1 · Alignment torqueA compass needle in a rotating field
Put a magnet on a shaft and surround it with a field you can rotate. The rotor's own field wants to line up with the applied one, and any angle between them produces torque. Turn the applied field and the rotor follows.
δ is the load angle — how far the rotor trails the field. Torque peaks at 90° and collapses beyond it, which is what "pulling out of step" means. This is alignment torque, and it is what a permanent magnet or a wound rotor field gives you.
1 · Reluctance torqueIron alone will do it too
Take the magnet away and leave a shaped lump of iron. Flux prefers the short route through steel over the long route through air, so the rotor twists to present its easy axis to the field. No magnet, no rotor current — just a preference for a lower-reluctance path.
Note the 2. Reluctance torque repeats twice per revolution of the field, because the iron does not care which end points at the north pole. It is weaker than alignment torque for the same size, and it needs saliency — a rotor that is magnetically much easier in one direction than the other.
1 · The rotating fieldThree phases, no moving parts
The rotating field itself is the trick the whole industry is built on. Three windings spaced 120° apart around the stator, carrying three currents 120° apart in time, sum to a single field of constant magnitude that sweeps around the bore at the supply frequency.
Nothing in the stator moves. Change the frequency and the field speeds up or slows down; change the pole count and it sweeps proportionally slower for the same frequency.
Two families follow from here: rotors that lock to the rotating field and turn at exactly its speed — synchronous — and rotors that must lag behind it to work at all — asynchronous, which means induction.
2 · InductionThe squirrel cage
The rotor is a stack of laminations with conducting bars around its rim, shorted together by a ring at each end. Aluminum or copper, cast or fabricated. No magnets, no windings to connect, no brushes, nothing to fail.
The rotating field sweeps past those bars. A changing field through a closed loop drives a current — Faraday — and a current sitting in a magnetic field feels a force. The rotor is dragged around after the field.
Magnet billNone. This is the machine that electrified the twentieth century, and it contains no permanent magnet at all.
2 · InductionIt has to slip
If the rotor ever caught the field, the bars would see no changing flux, no current would flow, and there would be no torque. So it never catches. The difference is slip, typically a few percent at rated load.
Slip is not a defect; it is the operating principle. But it has a price: rotor current means rotor I²R heating, inside a spinning part that is hard to cool. Roughly speaking, the rotor loss is the slip fraction of the power crossing the airgap.
The torque–speed curve that results is the reason these motors start themselves across the line: torque is available from standstill, rises to a breakdown value, then falls away sharply near synchronous speed.
2 · InductionWhat it buys and what it costs
The stator must supply magnetizing current to build the field, because nothing in the rotor brings its own. That current does no work but still has to be carried, which is why induction machines run at a lagging power factor and why they get relatively worse at part load.
- No magnets, no rotor connections, no position sensor required
- Starts directly on a fixed-frequency supply; tolerates abuse, heat and overload
- Cheapest torque per dollar in the world, at scale
- Rotor losses inherent to the principle; efficiency ceiling is real
- Lagging power factor; poor part-load efficiency
- Lower torque density than a comparable magnet machine
Used in: pumps, fans, compressors, conveyors, machine tools — and still in some traction drives, including BMW's Gen6 front axle and Tesla's light-vehicle induction machines.
3 · Surface PMMagnets on the outside
Glue arc-shaped magnets to the rotor surface and the rotor brings its own field. There is no magnetizing current to supply and no rotor current to heat — the rotor runs cool, and the machine is more efficient than an induction motor of the same frame almost everywhere on the map.
Because the magnets carry the flux and their permeability is close to that of air, the rotor looks magnetically the same in every direction. That means no saliency, and therefore no reluctance torque: all of the torque is alignment torque.
Magnet billThe highest of any topology for a given torque — the magnets sit in the airgap where the field is strongest, and there is no second torque mechanism to share the load.
3 · Surface PMBLDC or PMSM? It is the drive, not the motor
The two names describe the same class of machine driven two different ways. If the winding is arranged and the magnets shaped so the back-EMF is roughly trapezoidal, the drive can switch current in six blocks per electrical cycle with only three coarse rotor position signals. That is what the market calls BLDC.
If the back-EMF is sinusoidal and the drive controls current continuously with a fine position signal, it is a PMSM under field-oriented control. Same magnets, same iron; more expensive inverter and sensor, less torque ripple, less noise, better high-speed behavior.
Used in: drones, e-bikes, pumps, fans, cordless tools, hard drives and appliance direct drives — this is by unit count the most common motor built today.
3 · Interior PMBury the magnets
Put the magnets inside the rotor — most often as a V under each pole — and three things change at once.
First, the magnets are held by steel rather than by adhesive and a retaining sleeve, so the rotor survives far higher tip speeds. Second, the steel between and around the magnets shields them from the worst of the stator's reverse field. Third, and most usefully, the rotor is now salient: the path along the pole axis is blocked by magnet material, while the path between poles is clear steel.
That saliency is free reluctance torque, sitting on top of the magnet torque.
3 · Interior PMTwo torques, added
The magnet term peaks at 90°; the reluctance term peaks at 135°. Add them and the total peaks somewhere in between, at a current angle the drive has to find and hold. That is what maximum torque per amp control is doing.
The practical consequence is a smaller magnet bill for the same torque. In a well-designed traction IPM, a meaningful share of the output — often a quarter to a third — is reluctance torque, which is to say torque you did not buy any neodymium to get.
Magnet billRoughly 1–2 kg of sintered NdFeB in a typical light-vehicle traction motor, on the order of 7–8 grams per kilowatt. A modern hybrid does the same job with far less: the Prius magnet fell from about 1.2 kg in 2004 to roughly half that by 2017.
3 · Interior PMField weakening and the speed range
A permanent magnet's flux does not switch off. As speed rises, back-EMF rises with it until it meets the inverter's voltage ceiling and the machine can go no faster — unless the drive spends current opposing the magnet.
That is field weakening, and saliency makes it cheap: in an IPM the same current that weakens the field also produces reluctance torque. The result is the wide constant-power speed range that traction applications need, from a standing start to motorway speed on one gear ratio.
- Highest torque and power density in production
- No rotor losses; best efficiency across a wide map
- Mechanically robust rotor; wide constant-power range
- Rare-earth content, with the price and policy exposure that carries
- Needs an inverter and accurate rotor position — never line-start
- Spins as a generator whenever it turns, powered or not
3 · The magnet's limitsWhere a PM machine actually fails
The failure mode that matters is demagnetization. A short circuit, a stalled overload or a badly timed fault puts a large reverse field across the magnet at the worst possible moment — when it is hot. Intrinsic coercivity falls as temperature rises, so the margin you designed at room temperature is not the margin you have at the top of the duty cycle.
That is why traction grades carry dysprosium or terbium, and why grain boundary diffusion matters commercially: it concentrates the heavy rare earth at the grain boundaries, where coercivity is actually set, instead of alloying it through the bulk. Same protection, far less heavy rare earth.
The second hazard is uncontrolled generation. A PM machine dragged above base speed with a failed inverter pushes current back into the DC link whether you want it or not. Magnet-free machines simply switch their field off.
Background: what coercivity and the knee of the curve mean in detail is covered in Reading a B‑H curve.
4 · Synchronous reluctanceSaliency without magnets
Strip the magnets out of an IPM and keep the barriers. What is left is a rotor of nested curved air slots that make flux easy along one axis and hard along the other — a pure synchronous reluctance machine, running on the sin 2δ term alone.
It has the induction motor's rotor cost and none of its rotor loss: no bars, no rotor current, no slip heating. Fed by a standard three-phase inverter and controlled like a synchronous machine, it reaches efficiency classes an induction motor cannot.
Magnet billNone. This is the main commercially proven magnet-free answer to efficiency regulation, sold up to several hundred kilowatts at IE5, with IE6 now being marketed.
4 · PM-assisted SynRMA little ferrite in the barriers
Pure reluctance machines have a weak power factor, because every bit of flux must be paid for with stator current. Drop cheap ferrite into the flux barriers, oriented to oppose the q-axis flux, and the power factor improves, torque rises, and the inverter gets smaller — for a magnet that costs a small fraction of neodymium and contains no rare earth at all.
It is an honest engineering compromise, and it shows exactly where the magnet-free approach runs out of physics: at least one major pump-motor vendor sells a "magnetless" synchronous reluctance line whose smallest frames are, in fact, built with magnets, because pure reluctance cannot make the efficiency class in a small frame.
Magnet billFerrite only — strontium or barium hexaferrite. Cheap, abundant, no rare earth, but roughly a tenth the energy product, so it takes volume.
4 · Switched reluctanceSimplest rotor ever built
Now remove the rotating field as well. A switched reluctance machine has salient teeth on both members — commonly six stator poles and four rotor poles — and concentrated coils wound on the stator teeth. Energize one pair, the nearest rotor pair swings into alignment, switch to the next pair before it gets there, repeat.
The rotor is a bare stack of laminations. No magnets, no conductors, no insulation, nothing to demagnetize and nothing to melt. It tolerates temperature and fault conditions better than anything else on this page.
The price is paid in every other column. Torque arrives in pulses as each pole pair aligns, so torque ripple and acoustic noise are intrinsic rather than incidental. The drive is not a standard three-phase inverter, and it needs precise position feedback to switch at the right instant.
4 · Switched reluctancePerpetually the motor of the future
SRM has been three years from displacing the induction motor for about forty years. The physics is genuinely attractive and the commercial record is not: the venture that raised the largest round on an SRM-replaces-the-induction-motor thesis now leads with axial flux motors and keeps switched reluctance for a single HVAC line.
The technology is not dead — Honda's venture arm invested in a switched reluctance developer in late 2025, and modern control and wide-bandgap switches attack exactly the ripple and noise objections. But it remains a bet on control electronics solving a problem that is built into the geometry.
Used in: some appliance and HVAC drives, mining and traction niches where robustness beats refinement, and a long list of promising demonstrators.
5 · Externally excitedMake the rotor field with current
If the problem is the magnet, replace it with a coil. A wound-field or externally excited synchronous machine puts DC windings on the rotor poles and supplies them through slip rings and brushes, or — increasingly — through a rotating transformer with no contact at all.
The field is now a control variable. Weaken it at high speed for free, strengthen it at low speed for peak torque, and switch it off entirely when the machine is coasting, which eliminates both drag losses and uncontrolled generation in a fault.
Magnet billNone. This is the topology OEMs have actually put into volume production to get rare earths out of the drivetrain — BMW has used it since its fifth-generation eDrive, and its Neue Klasse rear axle is the second generation of it.
5 · Externally excitedWhat you give up
- No magnets at all; field controllable and switchable
- Excellent high-speed and constant-power behavior
- Inherently safe when unpowered — no back-EMF
- Copper on the rotor means rotor I²R loss and a rotor cooling problem
- Heavier and larger than an IPM of the same output
- Excitation path adds parts: slip rings and brushes, or a rotary transformer and its own converter
Several suppliers have contactless-excitation machines at various stages of readiness, and at least one major European program is targeting a 2027 start of production. The honest summary is that magnet-free traction is shipping, but it is a hedge against supply risk rather than an engineering victory.
5 · Brushed DCThe mechanical inverter
Before power electronics, the switching had to be done mechanically. A brushed DC machine puts the windings on the rotor and the field on the stator, and lets a commutator — a split ring of copper segments with carbon brushes riding on it — reverse the current in each coil twice per revolution, so the rotor's field is always held at the angle that makes torque.
It is beautifully simple to drive: apply a DC voltage and it runs, with speed roughly proportional to voltage and torque to current. It is also the only machine here with a consumable part.
Magnet billFerrite arcs in almost every low-cost application. Neodymium appears only where size matters — premium cordless tools, some servo actuators.
5 · UniversalThe same machine on AC
Wire the field winding in series with the armature and something useful happens: reverse the supply and both the field and the armature current reverse, so the torque does not change sign. The machine runs on AC as happily as DC — hence universal — and it does it at speeds no line-frequency induction motor can reach, because nothing ties it to the supply frequency.
That is why vacuum cleaners, routers, blenders and circular saws sounded the way they did for a century. What killed it was not a better AC motor but the battery: once a tool runs from a DC pack, the universal motor's one advantage disappears, and a brushless machine with an electronic drive wins on efficiency, life and noise. High-speed brushless motors now run appliances at speeds a commutator could never survive.
Used in: corded power tools, older vacuums and small kitchen appliances — a shrinking installed base, displaced application by application as products go cordless.
6 · Axial fluxTurn the machine on its side
Everything so far sends flux radially across a cylindrical airgap. Rotate the geometry ninety degrees and the flux crosses axially, between flat discs — a pancake instead of a can.
Two things improve immediately. The airgap area now sits at the largest radius the machine has, rather than being spread along a cylinder, and torque scales with radius squared rather than radius. And the magnets sit on a flat disc face, which is a much better shape for getting flux across a short gap.
The usual arrangement is a stator sandwiched between two rotor discs, so the magnet faces work against both sides of the stator and the axial pull on the bearings cancels.
6 · Axial fluxPower density you can measure, manufacturing you can't fake
The payoff is real: axial machines reach torque and power densities radial machines struggle to match, in a package a few centimeters thick — which is exactly what you want bolted to a gearbox face, inside a wheel, or on an aircraft.
The difficulty is that almost nothing about building one is standard. The stator core cannot be a simple stack of punched laminations; the magnets are large flat blocks under enormous axial attraction during assembly; and the gap must be held flat across a spinning disc. When a major OEM started series production of axial flux motors in 2026, it described a process of roughly a hundred steps, most of them new to the company and many new to the industry.
Magnet billHigh, and rising with power density. Axial flux is a more magnet-intensive answer, not a way around the magnet — which is worth noting given that it is the topology that actually reached a production line while several magnet-free programs slipped.
7 · SteppersA motor that counts
A hybrid stepper is a permanent magnet machine with fine teeth cut into both rotor and stator. The rotor is a magnet sandwiched between two toothed cups, offset by half a tooth, so energizing the phases in sequence advances the rotor one tooth pitch at a time — typically 200 discrete steps per revolution, and far more with microstepping.
Its virtue is that it holds position with no feedback at all. Count the pulses and you know where the shaft is, which is why steppers ran printers, plotters, machine tools and every desktop 3D printer ever built.
Its vice is the same thing: with no feedback, a missed step is silent. And it draws full current standing still, so it is least efficient exactly where it spends most of its life.
Also in this family: shaded-pole induction motors, where a copper ring on part of each pole delays the flux enough to make a crude rotating field — cheap, inefficient, and still in millions of fans; and hysteresis motors, which make torque from the lag of the rotor material's own B‑H loop.
8 · The mapTorque density against magnet content
Plot the families against what they cost in rare earth and what they deliver per kilogram, and the trade is clear. There is no topology in the upper left — nothing delivers magnet-machine density without magnets, and nothing ever has.
The magnet-free options are all real and all shipping. They are also all bigger, heavier, or more complicated to drive than the machine they replace. That is the trade being made, and it is being made for supply-chain reasons rather than engineering ones.
8 · What decides itThe choice is rarely about torque
Motor systems consume something over half of the world's electricity, so a percentage point of efficiency is a national-scale quantity, and efficiency regulation — not performance — is what has been reshaping the industrial end of this market. That pressure is what made synchronous reluctance a product rather than a paper.
At the traction end, permanent magnet machines still account for the overwhelming majority of light-vehicle motors, and the forecasts expect them to stay dominant with less rare earth in each one rather than be displaced. The magnet-free programs are real, funded and now in series production — but they are insurance.
The thing to price is not the magnet. It is the combination of export licensing on heavy rare earths and the floor price now underwriting Western neodymium capacity: a policy-set cost, not a market one, applied to a component that is a few percent of the motor's mass and a large share of its risk.