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Induction coil design can have a huge impact on process efficiency, part quality, and cost of manufacturing. Below are five ways to optimize your design and some induction coil basics.
How efficiently and effectively a workpiece is heated is determined by the induction coil. Induction coils are water-cooled copper conductors created from copper tubing which is readily formed into the shape of the coil for the induction heating process. As water flows through them, induction heating coils themselves do not get hot.
Work coils range in complexity from a coil that is precision machined from solid copper and brazed, to a simple solenoid- or helical-wound coil (made up of a number of turns of copper tube wound around a mandrel).
By producing an alternating electromagnetic field due to the alternating current flowing in them, coils transfer energy from the power supply to the workpiece. The coil’s alternating electromagnetic field (EMF) creates an induced current (eddy current) in the workpiece, which generates heats due to I Squared R losses (core losses).
The coil’s EMF strength correlates with the current in the workpiece. This transfer of energy is known as the eddy current effect or transformer effect.
As coils utilize the transformer effect, properties of transformers can help understand coil design. The inductor is similar to a transformer primary, and the workpiece is equivalent to the transformer secondary (assumed to be a single turn).
There are two key features of transformers which affect coil design:
As a result of the relationships outlined above, there are five conditions that must be considered when designing any coil for induction heating:
Closer to the coil turns themselves, flux is the most concentrated. It declines with distance from the turns. If a part was located off center in a coil, the area closer to the coil turns would intersect a larger amount of flux lines and so be heated at an increased rate.
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The section of the part away from the copper coil encounters less coupling and would be heated at a lower rate. In high-frequency induction heating this effect is more pronounced.
The coil does not have sufficient inductance needed for efficient heating if opposite sides of the inductor are too close. This effect can be offset by inserting a loop in the coil at the center. Then the coil will heat a conducting material inserted in the opening.
The magnetic field is weaker at the place where the coil and the leads join. This effect is most noticeable in single-turn coils. This condition becomes less important as the amount of coil turns grows and the flux from each turn is added to that from the previous turns.
The part should be offset slightly toward this area in static heating applications due to the impracticality of constantly centering the part in the work coil. The part should be rotated to enable uniform exposure if possible.
The coil should be coupled as near to the part as possible, so the largest viable amount of magnetic flux lines intersect the workpiece at the heating point in order to permit maximum energy transfer.
The flux lines are concentrated inside the coil, allowing the optimum heating rate at that location.
This information has been sourced, reviewed and adapted from materials provided by Ambrell Induction Heating Solutions.
For more information on this source, please visit Ambrell Induction Heating Solutions.
I am idly curious about induction heating; specifically for heating 80L-100L of water from 20C to 100C as efficiently as possible with an emphasis on as little waste heat going to the surrounding environment as possible. I have done some precursory reading ( http://en.wikipedia.org/wiki/Induction_heating among other related searches), but have more questions than answers, lol.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
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