using step-down transformer as step-up

** It is - with some provisions.

** Enough current would have to flow in the secondary to magnetise the iron core.

That level would be 12,000/220 times the normal level.

** Basically - you are wrong. ** Basically, they are not.

Problems can arise with small AC supply transformers that have high magnetising current levels and poor regulation factors.

These will still work OK in reverse, but at a somewhat lower voltages than when used the normal way.

..... Phil

My wild guess is feed "normal secondary voltage" into what is normally the secondary, with what is normally the primary being open, and the voltage at what is normally the primary will be low-90's % of "nominal", when the transformer is a bigger one of several to 10's of KVA. My experience is that ones of a KVA or two achieve upper 80's % in that area.

I consider this a fairly inefficient transformer. I do not have one handy to try. Though my guess is that running one of those in reverse needs several amps to maybe around 10 amps to supply "magnetizing current" from the low voltage side at "normal secondary voltage" (in addition to soldering tip load current), to achieve output voltage from the line cord maybe around 75-80 volts or so output voltage from the line cord. (Though I would not be surprised with the line cord putting out to open circuit or a voltmeter anywhere from 60 to 105 volts).

Loss, which is probably at least somewhat evenly split into the two directions.

Due to losses, I expect 12KV-to-220V (more like 240V) to have turns ratio a little less than 50 (12,000/240), probably more like 48 maybe 47.

Step-up from 240V to 12 KV means probably turns ratio of at least 53 even in 1-KVA capacity ballpark.

- Don Klipstein ( snipped-for-privacy@misty.com)

iron

Interesting....so does that mean that a transformer designed to step

220V up to 12kV would have more turns on the 220V side than a similar step-down transformer, so that the current drawn by the 220V winding in the absence of a load is smaller? Robert

Interesting....so does that mean that a transformer designed to step

220V up to 12kV would have more turns on the 220V side than a similar step-down transformer, so that the current drawn by the 220V winding in the absence of a load is smaller? ** Your whole notion is false.

Transformers are essentially symmetrical devices.

...... Phil

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so

he iron

No. It means that the wire used on the 220V side would be thicker (to carry the magnetising current without getting too hot) in a transformer designed to be used to step up a 220V source to provide a

12kV output, as compared with a transformer designed to step down a 12kV source to provide a 220V output.

The turns raito for either transformer may well be slightly higher than the nominal 54.5 to compensate for the voltage lost driving the magnetising current through the winding resistance, but this ratio should be the same for transformers driving the same output load in either direction. In practice, it isn't always possible to wind a high voltage winding with the optimal wire thicknesss - thin wire breaks easily.

-- Bill Sloman, Nijmegen

You ARE right. Transformers ARE essentially symmetrical. I think I'm starting to see that was troubling me had nothing to do with symmetry at all, but rather with the no-load current drawn by the winding used as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my

220V wall socket, it'll probably blow the fuse even if nothing is connected to the other side. But that's not asymmetry. It just means that the transformer wasn't meant to be used like that. If it was, it would have more turns on the 220V side (and a correspondingly larger number of turns on the 12kV side to maintain the ratio) so that the now greater inductance would draw less current while magnetizing the core.

So it's not the transformer itself that is asymmetrical in behaviour, but rather the requirements placed on the windings, depending on whether you use them as primary or secondary. A primary winding shouldn't draw much current when unloaded, and for a secondary, fewer turns is probably an advantage since it gives lower impedance so the voltage won't drop as much under load.

Does that make sense, or am I still wrong? Robert

You ARE right. Transformers ARE essentially symmetrical. I think I'm starting to see that was troubling me had nothing to do with symmetry at all, but rather with the no-load current drawn by the winding used as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my

220V wall socket, it'll probably blow the fuse even if nothing is connected to the other side. But that's not asymmetry. It just means that the transformer wasn't meant to be used like that. If it was, it would have more turns on the 220V side (and a correspondingly larger number of turns on the 12kV side to maintain the ratio) so that the now greater inductance would draw less current while magnetizing the core.

So it's not the transformer itself that is asymmetrical in behaviour, but rather the requirements placed on the windings, depending on whether you use them as primary or secondary. A primary winding shouldn't draw much current when unloaded, and for a secondary, fewer turns is probably an advantage since it gives lower impedance so the voltage won't drop as much under load.

Does that make sense, or am I still wrong? Robert

You ARE right. Transformers ARE essentially symmetrical. I think I'm starting to see that was troubling me had nothing to do with symmetry at all, but rather with the no-load current drawn by the winding used as the primary.

If I plug the 220V winding of that 25kVA step-down transformer into my

220V wall socket, it'll probably blow the fuse even if nothing is connected to the other side. But that's not asymmetry. It just means that the transformer wasn't meant to be used like that. If it was, it would have more turns on the 220V side (and a correspondingly larger number of turns on the 12kV side to maintain the ratio) so that the now greater inductance would draw less current while magnetizing the core.

So it's not the transformer itself that is asymmetrical in behaviour, but rather the requirements placed on the windings, depending on whether you use them as primary or secondary. A primary winding shouldn't draw much current when unloaded, and for a secondary, fewer turns is probably an advantage since it gives lower impedance so the voltage won't drop as much under load.

Does that make sense, or am I still wrong?

** You are still wrong.

When I say transformers are essentially symmetrical - I mean they CAN be used in both the normal and reverse directions.

Why not simply post a REAL question - ie one where YOU are not posing mad theories and making up false examples.

Eh ????

..... Phil

mad

I'm sorry. I didn't mean to upset anyone. This is a real question to me since I need a high-voltage step-up transformer for an RF amplifier and was planning to use a power distribution transformer backwards, but it's expensive so I wanted to make sure it will do what I want before I buy one. Again, sorry I'm being a pain. I won't ask any more dumb questions. Robert

Why not simply post a REAL question - ie one where YOU are not posing mad

I'm sorry. I didn't mean to upset anyone.

** Not upset, but you were going no-where with your absurd Q.

This is a real question to me since I need a high-voltage step-up transformer for an RF amplifier and was planning to use a power distribution transformer backwards, but it's expensive so I wanted to make sure it will do what I want before I buy one. Again, sorry I'm being a pain. I won't ask any more dumb questions.

** If you had just posted that simple Q right off - with a link to the tranny you are looking at - you would have very likely had your answer long ago.

..... Phil

Trying again with a "simple" question :-) Here's a link to pictures and some info about a transformer:

(You have to scroll down past some irrelevant stuff. Then there are pictures and a close-up of the nameplate.) It's a 25kVA 4160/7200/12470V to 240V 60Hz step-down transformer.

What I'm wondering is, if I use this as a step-up transformer by applying 240V to what is normally the secondary, what would be the no- load current drawn by the 240V winding?

I suspect there isn't enough information here to determine that, but perhaps someone who is familiar with transformers of this type would have some idea?

Robert

That's been done before with "pole-pigs"; typically hams using 3300/240 trannies (local dist. lines were often 3300V back in those days). They had no problem with excessive primary current - the magnetizing current in the core is sufficient either way - consider a neon sign transformer.

Have Fun! Rich

Are you entering the Darwin awards this year?... :)

--

John Devereux

** Such large transformers have low * percentage * losses and the iron and copper loss ( at full load) are about the same. The 240 volt winding is rated a tad over 100 amps - so I reckon I mag might be 5 to 10 amps or so on the 240 volt side.

But you will never be able to switch it on without using a big variac or a well designed "sort start "circuit and you will never get more out than you put in.

Clearly, you have some totally hair-brained, LETHALLY DANGEROUS and ILLEGAL scheme in mind.

Kindly GO STRAIGHT TO HELL.

Bloody lunatic.

...... Phil

g mad

o

I've built a few low power RF amplifiers in the past. I "rolled my own" impedance matching transformers for coupling between stages. I don't think that transformers designed for 50- 60 Hz are going to wrok well at RF frequencies. What frequency and power levels are you working at?

George Herold

If he is talking about a PSU for a big (1KW) SSB linear amp I've used microwave oven transformers to generate 3KV @ 1 amp. Its a low duty cycle application. The transformers are free from the local dumpster !

--
Best Regards:
                     Baron.

Yes, but only ALMOST a direct short; the secondary winds around a magnetic core, and has, thus, some series inductance. If one loads the primary, the effective impedance on the secondary will go down. If you short the primary, it acts like most of the series inductance vanishes, though the primary has significant internal resistance which will prevent the short circuit from being completely effective.

Actually, when speaking of running a step-down transformer backwards I was thinking of the power supply transformer. I'm thinking of making a high-powered induction heater with something like 100A at 240V input to the transformer, and a 20kW water-cooled power triode as an oscillator. Maybe my saying "RF" was a bit of an exaggeration; I was thinking of something under 100kHz.

As for the "Darwin awards" (thanks, John :-), I guess I must have been trying ever since I stuck my fingers into an energized light bulb socket at the tender age of 6, but somehow I'm still around :-)

As for Phil's lovely admonition to "go straight to hell", I hardly think this is "illegal", or even "lethally dangerous" if handled correctly. Units with far more power than I am proposing are used routinely commercially. For example, here's a 150kW unit:

I'm sure even that is small potatoes in some applications.

Also lurking in the back of my mind for some future day is a low- frequency radio transmitter for, say, 8kHz. I believe frequencies that low are unregulated, so there's no legal limit to the amount of power one could use. (Please no flame wars if I'm wrong...just a polite correction would be fine! :-) The only problem is antenna efficiency, but that's probably a topic for another thread....

Robert

Thanks for the tip. I'm looking for something more powerful, but this is a great idea for other projects. I see microwaves put outside on the curb all the time, and never thought of cannibalizing them for their transformers! I suppose the magnetron might be useful to some folks as well, but perhaps it would be burnt out in a discarded microwave. (Although, you'd be surprised how much perfectly functioning stuff gets thrown out by consumers and industry alike. I remember when I was a grad student, the amount of good stuff I scavenged from the university's garbage bin was astounding! And later when I worked at a big company too. Until they got wise and locked the garbage. To make sure the perfectly good equipment actually DID get turned into landfill or whatever. Sad, sad, sad....) Robert