Ive got a 480v primary to 120/208V secondary feed backwards, only feeding three phase pump motors, the problem is that the control boards for the pumps need a neutral, is there any way to reconfigure this transformer to make it produce a neutral on the primary side. This is a mission critical installation, and the possibility of getting the correct transformer is months away. Any suggestions.
Feeding a transformer backwards
- Thread starter CCDTHOMAS
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- Massachusetts
No you cannot get a neutral from that transformer. That XO should be left unconnected to anything.
You could add a small 480/120 volt trans for control power.
Is the 480 corner grounded or ungrounded?
You could add a small 480/120 volt trans for control power.
Is the 480 corner grounded or ungrounded?
chris kennedy
Senior Member
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- Miami Fla.
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- 60 yr old tool twisting electrician
What KVA are you looking for? I just got a Siemens 150 delivered in less then 2 weeks. Where are you located?
Pumps are 480 and you need a neutral there for 277v control ?
You sure ? Very unusual. Most either use 480 control or have a control transformer built in.
If you "need the neutral" for 120v control, a simple 480/120 control transformer will suffice.
You sure ? Very unusual. Most either use 480 control or have a control transformer built in.
If you "need the neutral" for 120v control, a simple 480/120 control transformer will suffice.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yeah, I'm a bit skeptical on this as well. Are you sure you don't have a situation where you can feed the "boards" with 120V from the 208Y120 side separately?
DUH!, Gus
Jraef, Thanks for not saying that
I am getting old. If you need 120v, there is no need for a control transformer, you should have it on the "feed" side.
Side note, Jraef. If the supply to the transformer is only 208 volt (no neutral installed with the supply) can you obtain 120 volt from one of the 208 legs and the not connected XO of the transformer ?
Jraef, Thanks for not saying that
I am getting old. If you need 120v, there is no need for a control transformer, you should have it on the "feed" side.Side note, Jraef. If the supply to the transformer is only 208 volt (no neutral installed with the supply) can you obtain 120 volt from one of the 208 legs and the not connected XO of the transformer ?
- Location
- Massachusetts
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Of course not. But he already said he had 120/208, so an X0 connection is implied in that statement. I interpreted his issue being that he did not have an X0 connection on the 480V side (he called it "primary" which is a little ambiguous in this case), and that would make sense of he has a Delta-Wye transformer. I'm just skeptical, like you, that the "control boards for the pumps need a neutral" which implies 277V if one is talking about the 480V side of the transformer. That would be highly unusual.DUH!, Gus
Jraef, Thanks for not saying that
Side note, Jraef. If the supply to the transformer is only 208 volt (no neutral installed with the supply) can you obtain 120 volt from one of the 208 legs and the not connected XO of the transformer ?
- Location
- Massachusetts
My understanding is that if you bond the XO (which if you where trying to supply 120 stuff you would have to) that under certain conditions that bonding jumper can become overloaded.
BUT ......... I am certainly no electrical theory guru so maybe one of the EEs can step in.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
Actually, a wye-delta transformer can be used to 'derive' a neutral. So in the case of a reverse connected transformer (wye primary, delta secondary) the line-neutral voltage on the primary will be nice and stable and could in theory drive a load.
If there is any unbalanced loading that 'pushes' this derived neutral toward one of the supply legs, you will get circulating currents on the delta side that will tend to rebalance things.
The problem with using this derived neutral is (as iwire notes) that if you want to legally run 120V loads, you need to bond the neutral. Even though this is a derived neutral, you have essentially set up multiple neutral bonds, and you can get current flow on your bonding jumpers. This is also why you do not connect X0 to the supply neutral when you reverse connect a transformer.
If you could legally treat this derived neutral as a hot conductor, and not bond it, then you would be able to run 120V loads with it.
The OP asked the same question in a different thread, and the above can be added to my answer here http://forums.mikeholt.com/showthread.php?t=126089#8
If the OP can get any delta to 480/277V transformer, in addition to the transformer that they are planning on reverse connecting, it could be used to derive a neutral (in the fashion of the zig-zag transformer mentioned in my other post.) But without knowing the size of the transformer and the size of the neutral loading, we can't be particularly helpful. Heck, the guy might be in Baltimore and only need 30KVA, in which case he should PM me
-Jon
If there is any unbalanced loading that 'pushes' this derived neutral toward one of the supply legs, you will get circulating currents on the delta side that will tend to rebalance things.
The problem with using this derived neutral is (as iwire notes) that if you want to legally run 120V loads, you need to bond the neutral. Even though this is a derived neutral, you have essentially set up multiple neutral bonds, and you can get current flow on your bonding jumpers. This is also why you do not connect X0 to the supply neutral when you reverse connect a transformer.
If you could legally treat this derived neutral as a hot conductor, and not bond it, then you would be able to run 120V loads with it.
The OP asked the same question in a different thread, and the above can be added to my answer here http://forums.mikeholt.com/showthread.php?t=126089#8
If the OP can get any delta to 480/277V transformer, in addition to the transformer that they are planning on reverse connecting, it could be used to derive a neutral (in the fashion of the zig-zag transformer mentioned in my other post.) But without knowing the size of the transformer and the size of the neutral loading, we can't be particularly helpful. Heck, the guy might be in Baltimore and only need 30KVA, in which case he should PM me
-Jon
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