[meeotch (OP)]

I've been trying out induction cooking on a small one-burner unit, with a large cast iron pan. I'm getting the expected hot spot in the center of the pan, and a serious temperature falloff toward the sides. No surprise there - the induction element is about 7" (I dismantled the unit to check), and the pan bottom is about 10.5".

So I'm wondering what can be done to even out the heat, other than cooking smaller food - and more broadly, why an induction burner would produce worse / less even results than my conventional electric burner of the same size. (Which it does - never had the issue to this extent with the same pan on my conventional stove.)

On another forum, someone suggested that a 220v unit will have a "different focus for the magnetic fields" and that a 110v unit is doomed to having a hot spot that's even smaller than the element itself. Which sounded a lot like hand-waving, though I'll allow that more expensive sometimes means better engineering, and if it's true that power transfer is highest in the center, more expensive units may try to compensate somehow. So, questions:

1) Can the heating of the pan be evened out with a diffuser, or clever timing, or some such?

2) What differences exist, if any, in a "high end" unit that relate to evenness of heat transfer?

[syhprum]

Is the pan you are using specified for use with an induction hod?
I would expect a proper pan to be to use part iron and part aluminium construction
I do not think the supply voltage is relevant as presumably there is a chopper type PSU before the induction frequency generation.

[meeotch (OP)]

The pan is cast iron - so, good for inductive properties, less good in terms of heat conductance.  I've noticed, though, that a lot of the induction "heat diffuser discs" that are sold for the purpose of adapting non-ferrous cookware are made out of... cast iron.  I guess if you're trying to approximate the "sandwiched" character of an induction-specific pot, and you've already got one of the layers (e.g. aluminum), then what you need is the other layer.  From this perspective, maybe an aluminum diffuser disc would be better for use with a cast iron pan.

I'm still not clear on the basic question of the inductive element itself - whether it by nature has a central hot spot, and whether units exist out there that are able to work around this with clever engineering.  I'd think that a constant winding density across the element would produce constant heat transfer, but it's honestly been 20 years since I've used my engineering degree for anything involving calculus.

[Colin2B]

......I've noticed, though, that a lot of the induction "heat diffuser discs" that are sold for the purpose of adapting non-ferrous cookware are made out of... cast iron.  I guess if you're trying to approximate the "sandwiched" character of an induction-specific pot, and you've already got one of the layers (e.g. aluminum), then what you need is the other layer.  From this perspective, maybe an aluminum diffuser disc would be better for use with a cast iron pan.

I'm still not clear on the basic question of the inductive element itself - whether it by nature has a central hot spot, and whether units exist out there that are able to work around this with clever engineering.

Yes cast iron is good for induction but not good for heat distribution, so if you heat the pan too quickly there is no time for heat to even out. If the pan is bigger than the element then you will obviously get uneven heating. Try heating up gently, medium not max.

The heat diffuser discs work by getting hot and transferring the heat to the pan. We were advised not to use one because the heat can build up next to the ceramic surface and damage it unless transferred quickly. Aluminium is no use next to the element as it's surface resistance is too low, would work as a sandwich on top of the cast iron diffuser but that is getting far too complicated.

I can only guess that windings in a quality units are designed to give more even heating, certainly hear more complaints with cheaper units.

[alancalverd]

A lot depends on the flatness of the pan base. Problem is that a thin cast iron pan can distort with use and develop a central hotspot, particularly if you (like me) do a lot of frying at max power. And some single induction rings are designed for wok use with a hot center anyway!

[meeotch (OP)]

I probably should've mentioned that my tests thus far have all been low temperature, and that I've tried turning the unit down to its lowest possible temp and waiting.  It's really sort of surprising to me that the induction unit should take longer to reach a reasonably even temp than my previous conventional electric unit (with a similar element size).  I guess the answer is that there really is a hot spot in the middle of the induction element - which, again, is a bit surprising if the only factor is the number of windings.

As far as diffusers go - most I found were cast iron, one was aluminum (but intended for use with a gas hob, for the reason you mentioned), and a couple were stainless steel - which sounds like maybe the way to go.

The last bit of surprising info is how little info I've actually been able to dig up on this topic.  Seems like hot spots in the center of elements would be a major concern, and that the technology is simple enough that there would be various solutions, with associated yelling from the manufacturers about how well they work.

p.s. - the pan itself is definitely thick.  It's difficult to lift with one hand, in fact.  But with my conventional (small, by no means overpowered) stove, I've always found that I could throw it on cold, and be eating a quesadilla inside of 10min or so.

[wolfekeeper]

220volt induction heaters ... everything else equal... have 2 times the power... otherwise... no inherent difference.

[syhprum]

I do not see any reason why 220v induction hobs should be higher powered than 110v ones, The power that devices can generate depends on the ratings the transistors that generate the high frequency current .
The electronics of these devices will consist of two units a chopper type PSU section that will take the incoming main AC and convert it to a DC supply suitable for the high frequency generating section and that section proper.
The amount of power consumed is relatively low and puts little strain on the mains supply.
American domestic mains supplies are normally  110v with one side grounded for light loads such as TVs lighting and such like or for heavier loads such as ovens or space heaters both 110v lines are used giving 220v to avoid feeding to much current into the grounded line.
We European's find this a little confusing as we are used to a 240v threephase system with three lines at 240v relative to ground.
 

[meeotch (OP)]

I suspect that the originally commenter two drew the distinction between 220v and 110v units was using voltage as a proxy for price, or "quality".  He pointed me at Cooktek.com as a source for "higher grade portable 220v" units - and I have heard of that particular brand in the context of high-quality commercial-grade induction units before.

The thing that gets me is that even Cooktek's own marketing lists things like "user friendly control knob" and "led display" as differentiators:  http://www.cooktek.com/cooktek-difference-0 [nofollow]  If central hotspots are indeed an expected problem, and if the problem is in fact solvable with more costly engineering, then where are the ads crowing about each manufacturer's Patented EvenHeatZone Technology, etc?

[wolfekeeper]

I do not see any reason why 220v induction hobs should be higher powered than 110v ones, The power that devices can generate depends on the ratings the transistors that generate the high frequency current .

The reason is that the 110 volt wall sockets don't provide as much power as 220 volt wall sockets for the same copper area.

[wolfekeeper]

You should try preheating the pan; cast iron is a poor conductor of heat, but give it time and the heat will spread out, particularly if it's quite thick; you can maybe set it on high to preheat, and then change the temperature down before you start cooking. Note that not all pans will allow you to heat them empty; so you should be cautious; and if it warps you need to take it off the hob immediately.

It's usually less of an issue with gas cooking because you can use aluminium pans, and they conduct heat a LOT better and are very even.

This is the reason why induction cookers come in different diameters, they work best when they're the same size as the pan.

You can get better pans; some pans have an aluminium layer on top of the iron layer, that spreads the heat around. Also adapters are available, which allow you to use an aluminium pan; it's just a plate made of iron you plonk the pan on.

[Davindram]

I got my induction hob this week and tested all my cookware (which is a considerable collection). It is 1800 watt which is the most you can get out of a 15 amp 120v circuit. I can't imagine needing more power for a single 8"  element.  The best test I found is to boil some water time it and watch the pattern. There is considerable difference for different pots and pans.

Cast iron skillet 12"-- heats quickly with noticeable hot spots. It doesn't matter how long you preheat the pan. The hot spots are still there. It is annoying. I tried 3 skillets a Wagner with a flame ring, a Lodge and a thinner Chinese one. They all heat quickly and are more controllable than with gas. A nice gas range can give much more even heat.

Allclad tri-ply - amazingly even heat. Everything I tried heated almost perfectly evenly. It does, however, heat more slowly than the cast iron. Very controllable as well. When you turn it down. It goes down. The largest pot I have is a 4qt.  I wonder what a stock pot or 16" skillet would do. This cookware costs more than the stove.

Carbon Steel flat bottom wok-  Dangerously quick. It will heat up to red hot in an instant. Fun in a violent sort of way.  The bottom is hot but the sides stay cool. This is what I am looking for in a wok.

Stainless stockpot--   will not workhttp://www.thenakedscientists.com/forum/Smileys/default/sad.gif

Copper bottom stainless-- will not work.

Welded aluminum disk stainless-- will not work

Le Creuset Cast iron-- Works reasonably well. About the same as a gas stove.

12" carbon steel crepe pan-- bad hot spots, I have not even tried a crepe.

[Luhan Monat]

I have that same problem.  After some research and experimentation, I came up with the following...
youtube.com/watch?v=o_BAxpendC8

It works great!

[evan_au]

I came up with the following...

Bundling the copper wire produces a ring of heat with a wider radius, which is better for a larger pan - but its still a ring of heating.

Of more concern is that the bundle of wires appear to be plastic-insulated. If the pan gets quite hot (eg while frying in oil), some of that heat will reach the coil of wires, potentially melting them and causing a short-circuit. The original coil used physical spacing between turns, and an insulator/spacer that did not look like thermoplastic.

On a lighter note, the video seems to be cooking with vocal fry.

[Bored chemist]

"How can I achieve even heating with induction cooking?"
Boil everything.

[Luhan Monat]

Of more concern is that the bundle of wires appear to be plastic-insulated.

The wires appear to be standard enamel insulation.  Also, the plastic tie-wraps are not showing any damage so far.