Whiffle
An induction coil that follows the wok
Induction heats a pan only where the coil sits close to it, and a few millimetres of extra gap noticeably cuts the power. Whiffle splits the coil into four concentric rings that move up and down to meet a wok’s curve. Two choices shape the design: what holds the rings, a lever linkage by default or else springs or counterweights, and what their tops look like, angled to match a wok by default or flat so the burner doubles as a flat cooktop. With nothing on the burner, the rings rest flush with the flat cooktop panel around them.
A linkage that shapes itself to the wok
A lever linkage called a whiffletree, which gives Whiffle its name, ties the rings together: an inner yoke joins rings 1 and 2, an outer yoke joins rings 3 and 4, and a main beam on a fixed pivot joins the two yokes. When a wok lands, its weight pushes some rings down, the beam tips, and the others rise until every ring touches. Older windscreen-wiper blades use the same kind of linkage to press evenly on curved glass, and large telescope mirrors rest on one.
With no wok on the burner, the rings rest level, flush with the cooktop panel around them. Angled tops sit with their outer edges in the panel’s plane, so the surface is level but ridged. Flat tops form a flat cooking surface that continues the panel. Either way, a wok sinks the inner rings and raises the outer ones until each ring meets its curve.
The linkage sits entirely above the base plate, and every part of it is rigid. Every joint is a plain pin, with no slots. Each ring hangs on three short rods pinned under the ring at the top and to the end of a straight yoke at the bottom. As a yoke tilts, its ends swing slightly sideways and the rods lean to follow, about 11° at most with the offset pivot the model uses and about 15° with a centred one, so nothing has to slide or stretch. The rods sit near the inner edges of rings 1 and 3 and the outer edges of rings 2 and 4, which makes each yoke long enough to tilt gently. The rods can only lean toward or away from the centre, and with three of them spaced around each ring, they hold the ring centred and level: the linkage alone keeps every ring in place. Each yoke is tilted so it slips under the low inner edges of the angled rings. The main beam sits to one side of the yokes, in a plane of its own, and is bent at its pivot. An axle through the middle of each yoke runs sideways into one end of the beam, and the beam’s knee turns on a post standing on the plate a third of the way along, so the inner rings sink less and the outer rings rise further into the space under the wok.
From the cooktop panel to the base plate, the burner then needs about 5 cm with angled tops and about 4 cm with flat tops, against about 5.6 cm and 4.6 cm with level bars, and about 18 cm with the levers hanging below the plate. Curved bars would save almost nothing more. What remains is set by how far ring 1 sinks.
A hollow cylinder hangs from the outer edge of ring 4: a skirt that moves with the ring and slides through the opening in the cooktop panel. At rest it reaches almost to the base plate. When ring 4 rises, the skirt rises with it and fills the gap that would otherwise open between the ring and the panel, so the linkage stays hidden and out of reach. It is just long enough to stay in the panel opening at ring 4’s highest point, and the whole linkage fits within its diameter. The housing around it is a compact box with crisp, square edges, only a few centimetres wider than the burner, with a black glass top. Below a razor-thin vertical band, its walls bevel sharply inward toward the base plate, most deeply at the front, and drop into shadow, so the burner reads as a thin slab. Every control sits on one side, in the front strip of the glass: two low, glossy black knobs for temperature and timer, either side of a small display. The display shows the four rings, each glowing orange in turn as the burner powers them one after another, along with the temperature and a timer that counts down and switches the heat off at zero.
Why link the rings
- Shape doesn’t depend on weight
- The rings settle wherever the wok’s curve puts them. Adding food makes the wok press harder but doesn’t change the shape, and each ring always carries a fixed share of the load, so none lifts off.
- It adapts to any wok
- Shallower and deeper woks each settle into their own shape, with nothing to adjust.
- Travel is split
- Rings move both up and down from level, so travel stays short: with a centred pivot, about 3 cm at most with flat tops and about 4 cm with angled tops. Moving the pivot a third of the way along the beam cuts how far the inner rings sink, to about 1.4 cm with flat tops and 2.2 cm with angled tops, while the outer rings rise up to 3.4 and 4.5 cm and the inner rings carry two-thirds of the wok’s weight. If every ring only sinks instead, the centre ring needs around 5 cm with flat tops and over 7 cm with angled tops. Shorter travel makes the seals easier.
- Flat pans, with flat tops
- A flat-topped burner is an ordinary flat cooktop for any pan wide enough to sit on all four rings, about 30 cm across or more. A smaller pan resting only on the inner rings would push them down and lift the outer ones around it. Angled tops rest level too, but a flat pan touches only the outer edge of each ring, so in practice they make the burner wok-only.
What still needs solving
- Holding the rings level
- With no wok, a free linkage tips toward whichever side is heavier. A light centring spring at the main pivot, plus stops, holds the rings level and gives way easily under a wok.
- Stirring and tossing
- The wok rests on a free linkage, so hard stirring can rock it. The inner rings carry two-thirds of its weight, which resists tipping, and friction and a little damping at the pivots let it settle quickly, but a cook will feel some give that a fixed burner doesn’t have.
- Three linkages per burner
- Each ring rests on three rods, so three identical linkages sit around the burner, one per rod position. A little give in their pivots stops small manufacturing differences from making them fight each other.
Flat or angled ring tops
A wok’s sides get steep near the rim: 15 cm from the centre, a typical wok rises about 8 mm for every centimetre outward. A flat-topped ring touches the wok along one edge, and the gap opens quickly across its width. Angled tops follow the curve instead.
| Wok curvature radius | Flat tops, rings 1–4 | Angled tops, rings 1–4 |
|---|---|---|
| 22 cm, deeper | 5, 13, 21, 27 | 1, 2, 3, 4 |
| 24 cm, nominal | 5, 12, 19, 23 | 1, 1, 1, 1 |
| 28 cm | 4, 10, 15, 18 | 1, 2, 3, 5 |
| 32 cm, shallower | 4, 9, 13, 15 | 1, 3, 6, 8 |
Worst gap under each ring, in millimetres. Angled tops are tilted to match a wok with a 24 cm curvature radius.
Flat tops
The burner rests as a flat cooktop, so it handles flat pans as well as woks.
Under a wok, the outer edges of rings 2 to 4 sit 1 to 2.5 cm away, where they add little heat. Narrower rings help only a little, because the trouble is the wok’s slope, not the ring’s width.
Angled tops
Every ring stays within about a millimetre of a nominal wok, and within about 1 cm even for unusually deep or shallow ones.
They rest level when the burner is empty, but they can’t form a flat surface: a flat pan touches only each ring’s outer edge, so the burner is effectively wok-only. They also need more travel than flat tops, about 4 cm against 3 cm.
Getting both would take rings that change their angle. A solid ring can’t, so each would have to be cut into segments hinged at their inner edge, like flower petals, that tilt as the wok presses on them. That works in principle, but it’s much more complex, and gaps open between the segments as they tilt. A separate flat burner beside a wok burner is the simpler answer.
Efficiency compared with a flat pan
The benchmark is a flat pan on an ordinary induction cooktop, sitting flat on the glass with the whole coil close to it. Two numbers matter. The first is how much of the coil’s power ends up in the pan rather than warming the coil’s own copper. The second is how much heating power the pan draws for a given coil current, and that drops quickly as the gap grows: 2 mm of extra gap costs about 11% of it, 5 mm about 25%, and 10 mm about 40%.
| Setup | Coil power reaching the pan | Heating power at the same coil current, by wok curvature radius | |||
|---|---|---|---|---|---|
| 22 cm | 24 cm | 28 cm | 32 cm | ||
| Flat pan on a flat cooktop | 94% | 100%, the benchmark | |||
| Round wok on a flat cooktop | 90–91% | 60% | 63% | 67% | 70% |
| Fixed concave wok bowl | 89–93% | 87% | 100% | 78% | 68% |
| Moving rings, flat tops | 90–91% | 73% | 76% | 79% | 82% |
| Moving rings, angled tops | 92–93% | 97% | 99% | 95% | 92% |
Heating power is relative to the same coil with every turn at the benchmark gap. The fixed bowl and the angled rings are shaped for a 24 cm wok.
The share of coil power that reaches the pan hardly changes. Every wok setup lands within about 5 points of the flat pan, and angled rings within about 2. Gaps cost heating power instead: the pan draws less power at a given current, so the inverter has to push more current to make up for it, or it reaches its current limit and the wok heats more slowly. Losses in the inverter’s switches also grow with current, so weak coupling costs a few more points at full power.
Angled rings stay within a few percent of full power on every wok tested, which a fixed bowl manages only for the wok it was shaped for. Flat tops give up roughly a quarter of the heating power and about 3 points of efficiency compared with a flat pan, the price of doubling as a flat cooktop.
End to end, conventional induction typically turns around 80–90% of the electricity it draws into heat in the pan. Angled rings should land close to that, perhaps 1–2 points lower because four inverters each carry some fixed overhead, and flat tops a few points lower again at full power. That assumes no metal in the mechanism sits in the field, since anything conductive near the coils heats up and wastes power, and that the four inverters run in sync so no power sloshes between them. For comparison, gas burners typically put only about a third of their heat into the pan, and high-power wok burners often less.
These figures come from a simplified model: coil loops over a steel surface at the local gap, 25 kHz, carbon steel at cooking temperature, the same 6 mm coil-to-pan stack for every setup, and no ferrite. Trust the comparisons more than the absolute values. Recomputing with a much finer numerical integration changed nothing, and varying the steel’s conductivity and permeability, the frequency between 20 and 40 kHz, the stack between 5 and 8 mm and the Litz wire’s resistance by half either way moved all the coil-efficiency figures together by at most about 3 points while changing the heating-power percentages by at most 1 point. A full simulation, for example in the free FEMM tool, or measurements on a test ring would firm them up.
Inside a ring
Each ring stacks a ceramic cap, the Litz coil and radial ferrite bars, all flat or all cone-shaped to match the ring top. The ferrite bars sit between the rod positions, so the rods never pass through them. The rods hang down rather than stick up, so nothing pokes above the cooking surface. In the linked version, all four rings share the same three radial lines, so one linkage can reach every ring on its line.
Seals bridge the gaps between rings, and they’re the hardest part of the design. A wok bottom passes 300 °C, oil that gets into the gaps polymerises into a varnish that would jam the rings, and neighbouring rings can end up a centimetre or more apart in height. A real build needs a high-temperature flexible membrane or small bellows here.
The rings move straight up and down, so they can’t follow a wok that’s tilted during cooking. When one side is lifted, the wok simply leaves those rings until it settles back.
Other ways to hold the rings
Two simpler variants drop the linkage. Each ring rests at the top of its travel and a wok pushes the rings down. Either ring top works, and the stops are set so the rings rest level. Both are built as compactly as the linkage, with nothing below the base plate, but they cost travel: the centre ring must sink by nearly the wok’s full depth over the rings, around 5.4 cm with flat tops and 7.3 cm with angled tops. From the cooktop panel to the base plate they need about 8.6 cm and 11 cm, roughly twice the linked version. Ring 4 never rises above the panel here, so its skirt shrinks to a short lip. Switch the model to Springs or Counterweights to see them.
Springs
These use the same compact levers as the counterweight version, with a coil spring pushing up under each lever partway along its long arm instead of a weight on its short arm. The wok sinks until the springs carry its weight, so adding or removing food changes how deep it sits, and the outer rings can lose contact when it’s light.
Spring force also grows with compression, so the ring that travels the most presses hardest. Long, soft springs keep that difference small.
Counterweights
Each ring hangs on three short pinned rods from three long levers lying just under the rings, so, as with the linked levers, the levers alone keep the ring centred and level, with no slots anywhere. Each lever pivots on a post outside the outermost ring, and a weight hangs from its short outer end under the cooktop panel. The inner arm is several times longer than the outer one, so each weight must be several times heavier than the share of the ring it balances. The upward force stays nearly constant through the travel.
That also means the weights can’t set the wok’s height themselves, so the wok needs a fixed rest, such as the edge of the cooktop panel, with the rings pressing up from below. The weights add inertia too, so a tossed wok landing makes them bounce unless they’re damped.
Wiring
One inverter per ring gives each coil its own power level and its own current sensing. The wiring is the same for flat and angled tops.
- Keep the moving part small
- Only the coils and their leads move. The inverters, capacitors and controller stay fixed on or below the base plate.
- Give every lead a service loop
- Leave a loose U-shaped or helical loop with slack for full travel plus a margin, and clamp the leads firmly at both ends. Flexing then happens only in the loop, never at a solder joint or where the wire leaves the coil. Litz wire tolerates repeated bending, but its fine strands break if the bending keeps happening at one spot.
- Run each pair together
- Keep a ring’s two leads side by side or twisted all the way to its inverter. Separated leads form a loop that radiates the high-frequency field into nearby parts.
- Insulate for heat
- Use PTFE- or silicone-jacketed leads, routed down along a rod and away from the hot cap and the ferrite edges.
- Synchronise the inverters
- Run all four at the same frequency and vary power by duty cycle or phase. Neighbouring coils at slightly different frequencies interact and produce an audible hum.
- Use per-ring current sensing
- It shows how well each ring is coupled to the wok, so the controller can catch a ring that’s stuck or not touching.
A simpler alternative wires all four rings in series through flexible jumpers to a single inverter. It’s cheaper, but you lose per-ring power control and sensing.
Before you build
The DC bus from 230 V mains sits around 325 V and is lethal. Test the mechanism with a commercial induction module or a low-voltage setup before building your own power electronics.
All dimensions on this page are illustrative, not engineered.
Weigh it against the version with no moving parts: a fixed concave bowl with several independently powered rings. The controller senses each ring’s coupling and sends more power where the wok sits farther away. It can’t fully make up for a large gap, but it covers the few millimetres of difference between typical woks without any seals.
Search patents before investing. Variations of adjustable-height induction coils have likely been filed.