Cooling of overloaded transformer

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Hi

When I go to the states, I usually buy white goods, half price than in Europe

So I have a 600W 230V to 115V transformer installed in a cabinet  

Now I have bought a 1300W toaster

Instead of buying a new transformer, I am seriously considering just overloading it, but then keeping the heat down by forced convection, namely a fan

Anyone tried overloading transformers before?

Regards

Klaus

Re: Cooling of overloaded transformer
On 2018-04-01 07:16, Klaus Kragelund wrote:
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Afraid it won't work. Before the core has a chance to heat up one of the  
windings will likely start to smoke. Or if the transformer has one it  
may irreversibly blow an internal thermofuse. It might even work with  
just one round of toasts but you know how it is. Guests come over and  
then "Oh, let's prepare 10 slices of toast".

There could also be some "discussion" with the adjuster of your home  
insurance about replacing the kitchen furniture after it has gone wrong :-)

What I have done in Europe, but that was not legit per code: I needed  
115V in my lab so I used a big 2kW transformer with center tap, not two  
separate windings. Those are smaller and in Germany they are called  
"auto-transformers" for some reason. However, I had to make sure that  
the 115V gear was connected between neutral and tap, never between hot  
(phase) and tap because that can blow Y-caps in there.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Cooling of overloaded transformer
Thanks Joerg

I suspected the same, better not to take any risk at all

Normal transformers are heat limited, and if the 600W was rated at 60 degrees ambient and I will be running it low temp

But secondary winding losses is on the square of current, so would probably not be wise

Re: Cooling of overloaded transformer
On 2018-04-01 08:18, Klaus Kragelund wrote:
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Depends on where they come from. Cheap ones may have no thermo-fuse at  
all, relying on a fire department not to be very far away :-)

Better ones have a self-resetting thermo-fuse but if this action happens  
a lot and under load that fuse can eventually fail. Medical grade  
transformers usually are not allowed to have a self-resetting  
thermo-fuse, it must be a one-time fuse and when that opens the  
transformer is, well, toast.


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The main thing is that windings heat up way faster than a core. They  
have no real thermal mass and because they are "cocooned" the heat has  
nowhere to go.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Cooling of overloaded transformer

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This is very interesting. Microwave ovens have the transformer running very  
close to saturation. If you want to use it for something else, you might need  
to remove the shunt that provides a limit to the core saturation when firing  
a magnetron. Other applications might include a spot welder, which usually  
only runs for a few seconds.

The oven may allow cooking for 99 minutes. With the shunt in place, what  
happens to the winding temperature?

Of course, anything I can imagine cooking in a microwave would catch on fire  
long before it timed out:)

  


Re: Cooling of overloaded transformer
On 2018-04-04 07:43, Steve Wilson wrote:
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As little as they often weigh these days I assume many microwave ovens  
now have switching power supplies.

People in Europe also talked about "coreless transformers" for low  
voltage lighting, from the days when LV halogen bulbs were popular.  
Those things would take in 230VAC and deliver 24VAC or similar, via a  
small ferrite core. Of course not at the kW power level, there'd  
probably be no market for that.

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Cooling of overloaded transformer

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Yes, you can buy switching microwave ovens in Walmart. They are usually much  
more expensive.

I womder if it would be possible to repurpose a switching power supply into a  
spot welder? Why not?

Re: Cooling of overloaded transformer
On 2018-04-04 08:05, Steve Wilson wrote:
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Simple microwave ovens have switchers as well, since a long time:

http://www.vk3hz.net/amps/Microwave_Oven_Inverter_HV_Power_Supply.pdf


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It's being done.

http://www.jatit.org/volumes/Vol51No1/11Vol51No1.pdf

--  
Regards, Joerg

http://www.analogconsultants.com/

Re: Cooling of overloaded transformer

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The control circuit looks complicated.
  
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I meant to convert a standard microwave oven switcher into a spot welder,  
like waht is done with a conventional microwave oven transformer. It looks  
like the control circuit might require considerable modification to handle  
the variable load. For example, there would be no requirement to monitor  
the load since there is no magnitron filament to keep warm. Rewinding the  
transformer to supply low voltage and high current might be a problem.

Probably easier to just find a junk microwave oven and go the conventional  
route.



Re: Cooling of overloaded transformer
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yeah, you can't have fewer than 1 turns.


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Re: Cooling of overloaded transformer
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Sort of.  You can use a multi-leg core as a turns divider.  You still have  
to have one turn on something, but the main winding can be effectively two  
or more turns per turn of that secondary.

The trick is to put a balancing winding between all the legs, to enforce  
equal flux.  Without, the legs act independently (given that the sum equals  
the flux through the center leg, the main winding).

For a spot welder, I suppose you'd have one turn on each limb, and wire them  
all in parallel; this would affect flux balancing, so you don't need  
anything extra.

Leakage inductance is worse, because the windings aren't on top of each  
other.  The textbook schematic of a transformer with windings on either side  
of a core, is a pretty awful transformer indeed.

It's interesting, for any low-voltage application: you only need as much  
core cross section as a single turn secondary requires, at the operating  
frequency.  You might design an isolated Vcore forward converter this way, a  
one-turn secondary that needs very little flux indeed, at a fairly high  
operating frequency.  The ideal core has a wide winding area with a thin  
center limb, allowing ample space for windings.  This would be a good  
application for a planar transformer, but available cores aren't usually so  
exaggerated in proportion -- they might be good for 5V or more on a single  
turn.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Cooling of overloaded transformer


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<https://www.cromweld.com/ac-vs-dc-welding/

Re: Cooling of overloaded transformer
On Thursday, 5 April 2018 01:33:05 UTC+1, snipped-for-privacy@notreal.com  wrote:
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That's all about arc welding. IMLE ac gives a lot more spatter. Spot welding is another matter altogether.


NT

Re: Cooling of overloaded transformer
Den torsdag den 5. april 2018 kl. 03.45.36 UTC+2 skrev snipped-for-privacy@gmail.com:
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no splatter with TIG, but yes spot welding another matter altogether


Re: Cooling of overloaded transformer
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assuming you mean Benefit of DC.

You can store it in capacitors.

so you can parallel a bunch of ultracaps and run the welder off a wall-wart.

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Re: Cooling of overloaded transformer
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I expect dome of the desktop unit work that way

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spot welding works by resistive heating of the metal, so there's no
ion effects.  DC is going to create a mgnetic field, I can't see that
being a good thing.

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this guy seems to be using regular low-ESR electrolytics:

http://rfscientific.eu/simple-capacitor-spot-welder

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4V so the turns ratio would be 55:1 assming 220V in.
have you measured the primary current?

there's probably a magnetic shunt, but I don't think that will effect
open-circuit voltage.

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yeah they're impressive.  

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Re: Cooling of overloaded transformer
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Yeah, cheaper ones are still iron cored.  They use copper clad aluminum  
windings.

Matter of fact, CCA, and copper plated steel, is very common for cheap  
Chinese wiring.  Even (especially?) piddly crap like breadboarding jumper  
wires.


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No, V/turn is too high, and the control is different.

I mean, you could adapt the control, and put a weird winding on there that  
reduces the voltage further, but you might as well start with a new  
transformer.

I wonder how skin effect affects spot welding.  Surely it would make the  
heating spot much larger (and stretched out along the direction of the  
electrodes), until it becomes hot enough (resistance becomes high enough)  
that it breaks through at the intended spot.

The inductance of the electrode arms would be a severe barrier above, say,  
10kHz.  It would look more like an induction heater (Q > 1), and you're  
wasting all your capacity on kVARs.

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Cooling of overloaded transformer
On Wednesday, April 4, 2018 at 8:05:43 AM UTC-7, Steve Wilson wrote:
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Yeah, mine is a Panasonic 'Inverter' model...  it's not notably lightweight, though.

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Some pliers-type spot welders are, nowadays, switching-power input, but
that's not the case for the cheapest Harbor Freight models...   the problem is,
you don't want or need DC for a spot welder, but that's what the inverter style has to
offer.   So, iron  being cheap, the welders use heavy 60 Hz iron transformers.

I hear a lot about Cuk converters, but does that still have a DC power storage
capacitor?  Such storage is wasted for welding.

Re: Cooling of overloaded transformer

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I think the inverters are AC. In any event, the question is can you convert a  
junk inverter microwave oven into a spot welder like you can with a MOT?

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Probably handles short circuit loads better than an inverter. Also, as Tim  
points out, less problems with skin effect.

Re: Cooling of overloaded transformer
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Yes, the capacitor is between the inductors as coupling.  It's optional, but  
the inductor is smaller with.

Incidentally, the ground return of a Cuk carries the sum of input and output  
currents.  Duh, but since the voltages are opposite, the current is the sum  
of the magnitudes, i.e., a 5V to -5V, 1A converter delivers 2A through its  
ground node.

All three nodes (in, out and GND) carry continuous current (just inductor  
ripple).

You might then suppose, well, let's ground the output and allow the GND  
return to lift.  Well, after some rearrangement, you can reduce this to a  
single winding inductor, and you've just invented a buck converter (and full  
switching ripple is back, because of the sum of input and "output"  
currents). :-)

I wouldn't think there would be anything wrong with DC for spot welding.

Just build a bigass multiphase buck, dump all the outputs together, don't  
even bother with output capacitors.  (You need a current mode controller to  
do this.)  A CPU VCORE controller might do the job.  You still need a  
shitton of 12V to get there, of course; if the controller doesn't mind what  
the supply voltage is, you might do 160VDC (rectified 120V, or higher for  
that matter), with a fairly low switching frequency and awfully low duty  
cycle.

Bonus points for tacking on PFC, so you can draw several kW in the seconds  
before the breaker pops.  Actually, just run it constant duty (with a fixed  
peak current limit to maintain protection) -- pulsating power is no worse  
than usual.

Alternately, use supercaps (and maybe a 24V bus instead) to deliver much  
more power than available (over the same time frame) from the outlet, and  
also enable "portable" applications.  Hmm, you'd only need about a dozen  
Boostcaps, that's not all that horrible.  It could actually be luggably  
portable (if not handheld).

Might also be good to use a current multiplier for the output:
https://www.seventransistorlabs.com/Images/Current_Multiplier.png
Just the series-parallel transformation of a Cockroft-Walton ladder.  
(Incidentally, the inductors have to be separate, you can't really share any  
on a common core.)  Doesn't seem to be any reference to it on the internet;  
perhaps I'll call it the Williams ladder.

Tim

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Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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