being a faithful account of the Axial-Flux Motor — its geometry, anatomy, operation & uncommon virtues — illustrated throughout with woolly mammoths
after the manner of the great paper engineers, with admiration — drawn fresh in ink-flavoured SVG, no tracing
From the notebook of the mammoth-keeper, wherein the entire secret of the motor is given away early.
Long before anyone wound a coil of copper, the keeper of mammoths knew the secret of turning things. Set your beast close to the millstone's centre and it strains all afternoon. Set the same beast at the end of a long bar — let it walk the wide circle — and the stone grinds as if possessed.
Nothing about the mammoth has changed. What changed is where the push is applied. A push far from the axle is worth more turning than the same push close in. Engineers call this turning force torque, and they will pay nearly any price to apply their forces at a generous radius.
The axial-flux motor, which occupies the remainder of this volume, is best understood as a machine that takes this old advice literally: it moves all of its pushing out to the rim, and grows wider rather than longer.
In which the familiar cylinder is turned ninety degrees and flattened into a pancake.
Nearly every motor the reader has ever met is a radial-flux machine: a rotor spinning inside a cylindrical stator, with magnetic flux crossing the gap sideways, pointing out from the axle like the spokes of a wheel.
The axial-flux motor commits one elegant heresy. It turns that gap ninety degrees. Rotor and stator become facing discs, and the flux crosses a flat gap running parallel to the axle — hence the name, and hence the pancake silhouette. The idea is as old as the motor itself; Faraday's first disc machines were axial. What kept the pancake waiting two centuries was the difficulty of making it strong, cool, and cheap — of which more in Plates IV through VI.
The machine submits to disassembly. Pull the brass handle below and it comes apart in an orderly fashion.
The high-performance arrangement sandwiches a ring of copper-wound iron segments between two magnet-studded rotor discs. Because each magnet faces its opposite across the stator, the flux travels a short, straight road from one disc to the other. A conventional motor must provide a thick iron yoke — a ring road — to return the flux; the sandwich simply doesn't need one. Less iron, less weight, fewer losses.
Looking straight down the axle while the machine runs. A small mammoth on a dynamo provides the electricity, as is traditional.
Energize the windings in sequence and each iron tooth becomes, briefly, an electromagnet — pulling the approaching rotor magnet and shoving the departing one. The rotor is dragged around in a perpetual, beautifully-timed tug-of-war across a gap of a millimetre or so.
The flux path here is short and straight — so straight that grain-oriented electrical steel, which conducts flux best in one direction (and which ordinary motors cannot exploit), serves happily in the teeth. What finally made the pancake practical at great power was the arrival of strong rare-earth magnets and fast transistor commutation — the electronic successor to our mammoth's crank.
The reader is invited to widen the disc personally, and to observe the consequences in mammoth-pulls.
| torque of the axial machine (grows as d³) | ×1.0 |
| torque of a radial machine (grows as d²) | ×1.0 |
| the pancake's advantage | ×1.0 |
Recall Plate I: turning force is push × radius. In an axial machine the working surface — magnets, copper, all of it — lives out at the rim where radius is generous. Widen the disc and you gain twice: more working surface, and a longer arm for every part of it. The arithmetic compounds to torque rising with the cube of diameter, where the cylinder of Plate II must settle for the square. A wider disc also presents more face to the cooling air, which is a second, quieter victory.
Wherein a subtlety omitted from lesser accounts is given its due: why the finest pancakes lean their iron.
A magnet that meets a straight tooth meets it all at once — a little magnetic doorstep. Crossing a whole ring of doorsteps in lockstep, the rotor thumps: engineers call it cogging, and passengers call it vibration.
The remedy is to lean the iron. Skew the teeth (or the magnets, or the windings wound upon them) so that each edge arrives gradually — the doorstep becomes a ramp, each magnetic hand-off overlapping the next, and the torque flows smooth as the keeper's best butter. The angled cores and angled copper of a fine axial machine are not decoration; they are the difference between a motor that hums and one that knocks.
Need more power? The kitchen analogy is exact: add another pancake to the stack.
Because each motor is a thin disc, modules stack along one shaft like records on a spindle: twice the motor, twice the power, scarcely any longer. Some designs go further and stack rotors and stators inside a single housing. The mammoth-keeper achieved the same result by harnessing beasts in file, though the gearing was less convenient and the feed bill considerably worse.
From supercars to the record books — and, in time, to the wheels themselves.
Axial-flux machines now power the hybrid systems of the Ferrari SF90 and 296 GTB, the Lamborghini Revuelto, and the McLaren Artura; three of them fly the Rolls-Royce ACCEL, the fastest electric aircraft yet built. Being flat and light, the pancake is also the first motor that can plausibly live inside a wheel — doing away with axles, differentials, and most of the drivetrain besides.
The keeper, shown here testing an early in-wheel arrangement, reports that the future arrives whether one is dressed for it or not.