Reversing action of a transformer

I always thought that a transformer that, say, was supposed to convert

480 volts to 240, or say 380 to 220, can always be wired in reverse and transform, say, 220 to 380.

I understand that there may be some practicalities involved, such as difference between nominal and actual no-load voltage to compensate for winding losses. But is there anything else that may prevent a transformer from being used in reverse?

I spoke to a customer, today, who believed that it should not be reversed.

i
Reply to
Ignoramus31086
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Ignoramus31086 fired this volley in news:2t6dnT snipped-for-privacy@giganews.com:

Ig, they cannot be reversed "practically". So-called "core losses" and winding resistances cause the actual load voltage on a load winding to differ from the actual windings ratio. A 2:1 load ratio only works 'one way' in reality, and is, in fact, seldom even close to 2:1 in actual ratio of turns.

If voltage regulation under load is not important AND one imposes only a fraction of the rated load on a step-down transformer, it can be used in reverse, but expect large variation from the 2:1 ratio when used thus.

Lloyd

Reply to
Lloyd E. Sponenburgh

Usually, sometimes, maybe. Here's what Acme has to say:

  1. Can Acme Transformers be reverse connected? ACME dry-type distribution transformers can be reverse connected without a loss of kVA rating, but there are certain limitations. Transformers rated 1 kVA and larger single phase, 3 kVA and larger three phase can be reverse connected without any adverse effects or loss in kVA capacity. The reason for this limitation in kVA size is, the turns ratio is the same as the voltage ratio. Example: A transformer with a 480 volt

source and thereby become the primary or input to the transformer, then the original 480 volt primary winding will become the output or

480 volt secondary. On transformers rated below 1 kVA single phase, there is a turns ratio compensation on the low voltage winding. This means the low voltage winding has a greater voltage than the nameplate voltage indicates at no load. For example, a small single phase transformer having a nameplate voltage of 480 volts primary and 240 volts secondary, would actually have a no load voltage of approximately 250 volts, and a full load voltage of 240 volts. If the 240 volt winding were connected to a 240 volt source, then the output voltage would consequently be approximately 460 volts at no load and approximately 442 volts at full load. As the kVA becomes smaller, the

attempts to use these transformers in reverse, the transformer will not be harmed; however, the output voltage will be lower than is indicated by the nameplate.

Reply to
Ned Simmons

I've been running my 440 3 phase plasma cutter through a transformer for years now, off my 220 generated three phase.

Karl

Reply to
Karl Townsend

Gunner Asch fired this volley in news: snipped-for-privacy@4ax.com:

On the other hand, I was in the business of (first) testing them, then later designing them. Florida Transformer Corporation, 1969... yeah -- long time ago. Plain'ol silicon iron-lam transformers have not changed a whit since then. Not even a whit's whit.

What's claimed for 'average' installations, and what works at full rated load are completely (totally) different animals.

Anyone who claims a 2:1 transformer of 'x' VA rating will work exactly the same in reverse has either 1) over-designed the thing badly or 2) lied.

The only other excuse I'll give them is if they designed it to go both ways... which wouldn't be 'over-designing' in that sense, nor a lie; because you can do that.

LLoyd

Lloyd

Reply to
Lloyd E. Sponenburgh

It is a violation of the 2014 NEC to install a transformer in reverse unless the data plate states that it is allowed.

John

Reply to
John

John fired this volley in news:Y_ snipped-for-privacy@giganews.com:

For reasons stated earlier. Lloyd

Reply to
Lloyd E. Sponenburgh

"Lloyd E. Sponenburgh" fired this volley in news:XnsA445CE8ED7A6Alloydspmindspringcom@216.168.4.170:

But... I should also say for some reasons NOT stated earlier.

Before, we were only talking about the ratings and voltages produced if a transformer of the correct relative voltages were reversed -- with the right voltage applied to whatever might be the 'primary of the day'.

But what if some mook connected a 4:1 backwards on a 440 circuit?

OOPS! LLoyd

Reply to
Lloyd E. Sponenburgh

That depends a lot on the transformer. The common consumer grade voltage converter transformers for international use of small appliances and home electronics are routinely used to convert either

120 to 240 or 240 to 120 - same transformer with the wires reversed. They work just fine. Mabee when you get into larger transformers it is no longer practical??
Reply to
clare

snipped-for-privacy@snyder.on.ca fired this volley in news: snipped-for-privacy@4ax.com:

Clare, why don't you tell ME what the differences should be... mmmm?

I'm open to hear your figures on 1) core saturation values and their effects, 2) IR losses, 3) hysteresis losses, 4) changes in effective turns ratios at different loads and DC levels (rectification of the output).

Any three will do.

LLoyd

Reply to
Lloyd E. Sponenburgh

For the last 3 years, I have had a transformer that I wired to be backfed. I have 240v and I need 480 to test various equipment.

It produces 486 volts without a load. I did have to pull the windings to tap into correct voltage, as the actual high voltage was too much.

Nuclear explosions, zombie invasion, fuses burning out, and other terrifying events have never occurred.

I do not really care very much about the inrush current, either, as long as it does not pop the fuses.

i
Reply to
Ignoramus31086

I am sure that his data plate says so!

i
Reply to
Ignoramus31086

Ignoramus31086 fired this volley in news:z_udnfEguPG snipped-for-privacy@giganews.com:

You tell me right there (with the lack of care about inrush) that it is a beast, and may have been designed for such duty.

Just the fact that it regulated as well as it did says a lot for the design.

A lesser one would likely have produced about 500V.\

But that you knew to polish off a couple of turns and tap down says more!

LLoyd

Reply to
Lloyd E. Sponenburgh

And he gets a Darwin Award. Fair enough, wot?

Reply to
Larry Jaques

I can't argue about transformer theory or design but 1:2 transformers were quite common over here. At one time Jakarta, Indonesia was 115VAC in some parts of town and 220VAC in other parts :-) Smallish thing, usually painted green for some reason, and you had one for your fan and another for your TV and so on.

Reply to
John B. Slocomb

Who knows. It is on a 100A circuit and is protected at the wall by 70A fuses.

I do not have 240v, I have more like 246 volts.

What connected in reverse, it produced 506 volts at the high side, which I deemed excessive.

I had to tap the output from the winding that produces about 486 volts and I am a happy camper now.

i
Reply to
Ignoramus31086

I don't know the engineering details - I just know that some will work perfectly, while others don't work so well. I suspect it has to do with core saturation and core permeability. Likely those with more iron work better.Eddy current losses will likely be part of the problem as well - so the quality and design of the laminations in the core will be somewhat critical. Design of the core, closed core and shell core, will make a fifference too - a shell core, or E-Core will have less flux leakage for a given size, This makes them more effective than a simple closed loop core (or "ring") A simple E-I or E-E core won't be as efficient as an interleaved core, which is a bit more expensive to build..

There is likely some difference in the windings of ones that work well compared to those that work poorly. Stacked, or sandwich coils will likely behave differently than co-axially wound coils, and if the coils are marginally sized there will be more copper losses as well.

A higher quality shell cored transformer will have better regulation than a lighter transformer or a closed core transformer and my suspiscion is there will be a difference between stacked and co-axial windings - not sure which will be better

Reply to
clare

We have a reversed 3-phase transformer to power our big convection oven. The plant supply is 208 three phase. The oven requires 480 three phase. The transformer is derated about 70% of the name plate KVA. Has been running 24-7 for perhaps 10 years.

Paul

Reply to
Paul Drahn

Some transformers are wired in figure 8 forms and when done they couple better. Some cheap windings have a central core and an outer winding on the 8. Better ones put both on the central core and use the outer loops for flux and thus power.

Both of you are correct - one has to test the transformer, and see if the power rating is working and not burning.

Consider a complex way - 220 to 12v. The low voltage winding is likely high current. The impedance of the 12 v circuit is likely low. Putting

12v ac on it and expecting 220 - might happen if the supply can deliver the wattage needed and the winding doesn't burn.

It comes down to VI of the input and VI of the output.

Can the winding take the power needed and not get hot.

Most can, some can't. Some transformers are made for it, others are marginal and some can't handle it.

Getting back to power only - e.g. your stuff - boosting a winding or bucking it or stepping up or down those power transformers are "Universal". Can be used in any configuration. Due to losses you might need to boost and due to gains (better coupling) need to buck (subtract). Power transformers tend to be universal. Electronic transformers can be either way.

You have to read the literature or just try it.

Martin

Martin

Reply to
Martin Eastburn

I do the like with my surface grinder. 220v to 220v 3 phase in a rotary, then transformed to 380 3 phase using boosts on each phase. Wild one is different then on to the surface grinders.

The boosters are 120/240 to 12/24v power units (forget the wattage) but big. With insulation voltage high. So the 12/24 could be stacked on top of the transformed leg of the 3 phase to make the step up value wanted.

My plasma cutter is on 220 single. Could go higher, but wasn't there when I connected the plug.

Martin

Reply to
Martin Eastburn

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