How does induction heating work?

Question

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

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

I'm interested in the 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. 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.

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

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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.

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.)

Inductance with steel pots is indeed a lot higher, and so is the resistance felt by the coil. What happens if we put on an aluminium pot? The resistance and inductance decrease. This affects the resonant frequency of the resonant LC circuit. The resonant frequency increases. Also because of the 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, then 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. 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 the resistance of aluminium is lower, the currents inside is 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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

There are several existing answers (including one that is accepted), but none talk about the role of the leakage inductance between the drive coil and the pan (though it is mentioned in a comment by John Birckhead).

Leakage Impedance

In a traditional transformer, there is a high permeability core threading both the primary and secondary coils, which loops back on itself to form a "magnetic circuit".

In most designs this leads to near unity coupling (to the 1st order) between the two coils. This means the primary effective impedance is turns ratio related to the secondary load, and shorting of the secondary effectively shorts the primary.

However in some designs (such as traditional magnetic neon sign transformers, some microwave oven transformers, and transformer based stick welders) there are magnetic "shunts" which provide a bypass path, allowing some of the primary flux to loop back without passing through the secondary coil.

The shunt path can be modelled as a "leakage inductance" in series with the primary coil, and this leads to a leakage impedance at the transformer operating frequency. This becomes the effective supply impedance.

If the secondary is shorted (say by a negative dynamic impedance plasma in a neon tube, or welding arc) the leakage impedance limits the short circuit current, with the primary flux preferentially flowing through the shunts rather than the secondary coil.

Load Matching

Maximum power transfer occurs when the load impedance matches the effective source impedance produced by the leakage inductance.

For instance, if you have a "10kV 50mA" neon sign transformer (10kV open circuit voltage, 50mA short circuit current), then the effective impedance is 200kΩ (10kV/50mA).

If the secondary is open circuit, no power is delivered. If it is shorted with a 0.1Ω load, 50mA will flow, but less than 1mW will be delivered to the load (though secondary winding resistance will probably lead to at least a few watts being dissipated in the secondary). However, if you apply a matched load of 200kΩ, then 25mA will flow and 125W will be dissipated in the load.

Leakage impedance between induction stove and pans

The relevance to the induction hob and pan of the OP, is that the coupling between the primary coil and the pan base (which is both the secondary coil and the load) is "core-less" (i.e. no high permeability core threading both the drive core and the pan base). This means there is a low coupling coefficient, a high degree of flux leakage, and therefore a high primary leakage impedance.

As with the example of the neon sign transformer, this means shorting of the secondary (say with a thick, highly conductive aluminium or copper pan) will lead to very little power delivery.

To maximize the power delivery you need a pan base with an effective impedance which matches (or at least approaches) that of the leakage inductance.

Use of ferro-magnetic pan bases

This matching is normally achieved through the use of a ferro-magnetic material (iron or some steels), which due to a very high-skin effect, show a high effective resistance (the eddy currents are contained in a thin surface layer, of what is already a relatively high resistance metal).

As noted in another question, you can also get limited heating of thin aluminium foil, because like with the ferro-magnetic skin effect, the thin section leads to a relatively high resistance.