Laminated Magnets

Eddy current is one of the biggest difficulties in the motor industry which will give rise to the temperature of permanent magnets and caused demagnetization, then affected the working efficiency of the motor. Laminated magnet is regarded as wise solution to solve the eddy current loss in the surge of high-speed rotating machinery development.

▲ Eddy Current Loss of Permanent Magnets:

In most cases, the eddy current loss of permanent magnets is much lower than iron loss and copper loss of the motor, but it will generate a large temperature rise in the high-speed motor and the high power density motor.

Ideally, stator magnetic field and rotor magnetic field of PMSM are rotating synchronously, or relatively static, thus permanent magnets without eddy current loss in such case. 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 noted that the harmonic magnetic field and eddy current loss will rise with increasing motor speed.

▲ About Laminated Magnets:

Laminated magnet is made by bonding several pieces of thin magnets together via special isolation glue which also be called magnet segmentation technology, and thus dramatically reduce the eddy flux loss of the magnets without changing the structure and performance of the motors. Either laminated SmCo magnets or laminated Neodymium magnets are available. Actually, puzzles existing in the development of laminated magnet are the assembly process and consequently high cost. We have collected much experience via some axial flux motor projects.

Bomatec AG has also launched their unique Snake Line Technology to achieve reduction of eddy current loss. According to the relevant conclusions, Snake Line Technology has more competitive advantages compared with laminated magnet, especially dimensional accuracy, mechanical properties, and cost.

▲ Laminated Neodymium Magnets:

Rare earth permanent magnets have greatly enhanced performance of permanent magnet motors and made it widely served in every area. Most o these rare earth permanent magnets are belonging to the sintered Neodymium magnet which have relatively high remanence, electrical resistivity, and low heat resistance. Eddy current loss of the sintered Neodymium magnet is relatively low compared with the copper loss and iron loss in the most cases. For high speed motors and high power density motors, eddy current loss of the sintered Neodymium magnet will lead to rotor temperature rise and irreversible demagnetization would take place in the sintered Neodymium magnet, then further affect motor performance. Laminated Neodymium magnet made by magnet segmentation solution are a preferred solution to find a balance between power and heat without changing composition of magnet, motor’s structure, and performance. Laminated Neodymium magnet is mainly served to axial flux motors in aircraft, motor sport, and automotive.

▲ Laminated SmCo Magnets:

Rotor temperature rise has not attracted enough attention in the initial designing of low power permanent magnet synchronous motors (PMSMs). Slot effect of stator windings, magnetomotive force of non-sine windings, or harmonic electrodynamic force caused by harmonic current will generates eddy current losses in the rotor magnets and rotor yoke. Eddy current loss of rotor is relatively small in comparison with stator’s copper loss and iron loss. With the increase in motor power, rotation speed, and permanent magnet’s volumn, eddy current in permanent magnets will result in the higher temperature rise, then lead to the demagnetization and then decrease motor performance. Lifetime of permanent magnets in the rare earth permanent magnet motor is mainly determined by electrical resistivity, but those magnets are always having high conductivity and low heat resistance. Samarium Cobalt magnets still have irreplaceable high temperature performance and stability compared with Neodymium magnets. Laminated magnets are proven to reduce eddy current losses in high efficiency motors besides some conventional solutions, then the combination of Samarium Cobalt magnets and magnet segmentation technology, laminated SmCo magnet, undoubtedly become the ideal choice for high temperature rotating machinery. 

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