Why is a high permeability desirable for induction heating?

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Discussion Overview

The discussion revolves around the desirability of high permeability materials in induction heating applications, exploring the relationship between permeability, conductivity, induced EMF, and heat generation in conductors. Participants examine theoretical aspects, practical implications, and the underlying physics of induction heating.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the resistance per unit length of a conducting wire is proportional to the square root of the ratio of permeability to conductivity, leading to the conclusion that higher permeability results in lower heat loss.
  • Another participant expresses uncertainty about the relationship between EMF, current, and heat generation, indicating that EMF influences current but is not a direct relationship.
  • A participant proposes a model treating the problem as two coupled circuits, suggesting that mutual inductance plays a role in determining the secondary current and its relationship to EMF.
  • Questions are raised about whether the changing magnetic field produces heat through the movement of magnetic domains in the material.
  • One participant notes that while some heat may come from magnetic domain movement, their primary interest lies in the skin effect and how it affects resistance in low permeability conductors.
  • A later reply emphasizes the distinction between treating the heating process as eddy currents rather than secondary currents in a transformer model, seeking clarity on the argument for high permeability materials.
  • Another participant reflects on the complexity of the issue, acknowledging the common argument that a high permeability pot is preferable for a given induced current.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and uncertainty regarding the mechanisms of induction heating and the role of permeability. There is no consensus on the explanations provided, and multiple competing views remain regarding the underlying physics.

Contextual Notes

Some discussions involve assumptions about the nature of induced currents and their relationship to resistance and heat generation, which may not be fully resolved. The complexity of the interactions between magnetic fields, currents, and material properties is acknowledged but not definitively clarified.

Who May Find This Useful

This discussion may be of interest to those studying induction heating, materials science, and electromagnetic theory, particularly in the context of practical applications like cooking and heating technologies.

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The resistance per unit length of a conducting wire is proportional to the square root of the ratio of permeability to conductivity.

The power generated as heat may be expressed as I-squared x R: also as V-squared / R.

In induction, the EMF induced is determined by the rate of change of the magnetic flux (Faraday). Thus, it would seem to me that the appropriate calculation of the heat produced in the conductor is given by V(induced) - squared / R. As a result, the heat is proportional to the square root of the ratio of conductivity to permeability (since R is in the denominator, above.) Thus, the higher the permeability (as in a ferromagnetic conductor) the lower the heat loss.

This is contrary to the case where the current is constant, determined by an external source of EMF. In that case, the heat generated is proportional to the product of the current squared and the resistance: I - squared x R. In this case, the heat generated is proportional to the square root of the ratio of the permeability to the conductivity.

In discussing why a high permeability is desirable in induction heating, the latter analysis is usually presented. But the induction current is not determined directly by an external source of EMF but instead by the changing magnetic flux from the primary current. It seems to me that the constant quantity is the induced EMF and not the induced current.

Please help me understand why this is not correct.
 
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I THINK the issue is that the EMF determines the Current in the material which determines the heat generated. But I am not sure.
 
Drakkith said:
I THINK the issue is that the EMF determines the Current in the material which determines the heat generated. But I am not sure.

Certainly, you are correct. The EMF does determine the current in the conductor being induction-heated. But, not directly. I have taken up the third-year college physics challenge by treating the problem as if it were two coupled circuits, coupled by their mutual inductance - as if it were a transformer situation. The two circuit equations can be solved for a periodic solution, and the secondary current determined in terms of V, M, L1, L2, mu1, mu2, the conductivities and the frequency, omega. From the secondary current, with reasonable approximations, I expect to see that the constant current is fairly well determined as you wrote by the EMF. However, I am not sure either. Thanks for your reply.

Meanwhile, if anyone has a quick answer that is convincing, please don't be hesitant!
 
Does the changing magnetic field produce heat by the magnetic domains in the material changing directions?
 
Drakkith said:
Does the changing magnetic field produce heat by the magnetic domains in the material changing directions?

In the application I have in mind, the heating is the ordinary resistance heating. Surely, some of the heat does in fact come from such losses. But my interest is in the skin effect, the confining of the currents to the surface of the heated conductor. That confinement raises the resistance per unit length. In conductors of low permeability the confinement is low.
 
Ah ok. Well, sorry I couldn't help!
 
Drakkith said:
Ah ok. Well, sorry I couldn't help!

Not at all! A little conversation is very motivating. Thanks!
 
The transformer equations are solved and presented on the net for the simple case I had in mind; saves me work. The result, in the form of the secondary current, does not help me in understanding the usual argument regarding induction heating and the need for a high permeability metal on the "burner". No doubt, that is true. Rather than treating the pan or pot like the secondary of a transformer, I should be treating the currents for what they are, eddy currents, rather than currents in a secondary. Therein may be the explanation I am looking for.
 
I have been making the issue more complicated than it is. The usual discussion of induction cooking simply says for a given current induced in the pot, a high permeability pot is desirable. Therefore one calculates the power using Isquare R in comparing rather than Vsquare / R.

I can't discard all my aluminum cookware; so I'll avoid induction cookers. Thanks to all!
Problem solved!
 

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