480V Delta to 480V Wye

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I have a customer that has a 208V Wye to 480V Delta transformer wired backwards to get 480V for a test station. They are an HVAC contractor and they want to be able to test motors in their shop. The problem is they need 277V to test some motors. With the delta transformer there is no neutral. What can I do (outside of replacing the transformer) to get them 277V?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have a customer that has a 208V Wye to 480V Delta transformer wired backwards to get 480V for a test station. They are an HVAC contractor and they want to be able to test motors in their shop. The problem is they need 277V to test some motors. With the delta transformer there is no neutral. What can I do (outside of replacing the transformer) to get them 277V?

You could install a 480 delta to 480Y transformer sized for the application.
 

mivey

Senior Member
If he's going to do that, may as well get a 208 delta to 480 wye transformer and replace the one they have. JMO.
The additional might could be much smaller. Also would not require re-working the existing test station wiring. Who knows? The Shadow knows.
 

electricalist

Senior Member
Location
dallas tx
I sent a msg to a friend of mine who does hvac and ask what would he need a neutral for when testing motors...Im only trying to help so bare with me when Im wrong.
The times i have seen a neutral needed were for dampers and control wiring and that was actually done within the unit its self..and the other times there was a 120v circuit ran for what was needed.
I dont know xfmrs so does the xfmr have a neutral tap. If not then its a 208 to 480 xfmr?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Unless the 208 to 480 transformer currently being used is particularly valuable, or installed in such a way as to make replacement impractical, I would recommend getting a proper 208V delta to 480V wye step up transformer. I assume that this is a 480V delta to 208V wye step down transformer being used with a reverse connection.

However...If you have a 480V delta source, you can use a 'zig-zag' type autotransformer to derive a neutral. You ground this neutral and you will have essentially created a 480/277V wye source.

Usually these are only used for grounding, and are sized for fault conditions. However see article 450.5; I believe that you can use them to derive a neutral to supply loads; I think need some sort of overcurrent protection scheme that turns off the _entire_ system in the event of an overload of the zig-zag transformer.

-Jon
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Might as well just by a step up transformer.

277V is basically single phase. I have not seen a 277V motor. What exactly is being tested?

Not sure some motors on a VAV I think

277 is the voltage source for the fan in a fan powered box with a 480 v heater?
As evidenced above, the only place I have ever seen 277V motors is in HVAC equipment, so it makes sense that someone building a test stand for HVAC equipment would want it. It is always single phase of course, and always relatively small. For a test stand then, I would just give them a separate 480-277V single phase transformer for use in testing those loads since you already have the larger transformer. It may mean separating out those 277V circuits if they are randomly tied to different phases of the 480V lines, but just for testing, that would be less costly.
 
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