Why do the magnets inside an electric motor get hot? The short answer: eddy currents. Sintered magnets are made of metal, and metal conducts electricity. When the magnetic field around a conductor keeps changing — which happens constantly inside a spinning motor — it pushes electricity to swirl around inside the material. Those swirling currents are eddy currents, and their resistance turns them into heat.
Think of water eddies
"Eddy" means swirl — the same word as the little whirlpools in a river. The currents loop inside the magnet like whirlpools, and the metal's resistance fights the flow, producing heat like a heating element. The faster the field changes, the stronger the currents: a motor driven by a modern inverter, switching thousands of times per second, heats its magnets far more than an old line-frequency motor ever did.
Why it matters
Heat is the enemy of neodymium magnets. They lose strength as they warm up, and if a magnet gets hot enough it demagnetizes permanently — the motor quietly loses power and never gets it back. So eddy heating is not just wasted energy; it is a reliability problem. See temperature and demagnetization.
How designers fight it
- Slice the magnet. Cut one big magnet into several insulated segments — traction-motor magnets are typically split into 3–5 pieces — breaking the current loops so the eddies cannot form.
- Block or divert the currents with insulating layers or conductive shields around the magnet.
- Use a poorer conductor. Bonded magnets — magnetic powder mixed with plastic — resist current a hundred to ten thousand times better, at the cost of magnetic strength.
Same physics, all metals
SmCo magnets are metals too, so they suffer the same eddy heating despite their excellent temperature rating. There is no way around the physics — only around the design.
Read on: magnets in motors → · resistivity and why it matters →