Forums

Cooling of overloaded transformer

Started by Klaus Kragelund April 1, 2018
On Sunday, April 1, 2018 at 6:25:40 PM UTC+2, tabb...@gmail.com wrote:
> On Sunday, 1 April 2018 15:16:27 UTC+1, 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 > > Of course, have overrun them hugely. Don't. You don't need a transformer, you need a dropper. >
Yes, but I have been unable to find one I could do one myself, but that's a project the better half won't wait for AC->DC->Inverter stage
On Sunday, April 1, 2018 at 6:37:08 PM UTC+2, John Larkin wrote:
> On Sun, 1 Apr 2018 07:16:21 -0700 (PDT), Klaus Kragelund > <klauskvik@hotmail.com> 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 > > Copper loss goes as current squared, so you can't overload it much. > > (1300/600)^2 is probably too much, even with extra cooling. > > How much does a good 240V toaster cost where you are? > > I like this one > > https://www.amazon.com/gp/product/B00A6Z6U30/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 >
My wife likes the kitchen aid stuff, so we have 115V mixer, toster, longtime cooker, etc Kitchen Aid is 50% price in USA. Amazon won't ship to denmark, so I pick it up when I am there on business. One time I had a big mixer with me, and I was pulled aside, they thought it was a bomb :-) Cheers Klaus
On Sunday, April 1, 2018 at 7:00:36 PM UTC+2, amdx wrote:
> On 4/1/2018 9:16 AM, 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 > > > > OK, I'm not suggesting this, but since it is a resistive load, Could > you put a non-polarized 260uv cap in series with the toaster. The > capacitive reactance would be equal to the resistance of the toaster, > or 11.3 ohms. (think speaker crossover caps,it may not be cost effective) > I would break this up into 13- 20uf capacitors to reduce the current in > each down to about 1 amp. > Also the voltage rating, I think it has to be at least 400V, but I'm > not sure how to calculate that. (hope it's 200V, they are available) > I once used this to drop about 40V down before a bridge for a 5V > regulator, where I didn't want anymore heat. I just substituted until I > had a reasonable input voltage. Harder to calculate because of gulps of > current at the peaks. > I would like some comments about calculating this for the resistive > toaster. > If you think it is a stupid idea, explain why. > Explain the pitfalls. > Mikek
Capacitive droppers are good, but in this case the load is all over the place (low resistance when cold, higher when warm, different power levels etc) But good point about thinking outside the box :-) Cheers Klaus
On Sun, 01 Apr 2018 07:16:21 -0700, 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
That's your answer: just buy a less powerful toaster. -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
On Sun, 1 Apr 2018 07:16:21 -0700 (PDT), Klaus Kragelund
<klauskvik@hotmail.com> 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?
Yeah, utilities do it all the time. A typical 20kVA pole pig that services one or two houses has a service factor rating of 2.3. That means that it can be overloaded 2.3X continuously, the only adverse effect being a shorter life. Under my bench is a 5kVA Topaz Ultra-isolation transformer and a 5kVA variac. I routinely run DUTs at up to 10kVA. Short duty cycle, of course. In your case, you're operating the transformer in what is essentially a long pulse mode. Therefore fan cooling is pretty worthless. What must happen is that the heat build-up in the inner windings must be conducted out and disposed of. I use oil cooling. Plain old mineral oil, available at your local farm store in gallon jugs. Over in the Tesla Coil world, it is quite common to melt the tar out of a neon sign transformer, solvent wash all the tar out of the windings and then immerse the transformer itself in mineral oil. A neon sign transformer is a stray flux constant current unit. The output current is set by the air gap of the flux shunting iron. Opening the air gap increases the current. Removing the magnetic shunt completely makes a regular transformer with moderate mutual inductance between the primary and secondaries. I've read reports of a properly prepared 15kV, 60mA transformer outputting around an amp of 15kV current when immersed in oil. Back to your question. Several things you need to check. First, determine if the windings are aluminum or copper. The cheap chicom autotransformers are frequently wound with aluminum wire with a varnish the correct color to look like copper. Aluminum windings will make the project iffy. It builds up heat so rapidly that not even oil cooling will wash the heat away fast enough. Next, look at the windings themselves. Are they solid masses or are there air gaps, usually created by winding over wooden wedges, in the windings. If there are air gaps then oil cooling will work extremely well. Maybe even with aluminum wire. Very important! Check to see if there is enough iron in the core. It's usual for the Chicoms to use the minimum necessary. The easiest way to check this is to gradually bring up the primary voltage with your load attached and measure the input current. If it suddenly rises then the core is saturated. The above is the cheap-skate in me talking. My practical side says that the Chicom transformer are so cheap that simply buying a larger one is the cheapest way out if you put any value on your time. John John DeArmond http://www.neon-john.com http://www.tnduction.com Tellico Plains, Occupied TN See website for email address
On Sun, 01 Apr 2018 09:53:18 -0700, tabbypurr wrote:

> On Sunday, 1 April 2018 17:49:38 UTC+1, John Larkin wrote:
>> I do most house wiring live, because getting bit by 120V isn't a big >> deal. 240 makes a lot of sense - less current, more power - but I bet >> it hurts a lot more. > > After 1 bite people are usually scared senseless.
Not me. I can barely sense it. One of the benefits of having very dry skin. One of the dis-benefits of same, however, is getting constantly zapped by static in the winter months. -- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.
On 4/1/2018 12:14 PM, Klaus Kragelund wrote:
> On Sunday, April 1, 2018 at 7:00:36 PM UTC+2, amdx wrote: >> On 4/1/2018 9:16 AM, 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 >>> >> >> OK, I'm not suggesting this, but since it is a resistive load, Could >> you put a non-polarized 260uv cap in series with the toaster. The >> capacitive reactance would be equal to the resistance of the toaster, >> or 11.3 ohms. (think speaker crossover caps,it may not be cost effective) >> I would break this up into 13- 20uf capacitors to reduce the current in >> each down to about 1 amp. >> Also the voltage rating, I think it has to be at least 400V, but I'm >> not sure how to calculate that. (hope it's 200V, they are available) >> I once used this to drop about 40V down before a bridge for a 5V >> regulator, where I didn't want anymore heat. I just substituted until I >> had a reasonable input voltage. Harder to calculate because of gulps of >> current at the peaks. >> I would like some comments about calculating this for the resistive >> toaster. >> If you think it is a stupid idea, explain why. >> Explain the pitfalls. >> Mikek > > Capacitive droppers are good, but in this case the load is all over the place (low resistance when cold, higher when warm, different power levels etc) > > But good point about thinking outside the box :-) > > Cheers > > Klaus >
How much lower is the R when cold? It would just heat up a little faster, a little more current through the caps for a short time, nothing unusual. Does your toaster actually have different power levels, or just different toasting times. My toaster is early 20th century technology.
"John Larkin" <jjlarkin@highlandtechnology.com> wrote in message 
news:m932cdpu2vh1v2p5cf2lba3ua6sgsj0787@4ax.com...
> On Sun, 1 Apr 2018 09:42:13 -0700 (PDT), tabbypurr@gmail.com wrote: > >>On Sunday, 1 April 2018 17:37:08 UTC+1, John Larkin wrote: >>> On Sun, 1 Apr 2018 07:16:21 -0700 (PDT), Klaus Kragelund >>> <klauskvik@hotmail.com> 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 >>> >>> Copper loss goes as current squared, so you can't overload it much. >> >>you can heatsink it a lot too. But don't. Life expectancy goes down a lot, >>the interior still gets hot. >> >>> (1300/600)^2 is probably too much, even with extra cooling. >> >>Memory is a bit fuzzy but I think I once pushed a mains transformer to >>about 3x output current long ago. It's terrible for regulation & MTTF, I >>wouldn't consider doing it today. >> >>> How much does a good 240V toaster cost where you are? >>> >>> I like this one >>> >>> https://www.amazon.com/gp/product/B00A6Z6U30/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 >>> >>> but a 240V toaster can go to 1800 watts, which would sure save time. >> >>I once had a 3.5kW one. I think you can still get 3.1kW. >> >> >>NT > > I do most house wiring live, because getting bit by 120V isn't a big > deal. 240 makes a lot of sense - less current, more power - but I bet > it hurts a lot more. > > > -- >
The "bite" goes up with the cube of the voltage. Or so it seams. Happy Easter all.
On 4/1/2018 11:04 PM, amdx wrote:
> On 4/1/2018 12:14 PM, Klaus Kragelund wrote: >> On Sunday, April 1, 2018 at 7:00:36 PM UTC+2, amdx wrote: >>> On 4/1/2018 9:16 AM, 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 >>>> >>> >>> OK, I'm not suggesting this, but since it is a resistive load, Could >>> you put a non-polarized 260uv cap in series with the toaster. The >>> capacitive reactance would be equal to the resistance of the toaster, >>> or 11.3 ohms. (think speaker crossover caps,it may not be cost effective) >>> I would break this up into 13- 20uf capacitors to reduce the current in >>> each down to about 1 amp. >>> Also the voltage rating, I think it has to be at least 400V, but I'm >>> not sure how to calculate that. (hope it's 200V, they are available) >>> I once used this to drop about 40V down before a bridge for a 5V >>> regulator, where I didn't want anymore heat. I just substituted until I >>> had a reasonable input voltage. Harder to calculate because of gulps of >>> current at the peaks. >>> I would like some comments about calculating this for the resistive >>> toaster. >>> If you think it is a stupid idea, explain why. >>> Explain the pitfalls. >>> Mikek >> >> Capacitive droppers are good, but in this case the load is all over the place (low resistance when cold, higher when warm, different power levels etc) >> >> But good point about thinking outside the box :-) >> >> Cheers >> >> Klaus >> > How much lower is the R when cold? > It would just heat up a little faster, a little more current through > the caps for a short time, nothing unusual. > Does your toaster actually have different power levels, or just > different toasting times. My toaster is early 20th century technology. >
Mine has a fixed 1200W power, with a timer based on a CD4541. The timer works nicely but the mains voltage varies widely and this results in varying degrees of toast.
There may be a much simpler solution.  Toasters have multiple
heating elements, so why not check whether they are all in series
or in a series/parallel arrangement.  If they are in
series/parallel, then converting to series connection might
allow correct operation at 230V.
Manufacturers might make the elements operate at 115V each and
adjust the internal wiring to suit different markets.

It may not work out, but it would be worth a look inside.

The solenoid in the "pop-up" mechanism might be a bit harder
to deal with. 

John