Transformer drawing 1 amp

Bob.Jones5400@gmail.com wrote:
> Thanks guys,
> 
> Could someone explain why and how a transformer limits its current?

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

> 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)

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).

> 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.

And you are back to the volts per turn problem.

> 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.

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)

On 14 Mar 2007 05:17:55 -0700, Bob.Jones5400@gmail.com 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.
>
>It hums like crazy when I connect power to it and everything seems to
>work fine with it except that its drawing 1 amp for no reason.   I
>seriously doubt this is power loss in the transformer but I can't
>figure out whats wrong. The only thing I can think of is that a few
>windings might have fused together somewhere but surely this would
>cause a huge current draw?
>
>The transformer doesn't look like there is anything wrong with it(no
>charred spots) and it came from a car battery charger. I'm trying to
>turn it into a variable power supply (about 5 to 12V) but I just have
>no clue if I should chunk the transformer or not.  I essentially have
>nothing connected to the transformer except a switch that works and
>the power cord that also works(they are not the issue here).  I
>connected an amp meter between the switch and one of the transformer
>leads and its reading about .8A AC after start up(there is a surge at
>the start but it seems to settle about there).
>
>
>------ SWITCH ------- ----- AMP meter ----- --- +
> 
>Transformer
>------ Neutral ------- ----------------------------------- +
>
>Anyone have any clues on whats going on or how I could farther
>diagnose the issue? Or is this natural for transformers to waste so
>much energy?  (I thought they were pretty efficient and being that I
>have nothing connected on the secondary I would expect it not to waste
>any power except maybe losses in the core)
>
>Thanks,
>Bob

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

John

On 14 Mar 2007 05:17:55 -0700, Bob.Jones5400@gmail.com 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

  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.

>
>It hums like crazy when I connect power to it and everything seems to
>work fine with it except that its drawing 1 amp for no reason.   I
>seriously doubt this is power loss in the transformer but I can't
>figure out whats wrong. The only thing I can think of is that a few
>windings might have fused together somewhere but surely this would
>cause a huge current draw?
>
>The transformer doesn't look like there is anything wrong with it(no
>charred spots) and it came from a car battery charger. I'm trying to
>turn it into a variable power supply (about 5 to 12V) but I just have
>no clue if I should chunk the transformer or not.  I essentially have
>nothing connected to the transformer except a switch that works and
>the power cord that also works(they are not the issue here).  I
>connected an amp meter between the switch and one of the transformer
>leads and its reading about .8A AC after start up(there is a surge at
>the start but it seems to settle about there).
>
>
>------ SWITCH ------- ----- AMP meter ----- --- +
> 
>Transformer
>------ Neutral ------- ----------------------------------- +
>
>Anyone have any clues on whats going on or how I could farther
>diagnose the issue? Or is this natural for transformers to waste so
>much energy?  (I thought they were pretty efficient and being that I
>have nothing connected on the secondary I would expect it not to waste
>any power except maybe losses in the core)
>
>Thanks,
>Bob

On Mar 15, 12:07 am, John Popelish <jpopel...@rica.net> wrote:
> Bob.Jones5...@gmail.com wrote:
> > Thanks guys,
>
> > Could someone explain why and how a transformer limits its current?
>
> 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.http://www.abiengr.com/~sysop/images/MOT-primary2.jpg
>
> > 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)
>
> 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).
>
> > 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.
>
> And you are back to the volts per turn problem.
>
> > 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.
>
> 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)

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

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

On Mar 15, 6:41 am, Bob.Jones5...@gmail.com wrote:
> On Mar 15, 5:01 am, The Phantom <phan...@aol.com> wrote:> On 14 Mar 2007 05:17:55 -0700, Bob.Jones5...@gmail.com 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



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

<Bob.Jones5400@gmail.com> wrote in message 
news:1173962139.672828.190300@p15g2000hsd.googlegroups.com...


** Smug stuborn,  Groper Alert !!


> 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.


**  Read my post to you   -  you ASS


> So it seems to work out. I guess this transformer is a
> current limiting one.


**  Bollocks.




......  Phil

Phil Allison wrote:

SNIP

>  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
> 
> 

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

Chuck

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On 15 Mar 2007 04:41:29 -0700, Bob.Jones5400@gmail.com wrote:

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

  Probably EI-125 lams:
http://www.laminationspecialties.com/EI/EI125.htm

>
>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.

  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.

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

>
>>   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?

  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.

>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