Permanent Magnet Motor: Specific Symptoms After Demagnetization

After a permanent magnet motor loses magnetism (including partial and full demagnetization), its core characteristic is a sharp drop in electromagnetic conversion efficiency, triggering a chain of performance and operational abnormalities.

In terms of output performance, load capacity drops sharply; input current increases significantly under the same load, and the motor easily loses synchronism or stalls under heavy load. The starting current can reach 2–3 times the rated value yet still fails to start properly. In cases of partial demagnetization, torque ripple becomes severe, making smooth operation impossible.

Electrical signals show typical anomalies: three‑phase current imbalance exceeds 10%, no‑load current is far higher than normal (even close to rated current), copper loss and heating increase sharply. Back EMF amplitude drops by more than 15%, with waveform clipping and glitches. Power factor falls below 0.8 (normally ≥0.9), and energy consumption rises by at least 8%–15, resulting in low efficiency and high power consumption.

Operational distortion is obvious: temperature rises much faster, stator winding temperature is more than 30℃ higher than normal. For neodymium‑iron‑boron permanent magnet motors, the operating area easily exceeds the critical temperature of 150℃, creating a vicious cycle: demagnetization → higher temperature → worse demagnetization. Magnetic field distortion often causes high‑frequency vibration at twice or six times the power frequency. In special applications, such as electric vehicle drive motors, demagnetization causes weak acceleration and greatly reduced range; for servo motors, positioning accuracy drops, response lags, and command tracking errors exceed limits.

Indirect failure risks cannot be ignored: long‑term overcurrent accelerates stator winding insulation aging and may cause turn‑to‑turn short circuits. Insufficient torque at high speed can lead to speed runaway, posing safety risks without mechanical braking. If untreated, partial demagnetization will gradually spread to the entire rotor, eventually causing the motor to lose driving capability. Repairs require replacing permanent magnets or the complete rotor assembly, at high cost.