Forums

Cooling of overloaded transformer

Started by Klaus Kragelund April 1, 2018
On 2018-04-01 08:18, Klaus Kragelund wrote:
> 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 >
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.
> But secondary winding losses is on the square of current, so would > probably not be wise >
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/
Joerg <news@analogconsultants.com> wrote:

> 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.
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:)
On 2018-04-04 07:43, Steve Wilson wrote:
> Joerg <news@analogconsultants.com> wrote: > >> 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. > > 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:) >
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/
Joerg <news@analogconsultants.com> wrote:

> As little as they often weigh these days I assume many microwave ovens > now have switching power supplies.
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?
On 2018-04-04 08:05, Steve Wilson wrote:
> Joerg <news@analogconsultants.com> wrote: > >> As little as they often weigh these days I assume many microwave ovens >> now have switching power supplies. > > Yes, you can buy switching microwave ovens in Walmart. They are usually much > more expensive. >
Simple microwave ovens have switchers as well, since a long time: http://www.vk3hz.net/amps/Microwave_Oven_Inverter_HV_Power_Supply.pdf
> I womder if it would be possible to repurpose a switching power supply into a > spot welder? Why not? >
It's being done. http://www.jatit.org/volumes/Vol51No1/11Vol51No1.pdf -- Regards, Joerg http://www.analogconsultants.com/
On Wednesday, 4 April 2018 15:43:52 UTC+1, Steve Wilson  wrote:
> Joerg <news@analogconsultants.com> wrote: > > > 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. > > 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:)
MOTs have a bimetal cutout on the core, and are normally able to cook for 15 minutes on full before the bimetal opens. Thereafter you only get low duty cycle cooking until the MOT cools down. That's why canning in a microwave is a no-no. I'm surprised to hear someone say that a kilo or so of copper has no thermal mass or conductivity. Heat sealers run transformers at many times rated P_out for seconds at lowish duty cycle. NT
On Mon, 2 Apr 2018 13:31:03 -0700 (PDT), tabbypurr@gmail.com wrote:

>On Monday, 2 April 2018 21:08:05 UTC+1, John Larkin wrote: >> On Mon, 2 Apr 2018 09:44:11 -0700 (PDT), Lasse Langwadt Christensen >> <langwadt@fonz.dk> wrote: >> >Den mandag den 2. april 2018 kl. 18.03.59 UTC+2 skrev George Herold: >> >> On Monday, April 2, 2018 at 10:56:46 AM UTC-4, mako...@yahoo.com wrote: > >> >> > > modify the toaster to run on 230 with a diode in series with the >> >> > > heating element ? >> >> > >> >> > bzzzt wrong >> >> > >> >> > I'll let someone else explain why. >> >> > >> >> > mark >> >> >> >> Huh, well except for screwing up the power factor on the AC line, >> >> what's wrong with it? >> > >> >P = V^2/R >> > >> >I must have been low on coffee >> >> It would probably be OK at twice the normal power, quick toast, but >> the controls probably wouldn't work. > >quick meltdown too.
https://www.youtube.com/watch?v=UUPSGvWr6xE -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"Steve Wilson" <no@spam.com> wrote in message 
news:XnsA8BA70D9580ADidtokenpost@69.16.179.23...
> Joerg <news@analogconsultants.com> wrote: > >> As little as they often weigh these days I assume many microwave ovens >> now have switching power supplies. > > Yes, you can buy switching microwave ovens in Walmart. They are usually > much > more expensive.
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.
> > I womder if it would be possible to repurpose a switching power supply > into a > spot welder? Why not?
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 Website: https://www.seventransistorlabs.com/
On Wed, 04 Apr 2018 09:14:49 -0700, Joerg <news@analogconsultants.com>
wrote:

>Simple microwave ovens have switchers as well, since a long time:
I think Panasonic is still the only company that uses inverters. I recently needed to replace the microwave oven in my motorhome. I was looking for a thousand watt unit with all-mechanical controls. I hit Wallyworld, Home Depot and Lowe's. No luck. All the mechanical timer units were 600 watts. Given the super-crappy line voltage inside an RV, electronic controls last anywhere from instant failure to a few months. While I was doing my shopping, I lifted up the right side of each. All except the Panasonics use transformers still. Since ChiCom Inc. makes probably millions of transformers, I can't really imagine how an inverter could be made any cheaper. I'm amazed that Panasonic's price premium is so small. Last year I bought the biggest, baddest Panasonic I could find. I think it's 1700 watts. Required a separate branch. It is wunderful! They've made several improvements since that PDF was written. The most significant is that the oven does not do the duty cycle control for low power anymore. It duty-cycle controls the magnetron all the way to power setting 1. My only complaint is that they include the 5 second fixed filament heating time in the set time. So if I want to nuke something for 45 seconds, I have to remember to set the oven to 50 seconds.
> >http://www.vk3hz.net/amps/Microwave_Oven_Inverter_HV_Power_Supply.pdf > > >> I womder if it would be possible to repurpose a switching power supply into a >> spot welder? Why not? >> > >It's being done. > >http://www.jatit.org/volumes/Vol51No1/11Vol51No1.pdf
They're not repurposing anything. They're designing from scratch. The design would be quite challenging. The current draw during the beginning of the weld is all over the place, as crud is burned away. Then as the weld is completed, the inverter must withstand essentially a short. Doable but challenging. Transformer design would be critical. I'm curious as to the benefit of DC. When I went through welding school many decades ago, all we had were AC machines. Anybody know what the benefit of an AC spot welder is? John John DeArmond http://www.neon-john.com http://www.tnduction.com Tellico Plains, Occupied TN See website for email address
On Wednesday, 4 April 2018 17:25:30 UTC+1, John Larkin  wrote:
> On Mon, 2 Apr 2018 13:31:03 -0700 (PDT), tabbypurr wrote: > >On Monday, 2 April 2018 21:08:05 UTC+1, John Larkin wrote: > >> On Mon, 2 Apr 2018 09:44:11 -0700 (PDT), Lasse Langwadt Christensen > >> <langwadt@fonz.dk> wrote: > >> >Den mandag den 2. april 2018 kl. 18.03.59 UTC+2 skrev George Herold: > >> >> On Monday, April 2, 2018 at 10:56:46 AM UTC-4, mako...@yahoo.com wrote: > > > >> >> > > modify the toaster to run on 230 with a diode in series with the > >> >> > > heating element ? > >> >> > > >> >> > bzzzt wrong > >> >> > > >> >> > I'll let someone else explain why. > >> >> > > >> >> > mark > >> >> > >> >> Huh, well except for screwing up the power factor on the AC line, > >> >> what's wrong with it? > >> > > >> >P = V^2/R > >> > > >> >I must have been low on coffee > >> > >> It would probably be OK at twice the normal power, quick toast, but > >> the controls probably wouldn't work. > > > >quick meltdown too. > > > https://www.youtube.com/watch?v=UUPSGvWr6xE
I doubt it'd have much mttf run like that, and there's fire risk to consider. There are plenty of things one can do. Whether they're good ideas is another question. We all know it. NT