OT- Connecting transformers "backwards"

katratzi · Apr 10, 2007

Is it "code" or legal to connect a transformer in reverse?
We have 6kva 460 to 230 volt three phase transformer. A customer has 208 volt three phase at his facility. He has a machine that
requires 400 volt 3 kva. This works out well because a 208 volt input on the secondary side will produce close to a 400 volt output on the primary. If it is fused correctly, is it o.k. to do this?

I have heard from some that this is not a good idea, that small kva units are usually built with a two winding "T" connection
(whatever that is) and should not be run backwards. What do you guys think?

Leadfoot · Apr 10, 2007

The only problem is that the efficiency of the transformer does go down. Since your transformer is rated 2x what is required, it should not cause problems. Make sure the output voltage does not rise too high. I have seen and done many secondary input and primary output transformer connections.

RussB · Apr 10, 2007

I agree, Ohms Law does not change when you turn the transformer around.

Jim Dungar · Apr 10, 2007

katratzi said:
Is it "code" or legal to connect a transformer in reverse?
We have 6kva 460 to 230 volt three phase transformer. A customer has 208 volt three phase at his facility. He has a machine that
requires 400 volt 3 kva. This works out well because a 208 volt input on the secondary side will produce close to a 400 volt output on the primary. If it is fused correctly, is it o.k. to do this?

I have heard from some that this is not a good idea, that small kva units are usually built with a two winding "T" connection
(whatever that is) and should not be run backwards. What do you guys think?

A T-connected transformer only has two coils rather than the normal 3 coils. These units often have extra windings on the high voltage/primary side in order to compensate for the internal losses (primarily due to the magnetic flux). Running Tee units backwards would mean that the output voltage would not be 480V and it will have poor voltage regulation.

katratzi · Apr 10, 2007

ok, thats two for and none against. I didn't see a problem with
it either.

jrwb4gbm · Apr 10, 2007

Make that three for. I have used this many times.

rsdoran · Apr 10, 2007

Jim Dungar said:
A T-connected transformer only has two coils rather than the normal 3 coils. These units often have extra windings on the high voltage/primary side in order to compensate for the internal losses (primarily due to the magnetic flux). Running Tee units backwards would mean that the output voltage would not be 480V and it will have poor voltage regulation.

In this case I do not see a problem, the T winding may actually work in your favor since the input will be 208v the output may be closer to the 400v you desire then 416v. That and the fact the power rating is twice what you need.

katratzi · Apr 10, 2007

sorry, Jim. you must have posted while I was typing!
how much voltage deviation are we talking about? the power draw
is minimal.

Jim Dungar · Apr 10, 2007

katratzi said:
sorry, Jim. you must have posted while I was typing!
how much voltage deviation are we talking about? the power draw
is minimal.

I don't think you can predict the deviation. This may not be a problem, but it is something you need to be aware of.

JesperMP · Apr 11, 2007

Is a T type transformer the same as an "auto transformer" ? In other words there is only one coil with a separate taps for the "primary" and "secondary" (*).
As far as I remember, these would only be used when there is a small difference between primary and secondary voltages (i.e 180V/230V). I think that 208V/400V is a strangely high ratio for an auto transformer.
Apart from that, using it for either step-up or step-down should mean no difference. Only, it must be rated for the highest voltage of course.

*: Edit. One coil per phase.

OT- Connecting transformers "backwards"

katratzi

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