LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Just remember that, if you ever feed a wye secondary as a primary, to leave the "neutral" floating.
Reverse transformer
- Thread starter Andy1099
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rhamblin
Senior Member
- Location
- Kaukauna, Wisconsin
I’m assuming they didn’t use a neutral. Why would you use a Y Delta if you needed a neutral.
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rhamblin
Senior Member
- Location
- Kaukauna, Wisconsin
And why must u leave it floating? And what do u mean by this?
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I'm not sure of the technical reason, but I understand severe overheating can occur in the wye-connected primary.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Yup, this is what happens. Delta-Wye transformer used in reverse with a bonding jumper left installed from XO to the transformer case, fed with 2" EMT.. Ticking time bomb. Lost a utility phase, caused a large imbalance, lots of current on the raceway.....
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The technical reason is that the delta winding imposes a constraint on the line to line (phase) voltages. Traversing the closed delta in either direction must lead to a total voltage change of 0V. Looking at it another way, the phasors must form a closed triangle when set end to end.
The output of a delta source must also automatically obey that same constraint.
But the line to neutral voltages in a wye system do not have any such constraint, and since each wye voltage transforms into a corresponding side of the delta the physical reality causes circulating currents on both the wye side and the delta side until the voltage drops on both sides allow the phasor equation to be satisfied. Those currents (particularly the neutral current) can be many times the design rating of the transformer.
The worst case happens when one POCO line to neutral phase is lost on the wye primary side.
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