Questions about induction heating

[Metallus]

Hi there,
I'm a material chemist, currently working on ceramics. I know the very basics of physics, but I have many doubts about electromagnetism.

How does induction heating exactly work? I understand that you use a hollow coil of copper with a cooling fluid circulating inside, through which you pass alternate current at high frequency. This generates a (oscillating?) magnetic field that points towards the center of the coil (according to right hand rule). If you put an object inside, called susceptor, it gets heated.

1) Why does this happen? What happens exactly at a microscopic level in the susceptor to justify the generation of heat? I always imagine alternating two magnets on a rod of iron and I can't fathom how this would heat it.

2) Does this work with any object or just metals/graphite and why? What would happen if I put, eg, my hand inside an induction coil powered to heat graphite at 1000C? What if I put an insulator?

3) I am amazed that the coil can heat graphite to 3000C without melting, considering that copper melts at 1083C. Yes, there is a cooling fluid circulating, but considering the graphite die is usually 1cm away from the coil, that seems really insane cooling. Or am I underestimating the effect of the insulator between the coil and the graphite die?

Thanks

 

[Windadct]

The material being heated needs to be conductive, the changing magnetic field creates eddy-currents in the material and the current creates the heat. Your thinking is correct about the magnets and a piece of Iron, but it needs to be a changing Magnetic field. - Example, if you look at the core of a transformer, it is assembled of laminated (insulated) steel plates, to reduce these currents in the core.

The coil does not need coolant flowing in it to make the effect work, it is only to keep the coil cool. To generate the strong magnetic field, the coil needs a lot of current.

 

[Metallus]

I understood that the coolant was for the joule effect, but if the material that is being heated is very close to the coil and it reaches 3000 °C, isn't there the possibility of the coil "softening" with just the radiation from that body? 3000 °C at 1 cm distance seem quite a lot to dissipate.

Also, if the material needs only be conductive, I assume I could safely put my finger inside a powered coil (granted I don't directly touch the coil and get shocked). Is that right or I'll still get "warmed"?

 

[jim hardy]

Also, if the material needs only be conductive, I assume I could safely put my finger inside a powered coil (granted I don't directly touch the coil and get shocked). Is that right or I'll still get "warmed"?

@Metallus
Test that with a hot dog before inserting your finger.

 

[Metallus]

@Metallus
Test that with a hot dog before inserting your finger.

Not planning of actually doing it :D, don't worry. I would be more scared of accidentally touching the coil and getting electrocuted, beside the fact that the coil itself heats up on its own. Just curious about what the effects would be on a human.

 

[jim hardy]

Why does this happen? .

Induction 'induces' current in the material to be heated. That current generates heat by simple I² X R_esistance of the material in the magnetic field.
It needn't be high frequency but it's way more practical to use it .
That's because rate of change of flux is what 'induces' and you well know that dsin(ωt) = ωcos(ωt) . So raising ω let's you use smaller transformer and wire.There are plenty of hobbyist sites making induction heaters. Modern "Inverter" microwave oven power supplies are a popular power source, they rewind the high frequency SMPS power supply transformer to give low voltage high current. Youtube will show you fun hobbyist videos.