Laminated Magnets

Eddy current is one of the biggest difficulties in the motor industry. Eddy current loss will result in the temperature of permanent magnets and caused demagnetization, then affected the working efficiency of the motor. Generally, stator magnetic field and rotor magnetic field of PMSM are rotating synchronously or relatively static, and permanent magnets without eddy current loss in such case. But in fact, there are a series of space and time harmonics are existing in the air gap magnetic field, and these harmonic components are stemming from cogging effect, non-sinusoidal distribution of magnetomotive force and phase current. The harmonic magnetic field will link with rotor magnetic field and hence generated eddy current and caused relevant eddy current loss. It should be also pointed out that the harmonic magnetic field and eddy current loss will rise with increasing motor speed. Laminated magnets are regarded as a wise solution to solve the eddy current loss.

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Magnet manufacturers can provide high resistivity magnets to improve eddy current loss performance, include bonded magnets and high resistivity sintered magnets. The electrical resistance of bonded magnets could be 102 to 104 times more than conventional sintered magnets due to the existence of binder, but the motor power and the maximum operating temperature are also limited. For sintered magnet, submicron fluoride can be added to the alloy, then fluoride will help magnet to form high resistivity layer on its grain boundary position and thus improve the electrical resistivity of the whole magnet. Laminated magnets which made by magnet segmentation technology are another very practical way to make high-resistivity magnets without changing composition of magnet, structure, and performance of the motor. Laminated magnets are made by bonding several pieced of magnets together via specific isolation adhesive, and it has been already applied to the high-speed rotating machinery. Either laminated SmCo magnets or laminated Neodymium magnets are available.

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