Induction hob: What is the effect of suddenly removing the pan? - Page 1
Forum section: electronics / projects

author: nali
author_rank: Frequent Contributor
timestamp: August 08, 2024, 03:21:02 pm
content: Electrically speaking of course. Not sure whether this belongs in Repair or here, but I'm more interested in the analysis than the actual repair.I've attached the reverse-engineered driver schematic of a hob I am looking at for a friend - 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?[Note there is no flyback diode in the schematic, the IGBT incorporates a power diode)This appliance has two identical PCBs each driving two rings, the issue has happened on each. Nothing in common apart from a fuse and power relay switching the line voltage. The only slight anomaly is the line voltage is a bit high here at 248~250V though still within UK spec.

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 05:12:18 pm
content: What is the model of the hob?There seems to be no detection of the no-load condition, which is bizzare.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 05:18:31 pm
content: I know those things get pretty goddamn cheap, but I had no idea there were actual open-loop unregulated and unmonitored versions out there.Well, I guess there are, they just don't sell well enough to show up that often.I wonder if there's a reason for that.Failure mode most likely overvoltage triggering avalanche destruction, and subsequent short circuit. There's no fuse either, it takes out the breaker.It's a danger to itself and it's a danger to others. Throw it on the heap ASAP.Tim

author: tooki
author_rank: Super Contributor
timestamp: August 08, 2024, 05:22:40 pm
content: Quote from: T3sl4co1l on August 08, 2024, 05:18:31 pmGiven that $40 induction hot plates (which actually manage 1800-2000W, exceeding many “real” stove burners) from IKEA or Aldi manage to have perfectly functional pan detection, I’d be surprised if it was designed to be open-loop, but rather that the thing’s circuit design sucks in some unexpected way.

author: nali
author_rank: Frequent Contributor
timestamp: August 08, 2024, 07:51:49 pm
content: Quote from: tooki on August 08, 2024, 05:22:40 pmYou're right guys - it isn't open-loop! I did think it was a bit crude (double check toroid to see if it was in fact a current txf - nope) but never looked inside one of these before. The PCB is single-sided and the IGBT emitter is connected to power return through what looked like a wire link, but is obviously a shunt (see pic, next to IGBT highlighted in red) as there is a connection to it leading to an analogue input on the MCU via some RC filtering.. updated schematic below.Nevertheless both identical PCBs have failed so there's something more fundamental than a simple fault, so I think the design is maybe borderline, exacerbated by running at 250V, 100% power and removing the load (pan). More than one corner case here.Peter-h the make/model is Gionien GIT470SP, it's a 4-ring 2.8kW unit.The owner has already decided to scrap the unit, I just don't like leaving things unexplained, they bug me

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 08, 2024, 08:37:20 pm
content: If you want to spend the time on it and have test gear **, , replace the IHW20N135 (1350V 20 A) (and probably at the 18V zener).Check that the normal IGBT gate ON voltage is at least 10VRun at reduced voltage ( 50%) using a variac. DSO Scope probes on V_drain ** and the current sensor, trigger on either rising high.Do the pan trick and try to distinguish which of voltage or current is closer to the datasheet max ratingsIt could be that the current feedback on both hobs is either failed, or never worked properly.**100:1 probe rated 1500V needed

author: nali
author_rank: Frequent Contributor
timestamp: August 08, 2024, 09:06:48 pm
content: Well there's 3 other identical channels to play with...This is the 3rd failure now; the original, my friend already replaced the IGBT before he gave it to me, then I re-replaced the IGBT plus the 18V zener as both had fused, and finally I replaced the Chinese bridge rectifier with a Vishay part as a precaution. The board I repaired is working, this is now on the other identical board.Sadly variac & HV probe are two items I don't have at home. I've been promising myself a variac for a little while now, I'll have to have a word with myself about the probe

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 09:19:20 pm
content: What frequency does this thing run at?It can't be in the audio range because you would hear it, due to magnetostriction. It could be 50Hz or 100Hz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 09:24:02 pm
content: I see 820k resistors near the output terminals as well. Are you sure there are not any other connections laying around?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 09:26:15 pm
content: Quote from: peter-h on August 08, 2024, 09:19:20 pmIt's class E, with 5-20uH coil inductance and 0.3uF capacitor. That gives say 90kHz, but the quasi-resonant class-E operation drops that (longer on-time in return for higher peak voltage).Tim

author: peter-h
author_rank: Super Contributor
timestamp: August 08, 2024, 09:34:00 pm
content: How do they get it through EMC approvals? It will be radiating 90kHz 180kHz 270kHz and all the way up

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 08, 2024, 09:37:44 pm
content: A typical "proper" induction heating inverter has a safe operating area surrounded by about 6 limit lines.( Capacitor Volt, Semiconductor volt Semiconductor current, Semiconductor temperature, rectifier power, turn off time)During startup and load changes and ramp down,, each of these limits might be approached.So there are 6 independent fast acting sensors/regulators. Any one can pull the firing frequency back into safe zoneWith your cooking units they seem to be using just Semiconductor current, Semiconductor temperature.When the pot is removed, for example, don't know, the drain voltage might fly up into danger zone while the Semiconductor current stays fairly constant.

author: Someone
author_rank: Super Contributor
timestamp: August 08, 2024, 10:24:48 pm
content: What does the instruction manual say?"cookware must not be removed while power is on" ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 08, 2024, 11:06:02 pm
content: Quote from: nali on August 08, 2024, 03:21:02 pmThe clamping scheme here is the resonant circuit and the reverse conducting diode, which is somewhat Hail-Mary.At low drive duty cycles, that limits to sine-ish ~ 2x Vcc, but as the ON time increases it becomes more like a TV flybackA half bridge driver always knows what the device swing will be, but a single device flyback part depends on the loading and release current.The worst case will be high drive current, with sudden load+damping removal, and you hit the jackpot hereI guess the manual says to turn offbeforeremoving the pot !

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 08, 2024, 11:25:18 pm
content: Quote from: T3sl4co1l on August 08, 2024, 09:26:15 pmPut those numbers into spice and this results, as we vary the damping/equivalent loadingESR Vpeak 10uS on time4R = 700V3R = 836V2R = 1.03kV1.5R = 1.15kV1R = 1.288kVIf we then vary the ON time, you can see a good response time matters, especially at high drive levels.1R 5us = 961V1R 15uS = 1.605kV

author: nali
author_rank: Frequent Contributor
timestamp: August 09, 2024, 09:48:41 am
content: Quote from: T3sl4co1l on August 08, 2024, 09:24:02 pmThose are directly across the coil terminal blocks, presumably to ensure the 0.33uF capacitor is discharged if a tech disconnects the coil.I measured the coil BTW, it's 90uH, and the working frequency is 30kHz which pans out [pun intended].The stuff mid-bottom is a small auxilary +5V SMPS for the logic.I'll probably shelve it for now until I get more suitable TE (I have isolation txf & dim bulb tester but that's about it for mains work) Annoying, because at my last job I had access to an abundance of HV diff probes, current probes, variacs etc etc..Even so it looks like trying to improve it is going to be a bit like putting lipstick on a pig as they say

author: peter-h
author_rank: Super Contributor
timestamp: August 09, 2024, 01:03:05 pm
content: 30kHz... I bet the cats and dogs love you

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2024, 06:23:28 pm
content: I measured my Nuwave at about 30 kHz with a big cast iron pan and the power is modulated to follow the line voltage so it presents a power factor corrected load to the AC line. It is suppose to be 1500 watts but only draws about 900 watts, making it underpowered for much cooking.

author: IanB
author_rank: Super Contributor
timestamp: August 09, 2024, 06:36:00 pm
content: Quote from: David Hess on August 09, 2024, 06:23:28 pmThat's interesting. I tried to measure the power delivery of my gas stove once, and found it to be remarkably low. I put a measured amount of cold water in my 3 kW electric kettle and measured the time to boil (2-3 minutes). I did the same with a stove-top kettle and with the gas burner on full power. It took much longer, I think 10-12 minutes to boil. I concluded that the actual power delivered to the kettle/pot/saucepan by the gas burner was far less than I would have guessed, much less than 1000 watts.Given that an induction hob directly heats the pan with no heat wasted, delivering up to 900 watts into the pan would seem reasonable.

author: soldar
author_rank: Super Contributor
timestamp: August 09, 2024, 07:23:49 pm
content: Quote from: IanB on August 09, 2024, 06:36:00 pmTechnology Connections

author: coppice
author_rank: Super Contributor
timestamp: August 09, 2024, 07:35:00 pm
content: Quote from: David Hess on August 09, 2024, 06:23:28 pmHave you tried with a stainless steel bottomed pan? They are usually what the makers optimise for, although different alloys can change performance there, too.

author: IanB
author_rank: Super Contributor
timestamp: August 10, 2024, 12:48:30 am
content: Quote from: soldar on August 09, 2024, 07:23:49 pmYes, actually, I have watched that.I just repeated the experiment.1 kg water (1 L), from room temperature (25°C, measured) to rolling boil (100°C).Electric kettle: 2m0s = 120 sGas ring, stove-top kettle: 6m15s = 375 sPower estimate for electric kettle: Power = 1 kg x 4.2 kJ/kg/degC x (100 - 25) degC / 120 s = 2.6 kWPro-rata power for gas ring: Power = 2.6 kW x 120 s / 375 s = 0.83 kW(My 3 kW kettle seems not to meet its specification.)

author: amyk
author_rank: Super Contributor
timestamp: August 10, 2024, 02:45:21 am
content: More relevant videos on this topic:

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2024, 01:14:35 pm
content: Quote from: IanB on August 09, 2024, 06:36:00 pmMy power measurement was made on the 120 volt AC side, which led me to investigate the output waveform with an oscilloscope. So power applied to the pan is 900 watts minus efficiency losses.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2024, 03:14:07 pm
content: Quote from: coppice on August 09, 2024, 07:35:00 pmI get identical results with aluminum and stainless pans which are designed for induction stovetops.I did just notice however that the Nuwave is indexing the power to the temperature control, even though it has closed loop temperature feedback. So if I set the temperature to the highest possible setting, the input power does go up to 1500 watts. (1) But this means that the recovery time at lower temperature is extended. The temperature control is very poor, for instance it overshoots the temperature when starting so it is best to wait almost half an hour before cooking, and then temperature recovery is very bad making cooking inconsistent.The design problem here is that the temperature sensor is measuring the temperature of the insulated cooktop, which is considerably delayed from the temperature of the bottom of the pan. It would actually be better without the closed loop temperature feedback.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.(1) Anything above 370F tends to burn stuff, limiting power to 900 watts at the plug.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2024, 07:29:45 pm
content: Quote from: David Hess on August 10, 2024, 03:14:07 pmNot 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

author: IanB
author_rank: Super Contributor
timestamp: August 10, 2024, 07:48:33 pm
content: 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?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 10, 2024, 08:35:33 pm
content: 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

author: ejeffrey
author_rank: Super Contributor
timestamp: August 10, 2024, 10:10:31 pm
content: Quote from: IanB on August 10, 2024, 07:48:33 pmDepends 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.QuoteYou 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

author: tooki
author_rank: Super Contributor
timestamp: August 10, 2024, 11:29:05 pm
content: Quote from: IanB on August 10, 2024, 07:48:33 pmI 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.

author: IanB
author_rank: Super Contributor
timestamp: August 11, 2024, 02:54:43 am
content: Quote from: tooki on August 10, 2024, 11:29:05 pmThat'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.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2024, 05:08:30 pm
content: Quote from: T3sl4co1l on August 10, 2024, 08:35:33 pmThe 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.

author: tooki
author_rank: Super Contributor
timestamp: August 11, 2024, 11:36:37 pm
content: Quote from: IanB on August 11, 2024, 02:54:43 amThe 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.Quote from: IanB on August 11, 2024, 02:54:43 amInstant 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.

author: radiolistener
author_rank: Super Contributor
timestamp: August 12, 2024, 01:30:54 am
content: Quote from: nali on August 08, 2024, 03:21:02 pmWhen 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.

author: radiolistener
author_rank: Super Contributor
timestamp: August 13, 2024, 04:37:38 am
content: Here is my measurements:Mains: 233V / 50 HzWater: 1000g ± 1gCondition: 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 WTime: 228 secConsumed Energy: 116 WhElectric kettle:Tefal KO299:Peak Power: 2110 WTime: 176 secConsumed Energy: 99 WhGas stove:Chinese 3500W:Stainless still pan (the same as for induction test).Gas components: Isobutane 65%, Propane 10%, Butane 25%Time: 302 secConsumed gas mass: 16.6gIt 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.

author: Phil1977
author_rank: Frequent Contributor
timestamp: August 13, 2024, 06:59:46 am
content: 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!

author: Someone
author_rank: Super Contributor
timestamp: August 13, 2024, 08:14:00 am
content: Quote from: radiolistener on August 13, 2024, 04:37:38 amCute 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.

author: Marco
author_rank: Super Contributor
timestamp: August 13, 2024, 08:33:16 am
content: The gas is around 225 Wh, excellent kitchen heater. For really big burners, gas is more than 3x better as a kitchen heater.

author: Marco
author_rank: Super Contributor
timestamp: August 13, 2024, 08:38:19 am
content: Quote from: IanB on August 10, 2024, 07:48:33 pmIgnoring losses for a moment, discontinuous mode boost circuits are rather trivial to control for constant power.