Neodymium Magnet

Overview

The neodymium magnet (NdFeB, neodymium-iron-boron) is the most powerful type of permanent magnet in commercial production, and it is the component that gives the Traction Motor (PMSM) its high torque density. Buried inside the Rotor Assembly, an array of these magnets creates the fixed rotor field that the stator drags around. The strength of those magnets is, more than almost anything else, what lets a compact motor produce 310 N·m of torque.

A neodymium magnet stores energy in the alignment of its internal magnetic domains. Its key figure of merit, the maximum energy product (BH)max, reaches around 400 kJ/m³, several times that of older ferrite or alnico magnets, which is why a small volume of NdFeB can replace a much larger and heavier alternative. A few kilograms of these magnets across the rotor's eight poles is enough to set up the field that lets the motor produce hundreds of newton-metres, making them by far the most magnetically dense part of the whole drivetrain.

Construction / how it's built

NdFeB magnets are powder-metallurgy products, not cast metal:

• The alloy is melted, then rapidly solidified (strip-casting) into thin flakes.
• The flakes are hydrogen-embrittled and milled into a fine powder of a few microns.
• The powder is aligned in a magnetic field and pressed into a block, locking in a preferred magnetic direction (this is what makes it anisotropic and so strong in one axis).
• The pressed block is sintered at around 1,080 °C to fuse the particles, then annealed.
• It is machined to final shape, the magnets for a rotor are usually flat rectangular bars sized to slot into the Rotor Core pockets.
• Finally it is coated (nickel-copper-nickel plating or epoxy) because raw NdFeB corrodes readily, and is magnetised to saturation.

The boron and the specific microstructure are what give the material its enormous coercivity; the neodymium provides the high magnetisation.

The microstructure is the secret of the material. NdFeB is not magnetic because of one big crystal but because of millions of tiny Nd₂Fe₁₄B grains, each a few microns across, isolated from its neighbours by a thin neodymium-rich grain-boundary phase. That isolation stops a reversal in one grain from cascading into its neighbours, which is what gives sintered NdFeB its high coercivity. Anything that coarsens the grains or thins the boundary phase, including running too hot, lowers coercivity, which is exactly the failure mode a motor designer must guard against.

Key specifications explained

• Remanence (Br ≈ 1.3 T) is how much magnetic flux the magnet retains on its own, directly proportional to the torque the motor can make per amp.
• Coercivity (Hcj) is its resistance to being demagnetised by an opposing field. This matters enormously in a motor, because during hard field-weakening the Stator Assembly pushes a strong opposing field at the magnets. If coercivity is too low at temperature, the magnet partially demagnetises and the motor permanently loses torque.
• Temperature ratings. Plain NdFeB loses coercivity as it heats. To survive the 180 °C+ found in a hard-working rotor, manufacturers add heavy rare earths, dysprosium (Dy) and terbium (Tb), which raise high-temperature coercivity. Grades are coded by temperature: N (80 °C) up through UH and EH (180–200 °C). EV traction magnets are typically UH/EH grades.
• Curie temperature (~320 °C) is where the magnet loses its magnetism entirely; the operating limit sits far below this.

Manufacturing & assembly

The strip-cast → mill → align → press → sinter → machine → coat → magnetise sequence is energy- and capital-intensive, and the supply chain is heavily concentrated, China refines the large majority of the world's neodymium and nearly all of the dysprosium and terbium. In the motor plant the magnets are inserted into the Rotor Core pockets, bonded with high-temperature adhesive, and frequently magnetised in situ after assembly so that fully charged (and dangerously attractive) magnets need not be handled. Each magnet's field strength is verified, because a weak or cracked magnet shows up as torque ripple or imbalance in the finished Traction Motor (PMSM).

Two refinements have become standard for traction magnets. The first is grain-boundary diffusion (GBD), in which dysprosium or terbium is painted onto the magnet surface and diffused inward at temperature so the heavy rare earth concentrates only in the grain boundaries where it raises coercivity, instead of being mixed uniformly through the bulk. This uses a fraction of the costly heavy rare earth for the same temperature rating. The second is segmentation: each pole is built from several thin magnet bars rather than one solid block, which breaks up the eddy-current loops that would otherwise circulate inside the conductive magnet and heat it from within at high motor speed. Cutting and coating more, smaller segments adds cost but keeps the magnets cooler and protects them from demagnetisation. Magnets are also handled with care because raw NdFeB is brittle and chips easily, and its dust is flammable.

Role / where it fits

In the assembly tree the magnets are leaf components of the Rotor Assembly; eight magnetic poles, often two or more magnet bars each, make up the rotor's field. Their cost and supply risk dominate discussions about EV motor design, which is exactly why magnet-free alternatives keep being explored.

Variants & alternatives

Within NdFeB, makers trade energy product against temperature and cost by varying the heavy-rare-earth content and using grain-boundary diffusion to concentrate Dy/Tb only at the grain surfaces (where it does the most good for the least material). Cheaper alternatives include ferrite (ceramic) magnets, far weaker but cheap and rare-earth-free, and samarium-cobalt (SmCo), which tolerates higher temperatures but is brittle and costly. The push to avoid these magnets entirely drives interest in induction and wound-rotor synchronous motors, neither of which uses any Neodymium Magnet at all. A separate class of bonded NdFeB magnets, where powder is mixed with a polymer and moulded, allows complex net shapes and small parts but reaches only a fraction of the sintered material's strength, so it serves sensors and small motors rather than traction. For now, sintered NdFeB remains the standard because nothing else matches its torque-per-kilogram in a passenger-car package. '.trim()