Induction Cooktop

Michael Faraday pushed a magnet through a coil of wire in 1831 and a needle twitched. That twitch is the whole burner.

Faraday's ring of iron and wound wire showed that a changing magnetic field pushes current through nearby metal, with no contact between the two. An induction cooktop is that experiment with the pan playing the role of the second coil. A flat spiral under the glass carries alternating current at 20 kHz to 100 kHz, tens of thousands of reversals a second. The field it throws off induces swirling loops of current, called eddy currents, in the pan's base. Metal resists those loops, and the resistance turns into heat inside the pan itself. James Clerk Maxwell later wrote the behavior into equations that engineers could design against. Without this, the coil would warm only its own electronics and the food would stay cold.

The heat sink under a cooktop is small for a reason. By 1985, inverters had learned to switch at the moment the voltage is already zero.

A wall socket delivers current at 50 or 60 reversals per second. The coil needs tens of thousands. Something has to chop the supply that fast, at kilowatt power, in a box thin enough to sit under a glass sheet. Resonant inverter designs solved it by timing each switch to the instant the voltage across it has fallen to zero, so the transistor turns on cold instead of tearing through a live current. A half bridge series resonant drive reaches about 96% efficiency at 2000 W across a working range of 50 to 3600 W. Hard switching at that rate would cook the transistors long before it cooked dinner.

Stick a fridge magnet to the bottom of a pan. If it falls off, the burner underneath will not heat it, whatever the electronics can do.

The pan is the second half of the circuit, and most good cookware was built to be the wrong half. Aluminum and copper conduct heat beautifully and let the magnetic field pass almost straight through, so the induced currents spread thin and generate little heat. Austenitic stainless steel, the common shiny kind, behaves much the same. Ferritic stainless and cast iron pull the field into a shallow layer where the current density is high enough to boil water fast. Conventional ferromagnetic cookware couples strongly but heats unevenly across the base under a flat inductor, and balancing those two is what the Zaragoza modeling work set out to do. Bonded ferritic base plates made ordinary pans work, and that is when induction became sellable.

A pot of stock that boils over is a control failure, not a heat failure. Guillermo Paesa and Gonzalo Lopez published a fix in 2011.

Induction dumps power into a thin steel base with almost no thermal lag, which is exactly why an open loop setting overshoots. Add cold water or a handful of vegetables and the load changes underneath the burner in seconds. Their adaptive simmering controller reads sensor feedback and retrims inverter power to hold the pan in a narrow band instead of swinging between rolling boil and nothing. An uncontrolled cooker burns 5.22 W·h more per standard water-heating cycle than one running optimized feedback. The energy saving is minor next to the control precision, which is what keeps a simmer from turning into a scorched pan.