Induction cooktops/ induction heating in general

This is an email i sent out, but i was thinkning some peolpe here could answerr as well

thanks



Dear Manufacturer
I am senior Design and Technology student and I am designing an induction-cooktop related product. I was hoping that you would be able to help, by forwarding me some information or directing me to a good source. Here is information i would apreciate:

• Information as to how induction actually works (All internet sources are useless)
  
  Specific features of your own induction cooktops (not secrets, just features)
  
  Specifications about your own induction cooktops (practical, not industrial)
  
  Cost of manufacture (though i dont fully intend on making an induction cooker at this stage) and approximate costs for main components. How does the power rating of the cooker effect the price?
  
  Any oppinions on the situation of induction cooking in Australia, where the market is not large at the moment?
  

Sencerely,

thanks,

stonerbill (though not stonerbill)

 

Induction cooktops work on the principle of electromagnetic induction, much the same as transformers or electric motors.

A high frequency AC current is driven through a coil of wire under the "burner" area, and it sets up a corresponding magnetic field over the burner. When a metal cookpot is placed in this field, the magnetic field sets up electrical currents (called eddy currents) in the metal, and the metal starts to get hot due to electrical resistance. Unlike gas or electric cooktops, the heat is actually generated in the pot material itself, not simply transferred through it from an external source.

Because of the way the system works, you are limited to using metal cookware on an induction cooktop. Glass and ceramics do not conduct electricity, and will not heat when placed on the coils. Metals like copper and aluminum have higher electrical conductivity (lower eddy current loss), and will generate less heat than poorer conductors like cast iron and stainless steel.

The same principle (induction heating) has long been used in industry for heating of metal objects for brazing, soldering, hardening, and heat treating. It only became practical for household use with the advent of cheap power semiconductors for generating the high frequency current. Lots of good info at:

http://www.radyne.com/faqs.htm

 

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Any of you folks know what frequency is typically used in consumer induction cooktops? Maybe just 60Hz AC is enough. I saw a levitation demonstration once where a professor placed a metal plate on top of a set of electromagnet coils plugged into the wall. The plate levitated on top of the coils, and was held in place there due to the mutual interaction between the coil fields and the induced eddy current fields in the plate. However, after several seconds of this, the plate got pretty hot. That was the first time I saw induction heating demonstrated so dramatically. I thought how cool it would be to levitate and cook your food at the same time!

 

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thanks but im talkin about a bit more technical, the sort of metals used, what sort of modification of power is needed, etc

maybe i should have made that clearer in theemail? lol i dont want to get replies back all telling me this more of this sort of thing

 

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By "modification of power" do you mean the generation of the high frequency?

The principle here would be much the same as a crude radio transmitter or switchmode power supply. Incoming AC power at 60 Hz would be rectified into DC using a diode bridge, and filtered using a bank of capacitors. The resulting DC is applied to a suitable switching device (IGBT, BJT, GTO, SCR, etc.) through the primary winding of a high frequency transformer. The switching device is gated on and off at the desired frequency by some sort of oscillator, resulting in a high frequency chopped current through the transformer primary. The secondary of the transformer would likely be a few turns of very heavy wire (or even copper tubing), which couples to the burner coil. This will result in a low voltage at very high current in the work coil. The number of turns in the transformer windings would be designed to make the transformer and work coil resonant at the operating frequency. Power control is achieved by either adjusting the voltage applied to the switching device, or varying the pulse width of the control pulses that gate it on and off.

 

I've used them before in restaurants that I've worked at. In my opinion, it is the best piece of cooking technology I've ever used.

 

fuckin expensive though, and you cant get the cheap ones here in oz. im gonna have to get one off the net and get a power adaptor (caus we run at higher voltage to america).

thanks, and yeh thats what i meant, ellis

do you recon i would be able to make a low powered, or compact induction coil? what sort of resources would that involve? im capable of the wiring up and all but im not sure about safety issues in contruction and all. My projexct doesnt focus on the cooker but the cookery (though i need an induction cooker for testing, and i dont want to have to keep going to a peer's place or bug the demonstration people in department stores), however, i would like to be able to include a low powered, warming device, which only needs enough power to keep food warm, not cook it.

 

How "low powered" can this thing be? The basic principle can be demonstrated easily enough (I heated a small piece of steel welding rod to red heat using an electronic type fluorescent lamp ballast as an RF source and a few dozen turns of insulated wire as a coil). But if you want to heat up a pan and actually cook food, you are probably talking about at least 1500W or so of power needed. Generating RF energy at these power levels is non-trivial. Unless you have experience working with high frequency, high power equipment (like multi-kW switchmode power supplies, broadcast transmitters or big ham radio amplifiers), you are probably better off trying to buy the RF generator already built. Surplus induction heaters and RF amplifiers show up all the time on eBay or at hamfests and the like. The parts needed to "roll your own" will cost at least as much as buying an old surplus unit. And working with this kind of power can be quite hazardous unless you have a lot of experience with it. Electrical shock or RF burn hazards are basically EVERYWHERE in the circuitry.

If you really want to homebrew one, I would suggest using vacuum tubes rather than semiconductors. Simpler circuit (self-excited oscillator), more forgiving of mistuning or wiring errors, cheaper parts (if you can scrounge a bit), and a long history of successful use for the application. An old RCA transmitting tube manual has a plan for a 450 kHz induction heater using 833A triodes (still made and relatively cheap) which would be a good starting point. I used the circuit as the basis of a Tesla Coil for a HS physics class project many years ago.

If you can't find a suitable circuit, I can scan the RCA manual and post it in the gallery.

 

Hmmm, I wouldn't recommend trying to build a vacuum tube circuit, StonerBill. You'd have to generate a lot of different high voltages to get all the biasing, screen voltages, etc. That means expensive power transformers, regulators, etc. And those tubes (833A) could have very high plate voltages (about 2000 volts = LETHAL) and you'd have to salvage a microwave oven to get that kind of voltage+power or else expect to pay over a hundred bucks for a plate transformer that big. Then you have to find another transformer to heat up the filaments with. Then you need to find yet another weird transformer to reduce the output impedance from your tube final down to the cooktop coil. These transformers are really expensive, heavy, and big.

I agree with EllisDTripp about trying to find a used induction cooktop. I see them on eBay going for about $100 or less, and that will cost about the same or less than trying to build one yourself. But if you really want the "learning" experience, here are some ideas on building one that is relatively safe:

1. Find an old 5-volt power supply they use for big mainframe computers. Get one that has a lot of current, like 100A or more. You find these all the time at surplus places and on Ebay and they are cheap.

2. Get some IGBTs, power FETs, or other solid state switching device. Make sure you find some that can switch big currents (100A or more).

3. Build a square wave oscillator, like using a 555 timer or something, and make it adjustable in frequency so you can experiment. Make it so you can easily adjust it from like 2kHz up to 200 KHz or more.

4. Build the circuit so you switch (chop) the 5-volt supply into the coil using the FETs or IGBTs being driven by the oscillator signal. The switch component goes in series between the power supply and the coil. Follow the data sheet for the switch device to figure out what you need to do to hook it up to your squarewave oscillator.

5. Build your coil. You need to use ohms law equations and inductance equations to make sure you get the maximum power transfer into the coil. Use a wire gauge and length that won't burn up, but not such that it overloads the power supply either. This is where you will have to do some math to get the optimized wire gauge, number of turns, and wire length. Start with planning the coil to be just a little smaller in diameter than the cookware.

6. Consider protecting the switching component by using a supression diode. Place the diode across the coil in reverse bias. It will absorb the flyback EMF. Since this is a big coil with lots of power, you will probably want to use a power resistor with your diode, and use a really big diode.

7. Connect the coil up to your "power oscillator" and viola you are done. Adjust the frequency for the maximum power transfer into the cookware.

You may want to consider using a higher voltage power supply, like 24V or more. That way you don't need as much current to get the same amount of power. Everything will depend on how you design your coil. You might even consider rectifying the AC with a bridge rectifier + filter capacitor and chopping that directly into your coil. If you design your coil right, it'll handle the voltage as long as you're giving it the right frequency. Then you won't need the 5-volt power supply. I think the consumer products do it this way- they look too small and lightweight to have any kilowatt transformers in there; even at tens of kilohertz they would be too big. If you do start connecting stuff directly to the AC line, just remember to put a fuse in line with your power cord!

Sounds like a fun experiment and I wish you the best of luck with it. After you do all this you might be able to melt some butter at least. Keep us posted!

 

Yes, vacuum tube circuits can be dangerous, but so can solid state equipment operating from rectified line voltage! The advantage of the vacuum tube circuit is the utter simplicity and the low parts cost, not to mention the resistance to "letting the magic smoke out", unlike banks of expensive power IGBTs. A self-excited triode oscillator only needs filament and plate voltage to function, both of which are easily obtained from discarded microwave oven transformers. The rest of the coils are simple air-core affairs which you can wind yourself. Here's a schematic and parts list for a 4 kW unit using an 833A:



As you can see, the parts count is very low, and the circuit is simplicity itself. Be careful with the high voltages, and you should be able to rig this pretty easily. The only somewhat tricky parts to get will be the HV mica caps, and they show up at hamfests or surplus dealers fairly often. If you require less than 4 kW, the same basic circuit will scale down nicely, using a smaller tube and a lower plate supply voltage. The "R" resistor value may need to be tweaked to get the tube biased to an efficient operating point.

 

thanks a heap guys,

ive looked on ebay but most suppliers only sell to america, though im trying to get in contact to them and see waht i can arange. 100$ for you is like 150-200 dolars for me remember too.

at least now i have some good startng points

 

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damn, didn't know they made them for the home. The ones that I was using in this restaurant, I purchased for 15 hundred a piece. Then again they were commercial grade.

 

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thanks but , uhh, you think i hadnt tried that already? lol

 

"Oh!!! it's COOK tops! Never mind."
- Miss Emily Lutella

 

besides the price.. i dont see why theyre not standard! i mean, theyre just such a great concept