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New for September 2023. This is not exactly a microwave topic, but it is topical and it might be something that you observe right below your microwave oven in your kitchen! A lot of cool engineering has gone into induction cooking, it is not as simple as placing a pot on top of a coil and running the house AC 50/60 Hz current through it. Maxwell's equations are involved, so keep reading!
Induction cooktops are form of "green" technology in that they provide efficiency advantages over radiant electric and gas stove-top heating (we are not talking about ovens here). Induction heating is 85% efficient, radiant electric is 75% to 80% efficient, and gas is 32%. If you want to boil water for pasta quickly, induction is the clear winner. Compared to gas cooking, with induction you don't have to worry about gas leaks and carbon monoxide in your home.
Note that induction is also used for wireless power transfer in mobile phones, and even in electric vehicle charging. In those cases, "wired" charging are always going to have higher efficiency.
A simple explanation of induction cooking is that it uses a flat coil (like a spiral inductor) under the stove top, driven at perhaps 40 kHz using an insulated-gate bipolar transistor (IGBT). Pulse-width modulation is used to vary the amount of heat that is transferred. The coil produces an oscillating B-field in the vertical direction. When this B-field impinges on a suitable vessel material (a frying pan for example), it induces eddy currents in the vessel which dissipate and transfer heat to the food. The material that is used in induction cookware matters... in practice, if a magnet sticks to your cookware, it is probably OK for induction cooking. But why not treat yourself to some cookware that is specificially designed for the task? Induction cookware could include a stack of different metals such as aluminum (for heat spreading) and magnetic steel (for heating), sandwiched between outer layers of stainless steel (for aesthetics and easy care)[1].
Video explaining induction cooking
Let's look at a video explanation of induction cooking, thanks to Silicon Soup on YouTube. The narrator points out that the Maxwell-Faraday equation predicts the induced E-field from a changing magnetic field, and the E-field is what cause eddy currents in the vessel (and ultimately heats it). Equivalent circuits for the heat transfer are reviewed, and a link to Google drive is provided so you can download the schematic of the cooktop. Measurements show that the coil is 95 micro-Henries with 0.05 ohms resistance, and the load (the pan holding your lunch) acts like 120 ohms. A resonant circuit is created by strapping a 320 nF cap across the cooking coil, and it "rings" at 33 kHz according to our Microwaves101 resonant circuit calculator. When a pan is not present, the ringing damps slowly compared to when the pan is present, due to de-Qing of the resonance (an inducation that power is being transferred). Varying the square-wave that pumps the resonator is what determines how much power is transferred. You'll get an appreciation for all of the contol circuitry that is required to make a reliable, idiot-proof product. You'll even learn a kitchen levitation trick! Thanks again to Silicon Soup!
"Induction cooker circuit, design, theory, must watch" by Silicon Soup
References
1. F. Moro, P. Alotto, M. Guarnieri and A. Stella, "Impedance design of cooking appliances with multilayer induction-efficient cookware," IECON 2013 - 39th Annual Conference of the IEEE Industrial Electronics Society, Vienna, Austria, 2013, pp. 5040-5045.