Rare earth permanent magnet synchronous motor (REPMSM) has the characteristics of small size, light weight and high efficiency. In theory, the rotor has no fundamental wave loss and the rotor temperature rise should be low, but the actual situation is not the case. Taking an increased safety rare earth permanent magnet synchronous motor developed by the author as an example, the rotor temperature rise reached 125°C during the test. If the rotor temperature is too high, it will cause the risk of demagnetization of the NdFeB permanent magnet and affect the normal operation of the motor. This paper analyzes the possible reasons for the excessive rotor temperature rise and proposes measures to reduce the temperature rise.
1 Rotor structure: The stator of REPMSM is the stator of asynchronous motor, and its structure generally refers to the structure of the rotor. The rotor of asynchronous starting REPMSM is composed of cage bars, shaft, rotor core and permanent magnets. The rotor core is made of laminated sheets, and NdFeB permanent magnets are filled in the rotor core. At the same time, cast aluminum forms a cage, as shown in Figure 1. Its starting process is the same as that of asynchronous motor. When three-phase symmetrical alternating current is passed through the stator armature winding, a circular rotating magnetic field is formed. At this time, the rotor is stationary, the rotor cage cuts the magnetic lines, and induces alternating current to form an alternating magnetic field, which acts on the stator magnetic field, and the rotor begins to rotate. When the rotor speed approaches the synchronous speed, no induced current is generated in the squirrel cage bars. Instead, the constant magnetic field formed by the permanent magnet rotates synchronously with the stator magnetic field, entering normal operation. Rotor structure diagram 2 Causes of rotor temperature rise: The heat generated by the motor during operation comes from the motor's losses. When the REPMSM is in synchronous operation, the rotor losses include permanent magnet losses and harmonic losses. 2.1 Permanent magnet loss: The resistivity of NdFeB is (1.44×10ˉ)Ω·m, which has a certain conductivity and will generate eddy current loss in the alternating magnetic field. The thermal conductivity of NdFeB is 7.7cal/m.h.°C, and the heat transfer is poor. NdFeB magnets are prone to rust and oxidation, making it difficult for heat to be conducted outward, exacerbating the temperature rise of the rotor. 2.2 Harmonic loss: Affected by factors such as the slot effect and the stator magnetic field, the harmonic magnetic field in the air gap of the motor is very complex. The harmonic magnetic field in the air gap moves at different speeds relative to the rotor, inducing current in the rotor core and squirrel cage bars, thereby generating harmonic losses and increasing the rotor temperature. 3 Measures to reduce temperature rise: Based on the above analysis, the corresponding solutions are proposed as follows. 3.1) Segmentation and layering of permanent magnets: The permanent magnets are no longer placed as a whole piece of material, but a piece of permanent magnet is divided into multiple small segments or multiple layers, as shown in Figure 2. And the surface of the permanent magnet segment (layer) is electrophoretically treated to reduce eddy current loss and reduce rotor temperature rise. 3.2) Increase the air gap: For asynchronous motors, increasing the air gap will increase the leakage magnetic field, increase the excitation current, and reduce the efficiency. For rare earth permanent magnet synchronous motors, increasing the air gap can increase the magnetic resistance of the high-order harmonic air gap magnetic field and the harmonic leakage reactance, reduce the degree of cross-linkage of its magnetic flux, weaken the harmonic current, reduce the surface loss of the stator and rotor and the harmonic loss, etc., thereby playing a role in reducing the temperature rise. 3) The rotor adopts semi-closed El slot or closed slot: This can reduce the surface loss of the rotor core and the pulse loss in the teeth, reduce the effective air gap length, improve the power factor, and at the same time reduce the pulse amplitude of the air gap magnetic permeability harmonics and reduce the harmonic loss caused by magnetic permeability harmonics. 4) Select appropriate slot matching: the lower the harmonic order, the more rotor slots there are, and the greater the loss; when the ratio of the number of stator and rotor slots is close to 1, the loss is minimal, so choose a close slot match as much as possible. 5) Double-layer short-distance distributed winding of stator winding: Double-layer short-distance distributed winding can select different spans according to needs, which can reduce high-order harmonics and reduce the fundamental electromotive force slightly, thereby effectively improving the waveform of the air gap magnetic field, reducing harmonic losses and reducing temperature rise. 6) Select high-quality NdFeB permanent magnets: In actual applications, it is found that the performance of the same grade of NdFeB permanent magnets produced by different manufacturers is quite different. Different grades of NdFeB produce different eddy current losses and different thermal conductivity. Selecting high-performance NdFeB permanent magnet materials with relatively high thermal conductivity is conducive to the conduction of heat on the magnetic steel, thereby reducing the temperature rise of the rotor. 4 Improvement measures and effects of the prototype rotor temperature rise: Based on the above analysis, the brand of NdFeB permanent magnets used in the prototype was changed from the previous 40SH to 33UH, and the temperature rise test was repeated. The results showed that the stator core temperature was 80℃, the temperature rise was 51℃, and the rotor core temperature was 140℃, the temperature rise was 110℃. After replacing the permanent magnets, the rotor core temperature rise dropped by 10℃, which shows that the eddy current loss of the permanent magnets has a great influence on the rotor temperature rise. 5 Conclusion: This paper discusses the reasons for the excessively high temperature rise of the rotor of rare earth permanent magnet synchronous motor, and analyzes and proposes methods to reduce the rotor temperature rise. After the original prototype was tested after the permanent magnet brand was replaced, it was shown that the eddy current loss of the permanent magnet has a great influence on the rotor temperature. Therefore, if measures such as segmentation or layering of permanent magnets can be taken during the motor manufacturing process, the rotor temperature rise will be reduced.
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