Induction hob: What is the effect of suddenly removing the pan?

[T3sl4co1l]

As an aside, I originally bought it for cooking candies and found it to be completely unsuitable.  Because of the way it operates, the minimum change in temperature (10F) is the difference between too little heat and way too much.  And an external temperature probe cannot be used accurately because the induction heats the probe directly.

Not completely useless, but needs to be done carefully.  I worked on one application where the client had embedded a thermocouple inside a bit of thin copper foil (1/2oz or less I think), and put that near the workpiece (still under a PTFE cover sheet, for chemical reasons -- a bonding process).  Surprisingly little heat is dissipated by the sensor, positioned in the middle of the work coil's pole piece.  (The idea to have the thermocouple sandwiched with foil, is peculiar; I think it did actually help more overall, reducing the thermal time constant by increasing conductivity to the otherwise-well-insulating PTFE cover sheet.  And it's just thin enough and small enough, and well positioned, to avoid much self induction.)  The bigger downside in that process was, since it's a thermal divider between pole piece, sensor and work, you never really know what the work temperature is, and you still need manually-crafted heat cycles; i.e. the process is partially open-loop, despite the feedback.

For sure, a cooktop without a sensor at the surface, or with a plug-in probe -- who remembers the days when microwave ovens had a probe inside them? -- is at the very least going to be a slow pig, but perhaps not even generally useful.

Tim

[IanB]

I think an ideal cooktop does not have closed loop control or feedback, but just delivers a fixed ratio of power between say 10 and 100% according to the position of a control knob.

So how do you do that with an induction heater? At any control setting, the total power delivered will vary from zero with no pan present to maximum possible with an ideal pan (iron, steel?)

If it were a resistive heater you could just use PWM or phase angle dimming, and removing the pan would be like disconnecting the resistance so no current flows.

How do you make the load on an induction heater look like a resistance, so it delivers a given fractional amount of power when the load is present, but does nothing when the load is removed? Surely it is still like resistance heating, with induced currents heating the metal of the pan. So to control heat generation, you just need to control the maximum power input to the induction coil, or to vary the frequency, for example?

Or is the challenge that different metals will heat differently, so for the same setting a cast iron pan may heat differently than a steel pan? Is the difference that significant?

[T3sl4co1l]

Well, the way I've done it is, run the control itself in a PID with per-cycle update rate, controlling to current or power setpoint, then an outer temp control loop at a lower update rate.

Current might be coil (rect. average, or peak) or switch (peak) current, or DC link (average inverter input).  The latter of course makes power measurement simple, independent of coil Q.

Otherwise, you'll need some calculation of coil V, I, Q..., which isn't the easiest.

Partly control partly protection, they also go to pulse-skipping or burst mode at low output, including at high Q (when power loss per cycle is small, i.e. inverter gain is high).  Which is partly how a pan is detected.

Tim

[ejeffrey]

I think an ideal cooktop does not have closed loop control or feedback, but just delivers a fixed ratio of power between say 10 and 100% according to the position of a control knob.

Depends on what you are doing.  Some tasks really would benefit from good temperature regulation.  It's hard to get right and poor temperature feedback is worse than none at all, which is probably why most don't bother.

So how do you do that with an induction heater? At any control setting, the total power delivered will vary from zero with no pan present to maximum possible with an ideal pan (iron, steel?)

You can certainly sense the dissipation factor of the coil to know how much power is being delivered.  Cooktops do this to detect when the pan is missing (or unclad aluminum) and shut off, but I think in practice this isn't used for regulation, they just use straight PWM.  But I don't think it matters.  At least in my experience there isn't that much of a difference between different induction-compatible pans.  Pan weight and thermal conductivity probably matter more than the differences in induction  efficiency between cast iron and stainless steel

[tooki]

I think an ideal cooktop does not have closed loop control or feedback, but just delivers a fixed ratio of power between say 10 and 100% according to the position of a control knob.

I think that is far too traditionalist view of it!

One of my favorite features of my cheap $50 Aldi induction hot plate is temperature control. It’s not terribly accurate, and it annoyingly can only be set in 20C steps, but the low accuracy doesn’t matter since I can work around its offset by feel. It’s so useful that I end up cooking on the induction hotplate 95% of the time, instead of on the real electric stove I have.

I use the temperature control mode all the time for deep frying: I put my pot of oil on it, set it to 200C, and then I can go about doing my prep work because it won’t keep heating the oil. I don’t know what actual control algorithm it uses, but the outcome is that it will heat the oil at full power initially, and then ramps down the duty cycle as it gets closer. In contrast, if I just used 100% power to heat it, I’d have to keep an eye on it because it wouldn’t take long at all to go between “hot enough” to “smoking heavily”.

Similarly, setting it to 80C or 100C is a perfect way to simmer or poach things.

In USA there’s some popular brand of induction hotplate which does quite accurate temperature regulation. I could see that being VERY useful.

[IanB]

I think that is far too traditionalist view of it!
...
I use the temperature control mode all the time for deep frying: I put my pot of oil on it, set it to 200C, and then I can go about doing my prep work because it won’t keep heating the oil. I don’t know what actual control algorithm it uses, but the outcome is that it will heat the oil at full power initially, and then ramps down the duty cycle as it gets closer. In contrast, if I just used 100% power to heat it, I’d have to keep an eye on it because it wouldn’t take long at all to go between “hot enough” to “smoking heavily”.

That's an interesting point!

The reason it doesn't occur to me is that I don't do deep fat frying at all (too much hassle to cool the oil, filter it and rebottle it afterwards, too annoying to clean the cookware after use). But if I were to do deep fat frying, there are dedicated fryers that have built-in temperature control. If I were to buy a dedicated plug-in hot plate, I would rather buy a dedicated plug-in deep fat fryer, since it comes with an integrated basket and a lid.

I cook with gas because I like the instant adjustment, and the cooking I do either involves boiling or shallow frying. Temperature control is done with an infrared thermometer and adjustment of the control knob.

[David Hess]

Well, the way I've done it is, run the control itself in a PID with per-cycle update rate, controlling to current or power setpoint, then an outer temp control loop at a lower update rate.

Current might be coil (rect. average, or peak) or switch (peak) current, or DC link (average inverter input).  The latter of course makes power measurement simple, independent of coil Q.

Otherwise, you'll need some calculation of coil V, I, Q..., which isn't the easiest.

Partly control partly protection, they also go to pulse-skipping or burst mode at low output, including at high Q (when power loss per cycle is small, i.e. inverter gain is high).  Which is partly how a pan is detected.

The Nuwave has proportional control of its output power, but I have watched it cycle on and off at a low rate which I suspect is caused by the poor thermal feedback loop.  About the only thing I could do is take it apart and see exactly how they mounted the thermal sensor, and see if it can be improved.

I found one inductive cooktop that pushes the sensor directly into the bottom of the pan, but I am not dedicated enough to spend $1500 on it.  GE apparently has some inductive cooktops which somehow embed the temperature sensor into the pan, but you have to use their custom pans.

[tooki]

The reason it doesn't occur to me is that I don't do deep fat frying at all (too much hassle to cool the oil, filter it and rebottle it afterwards, too annoying to clean the cookware after use). But if I were to do deep fat frying, there are dedicated fryers that have built-in temperature control. If I were to buy a dedicated plug-in hot plate, I would rather buy a dedicated plug-in deep fat fryer, since it comes with an integrated basket and a lid.

The problem with many deep fryers sold for consumers is that many are really poorly designed, with baskets far too large for the volume of oil, and with heating elements far too weak, so the temperature plummets when you put in useful amounts of food, especially if it’s frozen. But yes, a well-designed one is a good option for sure.

I cook with gas because I like the instant adjustment, and the cooking I do either involves boiling or shallow frying. Temperature control is done with an infrared thermometer and adjustment of the control knob.

Instant adjustment is a reason I love induction. You get the instant control of gas, but with arguably higher heating power compared to a typical gas burner. And unlike gas, where the majority of the heat is lost directly to the room (while producing combustion byproducts), with induction the heat is generated in the pan, so it doesn’t heat up the kitchen nearly as much as gas does.

[radiolistener]

the issue is that it will happily sit there boliing a pan of water, but if I suddenly lift the pan off the ring while it's going full power the IGBT violently lets out the magic smoke and all the lights go out.

As far as I can see the hob coil and 0.33uF act as a parallel resonant circuit when no load is applied, so high-Z. When a pan is on the ring it now becomes a low-z transformer. But what happens during the condition that triggers the smoke?

When there is pan, it get power from LC circuit. When you remove it, each new cycle add new amount of power to LC, but since the power loss is missing, the power remains in the LC circuit and accumulated. As result amplitude on LC circuit rise up and at some level can break semiconductor.


By the way is this a normal issue of modern induction hobs that it can't keep water boil at medium power?

Recently I bought Gorenje ICR2000SP induction hob and it's almost impossible to set medium power for boiling soup. It don't boil at all or boil too high, no medium power level.

Previously I had old HotPoint Ariston induction hob and it don't have such issue, it was able to boil the water at medium power with no issue.

[radiolistener]

Here is my measurements:

Mains: 233V / 50 Hz
Water: 1000g ± 1g
Condition: from 23°C to a high boiling state (referenced to electric kettle with automatic power off)
Peak power and consumed energy is measured with power meter.

Induction hob: Gorenje ICR2000SP:
Stainless still pan (induction compatible).
Peak Power: 1850 W
Time: 228 sec
Consumed Energy: 116 Wh

Electric kettle: Tefal KO299:
Peak Power: 2110 W
Time: 176 sec
Consumed Energy: 99 Wh

Gas stove: Chinese 3500W:
Stainless still pan (the same as for induction test).
Gas components: Isobutane 65%, Propane 10%, Butane 25%
Time: 302 sec
Consumed gas mass: 16.6g


It appears that electric kettle is 17% better efficient than induction hob.
And it appears that the real gas stove power is about 1400W.

The gas canister was not full, so it is possible to get a little-bit more power from this stove with higher gas pressure, but not much. I estimate it's max power is about 1500W instead of claimed 3500W.

[Phil1977]

The rated power of a gas burner usually is the calorific value of the burnt gas per unit of time.

What you measure is the heat that is transferred to the pot.

So around 50% of the flames energy go into your cookware - that´s quite a good value!

[Someone]

The gas canister was not full, so it is possible to get a little-bit more power from this stove with higher gas pressure, but not much. I estimate it's max power is about 1500W instead of claimed 3500W.

Cute toy stove and surprising amount of power packed into it. Benchtop/kitchen burners can go way off into many kW ratings but most are around 1-2kW so bang on.

[Marco]

The gas is around 225 Wh, excellent kitchen heater. For really big burners, gas is more than 3x better as a kitchen heater.

[Marco]

So how do you do that with an induction heater?

Ignoring losses for a moment, discontinuous mode boost circuits are rather trivial to control for constant power.