Induction cooktops, temperature control questions

Jan, since you did frying pan experiments, do you have a feel how much power transfer is lost at more distance from the coil? IOW would it matter on an induction cooktop if the steel is 3mm farther away from the surface? That's about the thickness of the bottom of my aluminum pot and I'd like to have the steel disc inside. The pot compatibility detection can be fooled, that's not a problem.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg
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I already have this controller:

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Currently used whenever I brew too much and need to use our old downstairs fridge as a fermentation chamber. You can even set the hysteresis on these controllers. For some reason mine came with a UK outlet so I use my homemade UK travel adapter.

For brewing a simple bi-metal thermostat suffices, you can get those to hold to within +/-2F.

I bought it for two reasons. Mainly to straddle two cooktops and then to reduce the change of a boil-over. Those are very messy in brewing.

It's filled with a little over 5-1/2 gallons of filtered water at the beginning, then malt extract, dry malt extract and other stuff is added but at least 1/2 gallon of water boils off as steam.

However, yes, the two cooktops underneath must be able to support a combined weight of up to 50lbs. I was considering brewing a double-batch a few times and then it would be almost 100lbs but so far haven't done it. Mostly because the cooktops lack heating power.

It also nixes all the advantages of induction, and then some. You'd incur roughly the same amount of contact losses plus the power losses in the electronics of the induction driver. If a steel plate must be put under the pot instead of inside I will stay with traditional electric cooktops.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Tue, 28 May 2019 07:57:27 -0700) it happened Joerg wrote in :

Good question, you make me curious. A few mm distance is OK I think, that is already the thickness of the insulation of the coils I use. But an alu plate in between? One moment... grabs some stuff

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It is logical, any metal or conductor will have Foucault currents induced

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Hoe that answers your question. From the same perspective at least some heating of your alu pot should happen. May even help to regulate the temperature better, warming up this alu plate to 52 C took almost a minute!

But the same principle is used to screen IF coils in radios as I am sure you know.

OK, that was todays experiment then :-) Bows to the public, curtain...

Reply to
Jan Panteltje

Thanks, but not really. It is good to know that the aluminum heats up as well. What I am more concerned about is whether, for example, instead of

1500W only 800W gets transferred because the coupling between coil and lossy material becomes too loose. IOW the other 700W not being lost but just not transferred and thus not taken out of the power outlet. All minus the usual circuit losses, of course.

Aluminum works quite well as a shield with real RF. Induction cookers work in the 20-30kHz range AFAIK.

:-)

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Tue, 28 May 2019 10:58:48 -0700) it happened Joerg wrote in :

Nothing much seems to be transfered past the alu plate Do I understand it right that you want a thick metal plate inside the vessel?

100 kHz here.
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The wikipedia entry gives the formula for the eddy current induced power:

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I see f square for the power also it is thickness square.

Anyways for a bit over 30$ you can get one from ebay and experiment, car battery ? This was on 15 V or so (2 x 7.5 V Meanwell in series, got those for 7euro50 each locally.

Why not use a thick steel plate under the vessel to heat it up?

Reply to
Jan Panteltje

tirsdag den 28. maj 2019 kl. 20.30.19 UTC+2 skrev Jan Panteltje:

then you have the same as a regular resistive heater and losses to the surroundings

Reply to
Lasse Langwadt Christensen

That would be bad news for my application.

Yes, because putting it underneath would be less efficient than the regular electric burners.

It's a bit more complicated when you have a stack of layers. Minor air gaps -> 3mm aluminum -> minor fluid gap -> steel disc.

Then I might as well stay with the electric burners I have now, they will be slightly more efficient. Only one area with minor gaps (burner surface to pot) and no electronics that introduce losses.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

you could solder the steel plate to the base of the vessel

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  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

On May 28, 2019, Joerg wrote (in article ):

power relay. Attach thermostat to center bottom of the pot and a concentric ring heater coil also to the pot. Use thermostat to control ring heater. Attach thermostat plate to pot using silicon rubber adhesive.

Need a jib crane for loading and unloading as well.

.

ra

can

Yep. KISS. Fixed thermostat and coil heater, discussed earlier. Maybe 220 Vac, for sufficient power for a double batch. No electronics.

Joe Gwinn

Reply to
Joseph Gwinn

That's exactly what I have now and the cheap electric burners even have an adjustable thermostat. Which is needed because later I must avoid boil-over and that requires a setting in the 210F region, adjustable depending on wind and outside temperature.

If I had a material that would improve the coupling between electric coil and the never 100% even pot bottom that would help a lot. Not for the thermostats in the cooktops because they are already sufficiently accurate but for heat transfer.

Nah, due to lower back issues I always siphon it into the fermenter. It sits on a foldable dolly for the trip to the temperature-controlled chamber.

Oh, if I could get a large diameter 240W heater at high power and reasonable cost I'd be happy. It would need to be 3500W or more to yield an advantage versus my current setup.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Wed, 29 May 2019 08:30:33 -0700) it happened Joerg wrote in :

I was wondering, I have a big steel induction cooking pan (thick bottom), just fill it with water and put the vessel in it? Can never overheat (100C it boils), automatic temperature control!

Reply to
Jan Panteltje

It would work but probably not efficiently because the water in the pan underneath would constantly boil off. I'd have to keep filling with pre-heated water.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On a sunny day (Wed, 29 May 2019 09:27:03 -0700) it happened Joerg wrote in :

Condensor (old car radiator), fan, water back to pan. :-)

Reply to
Jan Panteltje

What about a replacement heating coil for a 240V electric range? There'd have to be some available at 3500W, I think?

Clifford Heath.

Reply to
Clifford Heath

Pretty soon my brew place looks like the shop of Gary Gearloose :-)

I wouldn't mind but I have to schlepp everything back and forth when brewing.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

I thought about it but that turns the whole thing into a science project. They have no holders, no low-temp contacting, no reflector, no drip pan, nothing. The only way would be to find a discarded range or oven with built-in range and hacksaw a part of the range out of it, then make an enclosure.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Am 30.05.19 um 16:35 schrieb Joerg:

As a byproduct to make some noodles today, I put an still undrilled 2.5 mm aluminium 19" front panel between the pot and the induction stove.

The stove switched completely off as if there was no pot at all. It saw nothing that took any energy from the field.

cheers, Gerhard

Reply to
Gerhard Hoffmann

Dang. Thanks for the test. That pretty much ends my idea of using induction cooktops.

It wouldn't take energy away but could cause the coupling coefficient between induction coil and pot bottom to become too low. So a large chunk of the available isn't dissipated but just not being accepted. IOW the power draw from the wall outlet would drop a lot. Which defeats my purpose of trying to heat up beer brewing water faster than I can now.

Next up in 10 days is an American Amber Ale and a Belgian Witbier.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

You still could bolt the active area of an induction frying pan to the bottom of your beer kettle, including thermal paste. The thermal resistance might be quite small, just 2mm additional alu way with a huge cross section.

Reply to
Gerhard Hoffmann

On a sunny day (Thu, 30 May 2019 17:11:32 +0200) it happened Gerhard Hoffmann wrote in :

Some induction heaters have a metal sensor, a microswitch with magnet. If the object is too far away, for example with some alu in between that microswitch will not activate.

Reply to
Jan Panteltje

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