived from a three-phase system but so is a 120 volt load.

All single and double pole loads are single-phase loads regardless of their voltage. It's only a three-phase load if it uses all three poles from it's source.
I know this already! Not what I was asking.

I should have worded my question different. "If you use only the 3 hot legs of a 480/277 system to feed anything 3 phase (transformer, motor..etc.) is it considered Delta? I say no. I say it's still wye.

With that in mind, why do they call a stepdown transformer delta to wye. Even if you are using 3 wires of a WYE to primary. I think I got the answer from Murphy, "the primary coils in the transformer are delta".
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Old 09-08-2010, 06:58 AM   #11
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I know this already! Not what I was asking.

I should have worded my question different. "If you use only the 3 hot legs of a 480/277 system to feed anything 3 phase (transformer, motor..etc.) is it considered Delta? I say no. I say it's still wye.

With that in mind, why do they call a stepdown transformer delta to wye. Even if you are using 3 wires of a WYE to primary. I think I got the answer from Murphy, "the primary coils in the transformer are delta".

exactly.. wye and delta are just two differnt styles to connect threephase.

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