Permanent Magnet Motor Demagnetization Testing: High-Current Fault and Thermal Demagnetization Validation

Why Demagnetization Testing Is Non-Negotiable for PM Machines

Neodymium-iron-boron (NdFeB) magnets power the most energy-dense electric motors on the market—but they demagnetize. Every permanent magnet has a coercive field (Hc) below which the magnetisation is stable; exceed it with a sufficiently large opposing field and the magnet partially or fully loses its remnant flux. In traction motors, this threshold can be crossed during a three-phase short-circuit fault, a severe overcurrent event, or prolonged operation at elevated magnet temperature. Once demagnetized, the motor produces less torque per amp, runs hotter, and may never recover without being re-magnetized at the factory.

Validation testing must therefore demonstrate two things: first, that the motor survives the fault events defined in its specification without irreversible demagnetization; second, that it operates within the safe-operating region across the full temperature range it will see in service. This guide explains how demagnetization tests are set up and what the results mean for design decisions.

Physics Background: The Demagnetization Curve

The relevant material characteristic is the B-H demagnetization curve at the operating temperature. NdFeB grades are defined by their remnant flux density Br (typically 1.0–1.4 T at 20°C) and their intrinsic coercivity Hci. Both fall with increasing temperature—Br by about −0.11%/°C and Hci by −0.5 to −0.7%/°C for standard grades. Consequently, a magnet that comfortably withstands the peak d-axis current at 25°C may partially demagnetize at 120°C under the same current. High-coercivity grades (e.g., Grade N42SH or N42UH) push the temperature threshold higher but at higher cost and with slightly reduced flux density.

The motor designer computes the load line—the magnetic field seen by the magnet due to the winding’s magnetomotive force—and verifies that it never crosses the knee of the demagnetization curve. Test validation then confirms these calculations experimentally.

Core Test Items

1. Pre-Test Back-EMF Baseline

Before any fault injection, the test bench drives the motor at a defined speed (typically rated speed or 1,000 rpm) under no-load conditions and records the line-to-line back-EMF waveform at multiple temperature setpoints: 25°C, magnet operating temperature (80–120°C), and the maximum specified temperature. The fundamental RMS value of each phase and the harmonic spectrum are stored as the reference baseline. Demagnetization will reduce the fundamental amplitude; partial or asymmetric demagnetization will also introduce additional harmonic content.

Key metrics: back-EMF constant kE at each temperature; harmonic distortion THD < 3% for a well-designed machine.

2. Short-Circuit Pulse Injection

The most critical event for demagnetization is a sudden three-phase short circuit at the motor terminals. The bench injects a controlled short-circuit pulse while the motor rotates at rated or maximum speed, applying the worst-case demagnetising current (which peaks immediately after fault onset and then decays with the d-axis time constant). The duration is controlled to match the inverter fault-clearing time, typically 1–10 ms. The test is repeated at the maximum specified magnet temperature (using a thermal chamber or embedded resistance heaters in the winding to pre-soak the magnets). After each pulse, the motor cools to 25°C and the back-EMF is remeasured and compared to baseline.

Pass criterion: back-EMF fundamental reduction < 2% (or per OEM specification) after any single fault event at the specified temperature.

3. High-Temperature (Thermal) Demagnetization Test

In this test, the motor is soaked at the maximum rated magnet temperature (often 120–160°C depending on grade) with the worst-case demagnetising current applied continuously. The combination of elevated temperature—which reduces Hci—and the opposing field from peak d-axis current represents the most severe sustained operating condition. Temperature is measured with embedded thermocouples (K-type) at multiple magnet positions. The test is held for the duration corresponding to maximum sustained overload or the thermal time constant of the magnet, then the motor cools and back-EMF is remeasured.

4. Asymmetric Demagnetization Detection

Partial demagnetization of individual magnet segments generates even harmonics in the back-EMF that are absent in a healthy motor. The FFT of the three back-EMF waveforms is compared before and after fault injection; the emergence of 2nd and 4th harmonic content above the noise floor indicates localised magnet damage. Asymmetric demagnetization is particularly important for multi-pole machines with segment magnets because a single damaged segment creates rotor imbalance and can excite bearing frequencies.

Key metrics: 2nd harmonic amplitude < 0.3% of fundamental; asymmetry between phases < 1%.

5. Demagnetization Under Repeated Fault Events

The fault injection is repeated N times (where N is defined by the functional safety analysis or field reliability model) to assess cumulative degradation. A motor that survives a single fault may still show progressive demagnetization after 10 or 20 events. The test bench automates the pulse sequence, monitors back-EMF after each event, and plots the degradation trajectory. A motor whose back-EMF stabilises after 2–3 faults has a saturable degradation mechanism; one that continues to decline requires design revision.

6. Re-magnetization Verification

If the motor fails the demagnetization criterion, the magnets are re-magnetized using a pulse magnetizer and the motor is re-tested. This confirms that the demagnetization was not caused by a manufacturing defect (cracked magnet, wrong grade, incorrect magnetization direction) but by the operating condition exceeding design margins.

Test Bench Requirements

The demagnetization test bench requires a programmable AC or DC power source capable of sourcing the peak short-circuit current (typically 3–10× rated current for 1–10 ms), a high-bandwidth current measurement system (bandwidth > 10 kHz to capture the fault current peak accurately), and a thermal chamber or liquid coolant conditioning system to hold magnet temperature within ±2°C during the test. The back-EMF measurement system must resolve voltage amplitudes to 0.1% of the full-scale value across a wide frequency range (50 Hz to 2 kHz for high-pole-count machines).

What This Means for Test Bench Selection

Demagnetization testing sits at the boundary between electrical machine characterisation and safety validation. The test infrastructure must integrate power electronics capable of controlled fault injection with precision metrology for back-EMF measurement. If your motor programme uses high-performance NdFeB or ferrite magnets and operates at elevated temperature or under high d-axis current for field weakening, demagnetization testing is not optional—it is a required gate in the design verification plan.

We support demagnetization test programmes from small servo motors to large traction motors rated at hundreds of kilowatts. To discuss test configurations for your specific magnet grade and operating envelope, talk to our engineering team.