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
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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
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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
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On 2015-02-19, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

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
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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! <G>
LLoyd
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On 2015-02-19, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

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
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On Wed, 18 Feb 2015 16:53:07 -0600, "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??
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snipped-for-privacy@snyder.on.ca fired this volley in

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
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On Wed, 18 Feb 2015 20:07:28 -0600, "Lloyd E. Sponenburgh"

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
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On Wed, 18 Feb 2015 16:53:07 -0600, "Lloyd E. Sponenburgh"

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.
--
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On Wed, 18 Feb 2015 16:44:10 -0600, Ignoramus31086

Usually, sometimes, maybe. Here's what Acme has to say:
7. 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.
--
Ned Simmons

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

It is a violation of the 2014 NEC to install a transformer in reverse unless the data plate states that it is allowed.
John
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For reasons stated earlier. Lloyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> fired this volley in

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
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On Wed, 18 Feb 2015 19:25:14 -0600, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote: >"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> fired this volley in

And he gets a Darwin Award. Fair enough, wot?
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On 2015-02-19, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

    Probably instant beaker trip or fuse blow. That much mismatch should saturate the core and draw *lots* of current from the wall.
    But -- whatever was connected to the secondary, if it was turned on, or if there was enough voltage to jump the switch contacts, would almost certainly die, too. :-)
    Enjoy,         DoN.
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I am sure that his data plate says so!
i
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On Wednesday, February 18, 2015 at 8:05:48 PM UTC-5, John wrote:

Thanks for that, John, but many, many places don't even observe ... sometim es even PREVENT, yes prevent use of established commercial/residential bui lding codes. ===================================================="Texas didn't have a fire code and small counties are prohibited from havin g them."
"West, Texas, fertilizer plant blast that killed 15 'preventable,' safety b oard says" By Eliott C. McLaughlin, CNN Updated 9:58 PM ET, Tue April 22, 2014
The 2013 fertilizer plant blast that killed 15 people and wounded another 2 26 in West, Texas, "should never have occurred," the chairman of the U.S. C hemical Safety Board said Tuesday.
Though the board's report says that at least 14 people were killed, the dea th toll was updated to 15 people in the days after the blast.
The board's investigation, released a few days after the first anniversary of the explosion, indicates the incident was "preventable," Chairman Rafael Moure-Eraso said. The statement from his agency, which was given Tuesday t o reporters, blamed the company that owned the fertilizer plant, government regulators and other authorities for the incident.
"It resulted from the failure of a company to take the necessary steps to a vert a preventable fire and explosion and from the inability of federal, st ate and local regulatory agencies to identify a serious hazard and correct it," Moure-Eraso said.
McLennan County, for example, didn't have an emergency response plan in pla ce, and "the community clearly was not aware of the potential hazard at Wes t Fertilizer," the report said.
A lack of fire codes was repeatedly cited in the report, with investigators noting Texas didn't have a fire code and small counties are prohibited fro m having them. But, the chairman said, local fire departments need fire cod es to "hold industrial operators accountable for safe storage and handling of chemicals."
Texas town tighter than ever one year later
The board's supervisory investigator, Johnnie Banks, said all levels of gov ernment also failed to adopt codes to keep populated areas away from hazard ous facilities. This is not unique to West, Banks said.
"We found 1,351 facilities across the country that store ammonium nitrate," he said, adding that farm communities are just beginning to collect inform ation on the proximity of homes and schools to ammonium nitrate storage fac ilities.
The investigation determined that "lessons learned" from responses to simil ar incidents were not disseminated to firefighters, 11 of whom died when th e West plant exploded.
The probe said guidelines from the National Fire Protection Association and U.S. Department of Transportation recommend that firefighters evacuate the area surrounding "massive" ammonium nitrate fires and that the area be dou sed with water "from a distance." However, the report said, the guidance is vague because of the use of subjective words like "massive," "large" and " distance."
"All of these provisions should be reviewed and harmonized in light of the West disaster to ensure that firefighters are adequately protected and are not put into danger protecting property alone," Banks said.
U.S. guidelines for ammonium nitrate storage have been static for decades, the board said, but the United Kingdom in 1996 mandated that storage facili ties be one story, well-ventilated and constructed of concrete, brick or st eel.
Moure-Eraso lauded the Fertilizer Institute for recently establishing guide lines for the storage and transportation of ammonium nitrate, along with re commendations for first responders in the event of a fire. He further calle d on all states and counties to likewise update their guidelines.
"The state of Texas, McLennan County, (the Occupational Safety and Health A dministration) and the (U.S. Environmental Protection Agency) have work to do because this hazard exists in hundreds of locations across the U.S.," Mo ure-Eraso wrote. "However, it is important to note that there is no substit ute for an efficient regulatory system that ensures that all companies are operating to the same high standards. We cannot depend on voluntary complia nce."
Though the Chemical Safety Board investigates serious chemical accidents an d makes safety recommendations, it does not issue fines or citations. The B ureau of Alcohol, Tobacco, Firearms and Explosives and the state fire marsh al's office said last week that their investigation into the cause of the f ire remains active.
West's mayor, Tommy Muska, told CNN last week that doing more policywise, l ike instituting a statewide fire code, "would have been a wonderful thing."
"You don't want to overregulate," Muska said. "But you also have to look at what (could) make us safer."
Rep. Joe C. Pickett, chairman of the Homeland Security and Public Safety co mmittee in the state House of Representatives, said local authorities shoul d go beyond having every place subject to a fire code.
The Democrat from El Paso is pushing to give the state fire marshal's offic e more authority, particularly over unincorporated areas, where about 60 of the over 100 facilities storing ammonium nitrate, like what exploded at We st Fertilizer, are located. Unlike those falling within city or county limi ts, these facilities don't have to have things like sprinklers or other saf ety measures.
Other steps would include getting the word out about places that store ammo nium nitrate and other potentially explosive materials.
"We don't want this to happen again," said Pickett, noting that state autho rities took some immediate actions and that other, more deliberate ones are in the works. "There have got to be some changes ... But I don't want to r ush and do the wrong thing."
-- http://www.cnn.com/2014/04/22/us/west-texas-fertilizer-plant-explosion-i nvestigation/
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On 2/18/2015 6:03 PM, Karl Townsend wrote:

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
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