Odd transformer

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EEJake

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I have what I think is a somewhat unique scenario. I need to step down single phase 480V to somewhere in the voltage range of 415 to 380V but it has to maintain the same phase as the original 480V. The load it is supplying is nominally 3A with little inrush.

As I see it, to solve this issue I need to essentially use an auto transformer or a standard dual-winding transformer with the windings connected in a buck configuration. The problem is I cant seem to find an auto transformer or a dual winding transformer than I can accomplish this with. Would anybody happen to know where I could find one off the shelf, or would you have another solution?

Below I attached some crude examples, visuals seem to work better for me.

auto-transformer example.jpg buck winding config example.png

Thanks for your time.
 
Although it would be less efficient and more expensive, a straight isolation transformer with a 480V primary and a 400V secondary would do the job just fine too (in that it would not cause a phase shift). Not necessarily easy to find either, but just mentioning for completeness.

You might also be able to find a 480/120V transformer with a center tap on the 120V secondary, which would also do the job when the partial winding is wired as a buck winding.
(420 instead of 415 out, but....)
 
GoldDigger said:
Although it would be less efficient and more expensive, a straight isolation transformer with a 480V primary and a 400V secondary would do the job just fine too (in that it would not cause a phase shift). Not necessarily easy to find either, but just mentioning for completeness.

You might also be able to find a 480/120V transformer with a center tap on the 120V secondary, which would also do the job when the partial winding is wired as a buck winding.
(420 instead of 415 out, but....)
Click to expand...
Isn't most any isolation transformer going to have a natural "offset" which will mean the primary and secondary voltages are not "in phase" with each other? Isn't that offset usually about 90 degrees?

That said unless the load is something very sensitive and has to interact with something connected to the "out of phase" primary, why should the offset matter anyway?
 
A common misconception, made more confusing by the wide variety of phasing options available in three phase transformers.
The magnetizing current in the primary lags 90 degrees behind the primary voltage.
But the secondary voltage is proportional to the rate of change of the magnetic field, which throws in another 90 degree factor.
Another way to look at it is that the secondary load current and the primary load related current component will try to stay exactly opposite to each other so that the magnetic field in the core does not change from the no load value and phase. (An application of Lenz's law.)
 
140817-0936 EDT

Two Signal transformers ICT-H-A-20 connected to produce about 80 V from 480 can be used to generate a buck voltage for your desired about 400 V. If I interprete the information correctly the capability of the 500 VA rated transformer would be in the range od 10 A. You can get other buck voltages with a different combination of transformers.

Two ICT-H-A-24 provide 96 V, and one ICT-H-A-20 plus one ICT-H-A-24 provide 88.

http://www.signaltransformer.com/sites/all/pdf/ICT.pdf

You can also have a custom autotransformer wound by one of many companies that do this work. A Detroit example is:

http://osbornetransformer.com/
http://osbornetransformer.com/about-us/
http://osbornetransformer.com/products/transformers/auto-transformers/

.
 
If you have a transformer with a nominal 480 to 120V rating, and a 480V supply, there are _two_ possible buck connections that you can use.

One is drawn as your second diagram, with the 120V 'bucks' the 480V primary, and the voltage is from the 480V, to give a 360V output, too low for your use.

But the other connection puts the two coils in phase, as if you were going to boost the 480V to 600V. The '480V' terminal is an 80% tap between the common terminal and the high voltage terminal. You apply 480V between common and HV, and get 384V between common and LV.

The thing that you will need to confirm is that the 120V coil has the same insulation as the 480V coil. Transformers intended for buck-boost use have this explicitly stated.

The transformer is being used at only 80% of nominal voltage, so the KVA is reduced; but you can always use a transformer at lower than rated voltage as long as you can afford the loss in capacity.

-Jon
 
EEJake said:
I have what I think is a somewhat unique scenario. I need to step down single phase 480V to somewhere in the voltage range of 415 to 380V but it has to maintain the same phase as the original 480V. The load it is supplying is nominally 3A with little inrush.

As I see it, to solve this issue I need to essentially use an auto transformer or a standard dual-winding transformer with the windings connected in a buck configuration. The problem is I cant seem to find an auto transformer or a dual winding transformer than I can accomplish this with. Would anybody happen to know where I could find one off the shelf, or would you have another solution?

Below I attached some crude examples, visuals seem to work better for me.

View attachment 10791 View attachment 10792

Thanks for your time.
Click to expand...

I've looked at this a few times. Is there a problem in getting what you want, a 480V to 400V autotransformer, made to order?
 
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