Electric motors with no rare earths

Original Article ↗ Hacker News Discussion ↗

Motor Type & Basic Idea

  • Renault’s “magnet‑free” motor is an electrically excited synchronous machine (EESM / wound‑field synchronous motor).
  • It replaces rare‑earth permanent magnets in the rotor with an electromagnet energized via slip rings and carbon brushes (similar to an alternator).
  • Some confusion in the thread about DC vs AC; clarifications say these are electronically commutated AC machines with a controlled stator field.

Efficiency, Performance, and Trade‑offs

  • Excitation power for the rotor is reported as a small fraction of total power.
  • Claimed motor efficiency is ~92%, compared to >95% for top permanent‑magnet synchronous motors (PMSMs); several commenters stress this is a non‑trivial increase in losses.
  • Downsides: heavier, less mechanically robust rotor, more complex cooling, lower power density, and lower achievable maximum speed than PMSMs.
  • Upsides:
    • No dependence on rare‑earth magnets or their temperature‑dependent demagnetization.
    • Excellent field‑weakening and potentially higher efficiency at high speed / moderate torque (e.g., highway cruising).
    • Ability to fully de‑energize rotor for “idle” operation with minimal drag and some safety advantages.

Brushes vs Brushless & Maintenance

  • Current Renault/BMW‑style EESMs use brushes and slip rings; future “brushless excitation” via inductive transformers is mentioned but appears not in mass production yet (unclear/contested).
  • Brushes are described as wear items but cheap and long‑lived (order of 100k–250k miles), with some likening replacement to cabin‑filter level maintenance.
  • Others worry about access complexity (e.g., subframe drops on some cars) and note that PMSMs/induction motors avoid this failure mode entirely.

Comparisons to Other Motor Topologies

  • Induction motors and synchronous machines without magnets are historically common, especially at large scale.
  • Induction motors are simpler and robust but less efficient; some EVs use them as auxiliary/front motors that can be fully shut off to avoid drag.
  • EESMs sit between PMSMs (most efficient, rare‑earth dependent) and induction motors (no magnets, but less efficient) in the efficiency/complexity trade space.

Supply Chain, Policy, and Batteries

  • Strong theme: EESMs as part of an “ex‑China” strategy due to Chinese dominance in rare‑earths and related export controls.
  • European and Indian supply chains for EESMs and EV powertrains are discussed; there is disagreement over how much is truly “ex‑China” and how significant Indian sourcing is for EU EVs.
  • Pairing with Chinese sodium‑ion batteries is viewed as unlikely for European OEMs given trade and export‑control dynamics; instead, Japanese/Korean solid‑state batteries are mentioned.
  • Sodium‑ion batteries are discussed more broadly: lower energy density but cheap materials and good cold‑temperature performance; seen as promising for stationary storage and some low‑cost EVs, with early deployments in China.