Molten metal temperature vs Induction Coil Current

[gdritz]

TL;DR

Why does a coil current increase at the end of a cast iron melt?

Hi guys, I'm new here.
I have been analyzing the behavior of cast iron melts in induction furnaces and realized that at the end of the melt the current increases slightly. The power remains the same. Physically, does anyone know the reason?

I imagine to be related to the magnetic permeability of the metal (vs temperature), perhaps reaching a certain temperature (1540ºC is what we use) it somehow interacts with the coil. But I couldn't prove it.

Induction Furnace info:
4ton capacity
3000kW Power
Current between 2000~8000A during melt
Charge 50% steel 50% pig iron
Coil 1,70m x 0,8m , 20 turns

Excel graph attached with real data.

 

[Z0dCHiY8]

convection of molten metal could provide those changes of conductivity.

 

[gdritz]

convection of molten metal could provide those changes of conductivity.

Thanks, gave me some ideas..
But, do you mean metal magnetic conductivity or conductivity as electric current?
What would be the law of physics that applies to this phenomenon? Lorentz, Faraday? It's similar to an AC generator?

 

[Z0dCHiY8]

in solid phase, metal has static (low variable) properties of conductivity, but liquid one provides a lot of dynamics == for instance, convection divides molten metal into layers w/ different shares of impurities, so some layers get maximum of possible conductivity at given temp.

The material's electrons seek to minimize the total energy in the material by settling into low energy states; however, the Pauli exclusion principle means that only one can exist in each such state. So the electrons "fill up" the band structure starting from the bottom. The characteristic energy level up to which the electrons have filled is called the Fermi level. The position of the Fermi level with respect to the band structure is very important for electrical conduction: Only electrons in energy levels near or above the Fermi level are free to move within the broader material structure, since the electrons can easily jump among the partially occupied states in that region. In contrast, the low energy states are completely filled with a fixed limit on the number of electrons at all times, and the high energy states are empty of electrons at all times.
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from wiki

with rising temp, more atoms lose capability to capture free Electrons + ionized atoms become carriers too. However, electrical current cannot increase ad infinitum, because of effect of eddy currents. Even DC provides fluctuations of magnetic field thanks to flows/vortexes of molten metal.

Dynamic conductors are very tricky Beasts :)

 

[Z0dCHiY8]

But, do you mean metal magnetic conductivity or conductivity as electric current?

both == it's related to each other.

 

[Baluncore]

As the melt progresses, the surface area of metal is reduced until a single volume is present. The length of the surface path therefore shortens towards the end of the process.

I expect the electrical conductivity of liquid iron may be different to solid iron approaching 1530°C. Also the skin depth of the charge may change at the curie point near 770°C.

 

[Baluncore]

The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science
Series 7. Volume 44, 1953 - Issue 354. LXXX. The electrical resistivity of liquid iron, By R.W. Powell.
Pages 772-775 | Received 13 Mar 1953, Published online: 30 Jul 2010

Abstract

As a result of some preliminary experiments, which are described, the electrical resistivity of iron just above its melting point has been found to be 139 microhm cm2/cm. The resistivity of solid iron has also been measured and a value of 127·5 microhm cm2/cm is indicated at the melting point. Thus the increase in resistivity on fusion is only 9%, which is much less than would be expected on theoretical grounds and has been obtained for most other metals.

 

[gdritz]

Thank you Z0dCHiY8 and Baluncore
Can the phenomenon of increased current (as well as voltage) also be related to the Counter-electromotive force ?

 

[Baluncore]

Can the phenomenon of increased current (as well as voltage) also be related to the Counter-electromotive force ?

In electromagnetic systems everything is related. Which counter-electromotive force are you referring to?

In the original post you specified current increased slightly at the end of the cycle, while power stayed the same. That means voltage must fall. Do you monitor W = VA, and VAR ?

An induction furnace is a transformer where the furnace charge is both a lossy core and a single turn secondary.

 

[Tom.G]

Summary: Why does a coil current increase at the end of a cast iron melt?

at the end of the melt the current increases slightly. The power remains the same. Physically, does anyone know the reason?

Perhaps it is an impedance mismatch between the now-molten (no contact resistance between individual pieces) melt and the generator output impedance. Or to put it another way, the generator could power limited and as the melt resistance goes down the voltage- current product stays constant.