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Cooking with Magnets: An Intro to Induction

Posted August 31, 2009 11:43 AM

From Gizmodo:

Induction stoves may be making their way into restaurant kitchens, but for home cooks they're still a mystery. Fortunately, Wired product editor (and food geek) Mark McClusky volunteered to enlighten us: It took me nearly an entire evening in the the kitchen at Alinea before I realized what was weird about it. Sure, there's the stunning intensity of the chefs as they prepare Grant Achatz' intricate dishes, and the nearly-operating room level of cleanliness. But that's not what struck me one night at the end of service. What struck me is that I didn't know where the stove was. You see, in most restaurant kitchens—like most home kitchens—the stove is the focal point of the room, the place that all the action revolves around. If you're running the sauté station in most big restaurants, you're the man, the line cook who's banging out the most food in the hottest, most extreme environment. You're the alpha cook. Not so at Alinea. Of course there's a stove, but it's much smaller than you'd expect for a kitchen that puts out a couple of thousand plates a night, just four burners and a flat top. Instead, the chefs at Alinea do the vast majority of their cooking using induction burners, portable ones from CookTek. Induction is just plain cool. Instead of using a flame like gas, or radiant heat like standard electric burners, induction burners use a magnetic field. The field creates heat through the property outlined in Joule's first law—you do remember your thermodynamics, right?—in which current passing through conductive material generates heat. So what? Well, a couple of things. First, induction is super-efficient. Induction burners convert about 85% of the energy you pour into them into heat, compared to about 70% for electric burners and 40% for gas. That means you'll spend less to cook on induction. And since the burner itself doesn't create heat, it stays cool to the touch—take the pan off, and you can put your palm on it. That also means that they don't throw off ambient heat like gas or electric, so the kitchen stays much cooler. Then, there's the responsiveness of induction. Like gas, when you turn it off, there's no residual heat from the burner, just the pan. Plus, there's the flexibility of portable burners like Alinea uses. Frying something smelly? Got an outdoor power outlet? Set up a portable burner, and you can keep the stink out of your house. Want to keep soup warm at a party? Throw a burner on the buffet, and you're good to go. The one thing to keep in mind is that your pans do have to be magnetic. That might be a pain in the ass, especially if you're hip deep in anodized aluminum pots. But the good news is that some of the cheapest (and most fun to use) cookware around—cast iron—works amazingly on induction burners, as will all your fancy pots as long as they've got some stainless steel kicking around in them. If in doubt, grab a magnet from your fridge door to check.

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#1

Re: Cooking with Magnets: An Intro to Induction

09/01/2009 1:25 AM

I would imagine that the pot on an induction heater would be like the secondary of a transformer and that it is the ohmic loss that creates the heat. The electric currents induced in the pot and the electrical resistance of the material of the pot is the issue. So then why insist on a ferromagnetic material ? Aluminium does not work because it has a low resistance, not because it is not magnetic. Pyrex glass ware would not work because it not a conductor. Something in between which is a conductor, but has a high resistance would be best.

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#3
In reply to #1

Re: Cooking with Magnets: An Intro to Induction

09/01/2009 11:27 AM

For the frequency used, you must have a pot that a magnet sticks on.....

May I also refer to the old blog that we had that many contributed to:-

http://cr4.globalspec.com/comment/125379/Re-Temperature-Control-and-Induction-Cooking

It will clear up a lot of anomalies for you I feel.

We have been users for 4 or 5 years and are completely convinced that induction is the way to go, especially with regard to oil or fat fires as it simply switches itself off before the oil can catch fire.

Normal electricity or gas hobs or in fact any other type of hob do not achieve this level of safety ever....

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#4
In reply to #3

Re: Cooking with Magnets: An Intro to Induction

09/02/2009 11:54 PM

Thank you for all the information, but I am afraid you still have not cleared up my query about the physics of the induction heating process. My contention is that the electrical resistance of the material of the pot is more important than its magnetic permeability. Perhaps the thickness of the bottom of the pot too is a factor. I am in no doubt about the utility of the induction stove. Suppose we put a coil of enameled copper wire of (say) 0.1 mm diameter and lenght of a couple of meters with the ends shorted on an induction stove, what will happen ? My guess is that it will melt, especially if it is tuned to 50 kHz (if that is the induction stoves frequency).

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#5
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Re: Cooking with Magnets: An Intro to Induction

09/03/2009 4:06 AM

If a shorted copper coil melts due to induced magnetism (transformer effect) running a large current that is one thing, please do not confuse your physics here! Un-shorting the coil may show no heating whatsoever......in fact I would put money on that.

I believe that the frequency used in Induction cooking Hobs (50KHz rings a bell) is paramount in the effect on the metal concerned, eg. Its "tuned for steel/iron). I found a good web link that supports this fully and the first sentence says it all for me and for you too I feel:-

Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal. An induction heater (for any process) consists of an electromagnet, through which a high-frequency alternating current (AC) is passed. Heat may also be generated by magnetic hysteresis losses in materials that have significant relative permeability. The frequency of AC used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.

Look at the following link for more detailed information:-

http://en.wikipedia.org/wiki/Induction_heating

It appears that the reason that you need a pot that a magnet sticks on is because (as I mentioned before) Induction hobs are "tuned" for that (iron/steel) metal. Even many stainless steels are NOT heated by induction hobs. I always take a tiny magnet with me when buying cookware for the Induction Hob. I recommend it.....

Which is basically what I implied in all of my previous posts and blogs.....any further questions?

Have a great day.

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#7
In reply to #5

Re: Cooking with Magnets: An Intro to Induction

08/11/2010 4:29 PM

I have just discovered that not all magnetic pots work on all induction cooking units. I have a Presto model C601A stainless steel pressure cooker, made many years before induction cooking became available. A magnet will stick to the main pot quite well anywhere (except the Bakelite handles). On the lid, however, the magnet shows no attraction to the top surface. It attracts the rim of the lid weakly, but attracts the 'ears' of the latching cams quite well. It appears that this stainless is originally non-magnetic, but becomes magnetic when cold-worked. Although they look alike, I can't be sure the top and pot are made of the same kind of steel. They are of different thicknesses, so at the very least are from different batches of sheet stock.

In any case, we have been using the pressure cooker regularly on the induction side of our Electrolux EW30CC55GW2 hybrid (2 induction areas, 2 radiant burners) cooktop, ever since I installed it last December. When cooking artichokes, it would boil in one minute, and reach pressure in two minutes. Perfect! Unfortunately, last week the whole cooktop failed (appears to be the control unit that failed - the displays flashed on and off several times as it was dying; now no lights come on at all).

My wife was in the middle of a major cooking project, and I had loaned my only hotplate to a friend several months ago, so we went looking. We live in a small town, so there aren't many choices, but we did find a plumbing/appliance company that had a single Viking VICC120SS portable unit on hand. They only had it because it was a required part of a dealer package they had purchased around 2.5 years ago. Viking lists the unit at $550, and most places I saw on the internet list it at $495-500. Since they had not sold it, it had sat in their warehouse all that time, and they were happy to get rid of it; they sold it to me for $300. I know that cheaper units are available on the internet, but they would not be instantly available, and I do like to support local businesses, so I bought it. It worked fine for my wife's project of the day.

A day or two later, I went to cook a couple of artichokes, but the Viking unit just sits there with it's LED flashing - indicating that the pot is not accepted as appropriate. I changed to a different pot, and it worked fine, but since it was not a pressure cooker, it took a lot longer to cook, and in spite of adding water twice, it did cook dry.

I find it really interesting that the same pot works fine on one induction cooker and not on another! This makes it clear that they do indeed 'tune' the induction frequency (or waveshape or...) for specific metals. Once my Electrolux gets repaired, I plan on setting up my oscilloscope with a small coil, so hopefully I'll be able to see the frequency and waveshape of both units. I'll report what I find...

Among reviews I saw for the Viking, there were three very good ones and 2 very bad ones. One of the bad ones had been happy with it for just over a year, but just after the warranty expired, so did the unit. It would seem that extended warranties would be good investments for these devices. Or, just take Andy's advice and buy cheap ones and replace them when they fail. I couldn't do that this time because of the time constraints.

I do have to end with: regardless of the durability questions, I do LOVE cooking with induction! The speed, control, and ease of cleaning are superb!

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#2

Re: Cooking with Magnets: An Intro to Induction

09/01/2009 7:32 AM

How about something fabricated from metallic glass? Don't remember if that is the proper designation but there has been some advances lately in creating amorphous/non-crystalline metals. Maybe Santa will bring me a complete set of Wolfgang Puck ,tantulum/zirconium/hafnium, induction pots and pans. Can't wait for the future to get here so I can cook breakfast while commuting to work in my atomic-powered flying car.

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#6
In reply to #2

Re: Cooking with Magnets: An Intro to Induction

08/11/2010 2:58 PM

I just saw this forum today (8-10-2010), as I was preparing to do another post that will follow this one. I just checked, and the metallic glass I have available (AMS 4777) is actually LESS attracted to a strong magnet than a US $10 or $20 bill. In addition to the lack of magnetism, when it is manufactured, it must be cooled almost instantly from molten to near room temperature to prevent the formation of crystals. They claim that the stuff we use is cooled from 2000°F to near room temperature in one millisecond! As a result, they can't make it more than 2 or 3 thousandths of an inch thick, so I don't think you are going to see anything similar to a pot or pan made of it any time soon.

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