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

NOTE: I've removed the magnetic shunts that used to sit 
between the primary and secondary. (I cut off the secondary
with a chop saw, and used a punch to get the shunts out.)

Thanks,
Rich

<Bob.Jones5400@gmail.com>

>>
>> **  No law against using a small fan to blow on that tranny.
>>
>>  Easily triples the available power capacity.
>>
>
> Yeah, I already mentioned that I would do that. I didn't know it
> tripled the power capacity though. Do you mean that if it dissipates X
> watts then adding a fan will allow it to dissipate 3*X watt?


** No  -  I mean it can allow a tranny to deliver up to 3 times rated  power 
to a load.

Equates to  9 times increase in dissipation within the tranny.

Only limited by the ability if the tranny to regulate its secondary voltage.



......   Phil 

On Mar 16, 5:35 am, "Phil Allison" <philalli...@tpg.com.au> wrote:
> <Bob.Jones5...@gmail.com>
>
>
>
> > Thanks for all the help. I'm going to use the transformer as is for
> > now and get on with building the rest of the circuit. When I get some
> > more time I'll come back to messing around with the transformer
> > specific stuff. I can always drop in a better transformer if I need
> > to. If the only issue at this point is heat then I can deal with it.
>
> **  No law against using a small fan to blow on that tranny.
>
>  Easily triples the available power capacity.
>
> .......  Phil

Yeah, I already mentioned that I would do that. I didn't know it
tripled the power capacity though. Do you mean that if it dissipates X
watts then adding a fan will allow it to dissipate 3*X watt? Is this
true in general or just for things that heat up slowly and have
adequate ventilation for the fan?

Thanks,
Bob

<Bob.Jones5400@gmail.com>
>
> Thanks for all the help. I'm going to use the transformer as is for
> now and get on with building the rest of the circuit. When I get some
> more time I'll come back to messing around with the transformer
> specific stuff. I can always drop in a better transformer if I need
> to. If the only issue at this point is heat then I can deal with it.


**  No law against using a small fan to blow on that tranny.

 Easily triples the available power capacity.




.......  Phil

Thanks for all the help. I'm going to use the transformer as is for
now and get on with building the rest of the circuit. When I get some
more time I'll come back to messing around with the transformer
specific stuff. I can always drop in a better transformer if I need
to. If the only issue at this point is heat then I can deal with it.
Once I'm more comfortable with building a high current supply I might
invest more into quality components.

Bob

On 15 Mar 2007 23:05:12 -0700, "Bob.Jones5400@gmail.com"
<Bob.Jones5400@gmail.com> wrote:

>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?

  The suppliers usually want to sell hundreds of pounds at a time, but you
could try these guys:
http://www.laminationspecialties.com/

  Your best bet would be to try to find some surplus transformers where
they haven't welded the lams, or varnished them too heavily.  Then you
might be able to get the lams out and rewind the transformer.  You will
also need magnet wire; sometimes local motor rewind shops will sell small
quantities.  And, you will need insulating materials; Nomex is the material
of choice.

>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.

  Stainless steel won't work well at all.  The only thing that really works
well is real silicon steel transformer laminations.

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

<Bob.Jones5400@gmail.com> wrote in message 
news:1174025112.574864.250830@y66g2000hsf.googlegroups.com...
>
> 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.

If the laminations are welded together, it may cause more eddy currents and 
heating of the steel. It may be deliberate.
>
> 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.

Toroids are good for experimental transformers. You can often take apart an 
old Powerstat (variac), and wind a secondary on it. I have some 2 kVA units 
that were trashed because of burned brushes. I have used them with a few 
turns of very heavy wire (actually bus bar) to get several thousand amperes 
at a few volts. Usually you can get about 0.25 to 1 volt per turn on a 200 
VA to 2 kVA core. They are very efficient, and very quiet (acoustically and 
electrically). They are a bit harder to wind, however.

I have thought about making a small toroid from a length of steel strapping 
tape wound up like a clock spring. It should work, but the steel is 
probably not the best for the purpose. Might be an inexpensive learning 
tool, however.

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

You're welcome.

Paul 

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
>
>----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
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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

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