Cooling of overloaded transformer

On Wednesday, April 4, 2018 at 5:24:08 PM UTC-7, Lasse Langwadt Christensen wrote:
> Den torsdag den 5. april 2018 kl. 01.58.52 UTC+2 skrev mrda...@gmail.com:
> > On Sunday, April 1, 2018 at 7:16:27 AM UTC-7, Klaus Kragelund wrote:
> > > 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
> > 
> > 
> > Was this an April Fool's Joke?
> > 
> > If not, why not just toast your bread in a pan on the stove, over a flame?
> 
> he's got a wife ;)


I'm having trouble imagining a wife who doesn't want pan-toasted bread, but is okay with a 220-to-110V transformer with intercoolers, capacitors, and who knows what else sitting next to the toaster.

Michael

On 2018-04-04, Neon John <no@never.com> wrote:
> On Wed, 04 Apr 2018 09:14:49 -0700, Joerg <news@analogconsultants.com>
> wrote:

> 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?

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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On 2018-04-04, Steve Wilson <no@spam.com> wrote:
> Joerg <news@analogconsultants.com> wrote:
>
>> 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
>  
> The control circuit looks complicated.
>  
>>> 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
>
> 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.

yeah, you can't have fewer than 1 turns.


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"Jasen Betts" <jasen@xnet.co.nz> wrote in message 
news:pa4jdd$eko$2@gonzo.alcatraz...
> yeah, you can't have fewer than 1 turns.
>

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
Website: https://www.seventransistorlabs.com/ 

On Thu, 5 Apr 2018 07:30:04 -0000 (UTC), Jasen Betts
<jasen@xnet.co.nz> wrote:


>> 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?
>
>assuming you mean Benefit of DC.

Correct.

>
>You can store it in capacitors.
>
>so you can parallel a bunch of ultracaps and run the welder off a wall-wart.

Do you know of a welder manufacturer that is doing it that way?  I was
wondering more in terms of metallurgical benefits.

I'm also wondering about ultracaps that can discharge at that rate.

I have one of those handheld spot welders that Harbor Freight sells
that is a knock-off of the Lincoln unit.  The open circuit output is 4
volts.  I tried to measure the output current but it pegs my 5kA
Tong-tester.  It requires about half a second to weld 16 ga
sheetmetal.

I've seen the car body fabrication videos where several dozen spot
welders are at work simultaneously.  My little welder emits a jet of
sparks during the welding event just like on the videos.  Now I'm
really interested in what they use.

John
John DeArmond
http://www.neon-john.com
http://www.tnduction.com 
Tellico Plains, Occupied TN
See website for email address

On 01/04/2018 15:16, Klaus Kragelund wrote:
> 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
> 
Can you take it apart and rather than havong the elements for each slice 
in parallel rewire them to be in series.

On 2018-04-05, Neon John <no@never.com> wrote:
> On Thu, 5 Apr 2018 07:30:04 -0000 (UTC), Jasen Betts
><jasen@xnet.co.nz> wrote:
>
>
>>> 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?
>>
>>assuming you mean Benefit of DC.
>
> Correct.
>
>>
>>You can store it in capacitors.
>>
>>so you can parallel a bunch of ultracaps and run the welder off a wall-wart.

> Do you know of a welder manufacturer that is doing it that way?  

I expect dome of the desktop unit work that way

> I was wondering more in terms of metallurgical benefits.

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.

> I'm also wondering about ultracaps that can discharge at that rate.

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

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

> I have one of those handheld spot welders that Harbor Freight sells
> that is a knock-off of the Lincoln unit.  The open circuit output is 4
> volts.  I tried to measure the output current but it pegs my 5kA
> Tong-tester.  It requires about half a second to weld 16 ga
> sheetmetal.

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.

> I've seen the car body fabrication videos where several dozen spot
> welders are at work simultaneously.  My little welder emits a jet of
> sparks during the welding event just like on the videos.  Now I'm
> really interested in what they use.

yeah they're impressive. 

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