different voltages on secondary

Merry Christmas
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i have a portable transformer package 480v to 208/120. powered off a generator. voltage phase to ground on all 3 phases is 277 on the primary side, on the secondary i'm getting , a phase 76v, b phase 45v, c phase 120v to ground. why the different voltages?
 

charlie b

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Location
Seattle, WA
Occupation
Electrical Engineer
Is this a Wye-Wye transformer? If not, how are you reading primary voltage to ground?

Is this one of your company's products that is under development, or is this an item that is already being marketed?

Has this particular setup been working properly for some time, and is only now showing the problem, or was this set of readings taken as part of a QC test prior to the machine leaving the factory?

I am inclined to think that either there is a defect in the transformer windings or an error in the secondary wiring. (Edit: To correct a spelling error)
 
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crossman

Senior Member
Location
Southeast Texas
When you say "ground", what exactly are you talking about? The enclosure?

Are you certain that the neutral point of the secondary is attached to ground? If this connection is not present, you will get some "strange" voltage readings as measured with a high-impedance voltage meter.
 
this is an older product that has been working for some time now. it 480 delta to 208y120. the generator runs at 480y/277 but we do not connect neutral to primary side. the neutral is attached to ground at the transformer xo and at distribution panel on secondary.
 

charlie b

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Location
Seattle, WA
Occupation
Electrical Engineer
Did a client report a problem, and are you therefore in the middle of a troubleshooting effort? If so, can you tell us anything about the initial trouble report? For example, did they say that they tried to start some large load, and a breaker tripped?

Were the measurements taken under load, or with an open breaker on the secondary side of the transformer? If the measurements are no-load values, then I suspect it is time to "float test" the transformer (i.e., throw it in a river/lake/ocean (whichever is closest to your location) and see if it floats. ;) If the measurements are under load, then perhaps there is something wrong with the distribution system being served by this generator/transformer.
 
the ground is the equipment ground at the primary fused switch. the neutral is grounded at the x0 and at the secondary distribution panel. this is a unit that has been working for years. its a 480v delta to 208y120. it is run off a generator that is 480y/277. we don't connect the neutral on primary.
 
thanks for the reply. yes, i'm troubleshooting for client via phone. they are checking the unit before it goes out in the field.the voltages are no load. i'm pretty sure they've damaged transformer somehow. i'll recommend the "float test".
 
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