55 – Integrated Magnetization Analysis of an IPM Motor

Application Note / Model Data

Overview

Interior permanent magnet (IPM) motors often use strong rare earth magnets. They have poor workability, however, because the magnets are inserted into the rotor’s small gaps during the assembly process. After the magnets have been inserted the rotor generates a strong magnetic field, which means that the workability when embedding it into the stator gets worse, as well. This is why in some cases they assemble the magnets while still in an unmagnetized state and magnetize them after they have been assembled. This construction method is called integrated magnetization. Using this means of construction can improve the assembly process a great deal, but there is also the possibility that the magnets will not be completely magnetized.
Consequently, first one needs to confirm whether or not integrated magnetization is even possible, and then from there to estimate the electrical power that the equipment needs for magnetization.
Using a magnetic field analysis simulation with the finite element method (FEM) provides the ability to change the making current amount and yoke geometry as magnetization conditions, as well as to account for magnetic saturation and evaluate whether or not the magnets are completely magnetized.
This Application Note explains how to determine the changes that occur in a magnetizing field if the making current is changed during magnetization, as well as how to obtain the induced voltage and cogging torque in the motor using the aforementioned magnets.

Magnetization Field Distribution

The magnetization field distributions of the magnet during integrated magnetization at making currents of 1.0 kA, 1.5 kA, 2.0 kA, and 2.5 kA are shown in fig. 1. The strength of the magnetization field correlates to the size of the making current. On the other hand, the orientation of the magnetization field faces almost the same direction, regardless of the making current.

Magnetization Distribution inside the Magnet

Fig. 2 shows the magnetization distribution in two cases: An ideal magnetizing state with uniform magnetization through the entire magnet, and the magnetization field that occurs with making currents of 1.0 kA, 1.5 kA, 2.0 kA, and 2.5 kA. When the making current is 1.0 kA and 1.5 kA, the magnetization in the entire magnet is small. However, with a making current of 2.5 kA, there is almost no difference with the magnetization distribution in an ideal magnetizing state.

Induced Voltage Waveform, Cogging Torque Waveform

Fig. 3 shows the U-phase induced voltage waveform with uniform magnetization in the entire magnet as an ideal magnetization state, as well as when accounting for the magnetization field with making currents of 1.0 kA, 1.5 kA, 2.0 kA, and 2.5 kA. Fig. 4 shows the cogging torque waveform with uniform magnetization in the entire magnet as an ideal state, as well as when accounting for the magnetization field with making currents of 1.0 kA, 1.5 kA, 2.0 kA, and 2.5 kA. The magnetization distributions shown in fig. 4.1 affect the induced voltages and cogging torques. Figures 3 and 4 indicate that magnetization at 2.0 kA or higher provides mechanical characteristics equivalent to those when using a magnet with an ideal magnetizing state.

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