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The effect of surface grain reversal on the AC losses of sintered Nd–Fe–B permanent magnets

Moore, Martina and Roth, Stefan and Gebert, Annett and Schultz, Ludwig and Gutfleisch, Oliver (2015):
The effect of surface grain reversal on the AC losses of sintered Nd–Fe–B permanent magnets.
In: Journal of Magnetism and Magnetic Materials, Elsevier Science Publishing, pp. 43-48, 375, ISSN 03048853, [Online-Edition: http://dx.doi.org/10.1016/j.jmmm.2014.09.047],
[Article]

Abstract

Sintered Nd–Fe–B magnets are exposed to AC magnetic fields in many applications, e.g. in permanent magnet electric motors. We have measured the AC losses of sintered Nd–Fe–B magnets in a closed circuit arrangement using AC fields with root mean square-values up to 80 mT (peak amplitude 113 mT) over the frequency range 50 to 1000 Hz. Two magnet grades with different dysprosium content were investigated. Around the remanence point the low grade material (1.7 wt% Dy) showed significant hysteresis losses; whereas the losses in the high grade material (8.9 wt% Dy) were dominated by classical eddy currents. Kerr microscopy images revealed that the hysteresis losses measured for the low grade magnet can be mainly ascribed to grains at the sample surface with multiple domains. This was further confirmed when the high grade material was subsequently exposed to DC and AC magnetic fields. Here a larger number of surface grains with multiple domains are also present once the step in the demagnetization curve attributed to the surface grain reversal is reached and a rise in the measured hysteresis losses is evident. If in the low grade material the operating point is slightly offset from the remanence point, such that zero field is not bypassed, its AC losses can also be fairly well described with classical eddy current theory.

Item Type: Article
Erschienen: 2015
Creators: Moore, Martina and Roth, Stefan and Gebert, Annett and Schultz, Ludwig and Gutfleisch, Oliver
Title: The effect of surface grain reversal on the AC losses of sintered Nd–Fe–B permanent magnets
Language: English
Abstract:

Sintered Nd–Fe–B magnets are exposed to AC magnetic fields in many applications, e.g. in permanent magnet electric motors. We have measured the AC losses of sintered Nd–Fe–B magnets in a closed circuit arrangement using AC fields with root mean square-values up to 80 mT (peak amplitude 113 mT) over the frequency range 50 to 1000 Hz. Two magnet grades with different dysprosium content were investigated. Around the remanence point the low grade material (1.7 wt% Dy) showed significant hysteresis losses; whereas the losses in the high grade material (8.9 wt% Dy) were dominated by classical eddy currents. Kerr microscopy images revealed that the hysteresis losses measured for the low grade magnet can be mainly ascribed to grains at the sample surface with multiple domains. This was further confirmed when the high grade material was subsequently exposed to DC and AC magnetic fields. Here a larger number of surface grains with multiple domains are also present once the step in the demagnetization curve attributed to the surface grain reversal is reached and a rise in the measured hysteresis losses is evident. If in the low grade material the operating point is slightly offset from the remanence point, such that zero field is not bypassed, its AC losses can also be fairly well described with classical eddy current theory.

Journal or Publication Title: Journal of Magnetism and Magnetic Materials
Volume: 375
Publisher: Elsevier Science Publishing
Uncontrolled Keywords: NdFeB, Loss, Electric motor, Eddy current
Divisions: 11 Department of Materials and Earth Sciences > Material Science > Functional Materials
11 Department of Materials and Earth Sciences > Material Science
11 Department of Materials and Earth Sciences
Date Deposited: 17 Nov 2014 10:00
Official URL: http://dx.doi.org/10.1016/j.jmmm.2014.09.047
Identification Number: doi:10.1016/j.jmmm.2014.09.047
Funders: The work was funded by the ‘Forschungsvereinigung Antriebstechnik e.V. (FVA)’, Frankfurt/Main, Germany.
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