Wiring a Transformer backwards

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jonny1982

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Just out of curiousity, what would happen if a transformer was wired backwards?

example - Step down 480 to 208 transformer. Primary (highvoltage) is landed on X and Secondary (low voltage) is landed on H. Would it explode, or just trip fuse or breaker?

Thanks
 
I would think it would trip a breaker, not enough windings on the secondary side to handle the voltage. It would try to put out a lot higher voltage, on the other side. If it didn't trip the primary OCPD, whatever you have hooked up on the secondary side, would suffer. I would think. Never done it, don't know. Somebody will have a scientific answer, I'm sure.
 
Good question, you would end up with over 1100 volts not sure if the insulation would fail.
 
jonny1982 said:
Just out of curiousity, what would happen if a transformer was wired backwards?

example - Step down 480 to 208 transformer. Primary (highvoltage) is landed on X and Secondary (low voltage) is landed on H. Would it explode, or just trip fuse or breaker?

Thanks
Click to expand...

What will happen, since the load side is not tied to any source of that voltage (the transformer is the only source) is that the actual voltage on the "low" voltage side will be 480 times the turn ratio of the transformer, so 480 x 480/208 = 1,100V. That will blow up anything connected to the load side of the transformer and will probably exceed the insulation rating of both transformer and low voltage circuit.
As for what happens if no load is connected and the 480V winding can tolerate 1,100V, the idling current will be four times what it should be based on proper connection, with an even bigger inrush on connection, so it may just trip the supply circuit OCPD, whatever that may be. IF the core does not saturate. That would be the best possible outcome.
If that does not happen, the transformer core will surely be driven into saturation, increasing the current far beyond the factor of 4 and tripping the input OCPD or overheating the transformer to destruction.
 
The output voltage wont be correct and adjusting the taps in the xfmr wont correct it. For 480 the output can be adjusted to low 400 range or over 480.
 
It depends on the magnitude of difference, fault current and transformer size.

480 into 208 winding would saturate the core and lose the reactance. so it would essentially short out and blow a fuse. You may vaporize something if it's a small transformer.

If you hooked up a 240 to 208 backward, you'll get 277v on 208v side.
 
scrypps said:
I did it on a 240 - 24v xformer, didn't put it under load but it was on and there must have been 2400v on the secondary.
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Following EL's post, I would have expected it to trip the incoming CB. That's got to be saturated. The volts/turn ration is 1/10 what it should be.

ice
 
My take on this

Two conditions to consider:

1) No load on secondary......it would sit there at about 1100 volts, assuming the smoke did not escape from the insulation.

2) As it is loaded, the core would saturate (probably pretty quickly) primary current would soar, breaker would trip.

Just my SWAG, your mileage may vary.
 
Gary Glaenzer said:
My take on this

Two conditions to consider:

1) No load on secondary......it would sit there at about 1100 volts, assuming the smoke did not escape from the insulation.

2) As it is loaded, the core would saturate (probably pretty quickly) primary current would soar, breaker would trip.

Just my SWAG, your mileage may vary.
Click to expand...
Load or no load should have little effect on core saturation.
 
Not trying to hijack this thread, but...when I worked as a nuclear startup test technician, one of our jobs was to test MCC components for acceptability. One of the tests was to test the 480/120V control transformer. We figured we'd just apply 120V on the load side and measure the 480V on the high side. But almost every one failed. Ratio was wrong (I don't remember if was too high or too low). We reversed it and applied 480 high side (scary with a 480V source) and measured the 120V secondary and it was well within specs. What gives? Apparently back-feed of a transformer includes more than just the mathematical ratios.
 
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meternerd said:
Not trying to hijack this thread, but...when I worked as a nuclear startup test technician, one of our jobs was to test MCC components for acceptability. One of the tests was to test the 480/120V control transformer. We figured we'd just apply 120V on the load side and measure the 480V on the high side. But almost every one failed. Ratio was wrong (I don't remember if was too high or too low). We reversed it and applied 480 high side (scary with a 480V source) and measured the 120V secondary and it was well within specs. What gives? Apparently back-feed of a transformer includes more than just the mathematical ratios.
Click to expand...
From Square D:
Backfeeding is not allowed for any Industrial Control Transformers of any size, because windings are compensated and backfeeding will result in lower than expected output voltage.
Click to expand...
 
There is a subtle difference and s not so subtle difference between a power transformer and a power transformer.
The control transformer will be very conservatively designed with respect to core saturation and will have its turn ratio compensated for nominal load VD.
The power transformer will not be compensated except by primary tap settings and may be more vulnerable to surge issues because of relative winding positions.
 
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