Demagnetization analysis

Permanent magnets are used in many electrical machines and motors including various BDC motors, synchronous motors, loudspeakers etc. When subjected to external magnetic fields and/or temperature changes, the magnetic properties of permanent magnets may change leading to demagnetization which may affect the performance of such machines. It is therefore very important to take this phenomenon into account when designing such machines.

MagNet's Demagnetization analysis features allow the user to study the demagnetization characteristics of machines containing permanent magnets. It includes three probes of this phenomenon: the Demagnetization field, which reports in the computational domain the demagnetization values(a default value of zero is reported in the case of materials for which demagnetization properties are not defined) for each material, the Demagnetization prediction field which is a binary field that allows the user to visually probe the regions of the model that are susceptible to demagnetization, and the Demagnetization proximity field which allows the user to get quantitative measures of the level of demagnetization in the model.

In the next few sections we demonstrate how demagnetization properties are set in MagNet and then apply this to a simple BDC motor. To illustrate this, consider an 8 pole 48 slot brushless DC motor with round stator type and a rotor of type IPM with variable orientation. Such motors are of interest in designing hybrid electric motor vehicles. The permanent magnets of this motor are made of Neodymium Iron Boron: 48/11 material. It is shown here on the figure above.

Results

Click image to see full size version

MagNet's material editor allows the user to define and set properties of new materials as well as modify that for materials from its extensive library. All standard properties including demagnetization values can be specified very easily using the materials editor. A sample of this is shown in this figure which shows the materials editor demagnetization properties window for Neodymium Iron Born: 48/11. It is noted that demagnetization values are set as a function of temperature. In the examples below, demagnetization prediction capabilities are demonstrated for temperature and geometrical properties variations.

Click image to see full size version

To illustrate MagNet's demagnetization analysis capabilities we consider the motor introduced above and set it's temperature to 140 degrees. A static solve of the model is performed now and the demagnetization analysis fields are reported in an instant. First, the demagnetization field (defined above) is presented on the right. The permanent magnet materials for which demagnetization properties have been defined is clearly distinguished from the rest of the model in this figure.

Click image to see full size version

The Demagnetization prediction field is presented next. Even at 140 degrees, it is noted that a very small section of the permanent magnet is susceptible to demagnetization (as should be the case for an industrial motor such as this).

Click image to see full size version

The Demagnetization proximity field is presented next. From the definition of this field, the larger the values of this field (positive values being the threshold), the more susceptible the demagnetization will be and this is clearly verified in the results seen in the figure to the right.

MagNet's parameterization feature allows the user to compare geometrical and/or other variations of designs simultaneously. Consider varying the temperature for the motor considered here from 110 to 160 degrees. Clearly, within this range of temperatures, a variation in the demagnetization characteristics is expected. The animation presented below shows these variations.

Now consider varying the tooth gap angle. The effects of this at 140 degrees is shown in the animation presented below.

These are some simple examples of MagNet's demagnetization analysis capabilities. It is also possible to carry out detailed demagnetization characteristics analysis for thermal variations by coupling MagNet to ThermNet (Infolytica's thermal analysis software) and using MotorSolve (Infolytica's new Brushless DC motor design software).