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Anyone try running a step down transformer in reverse to power a machine?

Mr.Chipeater

Aluminum
Joined
Apr 20, 2021
I've got a new machine coming that requires 400V, the shop is wired for 208. We already have a 400 to 208 25kVA step down transformer lying around that we could use.

In theory we should be able to wire it up in reverse to convert our 208 to 400 for the machine but in practice has anyone tried this? Google seems to indicate that it would work.There does seem to be some talk of bonding vs grounding and possible lack of a neutral on the primary side.

Has anyone here reverse fed a transformer? Did you have any issues?
 
Has anyone here reverse fed a transformer? Did you have any issues?
In theory it can be bad but that never stopped me. Yes, ran a 460-to-230 set of three in reverse, only scary part was the giant CLANG ! noise when powered up. That just made me nervous but didn't stop me. Worked fine for several years.

Disclaimer : you should definitely not use me as a source for safe electrical practices. I'm just saying it worked, not that it's a good idea.
 
Some local codes now require transformers to be rated for step-up when they used to permit "reverse connection". This may have something to do with the large current inrush on powerup -- the loud "bang". Utilities hate those.

The other issue you may run into is having to run the separately derived ground to a corner of a delta winding. Not all localities allow this although I hear it's more common in areas with older industrial parks. Check with your machine manufacturer to confirm your machine will tolerate this if you go this route.

Personnaly I run an auto-transformer in reverse off my rotary phase converter.

RT
 
I think that I have 4 of them currently in line.
Had 5 up to a month ago.
Been dooing that for 30 years.

IDK what bang they are talking about.


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Think Snow Eh!
Ox
 
I'm running one on an injection molding machine, works fine. It doesn't bang, but it does hum rather ominously. And the tranformer sucks up more amps idling than the mills do actually machining.
 
IDK what bang they are talking about.
My setup was three separate transformers from 220 to 440, one for each phase, when you flipped the circuit breaker on the thing made a loud "clang !" noise. That was explained as inrush but why it would make a noise I dunno. And why yours doesn't I also dunno. There's lots of things I dunno, starting with the whole subject of females ....

Others also mentioned it tho so maybe I'm not totally off my rocker.
 
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Reactions: Ox
Mine is one transformer with three phases, so maybe that's the difference.

My confidence regularly outruns my competence with electricity.
 
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My setup was three separate transformers from 220 to 440, one for each phase, when you flipped the circuit breaker on the thing made a loud "clang !" noise. That was explained as inrush but why it would make a noise I dunno. And why yours doesn't I also dunno. There's lots of things I dunno, starting with the whole subject of females ....

Others also mentioned it tho so maybe I'm not totally off my rocker.


Well, mine are mostly "on", and sometimes "off" for months at a time.
Mine likely see more power cycles doo to the power going out, than me switching them.


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I am Ox and I approve this post!
 
I have one stepping up 240v to 400v after my phase converter running my lathe, and wired up many step up transformers in my past life doing industrial electrical
 
OK, that's several of you mentioning "400V".

What app doo you have for 400V?


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I am Ox and I approve this post!
 
OK, that's several of you mentioning "400V".

What app doo you have for 400V?


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I am Ox and I approve this post!
My old German Traub lathe was 380/400, and my current Masteel Maxcut slant bed with Fagor control is 380volt name plate, it’s running at 390volts off the 240/400 transformer with the taps adjusted to get me closer to the 380
 
So these machines must have been installed new with dedicated external X's.

Usually those X's it seems were embedded into the machine.


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Think Snow Eh!
Ox
 
So these machines must have been installed new with dedicated external X's.

Usually those X's it seems were embedded into the machine.


-----------------

Think Snow Eh!
Ox

Yes, I needed to buy a transformer with the Maxcut lathe, they luckily buy them in bulk and sell it with the machine at a decent price
 
I have 3 transformers in use wired that way. Never heard a bang or clang. I did have to feed them with a breaker that was rated for twice the max amp draw of the transformer to keep the inrush current from tripping the breaker on startup.
 
I have 3 transformers in use wired that way. Never heard a bang or clang. I did have to feed them with a breaker that was rated for twice the max amp draw of the transformer to keep the inrush current from tripping the breaker on startup.


Well, the rated amps is rated at the high voltage rate, since it is sold as a "step down".
Since you are feeding at the low voltage - the amps would be doubled.

KVA is KVA. (1000 volt/amps)
30KVA transformer:

30,000 / 240V = 125A
30,000 / 480V = 62.5A


------------------------

Think Snow Eh!
Ox
 
Some local codes now require transformers to be rated for step-up when they used to permit "reverse connection". This may have something to do with the large current inrush on powerup -- the loud "bang". Utilities hate those.

The other issue you may run into is having to run the separately derived ground to a corner of a delta winding. Not all localities allow this although I hear it's more common in areas with older industrial parks. Check with your machine manufacturer to confirm your machine will tolerate this if you go this route.

Personnaly I run an auto-transformer in reverse off my rotary phase converter.

RT
Here in the UK domestic power's 240 single-phase, industrial's 415 3-phase, so I put the transformer (take a 9KVA oil-cooled welder, strip off the welding winding, 3D print a bobbin and wind on root3 times the primary turns, pot with epoxy) BEFORE the rotary converter, works pretty well (have a 5HP spindle mill with 1.5HP feed motor, mag brake and clutch, and big drill press, bandsaw, 300A TIG that I can plug in). UK electrical regs are *different*, so an isolating transformer's permitted in step up/down, as long as there's a safety "protective conductor" earthing the accessible parts, seems logical to me :)
 
Here in the UK domestic power's 240 single-phase, industrial's 415 3-phase, so I put the transformer (take a 9KVA oil-cooled welder, strip off the welding winding, 3D print a bobbin and wind on root3 times the primary turns, pot with epoxy) BEFORE the rotary converter, works pretty well (have a 5HP spindle mill with 1.5HP feed motor, mag brake and clutch, and big drill press, bandsaw, 300A TIG that I can plug in). UK electrical regs are *different*, so an isolating transformer's permitted in step up/down, as long as there's a safety "protective conductor" earthing the accessible parts, seems logical to me :)
I needed to step up from 240 to 600. 600 V rated components for an RPC are more expensive and harder to find than transformers.

The rule changes in the electrical code requiring purpose-designed step-up transformers has more to do with inrush current spikes being troublesome for the utility. Reverse-connected transformers installed in accordance with the electrical code are safe.

RT
 








 
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