Electric motors with no rare earths
Carmakers are turning to electrically excited synchronous motors (EESMs) that replace rare‑earth permanent magnets with controllable electromagnets, trading a few percentage points of peak efficiency and added rotor complexity for supply‑chain resilience and lower materials risk. Commenters compare these motors with permanent‑magnet and induction designs in terms of efficiency at highway speeds, power density, cooling, maintenance of brushes/slip rings, and the feasibility of brushless excitation. The shift is framed within a broader push in Europe and India to decouple EV powertrains from Chinese rare‑earth and battery dependencies, even as North America and China largely stick with permanent‑magnet motors for now.
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.