How does inductive heating with magnets work?

[AgainstOdds]

I've been trying to learn more about inductive heating to try and answer a few questions of mine concerning the amount of heat generated by magnets passing through opposite magnetic fields at a high frequency. Does inductive heating heat from the inside of a conductive material outward, or does it heat from the surface in, like conventional methods?

 

[sreid]

The heat is created throughout the material. The heating is caused by eddy currents in the material.

 

[EdStainless]

An electrically conductive material moves through a magnetic field. The result is an electrical current flow, the Eddy currents in the material. These cause heating because of electrical resistance. The heating tends to be greater on the surface since the material will shield the filed, the field strength internally will be lower than on the surface, so less eddy current.

The material does not need to be a magnet. Any electrically conductive material moving in a field will heat.

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[AgainstOdds]

So the degree of heating taking place is dependant upon the strength of the magnetic field and the frequency at which the conductive material being heated is passing through the magnetic field, correct? Does the positioning of the magnetic fields have much effect? For example, from what I've seen, generally processes using inductive heating place the object to be heated on the inside of a coil with a high frequency electrical current passing through the coil. Could the object still be heated if it were placed outside the coil, rather than inside?

 

[MJR2]

The field is higher and more controlled inside the coil.

Your design in a previous post is similar to the rotor of an eddy current separator for municipal refuse. The eddy current separator has a number of PM's arranged like your inner cylinder. It rotates at speeds of up to say 3000 RPM. Hold an aluminum can near the rotor for a few seconds and your bare fingers WILL let go. Really neat effect.

Mike

 

[dgallup]

The amount of heating is also dependent on the materials conductivity.

 

[AgainstOdds]

MJR2 - thanks for remembering my previous post. I wasnt quite sure how to relate the two questions without being extremely wordy or confusing. I'd like to change the scenario a bit though. Same inner cylinder, roughly 12" in diameter and 70-80 magnets attached to the circumferance. But this time only a few, say 4 magnets on the outer cylinder, evenly spaced at 0 degrees, 90, 180, and 270, and instead of spinning the outer cylinder, this time the inner cylinder is spun at a lower speed of say 1000rpm. In this scenario, the inner magnets are only passing through 67 fields per second, while the outer magnets (though not moving) are passing through 1,166-1,333 fields per second (depending on the exact number of magnets attached to the inner cylinder. If I'm understanding inductive heating correctly, this should cause only a slight temperature increase in the inner magnets compared to a considerable temperature increase in the outer magnets, correct?

 

[MJR2]

Metal moving in a magnetic field or PM's moving near metal will cause heating. Your numbers suggest about 3100 FPM surface speed. If your rotors or shell are metal (steel, stainless steel or alumninum for instance) you will have heating. I have had signifcant heating of SS at 1000 FPM surface speed with ferrite magnets. Like cooking eggs.

Your rotor at 1000 rpm will require siginificant engineering to not come apart.

Think carbon fiber to minimize all of these problems.

You have come back to your earlier post where you are asking for actual design values. You need to do the work or hire someone to do it for you. Or make prototypes and probably watch a bunch fail. You should have fun either way.

Mike

 

[AgainstOdds]

Thank you all for your help. I am still struggling to understand the concept of inductive heating. I can see how alternating magnetic feilds could be a problem, but as in my application, all magnets will be oriented the same way. That is, one magnet will pass another, and the next magnet that passes will be oriented with the same pole facing the stationary magnet.

For Example: stationary magnet: N
S

passing magnets: N N N N N (direction of
S S S S S motion ->)

Why wouldnt this setup act as if the passing magnets were all one continuous magnet passing by the stationary magnet? Its possible I'm just not grasping the concepts here. I trust you all know more about magnets than I, and if you say this will still generate heat then I'll believe you. Magnets are not my specialty.

 

[IRstuff]

It probably doesn't generate as much heat as a standard inductive configuration, since the energy transferred to the opposing pole approaches is released when the opposing pole recedes. In a normal inductive heating situation, the dipoles probably simply get cranked back and forth, with all the energy going into rattling the molecular bonds.

TTFN

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