The Whole Argument, Compressed
Ferromagnetism is electrostatics wearing a disguise, a permanent magnet needs two properties no element supplies together, and the binding constraint at the end of it all is not physics.
Magnets and Motors · Part 5 — Reference · Chapter 12 · 11 min read
Part 5 — Reference
The whole argument, compressed
Seven claims, one table, and the two other series this one sits alongside.
A reference chapter rather than an argument. Everything this series rests on, stated once.
7
Claims
8
Rows in the datasheet table
18
Figures across the series
3
Companion series
12.1 — Seven claims
12.1.1 — Ferromagnetism is electrostatics wearing a disguise
The exchange interaction — the Pauli principle applied to Coulomb repulsion — holds moments parallel a thousand times more strongly than magnetic forces could, and only iron, cobalt and nickel sit in the narrow window where it works at room temperature.
12.1.2 — A permanent magnet needs two properties no element supplies together
High Curie temperature and large magnetisation come from 3d exchange; high anisotropy comes from the aspherical, spin–orbit-coupled 4f shell. Nd₂Fe₁₄B is a division of labour: iron holds the order, neodymium holds the direction, boron holds the structure.
12.1.3 — Coercivity is microstructure, not chemistry
Every real magnet reaches only a fraction of its theoretical anisotropy field, because reversal nucleates at grain surfaces. That is why grain boundary diffusion works, and why it cut heavy rare earth consumption by most of itself without a new material.
12.1.4 — A motor makes torque two ways
A well-designed traction rotor gets a third to a half of its output from geometry alone — reluctance torque, available with no magnet involved, which is why the magnets are buried rather than surface-mounted.
12.1.5 — Torque is a surface shear stress
So it scales as D²L, and power is torque times speed. That is the whole reason traction motors are small, fast and geared rather than large, slow and direct.
12.1.6 — The magnets set the thermal limit
Not the insulation — 80 °C for a plain N42 against 180 °C for Class H copper. And they fail on a combination rather than a single cause: hot and reverse-driven at the same instant, crossing a knee that moves five times faster with temperature than the strength does.
12.1.7 — And the binding constraint is not physics
Neodymium is more abundant than lead. What is scarce is the separation capacity to tell fifteen chemically near-identical elements apart — a consequence of the lanthanide contraction, and a plant problem rather than a geology one.
12.2 — The datasheet table
Representative standard-condition literature values. Verify against the specific magnet datasheet in use.
| Family | (BH)max kJ/m³ | B_r T | H_cJ kA/m | T_C °C | Max op °C | α(B_r) %/°C | β(H_cJ) %/°C |
|---|---|---|---|---|---|---|---|
| NdFeB N52 | 398–422 | 1.43–1.48 | ≥876 | 312 | 80 | −0.12 | −0.60 |
| NdFeB 42SH | 318–342 | 1.28–1.32 | ≥1,592 | 340 | 150 | −0.11 | −0.55 |
| NdFeB 35UH / 33EH | 247–287 | 1.13–1.21 | ≥2,388 | 350 | 180–200 | −0.10 | −0.50 |
| NdFeB bonded | 40–80 | 0.55–0.70 | 600–800 | 312 | 120 | −0.13 | −0.40 |
| Sm₂Co₁₇ | 200–240 | 1.05–1.12 | 600–2,000 | 920 | 300–350 | −0.030 | −0.20 |
| SmCo₅ | 120–180 | 0.85–1.00 | 1,300–2,400 | 750 | 250 | −0.045 | −0.30 |
| Sr ferrite | 26–40 | 0.38–0.45 | 200–350 | 450 | 250 | −0.20 | +0.40 |
| AlNiCo 5 | 40–52 | 1.25–1.35 | 50–60 | 860 | 500 | −0.02 | −0.02 |
12.3 — Where to go next
Important
This series covers one half of the powertrain. Its companions cover the other two: The Periodic Table of the EV on why a cell is made of what it is made of, and Ore to Vehicle on where all of these materials actually come from — including the rare earth separation cascade that chapter 10 stops at the edge of.
Quick check: test yourself
1.What is the one-sentence version of why Nd₂Fe₁₄B works?
Show answer
2.Which single number best predicts whether a magnet survives in a motor?
Show answer
Chapter summary
- ✓Ferromagnetism is the Pauli principle applied to Coulomb repulsion, and only three elements sit in the window where it works at room temperature.
- ✓A permanent magnet needs 3d exchange and 4f anisotropy together, which is why every good one is a compound.
- ✓Coercivity is set by microstructure, which is why grain boundary diffusion cut heavy rare earth use without a new material.
- ✓Torque comes from magnets and from geometry, scales as D²L, and is why traction motors are small, fast and geared.
- ✓The magnets set the thermal limit and fail on a combination of heat and reverse field — and the supply constraint at the end of it all is a separation plant, not a deposit.
Frequently asked questions
What is the one-sentence version of why Nd₂Fe₁₄B works?+
It is a division of labour. High Curie temperature and large magnetisation come from 3d exchange, high anisotropy comes from the aspherical spin–orbit-coupled 4f shell, and no element supplies both — so iron holds the order, neodymium holds the direction, and boron holds the structure that lets them coexist.
Which single number best predicts whether a magnet will survive in a motor?+
Intrinsic coercivity at the maximum operating temperature, read against the load line the geometry imposes. Remanence tells you how strong the magnet is; H_cJ at temperature tells you whether it will still be a magnet after the worst fault the drive can produce. It falls roughly five times faster with temperature than remanence does, which is why the knee, not the strength, is what kills magnets.
What limits a traction motor’s continuous rating?+
Heat removal, and specifically heat removal from the magnets. Class H insulation tolerates 180 °C while a plain N42 magnet is finished at 80 °C, so in a permanent-magnet machine the magnets set the limit. That is why hollow-shaft and direct-oil cooling exist, and why grade selection is settled by the thermal model rather than the catalogue.
Reviewed by
Magnets and Motors is an original educational series on permanent magnets and electric machines. Datasheet ranges, temperature coefficients and efficiency figures are representative standard-condition literature values, not measurements of a specific product, and sources differ on several. Always verify against the specific magnet and lamination datasheets in use before making design, procurement or certification decisions.