Author Topic: Induction hob  (Read 1034 times)

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Online PlainName

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Induction hob
« on: May 19, 2022, 09:15:34 am »
Wow! An actual electronics question in cooking  :-DD

OK, so we (that is, I, but hopefully there won't be an argument) are looking at a portable induction hob. Reason: to see how it goes and maybe upscale to a full 4-hob panel. A browse of Amazon and John Lewis (a UK shop with good rep, usually sells quality kit) shows many such single-hob products all very similar.

Now, my understanding is that induction is better than gas or electric (plates - I know induction is electric!) is that the heat is more instant and controllable. All the ones I have seen in this price range, which is <£100 and typically <£55, don't have the fine control. They have steps, such that you pick 1 of 10 power levels, say, or similar temperature settings (although the temperature appears to be quite, ah, loose).

OK, I can go with that but it seems that at the two lowest levels the PWM is very slow. Slow enough that on boiling a pan you can see the water bubble, then not bubble, then bubble, not bubble. This is how my partner's folks' halogen  hob works normally, and it's horrible - no-one likes cooking on that. Anyway, I am wondering why all the different brands are subject to this low-power stuttering.

Am I making a mistake in going for one of these to discover the delights of induction?
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Offline SeanB

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Re: Induction hob
« Reply #1 on: May 19, 2022, 12:34:57 pm »
Induction normally does PWM of the actual oscillator, no real power control is possible with the cheap self oscillating designs they normally use, and thus at low settings they implement pretty coarse PWM to get low average power, and also to avoid harmonics being impressed on the mains, so the drive has to be an integral number of full mains cycles, and thus the low PWM frequency. Hard to do single pulse control that way, better to have a longer time, say 10s total, and average over that.

Yes the single plate ones all are pretty much the same electronics inside, similar controls for power as well, and thus similar characteristics. Only a few power levels as well, some have more, some have less, but pretty much only a full stove will have any form of temperature feedback, simply because of the very poor coupling between the pot and the glass top, deliberate though it is, and also because your sensor wiring will also have massive common mode voltage in it. About the only feedback is detecting pot size on the built in types, using a few separate drivers, and sensing the open circuit voltage and current on them, so as to detect pot size, and presence on the top every cycle, so that the power can be turned off before it destroys the oscillators.
 
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Online PlainName

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Re: Induction hob
« Reply #2 on: May 19, 2022, 12:45:12 pm »
Quote
so the drive has to be an integral number of full mains cycles, and thus the low PWM frequency

Ah! That explains it perfectly, thanks  :-+
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Online PlainName

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Re: Induction hob
« Reply #3 on: February 02, 2023, 05:37:23 pm »
Update: finally acquired a Lidl (aka Silvercrest) induction hob for £29, perhaps the cheapest available without going to Ebay used. Lidl have removed the advert in the UK but I found this: https://www.lidl.de/p/silvercrest-induktionskochplatte-sikp-2200-b1/p100354554

Anyway, I was pretty impressed. I put a pan on it with oil in, set to 180C and then got some garlic out of the pantry. When I turned around the thing was already smoking! On our electric hob you'd be waiting several minutes for that. So, pushing on I turned it down to 140C and did the onions, and at that temperature they were on the verge of burning but not quite there. And stayed like that. I couldn't manage to do that on the electric hob without fiddling with the control all the time.

For soup, I could set it to simmer and it would jolly well stay simmering. No setting the control to 2 then 3 then 2, the frequency depending on whether the door is open or not. So I like this and can see a full stove top in the future.

One thing though: I am told it makes an awful racket. I initially thought that was the cooling fan, but thinking about it the kettle makes a racket as well, even when it's just started and not yet boiling. Is the noise an unavoidable part of the process?
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Offline helius

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Re: Induction hob
« Reply #4 on: February 03, 2023, 02:25:10 am »
Doesn't induction cooking require pans with a "diamagnetic base" to facilitate eddy currents?
 

Offline Alti

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Re: Induction hob
« Reply #5 on: February 03, 2023, 04:41:00 am »
Now, my understanding is that induction is better than gas or electric (plates - I know induction is electric!) is that the heat is more instant and controllable.
More instant than resitive heaters - yes, if same powers are considered and low quantity of food is prepared. If you want to compare 2kW inductive and 3kW resistive, the higher the quantity/mass, the less of an advantage inductive heating has.

Controllable - depends on the definition. Both are open loop. The resistive heaters usually use infinite switches and come with infinite step count. Inductive - usually 10 steps 0-9.

As for "inductive is better than electric" - maybe for part of the users but not for me. My power supply connection is limited to 3-phase 230/400V and 20A. My electric stove is also 3x16A. Then there is 11kW (3x16A) tankless electric water heater. Also there are other loads, lights, kettle, microwave oven etc. Guess what would happen when I started cooking and someone used hot water at the same time, had this been a inductive stove? Yup, disconnection.

My water heater comes with shedding circuit that allows connecting the stove via water heater. This way, once the water is drawn, it first disconnects stove and then enables heater. This is only possible with resistive stoves, "better inductive stoves" won't work with shedding.
 
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Offline aeberbach

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Re: Induction hob
« Reply #6 on: February 03, 2023, 05:21:03 am »
Doesn't induction cooking require pans with a "diamagnetic base" to facilitate eddy currents?

The man at the Miele shop told me that if a magnet sticks to it, it works - cast iron, stainless, just not aluminium or pure copper or other non-ferrous. Apparently there are even heating pads that you can buy that will stick to the bottom of non-ferrous pots to make them compatible.

I'd like the Miele - they have one continuous large base where the top detects where the pot is and applies heating to that exact area. It even illuminates LEDs under the pot so you can see that it has detected the right area. Move the pot and the heating moves too. Only trouble is they approach $10k...
« Last Edit: February 03, 2023, 05:23:36 am by aeberbach »
 

Offline wilfred

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Re: Induction hob
« Reply #7 on: February 03, 2023, 06:53:37 am »
Am I making a mistake in going for one of these to discover the delights of induction?
I don't think so. I also bought a cheap hotplate to test induction cooking out. I paid AU$29 at a large Australian Supermarket chain. I think these cheap induction hotplates are pretty much like toasters, basically all the same. The fan noise was unexpected. It's not loud. The ability to set a low simmer is something I liked. And it does boil a pot quicker than a gas stove without heating up the kitchen as much on hot days.

On conventional electric or gas stoves you lose a lot of heat outside the pot underneath and up the sides. I'm confident Induction is only going to increase in popularity. Aside from needing suitable cookware I can't see a downside. I have a 4 hob stove and I have never used more than two simultaneously so my next stove will be induction although this single hotplate would serve me well 95% of the time.
 

Online PlainName

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Re: Induction hob
« Reply #8 on: February 03, 2023, 01:17:21 pm »
Doesn't induction cooking require pans with a "diamagnetic base" to facilitate eddy currents?

'Compatible'. Like aeberbach says, you can get plates and stuff to put under non-compliant pans that basically simulate an electric hob. We didn't get any of our pans with induction hobs in mind, but even our cheapest non-stick frying pan (a fiver or so from Lidl a couple of years ago) works well. Obviously, our glass ones won't but I don't like using those anyway ;)
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Offline Alti

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Re: Induction hob
« Reply #9 on: February 03, 2023, 01:24:04 pm »
On conventional electric (..) stoves you lose a lot of heat outside the pot underneath and up the sides.(..)
I do not know what kind of "conventional electric stoves" you have down there. I must assure you there is no measurable heat loss "underneath and up the sides" with glass-ceramic electric stove, for me this is conventional.
I'll also add such electric stove has nearly 100% efficiency, power factor 1.0, does not generate harmonics, it does not include rectifier, filters, IGBT, coil that all generate losses. Nor the fan to dump the losses into kitchen air.
 

Online PlainName

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Re: Induction hob
« Reply #10 on: February 03, 2023, 01:24:57 pm »
Now, my understanding is that induction is better than gas or electric (plates - I know induction is electric!) is that the heat is more instant and controllable.
More instant than resitive heaters - yes, if same powers are considered and low quantity of food is prepared. If you want to compare 2kW inductive and 3kW resistive, the higher the quantity/mass, the less of an advantage inductive heating has.

I am thinking that, like with microwaves, the energy is going into the internals of the target rather than being conducted from the outside. So instead of waiting for the heat to work it's way up through the heating element and then the pan base, it's starting off in the base.

Quote
Controllable - depends on the definition. Both are open loop. The resistive heaters usually use infinite switches and come with infinite step count. Inductive - usually 10 steps 0-9.

Similarly, there is no thermal inertia from the cooker. Turn off the power and the temperature should start dropping straight away. With the resistive heater, even though the element will start to cool, it is still imparting energy to the pan and will increase the thermal mass.
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Online PlainName

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Re: Induction hob
« Reply #11 on: February 03, 2023, 01:29:46 pm »
Quote
Aside from needing suitable cookware I can't see a downside.

Fragility. You wouldn't want to be banging your wok against the glass like they do in the YouTube 'Asian streetfood' videos. And the glass is easily marked, and it's slippery. You can get silicone disks to go under the pans to counter the marking and slipperiness, so maybe that's the answer, but it's amazing how much you bang pans on the stove without realising.
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Offline Marco

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Re: Induction hob
« Reply #12 on: February 03, 2023, 01:41:03 pm »
Controllable - depends on the definition. Both are open loop.

There are hobs with temperature sensors for closed loop control, but if you want a smooth surface and not mess about with external probes they need to work through the glass and with pans made from wildly different materials, so I wouldn't 100% rely on it working all the time.

For portable hobs Xiaomi/Tokit has one with a nub which contacts the pan for temperature control, but it's a smart device and you can only do temp control through your mobile phone for some obscure reason.
« Last Edit: February 03, 2023, 01:56:54 pm by Marco »
 

Offline Alti

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Re: Induction hob
« Reply #13 on: February 03, 2023, 03:21:36 pm »
(..)If you want to compare 2kW inductive and 3kW resistive, the higher the quantity/mass, the less of an advantage inductive heating has.
I am thinking that, like with microwaves, the energy is going into the internals of the target rather than being conducted from the outside. So instead of waiting for the heat to work it's way up through the heating element and then the pan base, it's starting off in the base.

BTW, 2kW in case of induction is 2kW input. It has some drop in efficiency, lets say 5%. Are we comparing 1.9kW resistive with 2kW inductive for boiling 3 liters of soup or are we comparing 3kW resistive and 2kW inductive for boiling 1 spoon of oil?

I think it should be possible to estimate thermal inertia step response of ceramic glass + steel pot and when exactly it reaches 90% of setpoint of power. For sure this takes some time.
Boiling 1 liter of water 10oC-100oC requires 380kJ and that is 200s with 1.9kW delivered and no heat losses. As you can see - this is quite some time so either 1.9kW directly or 3kW through the glass and the answer is not that straightforward. Besides, you need to cut out power for resistive heater before water starts boiling (some experience needed, or controller) or this leads to losses.

Off the base with 2kW or with thermal inertia of ceramic glass with 3kW. Pot stays the same. The higher the quantity/mass, the less of an advantage inductive heating has. Those resistive ceramic heaters are rarely 2kW, maybe the smaller ones. The bigger the diameter the higher the powers these offer. And, you also need to have special pots with flat bottom for low thermal resistance. The cheapest crap that does not lay flush won't be fast.
 

Online PlainName

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Re: Induction hob
« Reply #14 on: February 03, 2023, 04:40:05 pm »
Quote
BTW, 2kW in case of induction is 2kW input. It has some drop in efficiency, lets say 5%. Are we comparing 1.9kW resistive with 2kW inductive for boiling 3 liters of soup or are we comparing 3kW resistive and 2kW inductive for boiling 1 spoon of oil?

Sorry, I don't have a clue. The only reason I know the induction hob is 2KW is because it says so in the manual (and is part of the name). I didn't buy it because it was rated thus - I would have got it had it been 1.8kW or 3kW. As to the traditional resistive hob... there are two sizes: big hot ones and small not so hot ones. That's the sum of my knowledge, I'm afraid :)
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Offline aeberbach

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Re: Induction hob
« Reply #15 on: February 03, 2023, 11:24:53 pm »
Am I making a mistake in going for one of these to discover the delights of induction?
The ability to set a low simmer is something I liked.

For this reason I am going to get a single element cheap induction to try. My gas can't simmer bolognese or other saucy things without burning the base, the lowest setting is just too hot.
 


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