Transformer drawing 1 amp

I have a transformer that is drawing about 1 amp when the secondary is

** No.

** Yes it would.

Dump it.

....... Phil

You didn't mention heat. Is the transformer as hot as a 100 watt light bulb?

Chuck

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All transformers must draw some primary current, even when nothing is connected to their secondaries. This is because a transformer generates voltage across each turn in proportion to the rate of change of flux through the core. It takes some current to swing the core flux back and forth to get that rate of change. Your transformer has a full load rating of about 12*15= 180 VA, so with 120 volts across the primary, that load would require 180/120= 1.5A, in addition (sort of) to the magnetizing current I spoke of.

But the magnetizing current is normally a small fraction of the full load current, so I suspect that something is wrong. Either you are applying more voltage than the winding is intended for, driving the flux swing into saturation, or there is a partial short, somewhere inside the transformer, which could take quite a while to smoke or produce an obvious high temperature on the outside, where you can feel it.

May be a shorted turn or two that would cause the symptoms. A single turn may only have a voltage potential of a fraction of a volt to the turns on either side - the voltage being so low it doesn't immediately blow fuses - but may be causing the hum.

Like someone already suggested 1 amp 120 volts is 100 watts and in a short time you should feel it getting warm then hot.

Or being a reactive load (the inductance of the winding) is causing some hinky reading with your meter.

I used to work for a place that wound their own PS transformers - we measured them with an old fashioned moving vane meter for excitation current and a few milliamps, maybe as high as 10-20 ma in that power range would be normal - depending on how efficient the client wanted the supply to be.

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No.

It may be designed with high leakage. This is a form of current regulation.

A thermostat.

What do you want to do with this transformer? It's probably only good for battery charging.

This would be abnormally high for most transformers. However, one designed for a battery charger may be designed with a lot of reactance so it is current limited and inherently short circuit protected. If it does not get very hot after about 30 minutes, it is probably drawing mostly reactive current, and may be safe. For an ordinary power supply, it would be better to get a transformer designed for a well regulated output voltage.

Paul

Thanks guys,

Could someone explain why and how a transformer limits its current? They all look the same to me. I know there is saturation of the core that limits stuff but I don't know any details about it. It seems that the only parameters that one can change(the main ones at least) are the winding ratio and total windings. (assuming standard transformer design) I suppose if you just have one winding on the secondary and

100 on the primary its not going to function as well as if you have 100 windings on the secondary and 10000 on the primary.

In any case, I'm testing the transformer as we speak. Its been running for about 40 mins and now has a temperature of about 37C. For the first 20 mins its was quite cool and maybe rose about 5 degree's above ambient. My meter is telling me that its drawing 0.8A but another meter said it was only about 0.6A when I tested it. Not sure if the meters are bad, if it has something to do with the inductance, or I just made a mistake... or maybe the meters are not calibrated properly.

I had another transformer that is 25VCT @ 2A and it was drawing about

0.1A without any load. I suppose if everything scales up linearly then the 0.8A is reasonable. Its already at 40C and drawing about 0.90A (it slowly has seemed to creep up from about 0.84A at the start to 0.9A now) so is I draw another 1.5A from it then I'm sure it will get to hot.

It seems to bee peaking out at about 42C(the current has dropped to about 0.84A) but I could be wrong. Its hot to the touch but not close to burning but if one presses pretty hard then it does get uncomfortable and feels like it might burn after a short time. I can only imagine that having a full load on it will make it impossible to use and will most likely burn it up? Also I'm not sure how well my thermometer is measuring the actual temperature. I do not smell any smoke or fumes of any kind either.

What should I do? Chances are I will run the power supply for long durations and rarely will I draw more than a few amps and when I do it will be for short periods of time(in most circumstances). I just need something with a little more juice than my current ps for those rare occasions when I want to play with things like electromagnets and motors. Although I don't want to take any chance have the thing burn down the house either.

If some transformers are designed in this way(to run "hot") could someone please explain why? I'm at a loss as to explaining the behavior of this transformer if its not broke but besides the extra current everything else seems, at least so far, to be ok. Although when that extra 15 amps goes through the secondary and extra 1.5A going through the primary probably spells disaster?

Thanks, Bob

** Resistance in the windings is by far the main limiting factor on output current.

It is also by far the main cause of self heating.

** Saturation limits the max applied primary voltage and the lowest operating frequency. ** That tranny is probably OK. 0.6 amps off load is very high but not fatally so.

** When you apply a load to the secondary, the primary magnetising current FALLS.

Some trannys, like those in a microwave oven, are never meant to be run off load.

** Bet that does not happen.

BTW

You MUST use a "true rms" meter to test AC current transformer windings.

Ordinary ( average responding ) meters will be way under the real number in most cases.

...... Phil

How much is peace of mind worth? (New worry for the new century: it's no longer "Did I turn off the gas" as our grandparents used to ask each other in the movie theatre, but now "Did I turn off my power supply.")

Buy a new transformer.

--
John English

When there is no secondary load, the inductance of th primary limits its current. That inductance drops precipitously, twice a cycle, if the core flux reaches saturation. This occurs if you apply excessive voltage to the primary.

The transformer limits current to the secondary by the resistance of both primary and secondary windings (that waste some of the available voltage, so less is there to drive current through a given load resistance). But if there is a significant flux path around the primary that does not also pass through the secondary, then there is an additional inductive current limiting effect that acts like having an inductor in series with the secondary.

Some transformers intended to survive large secondary overloads (like welders, neon sign, microwave oven and large battery chargers) provide this flux path by separating the primary and secondary coils and adding blocks of core material between them, with a small air gap.

In this picture of a microwave oven transformer, the secondary has been removed (it wound through the holes where the coin lays) and you can see the two blocks of laminations that almost close the flux path around the primary winding.

The critical spec that involves saturation is volts per turn, so for a given number of primary turns, the voltage you apply to the primary. Are you sure the winding you are connecting to the line is really one designed for that much voltage? If you have a variac you can use to turn down the line voltage, smoothly, you can use it to see if the high no load primary current abruptly goes away at some reduced voltage. That would prove that the current is not a result of a short (that would cause the current to drop in rough proportion to the applied voltage).

And you are back to the volts per turn problem.

If the current is a result of a few turns shorted, that current will fall as these turns get hot (till that hot spot causes more turns to short).

(snip)

Does it get hot after, say, an hour? If not, it's probably just a really cheap transformer but OK to use.

John

What are the dimensions of the transformer core? I happen to have a transformer rated 8 amps @ 24 volts, about the same rating as yours. It's wound on a 2" stack of EI-137 laminatinos:

A really good way to detect shorted turns is to measure the unloaded loss of the transformer in question. This requires a wattmeter, which you probably don't have, but just to show some typical measurements, I slipped a single turn of 21 gauge wire around the center leg of my transformer, and also a single turn of 14 gauge wire.

I then measured the current with 120 VAC applied, and with all secondaries open and no shorted turns. Then I successively shorted the single turn of 21 gauge wire and the single turn of 14 gauge wire.

The results were as follows:

Primary current Wattmeter reading

No shorts .187 A 10.5 watts

21 ga shorted .280 A 25.0 watts

14 ga shorted .65 A 67.0 watts

With only 10.5 watts dissipated, the transformer should only get a little warm after an hour or so. With 67 watts dissipated, I would expect it to get hot. The description you give in another post sounds like it's hotter than it should be with no load.

If it's about the same size as my exemplar, then the primary current of .6 amps with no load, plus the temperature rise you are seeing, would seem to indicate a shorted turn.

I also did a short circuit test by placing an ammeter across the secondary and bringing up the primary voltage slowly with a variac. With 8 amps (rated current) in the ammeter connected to the secondary, the wattmeter indicated 10 watts dissipation. This means that at rated load, the copper loss in the transformer is 10 watts, and with a core loss of slightly less than 10.5 watts, the total loss (core + copper) is about 20 watts at full load, substantially less than the 67 watts shown above. If this transformer is similar to yours, and it were operated at full load, you would have more loss due to a shorted turn than due to the load.

The insulation system is probably not designed for this much temperature rise, especially in the immediate vicinity of the shorted turn, The transformer would smell and more shorted turns would eventually result. :-( Without the protective overtemp cutout you removed, there would be danger of a fire.

winding.http://www.abiengr.com/~sysop/images/MOT-primary2.jpg

With the variac I have the following

20V 0.1 A 40V 0.13 A 60V 0.21 A 80V 0.31 A 100V 0.47 A 120V 1.07 A 125V 1.48 A 130V 1.78 A

This looks almost exponential although I can't tell as the first 5 measurements are approximately linear and there is a huge jump when going from 100 to 120.

What does this mean? There definitely seems to be something happening about 115V. (looks almost linear up to that point then linear again but with a much larger slope)

I can't tell what the "core" is inside the bobbin but strangely there is a wood chip(looks like a wedge) that is sticking out on one side up above the bottom. It looks like someone wedged it in there but it was made like that(its obvious). Not sure if this could mean the core is changed or what. Although looking at the bobbin The two coils look like the are stacked on top of each other and are of the same width.

It sounds like this transformer might be current limited like you guys were mentioning. If so, does this mean that I cannot use it for my purposes? The reason I want to use it is because its 15 amps and my current ps is only about 0.75A and every time I use it above 1A I get nervous. (it has a fan in the case though and seems to stay cool but I never run it long above 1A... sometimes though I run it at 2-3 amps for short periods of time( in seconds)). I don't actually need the full 15 amps out of the thing and ATM I only have a 4A bridge rectifier but I was plan on using that and if I ever have the need up it then I'll just replace the rectifier. Also I couldn't find any power mosfets larger than about 8A so I'm limited by that ATM too.

I just don't see any reason to waste the transformer if it will work for my needs. Later on I want to try and build my own transformer when I get some more time. I've got a book on transformer theory and design but haven't got around to reading it yet. (trying to get this PS built so I can play with some motors).

Does this current limiting have anything to do with resonance? If I were to keep increasing the voltage with the variac would I eventually see a resonance curve?

Thanks for your help. I appreciate you taking the time out to explain things(and the other guys too. Everyone has been most helpful).

Bob

wound

laminatinos:

Its about 4x3 but a little smaller

Its 3 4/5 x 3 1/5 x 2

So maybe its just a cheap transformer? Seems to be about the same size as yours but rated at almost twice the current. Of course even if I used it only at 8A then it would still be great.

turn

How did you short the turns? Did you manually rub off the insulating layer and short them?

warm

Yeah. I'm not sure. It seems like it might have been designed to do that. I guess its just not really the best transformer for what I want but maybe it will work if its not shorted in any way.

But if its current limited then this seems natural? Personally if it is current limited it seems like a huge waste of power but maybe its the easiest way. I'm thinking about adding a load to is and see what happens after an hour or so. Try and push and see if it smokes or anything(since its rated at 15 amps maybe I'll just push it to 10A since I'll probably never use it above that).

and

rise,

I added a new 100C thermistor but maybe that was a waste of 1$ if this transformer is bad. I think I'll try and add some load to it and have it run for a while and see what happens. This should give me a good indication if its good or not. This being a car battery charger was surely designed to run for several hours straight? Although maybe the thermistor would cause it to turn off and cool down when it overheated so it wasn't really for continuous use? Is there any max temperature for transformers operating at full load?

Thanks for taking the time out. Its been really helpful in that now I do have an idea, at least for some transformers, about there current draw and what happens when they are shorted. I'm not sure if it applies to transformers in general though but still is helpful(if I can remember the results ;).

Thanks, Bob

05:17:55 -0700, snipped-for-privacy@gmail.com wrote:

wound

laminatinos:

of

turn

turn

warm

should

and

10

watts,

rise,

would

Now that I think about it maybe the transformer is not 15 amps. I thought because the charger was 15 amps that the transformer should be but maybe it was peak? since its a battery charger maybe the peak current was many amps but the average sustained current was only maybe

6-8 amps. (it seems like the same size as your transformer so maybe this is the case).

In this case the 1.0A with no load seems awful large as the turns ratio is 10:1 and that means that its only drawing about 0.8A with full load. Although, again, maybe the transformer was designed this way?

Bob

On the case(of the charger, not the transformer) it says

Input: 120VAC 50/60 Hz 2.8 Amps Output: 12VDC 15 Amps

This is why I assumed it was 15 Amps.

Now since the ratio is 10:1 that means that it would draw a max current of 1.5 Amps at full load on the primary due to the load. Since I'm drawing 1A without any load this is about 2.5A on the primary at full load. So it seems to work out. I guess this transformer is a current limiting one.

Now if I understand a current limiting transformer it is made in such a way as to limit the max current drawn on the primary? It does this by sacrificing power? So at full load of 15A the primary will be pulling much more current than it should but if I tried to pull 20A it would not let me? Sorta like some sort of over current protection? A "normal" transformer would not get as hot but it seems to be ok for this one?

Thanks, Bob

** Smug stuborn, Groper Alert !!

** Read my post to you - you ASS

** Bollocks.

...... Phil

SNIP

Only if there is core saturation or a non-linear load. Otherwise, the two meters ought to read identically.

Chuck

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wound

laminatinos:

Probably EI-125 lams:

Notice that my transformer is 24 volts @ 8 amps, while yours is 12 volts at 15 amps. My voltage is higher and current is lower, but the product of the two is about the same as yours. This is why I say the ratings are similar. I notice in another post that you say the 15 amp rating is the rectified output. This is not the same as the AC current rating of the transformer, so the comparison is somewhat off.

turn

turn

I used a piece of magnet wire and scraped off the insulation at the ends of the wire so I could short them together.

warm

should

Can you post a digital picture of it over on alt.binaries.schematics.electronics?

If it is indeed constructed of standard EI laminations, concentric wound, then is isn't current limited. I suspect that it is a "cheap" transformer that is designed with core flux density somewhat high to help lower the initial cost. This can cause the no-load current draw and temperature to be high. I'll respond to another of your posts where you measure the current vs. input voltage.

Also, can you measure the resistance of the primary and secondary with your ohmmeter? When you do that, first short the ohmmeter leads together and measure the resistance of just the leads. Then subtract that value from the resistance you measure; this is especially important for the measurement of the secondary. Measure the secondary from the outside connections of the secondary, ignoring the center tap.

and

watts,

rise,

would