Step-down transformer backward?

I have this transformer:

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model M4L-2-10, 210/230/250 primary with 115 secondary.

Is it safe to use it in reverse to get 230v from 115 mains?

This is in N. America.

Thanks.

Reply to
Fester Bestertester
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Fester Bestertester wrote in news: snipped-for-privacy@news.eternal-september.org:

Should be no problem. A transformer can be used in both directions.

BTW 1000VA is quite a heavy signal :)

petrus bitbyter

Reply to
petrus bitbyter

Be ready for the voltage out to be around 90 - 92% of expected.

I found out the hard way once when I wanted to make an 'isolation transformer. I had two 'bell' transformers so thought aha! put them together 115 down to 24 V then 24 v back up to 115! WRONG! only got 100V, why? seems the transformer companies add some turns, some oomph, to make up for the losses. Draw out the model and you'll see.

So conclusion is may work may not. but is safe to do. The 'shell' of the transformer is referenced more to the secondary, so just watch your isolation.

Reply to
RobertMacy

I once described a setup using a transformer backwards, Phil Allison, said it was a bad idea, something regarding magnetizing current is specified and wound for the primary. The secondary may not have the turns needed for the proper magnetizing current. My setup worked short term, that's all we needed, might have been a problem if we tried to run it 24/7. I am accepting Phil's input, even though I always thought transformers were reciprocal. One caveat, my setup was using two transformers for isolation and looked like this 120//12 to 12//120 driving a many, many section Cockcroft?Walton generator. I think we were up near 10kV.

Mikek

Reply to
amdx

This sort of thing was done in amateur radio applications all the time.

AFAIK the only real caviat is that with losses going the "wrong way" you may not get a full 2:1 step-up ratio, and you may not be able to run it at full rated power. If it's operating in a satisfactory manner and not getting too hot, then you're OK.

Phil knows a lot of things. He just thinks that he knows a lot more.

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Reply to
Tim Wescott

One item on the list to watch out for is the L value on the secondary side.

Transformers do not need to worry a lot about the L on the secondary side since it isn't being connected to a source.

The gage of the wire will be larger of course..

Most of the time they don't put a lot of copper on the secondary side and when you reverse it the coil may saturate to a higher no load current than expected.

What I would do if you have one handy, is put a LCR meter on the secondary to measure the X and then apply that to basic ohms law to find out what the no load current is going to be.

It'll most likely work for you but be less efficient power usage wise.

Jamie

Reply to
Maynard A. Philbrook Jr.

That tranny has several sec taps, including 250, so he can probably get close to 230.

Should work fine.

Reply to
John Larkin

1.2 H. Which works out to 450 K-ohms, 2 mA. Seems pretty small...
Reply to
Fester Bestertester

Yes you can. Select the right tap under load. If used unloaded, the voltage will be higher. Take care not to select a too high secondary under no-load conditions. Under full load the voltage will decrease.

w.

Reply to
Helmut Wabnig

So: higher voltage unloaded; lower voltage loaded; less total watts available.

Sounds like it will work for me within these limitations.

Thanks to all.

Reply to
Fester Bestertester

If you use an LCR meter on an AC power transformer you may get inductor values MUCH lower than expected.

Why? because these transformers are designed for a lot of voltage on them so they don't care about using highly coercive material, translates to ...the material does NOT come 'on' magnetically unless driven much harder than most LCR meters are capable of. So, down in the lower permeability region the inductance can look even 1/10th what it really will be when properly driven.

Reply to
RobertMacy

on most LCR the larger scales are in the 1Khz, i have one that will use

100Hz. take your pick.

Hasn't failed me for looking for defective or just measuring the value, yet.

Jamie

Reply to
Maynard A. Philbrook Jr.

I (OP) think i'll just use an in-line amp meter.

Reply to
Fester Bestertester

This was one of my early conundrums while in a first years electronics class lab. Everyone was given a 0.5 Henry choke and told to measure it using (been a long time) but, as I recall, a sig gen, a big resistor and meters. The class all wrote up their reports with some silly results like 0.017 Henries. No one cared that the chokes were labeled 0.5 Henries. Of course the TA who spoke very poor English cared even less. I went on a mission to figure this out, and learned about B/H curves and initial permittivity. I also learned how low the current was for our class experiment. It was an exiting time for me. :-) 40 years later this pops up when I think of the 4 times rule for winding transformers, specifically for the input to a radio. A radio signal of 1uv barely excites the transformer core, I suspect we are way below the signal level where any relative permeability specs were made. I have never seen any allowance for that fact when winding transformers. Mikek

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Reply to
amdx

That is particularly true of small low current transformers, and in the case of bell transformers, you are talking about a transformer that may have a loosely coupled primary to secondary or high resistance secondary so it can withstand the kind of abuse expected in that kind of service.

Ever wonder why they are so large for such low VA ratings?

Reply to
default

The 3A meter fuse is blowing when i try to measure the quiescent curent.

When i run the transformer from a variac it gives expected output with no indications of shorted turns (i.e., runs cool).

isn't this unusually high even for a "backward" configuration?

have to dig out my 10A meter...

Reply to
Fester Bestertester

A primary core current of 0.25A referred to the secondary would only be

0.5A and even with some amount of extra say up to 0.75A, yeah that does sound large! Perhaps, the 'connection' technique? and surge currents?

I'm super skittish about doing current measurements, since to measure current the meter is a short and I found that if you accidently leave the test leads in the meter configured as current measurement and switch to voltage reading and then try to read a voltage; the meter is still a short! So now I always test with a small resistor in series measuring the voltage drop first, then maybe a current meter. Hey, fuses here are $1.25 ea!

Reply to
RobertMacy

amdx wrote in news:ltcrgj$14d$ snipped-for-privacy@dont-email.me:

Nobody is perfect. Even I am not, neither is Phil. Transformers are not perfect either. But you can optimize a transformer to do do better in one direction, As a result it will do less good in the other. Not all transformers are designed this way. But when enegy saving is an issue, even little bits will help. 0.1% of 1MW is still a lot of power. Nevertheless, unless you have an extreemly special transformer it will do in both directions and not only in hobby circuits. (That's not to say it will do in all professional applications.)

petrus bitbyter

Reply to
petrus bitbyter

it's in rush current..

cross your leads, fire up the tranny and then uncross them..

Jamie

Reply to
Maynard A. Philbrook Jr.

Measured: 0.92A primary steady-state current.

Not unreasonable for my needs.

Thanks to all who helped me. Much appreciated.

Cheers.

Reply to
Fester Bestertester

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