How induction heating works

On the internet there is unbelievable amount of contradicting information. The textbooks don't explain it in depth enough, so I have to ask here.

I have an induction heating system which has a coil trough which there is applied an AC voltage which frequency I can regulate and I use cooking pots as load.

I'm interested in power consumption of different cooking pots. I'm most interested in why aluminium pots can't be heated at the same frequencies as pots with higher permeability.

One reason could be the hysteresis losses, but many papers say that hysteresis losses only contribute up to 10 % of total power losses inside the pot. So if this is true, this couldn't be the reason.

They then say that the losses increase if the resistance of the pot increases. But that doesn't make sense because then plastic pots would have the highest losses because of high resistance. My thinking is that if the pot has a low resistance, then the currents will be higher which will increase the losses. If a pot has high resistance, less current will flow and the losses will be lower. But this is not what we observe.

So what is the reason for this? Low resistance materials induce poor losses and high resistance materials induce poor loses. If resistance is not what determines the losses, then what is?

EDIT for mkeith: I am more interested in how the eddy currents are induced inside the material and how it leads to losses, than an actual product.

But to answer your question, induction cookers can only heat aluminium pots at high frequencies (around 100 kHz), while steel can be heated equally well at 18 kHz and lower (they use 18 kHz+ to avoid noise in the audible range).

Now Inductance with steel pots is indeed a lot higher, and so is resistance felt by the coil. Now what happens if we put on an aluminium pot? The resistance and inductance decrease. This effects resonant frequency of the resonant LC circuit. The resonant frequency increases. Also because of decrease in resistance, the current increases (if we increase the frequency to match the new resonant frequency). This would mean that because we have a low resistance, the current drawn would be very big and because of so low resistance the heating would still be smaller. Big current increases the negative losses because of internal resistance of the coil and the switching elements can only take a limited amount of current.

This explains why we can't heat aluminium at low frequencies. We increase the frequency to increase the resistance of aluminium pots because of increased skin effect which allows us to have lower current running through the switching elements for the same amount of heating power.

Now I understand this, but the real question I'm interested in is the following. If we have a current source that maintains 10 A at a fixed frequency. And in one case we have an aluminium pot causing 0.3 ohms increase in resistance over primary and in the other a steel pot which adds 5 ohms of resistance over primary. In both cases the internal resistance of primary is neglectable.

We see that aluminium pot is taking 30 W of power while steel pot is taking 500 W. But how does this make sense from the following perspective?

The current is the same in both cases, so the electromotive force created in both pots is the same. Because the pots have different electrical resistances, the eddy currents inside of them should be different as well. The losses are calculated by current flowing inside the pots squared times the pots resistance.

Because resistance of aluminium is lower, the currents inside them is larger, which means more losses should be created in the aluminium pot. But the actual result is just the opposite. This is the main part of my confusion.

• High enough resitance is neccessary but of course only until some limit (which is still so low that only materials we call "conductive" will work at all). It is similar to the maximum power transfer theorem: if resistance is too low or too high only little powre is transfered. In insulators no power is transfered because there is no current; in good (or super-) conductors no power is transfered because there is no/too little voltage. And: of course conductivity is only one factor. Another one is permeability. – Curd Oct 24 '17 at 13:16
• I've read it all numeros times. And what you say is wrong. Eddy currents Will be induced even if the material is not ferrous. What the permeability Will do is decrease the skin depth, increasing the effective resistance. But increase in resistance decreases the losses because it decreases the current which contributes more to the losses than the resistance. The only way this could make sense is if the increase in resistance because of decreased skin depth would not effect the ammount of current induced. But I think this is not the case. – MaDrung Oct 24 '17 at 13:23
• Many papers say that in induction cooking, hysteresis losses represent only up to 10 % of total power loss. This is not the case then if this is correct. – MaDrung Oct 24 '17 at 13:36
• @MaDrung Please stop writing will with capital w. – winny Oct 28 '17 at 7:17
• @MaDrung No, it's on your device. I've fixed it now along with some spelling and grammar errors. – winny Nov 2 '17 at 11:25

I am considering your edited case, where the frequency and current are held constant. You observe that the EMF should be the same in both pots, therefore the pot with a lower resistance should heat up more. However, I believe you are assuming something that is not true. The EMF will not be the same in the steel and aluminum pots because the magnetic flux density (B-field) will not be the same.

The primary coil, driven at constant current, will give rise to a time varying magnetic field (H-field) which will be the same in both pots. But due to the high magnetic permeability of the steel pot, the magnetic flux density (B-field) in it will be much, much higher than in the aluminum pot.

As you know, Faraday's law relates the rate of change in the B-field to EMF. Since the steel pot will have much higher B-field, it will also have a much higher EMF, and thus a much higher power dissipation despite its high resistance.

If the EMF in the two pots really were the same, then you can easily see from the equation P=V^2/R that the pot with lower resistance would have higher dissipation.

To be just a bit more numerical, note that the permeability of steel is around 100x higher than aluminum, so the EMF in steel is actually around 100x higher, and V^2 is 10,000x higher. Aluminum is a better conductor than steel, but not 10,000x better, so the net result is that the steel gets much hotter under the same H-field magnitude.

• I have another question. If we use 2 pots that both have permeability equal to that of air and we have a current source that maintains 10 A. The only difference between the two pots is their resistance. We would then expect that the same EMF will be induced in both pots. This would mean that inside the pot with lower resistance, there will be more losses because less resistance means more eddy currents. But if you calculate the losses on the primary, because the pot has a lower resistance, voltage required is smaller to maintain the current. this means that losses should be smaller, not bigger – MaDrung Nov 2 '17 at 9:43
• Is it possibly, that because the eddy currents inside the pot increase because its lower resistance, that those eddy currents create bigger counter voltage to the coil, which means that the voltage of the current source actualy had to increase instead of decrease after the transient period? – MaDrung Nov 2 '17 at 11:29
• We may be abusing transformer equations somewhat. Also, I am not an expert on transformers. It is an interesting question to be sure. Basically what you are saying, I think, is that if we transform the load impedance back to the primary, a lower resistance should give rise to a lower voltage for a fixed primary current. Thus power in is less. Yet it is heating up more. So what gives? I don't know. – mkeith Nov 2 '17 at 19:09

I think that a different perspective is required. We need to see it from a power transfer point of view.
There are at least 3 components to the "induced" power transfer. 1 - transformer action, 2 - hysteresis, and 3 - resonance.
Most of the power is transferred by transformer action. The metal pot acts as a shorted, "1 turn" secondary, as well as the transformer's core.
As is well known, iron has a better permeability than other metals (aluminum), therefore it is better at transferring the primary power to the secondary.
Since the secondary (the core) is "shorted," the power transferred gets dissipated in/by the core.
The second component, hysteresis, has already been mentioned and contributes an additional 10%.
Resonance is also an important factor because different metals (materials) will have different resonant frequencies, and the resonant frequency is very important to achieve maximum power transfer.

A induction heater is really a transformer where the secondary is the thing being heated. One important advantage of this is that there can be isolation between the power input and the thing being heated. Magnetic fields can pass thru electrical, thermal, chemical, and other barriers.

As with any transformer, there is a equivalent impedance presented at the secondary. For maximum power transfer, the load has to match this impedance. Power is voltage times current. At 0 impedance, the voltage is 0, so the power is 0. At infinite impedance, the current is 0, so the power is 0.

The induction heaters you are describing are apparently set up to deliver maximum power to something of iron or steel. Aluminum is more conductive, and apparently has too low impedance to work efficiently with the induction heater you have. Plastic won't work at all because the impedance is very high, effectively infinite for practical purposes.

• Can you read trough my EDITed part to answer my question? Also how come if plastic has very high resistance, the resistance of the primary in presence of plastic is also not very high? It seems that the primary just ignores the materials that aren't good conductors. What effect causes opposition to current in primary then? – MaDrung Oct 27 '17 at 11:37
• Whoever downvoted this, what exactly do you think is wrong, misleading, or badly written? – Olin Lathrop Oct 27 '17 at 21:30
• @Olin Lathrop Hi Olin. I am not the guilty party - I never downvote, but I do think both answers have the same problem. Inductive heaters are not analogous to transformers; they are more like poorly designed transformer cores in a transformer with no secondary. Does this make sense? – John Birckhead Oct 27 '17 at 21:43
• I am not the guilty party either, but I don't really understand your answer. What two things are you saying should have their impedance matched? – mkeith Oct 28 '17 at 5:02

If we have a current source that maintains 10 A at a fixed frequency ... In both cases the internal resistance of primary is neglectable...

... We see that aluminium pot is taking 30 W of power while steel pot is taking 500 W. But how does this make sense from the following perspective?

The aluminum pot is reflective. It reflects most of the energy. Since that reflected energy is not absorbed by the Aluminum pot, it is mostly not taken from the 10 Amp low-impedance power supply you postulate. So the voltage on your theoretical 10 Amp power supply is lower: if you create that with a switching supply connected to the mains, that will take less current from the mains. If you create it with a linear supply connected to the mains, the linear supply will get hot.

The mechanism of reflection involves the induced currents, so you can get there from here, but to do so you have to accept the observable facts: lots of current flows in the aluminum, not much power is lost in the aluminum.

The aluminum is in the near field of the induction coils so you can't call it 'radiation', so you don't have 'radiation' and 'reflection', but at the atomic level you have EM field which, through interaction (mostly) with electrons is either transferred to thermal energy or retransmitted. In the case of aluminum, the mechanism for transfer to thermal energy at low frequencies is not efficient, and the mechanism for re-radiation/reflection is efficient.

• What would happen if you had 2 pots with permeability equal to air with 2 different resistances and a current source that kept 10 A trough the coil. I would say that the power is greater in the pot with lower resistance, since it induces more eddy currents because of the lower resistance. But because of lower resistance, the voltage required to run 10 A is smaller, so the power should be smaller as well for a fixed 10 A current. This contradicts itself. – MaDrung Nov 2 '17 at 12:21