Step-down transformer backward?

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

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.

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

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.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

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

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

Should work fine.

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

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.

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

Sounds like it will work for me within these limitations.

Thanks to all.

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.

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

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

ctuallly why don't you just take whatever transformers, piut the primaries in parallel and the secndaries in series ?

** So you have about 120V AC at 60Hz available.

Problems regularly occur with such trannys wired in reverse if the turns ra tio is wrong or the AC frequency is too low.

Luckily, linked tranny is speced for 50Hz as well as 60Hz with a 115V rated secondary. Using it at 60Hz implies the input voltage can be increased by

20% for the same magnetising current when used at 50Hz. It is rated at 1kW so regulation is gonna be close to 5%.

The turns ratio you need is exactly 2:1 so 120V becomes 240V at no load, dr opping to 230 or so under full load.

The ratios available are 1:1.9 and 1:2.08 for the 230V and 250V windings re spectively.

It should work OK with the 230V winding giving about 220V under load and th e 250V winding giving about 240V under load.

.... Phil

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

--
This email is free from viruses and malware because avast! Antivirus protection is active. 
http://www.avast.com

Thinking of this, wouldn't it seem logical that the primary is outside the secondary ? You are trying to achieve a desired output. That is al that matters.

** Wanna try that again ? 1.2 x 2 x pi x 60 = 452 120 / 452 = 0.265

.... Phil

Good one! ...Hanging head in the shame of NOT THINKING OF THAT! In defense, was used to working with 120/24 transformers where two wouldn't do it.

But, as you so rightly pinted out, 240/120 takes two to do it, and done. As long as the secondary can take that voltage. And most transformers are rated, for what? 3kV? or at least 2kV isolation so should work safely.

Yes, that's correct.

One should remember that H does not invert, which would give the answer of 2 mA, if one did. :)

Jamie