Re: Transformer drawing 1 amp

On Mar 15, 5:01 am, The Phantom <phan...@xxxxxxx> wrote:
On 14 Mar 2007 05:17:55 -0700, Bob.Jones5...@xxxxxxxxx wrote:

I have a transformer that is drawing about 1 amp when the secondary is
not in use. Is this normal? Its a 15amp 12V CT transformer. The
secondaries read 12 volts from the center like it should(its actually
about 13V). It had what I guess what a thermistor(a rectangular box
that as stuck inside the transformers windings) but I took it out
because I thought it might be the problem.

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:http://www.laminationspecialties.com/EI/EI137.htm

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.

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.


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

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.


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.

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.


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).

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.


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

.

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