Induction hobs: spawn of the devil
7th June 2022
Recently, we moved house, into another property whose kitchen was fitted with an in-built induction hob. As you may have gathered from the title, I don't like it. These notes will try and describe why.
First, I should say that I've always up to now cooked on gas and so the transition to an electric hob was going to be traumatic. But there are some particular aspects of the hob which we now have that exacerbate this.
It's a Beko HNI64401AT. I had no choice in this, it was in the house already, but reviews I've seen of this model don't appear to be all that bad. Plus, it doesn't seem to have been heavily used previously, and I have no reason to suspect it's faulty. But most, if not all, of what follows will be applicable to any make of this type of hob.
The reasons against can be headlined as follows:
Inconvenience of operation
Refusal to work with many pans
Uncontrollability of heat
With a gas hob, you have two particular advantages: each burner is controlled by a knob which gives a continuous, volume-control-like setting; and you can see by looking at the flame just how much heat you are getting, and turning the knob changes this immediately.
With the induction hob, you don't have any of this. It's a flat, black plate of glass with markers on it where you should put the pans; there's no knob, you have to use a touch panel for all operations. There is no way of seeing immediately how much heat is going in, and you can't instantly turn the heat up or down. Instead, you must identify which point on the touch panel refers to the ring you want to adjust, select it, then touch another point on the panel to increase or reduce the heat in steps (1-9, see later). And when I say "touch", this is misleading; the touch points usually don't respond immediately and you need to hold your finger on the point for a while before anything happens. So, it is nothing like as easy to control as a knob for each ring.
Placing anything on the surface of the glass is fraught with hazard. If by chance you cover the touch panel with anything metallic it will squeal at you till you remove it. The glass itself is susceptible; the instructions have numerous warnings, among which are
DANGER: Do not allow any object to drop on the hob. Even small objects such as a saltshaker may damage the hob. Do not use cracked hobs. Water may leak through these cracks and cause short circuit. In case of any kind of damage on the surface (eg., visible cracks) switch off the product immediately to minimize the risk of electric shock…. Always keep the control panel clean and dry. Having damp and soiled surface may cause problems in the functions…. The surface might get damaged by some detergents or cleaning materials. Do not use aggressive detergents, cleaning powders/creams or any sharp objects during cleaning… If the surface of the touch control panel is exposed to intense vapor, entire control system nay become deactivated and give error signal….Sugar-based foods such as thick cream and syrup must be cleaned promptly without waiting the surface to cool down. Otherwise, glass ceramic surface may be damaged permanently.
What conventional hob is so finicky? After all, when not in use the glass is a big flat surface which invites you to stand things on it, and in a small kitchen with few other work surfaces, that's almost a necessity. And, you don't dare slide a pan from one point to another on the surface, for fear of scratching it. You must keep a pan centralised on a ring which suits its size, otherwise it doesn't want to work. And this leads me on to…
When we moved to this house we had a large collection of cooking pots and pans, all of which worked with the gas hob. Many of them were stainless steel, some were aluminium.
Of all of these, the only ones which actually caused this induction hob to function were a set of Le Creuset cast iron enamelled pans – which are magnificent, but which we hardly ever used before because they're so heavy – and a steel egg poacher. Nothing else would work. The device would simply not recognise that it had a pan sitting on it, and would refuse to even contemplate providing any heat. We have had to invest in a new set of "induction-ready" pans to avoid the threat of wrist injury from the Le Creusets, and to be able to consider such straightforward jobs as frying an egg.
But even with the new pans, the hob is still unable to function with a pan which is too small or too large for its rings. There are only two sizes of ring, 18cm (large) and 14.5cm (small). Try to use the wrong size pan for each ring, or move it too far away from its allotted position, it won't be recognised and the controller will barf.
To be fair, all of this is stated in the user instructions:
In induction cooking, only the area covered by the vessel on the relevant cooking zone is energised. The vessel's bottom is recognised by the system and only this area is heated automatically. Cooking stops if the cooking vessel is lifted from the cooking zone during cooking. … For induction hobs, use only vessels suitable for induction cooking. … Only ferromagnetic pans can be used for induction cooking. Ferromagnetic materials include: Enameled steel, cast iron, stainless steel designed for induction cooking. Non-ferromagnetic materials include: These types of pans will not work with induction cooking: Stainless steel, glass, ceramic, copper or aluminium pans. … Use flat bottomed saucepans or vessels only. … The diameter of pan's ferromagnetic area should match the size of the cooking zone. If the pan is not detected by the cooking zone, try it on the next smaller cooking zone. … Operating a cooking zone without a vessel or saucepan on it will cause damage to the product. Turn off the cooking zones after the cooking is complete.
Let's say you've got the right pan, you've put it on the right zone, and you have negotiated the touch panel to select a cooking level. The cooking heat can be selected in steps, from 0 (off) through 1, 2, 3 etc. to 9 (full on). You would think, wouldn't you, that this would apply the power to the bottom of the pan in increments of 10% from zero to full, no?
You'd be wrong.
The user instructions merely state
Touch "+" or "-" keys to set the temperature level between "1" and "9" or "9" and "1"
implying a semi-continuous increase of heat as you go up the scale. In fact, the heating control is anything but semi-continuous in steps of 10%. To find out what is actually happening, I have measured the application of the induction field (with an oscilloscope connected to a near field probe next to each ring while it's operating with a saucepan full of water) at each level setting on each ring. This shows the following features:
the field is applied on an on-off basis with the effective heating being controlled by duty cycle variation – what in engineering terms would be called "pulse width modulation". You can easily tell this just by listening to the hum generated when the field is on, and the obtrusive clicking noise every time it switches on. The figure of “percent of full power” below is taken by dividing the on time duration by the duty cycle period.
There are two periods over which the duty cycle variation is applied, 12.28 seconds or 4.1 seconds. Which period is used depends on the setting; settings 4-7, shown with a blue shading below, have a 4.1 second period, the rest have 12.28 seconds.
There is no difference between setting 8 and 9; they are both full power, with just a 60ms break every 12.28 seconds.
The settings up to 7 are non-linear. The actual duty cycle variation, translated into percent of full power, is shown in the graph below. The lower levels, up to 3, use the 12.28 second period, i.e. a short period of full power is applied every 12.28 seconds; the higher levels from 4 to 7 use the 4.1 second period.
Notice
particularly the reduced levels of energy as a percentage of full
power below setting 6; and even more particularly, the massive jump
between 6 and 7. This translates into a cooking regime, as
experienced every time I use this thing, of either "boiling too
fast" (anywhere from 7 to 9) or "not boiling at all "
(anywhere from 6 to 1). There is no happy middle ground of about
half-power, on any ring. This is why I regard the machine as
"uncontrollable".
Controlling heating by this method of duty-cycle control is widely used for all manner of appliances, and of itself is not bad practice. A duty cycle of several seconds, though, relies on the thermal time constant of the heated object in order to give an approximately steady input of heat. It's evident from experience that the thermal time constant of typical cooking pans is less than several seconds,with the result that the boiling at settings of 6 or 7 occurs in spasms. This is stated in the user instructions:
On levels from 1 to 7, the water or oil in the pot may boil or stop. This may lead the user to think that the product turns on and off intermittently. This case, which is observed especially when water or oil is little, is not a failure; it is just the way the product operates.
The frequency of the induction field used in this model is around 30kHz. It's not a smooth or sinusoidal 30kHz, though; the waveform seen at full power on one of the rings is shown below, at two timebase settings. From this you can see that the 30kHz is quite distorted, and is modulated by a full-wave-rectified 100Hz period, from an unsmoothed mains supply. It doesn't need to be any smarter than this to achieve heating, and the fact is that leaving it this way allows the lowest cost circuitry for actually generating the field. On/off control of each zone, detection of the presence of a pan, and interfacing with the touch control can all be easily and cheaply achieved with software in a small microcontroller. The total electronics "under the bonnet" would not need to be sophisticated or expensive.
(a) 100Hz modulated 5ms/division
(b) 30kHz waveform, 10us/division
As said at the beginning, I have no reason to believe that the product is faulty: it appears to be working entirely in accordance with what the user instructions say (and, for that matter, don't say). As someone who has had a lifetime's experience cooking on gas, I was interested to see how different an induction hob would be. The answer: it's very different; if you're a gas user and you want to keep your sanity in the kitchen, do not under any circumstances invest in an induction hob. You have been warned.