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Fish4Fun

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Assuming a thin ferrous skin mechanically attached to a multi-layered aluminum/stainless pot designed specifically for induction or resistive heating, would it be practical to use a rotating disk with permanent magnets embedded in it to heat the water inside the pot? Obviously the amount of heat in watts delivered to the water will be less than the amount of energy used to rotate the disk, but this does not preclude the efficiency of heating the water via induction being higher than using mechanical energy to create electricity that is in turn used to drive conventional resistive heating elements. That is to say, if the available energy source is an IC, wind, hydro or other mechanical energy source, can it be more efficient to use this energy to drive a rotating disk populated with permanent magnets than an alternator/generator than a resistive heating element?

Given a roughly 18 inch diameter pot and a similarly sized rotating disk and a practical rotational speed of 2000 to 4000 RPM, is it generally better to have more magnetic pole pairs, or stronger magnetic flux densities? I assume, like everything in engineering, there is a min/max relationship where increasing the magnet pairs (increasing the "frequency") is overshadowed by the physically related decrease in magnetic field density per pole.

I am not asking for anyone to work out the maths for me, but rather a bit of guidance on the practicality of an engineering solution and some tips on a good starting point based on either engineering knowledge, or real world experience. If the idea of using permanent magnets to heat water in a pot is simply impractical for any reason I would just as soon abandon the idea to folly w/o building any prototypes or agonizing over the math; on the other hand, if the idea has some merit, a bit of guidance on a starting point would be much appreciated. For instance, if 1/4" Neodymium disk magnets were placed in concentric circles of 184 magnets, 178 magnets, 170 magnets etc, etc would this be more or less effective than say concentric circles of 1" disk magnets beginning with 50 magnets, then 46 magnets, 42 magnets etc, etc. For the outer circle, at 3000 RPM, we would have an effective "frequency" of 270kHz (1/4" magnets) vs 75kHz (1" magnets). Obviously as we move inward on the disk the effective frequency will decrease. Might it be more efficient to use 1/4" x 1/4" x 1" rectangular magnets than circular magnets?

Again, I am not looking for someone to work out the actual math, I would just like some general "off-the cuff" thoughts on where to begin if the idea is viable enough to move to prototyping. I would begin prototyping by starting with a much smaller model and a much larger rotational speed range (~4" disk, 1000 -20,000 rpm) perhaps three disks: one with 1/8" diameter disk magnets, one with 1/2" diameter magnets and the final one with 1/8" x 1/8" x 1/2" rectangular magnets.

Things I am assuming that may or may not be true:

1) The heat gain to the magnets/disk will be minimal and meliorated by air cooling.

2) It is practical and safe to construct an 18" disk capable of maintaining integrity @ 2k-4k RPM ( @ 4k rpm, the outer edge of an 18" disk would be rotating @ ~314 sfps ==> 1.5 * pi * 4000 / 60 )

3) The heat delivered to the pot will be = to the mechanical energy input to rotate the disk less frictional losses.

4) Mechanical input to the disk of ~10hp = ~7.5kW ==> Actual heat gain of the pot should be 6.7kW (90% efficiency) to 3.7kW (50% efficiency) for pot temperatures close to ambient.

Again, thanks for any insights or thoughts, not asking anyone to solve the math, just guide me to determining if it is worth the time and money to investigate. It would take me weeks to muddle through the math, days to build the prototypes, I just don't want to start down either path if the general idea is flawed or impractical. Obviously I have a specific purpose in mind, I am not limited to inductive heating with permanent magnets, there are other, simpler ways to heat water to 100C, but if the idea is practical, offers efficiency gains and/or is more cost effective than resistive heating elements then I would be interested in pursuing at least a prototype.

Thanks in advance!

Fish