Permanent magnet tolerances
Permanent magnets do not only vary in magnetic moment and magnetization direction, their properties may also differ considerably depending on the size and shape of the actual magnet.
Permanent magnets are never one like the other regarding their magnetic properties. The Magnetic Materials Producers Association has put together standard specifications for permanent magnet materials and permanent magnet guidelines on this topic. According to these publications, individual manufacturers can generally hold magnetic property tolerances of ±5% for residual flux density, ±8% for coercive force, and ±10% for the energy product of a permanent magnet. In addition to these tolerances, the magnetization direction may also vary in the range of ±5°.
Based on the above mentioned permanent magnet tolerances, two magnets next to each other can in the worst case be 10% weaker/stronger with a difference in magnetization direction of up to 10°. This can seriously deteriorate the performance of an electrical machine as compared to the simulation results with perfect magnet properties. Unfortunately, The permanent magnet tolerances may cause effects such as increased torque ripple, unbalanced forces, bearing currents, circulating currents, etc.
For manufacturing considerations, it becomes increasingly important to measure the magnetic moment and magnetization direction of every permanent magnet during assembly. Up to now, this was just done for prototype and research applications, since the required Helmholtz-coil setup cannot be easily integrated into the process lines and has a very slow and tricky handling.
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