In order to carry out a precise evaluation of demagnetization, it is necessary to get a definite grasp of areas where a reverse magnetic field occurs and the materials’ demagnetization properties. With magnetic field analysis simulation using the finite analysis method (FEM), it is possible to calculate the reverse magnetic field and determine whether magnets and other parts demagnetize due to reverse magnetic field, taking material demagnetization properties into account.
This Application Note presents how to change the temperature of permanent magnets in an analysis, and then evaluate the effects on the torque waveform, magnetic flux density distribution, etc.
The average torque is reduced due to thermal demagnetization when the magnet is at a temperature of 140 deg C, as indicated in fig. 1. Also, the irreversible demagnetization that occurred at 140 deg C is apparent in that the average torque is 16% lower when the temperature of the SPM motor is returned to 60 deg C.
Magnetic Flux Density Distribution
From the figure, it can be seen that the overall magnetic flux density is lower at 140 deg C compared to before the temperature is raised. It can also be seen that it does not return to its original state even after the temperature is returned to 60 deg C.
Demagnetizing Ratio Distribution
It can be seen from the figure that even when the magnet is returned to 60 deg C, there is demagnetization across a wide range compared to before.
(*1) The demagnetization ratio shows whether demagnetization has occurred from the specified standard magnetization.
The measuring points selected to confirm operating points are shown in fig. 4, and the operating points are shown in fig. 5.
In fig. 5, it can be seen that irreversible demagnetization has occurred because the operating point exceeds the knee point at measuring point 1 after heat is added and the operating point does not return to the original B-H curve after the temperature is reduced to 60 deg C. The size of the reverse magnetic field acting on each element can also be seen on the horizontal axis.