205 - Analysis of IPM Motor Characteristics using Thermal Equivalent Circuit
To realize high output and efficiency in a motor, it will be important to understand temperature increase in each part of the motor. This is because coil resistance and magnet characteristics change with increase in temperature, and this may cause a large impact in the motor characteristics. It is insufficient with a magnetic field analysis of constant temperature, and it will become clear that magnetic field analysis accounting for temperature increase will become necessary.
As a method to account for temperature increase, there is a method of running a coupled analysis of magnetic field and thermal analysis. With this coupled analysis, detailed predictions such as evaluating temperature is possible, but apart from magnetic field analysis, it will become necessary to create a thermal analysis model, and this results in extra work.
On the other hand, a magnetic field analysis using heat equivalent circuit can add a heat equivalent circuit to the circuit of magnetic field analysis, and easily run analysis accounting for temperature increase. This method does not create a complex 3D thermal analysis model and it easily and quickly obtains results. Here, we confirm a situation where temperature increases due to eddy current of the iron loss and magnet, which causes demagnetization of the magnet and torque reduction.
Temperature variations for each part are shown in fig. 1. For the first step of the analysis, a transient magnetic field analysis is performed, and average time losses are calculated. Next, steady values for temperature rises of each part are obtained from the average losses in a certain time period. One set of the process of coupled magnetic field and thermal analysis corresponds to one step of the graph below. Material properties are updated when the temperature rises, so a transient magnetic field analysis is performed again, the average time losses are calculated, and the steady values of the temperature of each part are obtained. This corresponds to step 2 of the graph below. To grasp the process of temperature rises, a 5 step process is run.
Torque waveform is shown in fig. 2. A decrease in torque can be verified due to magnet thermal demagnetization. Furthermore, this analysis model is running with a specified current, so changes in resistance due to temperature rises do not affect torque. For this analysis, temperatures are obtained in the thermal equivalent circuit every 16 steps in the magnetic field analysis, which is one period of eddy current in the magnet, and the temperature information is updated.