Induction heating of non-magnetic metal or past curie point?

[Artlav]

Hello.
I'm experimenting with induction heating, and put together a small unit at about 100-200W (measured at input).
http://orbides.1gb.ru/img/ih_overview.jpg

Royer oscillator type, 150kHz frequency.
Pretty much as expected, it gets a steel blade to glow in about 10 seconds.
http://orbides.1gb.ru/img/ih_glow_vis.jpg
http://orbides.1gb.ru/img/ih_glow_ir.jpg

However, the glow does not get brighter and only spread out, suggesting that the temperature is stuck around curie point.
But with steel it at least works.
If i put in a similar strip of aluminium, nothing would happen at all - the coil would overheat in about 5 seconds, and the aluminium strip is not even getting warm.
The oscillations are present in both cases, so it's not an equipment failure.

Now, i've seen videos of 2kW units melting aluminium, how is that possible if at just 10 times less there is no noticeable effect?

So, how can i get it to heat up aluminium?
Does it need different frequency/design, or it's just a case of needing more power?

 

[BradtheRad]

It may have to do with steel being a ferrous metal. It attracts and channels the magnetic flux lines.

Aluminum does not. It may develop eddy currents, but no doubt it's at a lower intensity than steel.

 

[Artlav]

It may have to do with steel being a ferrous metal. It attracts and channels the magnetic flux lines.

Aluminum does not. It may develop eddy currents, but no doubt it's at a lower intensity than steel.

That much i'm aware of.
But what are the conditions for induction-melting aluminium?
Simply adding more power, or some specific design considerations as well?

 

[BradtheRad]

It seems that skin effect plays a large part. It allows ferrous metals to be heated by induction.

To melt aluminum may require that it be in contact with steel which itself is being heated by induction.

Besides increasing wattage, it may also help if you change the frequency of operation, or the thickness of the aluminum, or alloy content (perhaps nickel which is magnetic), etc.

Wikipedia is informative on this topic:

"Aluminum or copper alone does not work on an induction stove because of the materials' magnetic and electrical properties.[3] Aluminum or copper cookware is more conductive than steel, and the skin depth in these materials is larger since they are non-magnetic. The current flows in a thicker layer in the metal, encounters less resistance and so produces less heat. The induction cooker will not work efficiently with such pots."

 

[jiripolivka]

As you mentioned 150 kHz as the induction source frequency, this causes the induced eddy current to stay on metal surface, not going deep. For induction-heating of metal volume, low frequency is preferred. Induction-heated ovens often use ~15 Hz AC to heat up a larger volume.

Try simply 60 Hz AC, by using a large transformer coil and core, open magnetic circuit towards the heated metal. Use primary voltage regulator as the magnetic circuit may not load the primary coil and cause an overcurrent.

 

[FvM]

But what are the conditions for induction-melting aluminium?
Simply adding more power, or some specific design considerations as well?

Different frequency, coil design, impedance matching, whatsoever (need to be calculated). You have observed the effect of aluminium effectively shorting the induction cooker coil yourself:

If i put in a similar strip of aluminium, nothing would happen at all - the coil would overheat in about 5 seconds, and the aluminium strip is not even getting warm.

 

[jiripolivka]

Different frequency, coil design, impedance matching, whatsoever (need to be calculated). You have observed the effect of aluminium effectively shorting the induction cooker coil yourself:

Please check: "www.rdoinduction.com" and "www.atlasfdry.com" for details of induction furnaces.