GB/T 40794-2021Measurement method of irreversible magnetic flux loss due to high temperature of rare earth permanent magnet (English PDF)

3.1 Irreversible magnetic flux loss due to high temperature

The part where the magnetic flux cannot be restored when the magnet is heated from normal temperature to high temperature and then returned to normal temperature.

NOTE: It is measured in percentage (%).

3.2 Load line

The locus of the operating point of a permanent magnet forming part of a given magnetic circuit when the magnitude of its magnetization varies.

NOTE: The operating point of a permanent magnet is a point on the demagnetization curve or return line of a permanent magnet forming part of a given magnetic circuit. Its coordinates are the working magnetic flux density and magnetic field strength. [Source: GB/T 9637-2001, 221-04-11]

3.3 High temperature aging

The test that the permanent magnet is kept at a certain temperature in the high temperature test box for a period of time, and the high temperature environment is simulated to test the thermal stability.

8.1 Initial flux test

Place the sample at room temperature for no less than 2 h after magnetization and saturation. Make sure that the specimen temperature is consistent with room temperature. The sample is placed in the homogeneous space of the Helmholtz coil. The easy magnetization direction of the sample is parallel to the axis of the Helmholtz coil.

Then measure the magnetic flux according to the provisions of GB/T 38437. Repeatedly measure each sample for 3 times. Take the average value of the displayed values of magnetic flux (magnetic dipole moment) as the measured value before the test.

8.2 High temperature aging test method and sample placement

8.2.1 Open-circuit mode

The test sample needs to be placed in a non-ferromagnetic sample frame. During the test, avoid the sample close to the magnetic material. In order to avoid the mutual adsorption of the samples during the test operation, causing damage to the samples and injuring the test personnel, it is necessary to customize a special sample frame or a fixing scheme with the same effect. If the aluminum alloy plate is fixed with high-temperature-resistant adhesive tape, the distance between the specimens shall not be less than 8 times the maximum side length of the specimen.

8.2.2 Semi-open-circuit mode

8.2.2.1 The sample is absorbed on a pure iron plate with a thickness of 1.0mm±0.1mm.

The easy magnetization direction of the specimen shall be perpendicular to the surface of the iron plate. The distance between specimens shall not be less than 4 times the side length of the same direction of the specimen. The direction of the N poles of the samples shall be the same.

8.2.2.2 The inner arc of the tile-shaped magnet is adsorbed on the iron plate downward.

For samples that are inconvenient to be placed by magnetic pole surface adsorption or samples with special shapes, place them on the principle of safety and convenience. Indicate in the test report.

8.3 Determination of high temperature aging heating and heat preservation

A.2 Variation law of operating point of rare earth permanent magnet and position of inflection point of B-H curve

A.2.1 The reason for the irreversible loss of magnetic flux is the relative change of the operating point of the permanent magnet relative to the position of the inflection point of the B-H curve of the magnetic material. Since the temperature coefficient of remanence and intrinsic coercive force of rare earth permanent magnets above room temperature is negative, and the absolute value of the temperature coefficient of coercive force H is larger than the absolute value of the temperature coefficient of magnetic flux density B, the higher the temperature, the higher the corresponding B-H curve inflection point. See Figure A.2.

Example 1: In Figure A.2, when the temperature rises from 100°C to 120°C, the inflection point of the B-H curve rises from point C1 to point C3.

A.2.2 In a static magnetic circuit, the permanent magnet operating load line is a straight line. The operating point of the permanent magnet goes down along the load line with increasing temperature.

Example 2: In Figure A.2, the 40SH phi10.055mm×2.017mm sintered NdFeB magnet is in the open-circuit state. When the temperature rises from 100°C to 120°C, its operating point moves downward from point A1 to point A3. That is, move down from above the inflection point C1 of the B-H curve to below the inflection point C3 of the B-H curve. […]