Transformer drawing 1 amp

On 15 Mar 2007 04:26:09 -0700, Bob.Jones5400@gmail.com wrote:

>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.

  Could you take a few more measurements at 5 volt intervals between 100
and 120 volts?

>
>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)

  It means the core is saturating.  Do you have an oscilloscope?  If you
do, the waveform of the current in the primary can tell us something about
what's going on.  I've posted some screen captures from my transformer over
on alt.binaries.schematics.electronics.

>
>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.

  This is a standard construction practice.

>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.

  This is a standard "concentric wound" construction, and if these are
ordinary EI laminations, then it's not a "current limited" transformer.

>
>It sounds like this transformer might be current limited like you guys
>were mentioning.

  I'm pretty sure it's not current limited.  I think it's just a low-cost
transformer that has been designed to push the core well into saturation.

>If so, does this mean that I cannot use it for my
>purposes?

  As long as it doesn't overheat, you can use it.

>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?

  It has nothing to do with resonance.

>
>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

Bob.Jones5400@gmail.com wrote:

> 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.

That sure looks like the onset of saturation.

> 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.

The wedge is probably just an assembly aid, to hold things 
together.

> 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?

If it was a current limiting design, there would be a space 
between the primary bobbins, and a chunk of core material 
slid into the space between them, to detour some flux around 
the secondary.  Yours just sounds like a fairly high leakage 
inductance design to make the output voltage sag a bit under 
the full load of an uncharged battery, and made with the 
absolute minimum core material and copper to minimize the cost.

> 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?

I don't think so.

> If I
> were to keep increasing the voltage with the variac would I eventually
> see a resonance curve?
(snip)

No, it would just catch fire.

Speaking of which, I think you can get some use out of this 
transformer, as long as you fuse the primary at about 2 amps 
(in case the heat shorts out the winding) and don't mount it 
near anything flammable.  If it is insulated for high 
temperature, it may work well enough for your purpose.  If 
there is some room in the primary bobbin, and you could 
slide a few more turns of magnet wire in there, to raise the 
turns count a little (lower the volts per turn), you might 
be able to lower the idle current quite a bit.  This will 
also reduce the secondary voltage a little.

Bob.Jones5400@gmail.com wrote:
> 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.

I think it is better described as a cheap transformer 
(minimum copper and iron, intended to run hot) that has a 
soft output regulation.  In other words, you can expect a 
fairly large change (say, 10%) change in secondary voltage 
between zero load and full load.  A true current limiting 
transformer would have the secondary voltage go to almost 
zero at something like full rated to twice full rated 
current.  This gets into what "full load rating" means.

> 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?

Not so much.  It does that by shifting the phase between 
voltage and current.  Current through an inductance is 90 
degrees delayed with respect to the applied voltage, so that 
almost half the time, the current is in the reverse 
direction, compared to the voltage.  During those parts of 
the cycle, the energy previously stored in the magnetic 
field (when the current and voltage had the same sign) are 
being sent back to the power line.  There is current, but 
the average power (the average of the positive instantaneous 
power when the magnetic field energy is increasing and the 
negative instantaneous power when the magnetic field energy 
is decreasing) is quite a bit less than the product of the 
RMS voltage and RMS current (VA).  So a 1 amp primary idling 
current does not mean that the transformer is soaking up all 
of 120 watts, but only some fraction of that.

> 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?

If it was a current limiting design, the secondary voltage 
would severely collapse as you passed through full rated 
current.  If you shorted the secondary (collapse the voltage 
all the way to zero) the current would not be limited only 
by the resistance of the windings, but also by the effective 
inductance the design puts in series with the secondary.

> 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
> 

On Thu, 15 Mar 2007 00:07:25 -0500, John Popelish wrote:

> 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
> 

Whoa! Dooood! I've been cited! ;-) ;-) ;-)

Guess I'll throw myself a little party. ;-)

Cheers!
Rich

On Thu, 15 Mar 2007 05:35:39 -0700, Bob.Jones5400 wrote:

> 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?
> 

It's probably current-limited. At no load, it's wasting power, probably
by saturating the core. But when you draw current off the secondary,
the back EMF will limit the flux and it can come out of saturation -
or the leakage flux doesn't leak so much, something like that.
(Transformers are like Black Magic, AFAIC. ;-) )

Find some kind of load, like some automotive light bulbs - I don't
know how many amps a headlight takes, but it'd probably be a dandy
load. (Does a headlamp take 15 amps?)

Use the Variac, and measure the voltages and currents on both sides,
and write up a chart like the other one, but with various loads (a 
couple of brake lights, a BMF power resistor, whatever).

Another verrrrry interesting thing you could do, if you have access
to a dual-trace scope, would be to watch the primary voltage and
current simultaneously. This will tell you how reactive it is. You
could be drawing 1 amp with a power factor of, maybe 10% (or whatever -
I'm sure someone will correct me if this is wrong.)

Good Luck!
Rich

"John Popelish" <jpopelish@rica.net> wrote in message 
news:rYKdnSzX3OWl72TYnZ2dnUVZ_oKnnZ2d@adelphia.com...

> The wedge is probably just an assembly aid, to hold things together.

No, it lets you use less iron for the bobbin size. CTs are similar.

   You saw in the scope pix I posted that a single shorted turn overwhelms
the peaky current peaks caused by saturation.  It still is possible that
you have a high resistance short, but that wouldn't produce much heat if it
were the case.

  If you have some small wire you could slip through a small space (if
there is any!) and get a turn around the center leg of the transformer, you
could do what I did and look at the primary current when you short a single
turn.  If there's not quite enough space, take a slender wooden
shish-ka-bob skewer and try to force open a space for a wire.

  Based on the scope pix, I don't think you have a shorted turn.  I think
you have a cheap transformer.

  In addition to running the core into saturation, it's probably a cheap
grade of iron, such as M19 instead of M6 (the number refers to the watts
dissipated per pound of laminations).  This could help account for the high
current draw and the high temperature of the core even at no-load.

Ok, Thanks again, Hopefully I'll understand more of this stuff when I
play around with transformers. I have some idea from physics but I
don't have any working knowledge and I've forgot a lot of stuff.

Bob


On Mar 15, 12:29 pm, John Popelish <jpopel...@rica.net> wrote:
> Bob.Jones5...@gmail.com wrote:
> > 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.
>
> I think it is better described as a cheap transformer
> (minimum copper and iron, intended to run hot) that has a
> soft output regulation.  In other words, you can expect a
> fairly large change (say, 10%) change in secondary voltage
> between zero load and full load.  A true current limiting
> transformer would have the secondary voltage go to almost
> zero at something like full rated to twice full rated
> current.  This gets into what "full load rating" means.
>
> > 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?
>
> Not so much.  It does that by shifting the phase between
> voltage and current.  Current through an inductance is 90
> degrees delayed with respect to the applied voltage, so that
> almost half the time, the current is in the reverse
> direction, compared to the voltage.  During those parts of
> the cycle, the energy previously stored in the magnetic
> field (when the current and voltage had the same sign) are
> being sent back to the power line.  There is current, but
> the average power (the average of the positive instantaneous
> power when the magnetic field energy is increasing and the
> negative instantaneous power when the magnetic field energy
> is decreasing) is quite a bit less than the product of the
> RMS voltage and RMS current (VA).  So a 1 amp primary idling
> current does not mean that the transformer is soaking up all
> of 120 watts, but only some fraction of that.
>
> > 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?
>
> If it was a current limiting design, the secondary voltage
> would severely collapse as you passed through full rated
> current.  If you shorted the secondary (collapse the voltage
> all the way to zero) the current would not be limited only
> by the resistance of the windings, but also by the effective
> inductance the design puts in series with the secondary.
>
> > 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

On Mar 15, 4:38 pm, The Phantom <phan...@aol.com> wrote:
>    You saw in the scope pix I posted that a single shorted turn overwhelms
> the peaky current peaks caused by saturation.  It still is possible that
> you have a high resistance short, but that wouldn't produce much heat if it
> were the case.
>
>   If you have some small wire you could slip through a small space (if
> there is any!) and get a turn around the center leg of the transformer, you
> could do what I did and look at the primary current when you short a single
> turn.  If there's not quite enough space, take a slender wooden
> shish-ka-bob skewer and try to force open a space for a wire.
>

Ok, I'll try it when I get some time.  There seems to be a gap in some
spots that I'm sure I could get some wire through.

>   Based on the scope pix, I don't think you have a shorted turn.  I think
> you have a cheap transformer.
>
>   In addition to running the core into saturation, it's probably a cheap
> grade of iron, such as M19 instead of M6 (the number refers to the watts
> dissipated per pound of laminations).  This could help account for the high
> current draw and the high temperature of the core even at no-load.

Yeah, it  probably is just a cheap transformer. I didn't know about
the differences in the material but now I do ;)  I know they
"welded"(looks like its bee soldered) the laminates together so I
can't take it apart.

Do you know if its easy to come by the laminates? I'd like to try to
make a few transformers for fun but I don't want to spend to much
money on it. I've seen bobbins for sell but not laminates.  I was
thinking about trying to make some but I don't really have that thin
metal. well, I have some stainless steel sheets that might work? I'd
have to coat them with something so that there are no eddy currents.
Maybe I can play around with this and see what works best.

Thanks for your time and energy, You've been quite helpful.

Thanks,
Bob

On Thu, 15 Mar 2007 08:07:13 -0500, chuck <nospam@nospam.org> wrote:

>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.

  There *will* be saturation.  The current in the primary of an unloaded, iron
cored transformer is essentially just the magnetizing current.  In a properly
designed transformer with rated voltage applied to the primary, this current
will not be sinusoidal, because the core will be near saturation.  See the
waveforms Bob Jones and I posted over on ABSE.  With waveforms that peaky, an
average responding meter will be substantially in error.

  Only if you are measuring a sinusoidal current waveform will an average
responding meter give the same reading as a true RMS meter.

>
>Chuck
>
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