reversing transformer

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wireperson

Senior Member
Location
Florida
I need 3 phase 480V to feed a condensing unit, I have 120/208 3 phase delta high leg service , the people I need to install this transformer for have a step down transformer 480 V delta 120/208 Y , is it possible to reverse the step down transformer and make it a step up? , no neutral needed at the CU, explain why yes or not please.
 

wireguru

Senior Member
you stated 120/208 delta high leg. Isnt that 120/240 delta? In which case your 208Y120 transformer isnt going to work.
 

tom baker

First Chief Moderator
Staff member
I say its not allowed to reverse a transformer unless the transformer is listed for that use:
450.11 Marking.
Each transformer shall be provided with a nameplate giving the name of the manufacturer, rated kilovolt-amperes, frequency, primary and secondary voltage, impedance of transformers 25 kVA and larger, required clearances for transformers with ventilating openings, and the amount and kind of insulating liquid where used. In addition, the nameplate of each dry-type transformer shall include the temperature class for the insulation system.

Typically if you take a step down - its delta wye and reverse it, it becomers a wye delta and then you have to corner ground it.

I don't know how this will work in your case with high leg as primary.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
You will not get 480 using it on 240 volt delta system, you will get 575-600 volts, i've done this before to feed canadian equipment. You will need a transformer set up for this voltage combination.
 

hurk27

Senior Member
H'mmm with a 2.301 ratio, at 240= 554 volts out the 480 side, should have some smoked equipment at an increase of over 70 volts, maybe you had a large enough voltage drop that allowed it to work?

Sorry didn't see your post #7 but the turn ratio is still wrong for this voltage, and will cause over heating if ran at capacity
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
H'mmm with a 2.301 ratio, at 240= 554 volts out the 480 side, should have some smoked equipment at an increase of over 70 volts, maybe you had a large enough voltage drop that allowed it to work?

The equipment it feed were rated at 575 volts,(Canadian) so no it would not smoke the equipment. Also I had to back the taps down, because it actually put out 600 volts when I first turned it on! I too was expecting somewhere around the 550-560 volt range!
 

templdl

Senior Member
Location
Wisconsin
It may appear to be a simple thing as ratio be applying a higher voltage to a transformer than it is rated for such as applying 240 to a 208v winding will most likely cause the tranformer core to go into saturation, the transformer stops acting like a transformer. The result will be that the input current will shoot up and the secondary voltage will collapse.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
I have the same question.
How does one supply a 208V transformer with 240V and still adhere to Code???

There are many things that the code will not allow that work perfectly fine, The code has to cover a large scope of work, and is intended to provide safety standards that does not always fit all situations. Manufacturing is one of those cases where code does not always practical. The utility company wanted $25,000 to supply the plant with 575 three phase, just for 3 welders and 1 crimper. The transformer was properly protected with the correct AIC short circuit protection, the location of the transformer and panel would not create a fire or safety hazard if it failed.
 
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