The influence of the eddy current loss of the magnet on the temperature rise of the permanent magnet motor's rotor
- Time of issue:2021-10-20
- Views:0
The influence of the eddy current loss of the magnet on the temperature rise of the permanent magnet motor's rotor
The rare earth permanent magnet synchronous motor (REPMSM) has the characteristics of small size, light weight and high efficiency. Theoretically, the rotor has no fundamental wave loss, and the temperature rise of the rotor should be low, but this is not the case in practice. Taking an increased-safety rare earth permanent magnet synchronous motor developed by the author as an example, the temperature rise of the rotor was as high as 125°C during the test. If the rotor temperature is too high, it will cause the danger of demagnetization of the neodymium iron boron permanent magnets and affect the normal operation of the motor. This paper analyzes the reasons that may cause the excessive temperature rise of the rotor, and proposes measures to reduce the temperature rise.
1 Rotor structure: The stator of REPMSM is the stator of an asynchronous motor, and its structure generally refers to the structure of the rotor. The rotor of the asynchronous start REPMSM is composed of a squirrel cage bar, a rotating shaft, a rotor core and a permanent magnet. The rotor core is formed by stacking punched sheets. The rotor core is filled with powerful permanent magnets and cast aluminum to form a squirrel cage, as shown in the figure. 1 shown. The starting process is the same as that of an asynchronous motor. When a 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, and the rotor squirrel cage cuts the magnetic lines of force, and the alternating current is induced to form an alternating magnetic field, which is combined with the stator magnetic field. Function, the rotor starts to rotate. When the rotor speed is close to the synchronous speed, no induced current is generated in the squirrel cage bar, but the constant magnetic field formed by the permanent magnet rotates synchronously with the stator magnetic field and enters normal operation.
2 The cause of the temperature rise of the rotor: the heat generated when the motor is running, all come from the loss of the motor. When REPMSM runs synchronously, the rotor loss includes permanent magnet loss and harmonic loss.
2.1) Permanent magnet loss: The resistivity of neodymium iron boron is (1.44×l0ˉ)Ω·m, which has a certain degree of conductivity and will produce eddy current loss in an alternating magnetic field. The thermal conductivity of NdFeB is 7.7cal/mh°C, and the heat transfer is poor. The neodymium iron boron magnet is easy to rust and oxidize, which makes it difficult for heat to conduct outwards, which intensifies the temperature rise of the rotor.
2.2) Harmonic loss: Affected by factors such as cogging effect and stator magnetic field, the harmonic magnetic field in the air gap of the motor is very complicated. The harmonic magnetic field in the air gap moves relative to the rotor at different speeds, and induces current in the rotor core and squirrel cage bars, thereby generating harmonic losses and increasing the temperature of the rotor.
3 Measures to reduce temperature rise: Based on the above analysis, the corresponding solutions are proposed as follows.
3.1) Permanent magnet segmentation and layering: The placement of a strong magnet is no longer the entire section of material, but a section of permanent magnet is divided into multiple small sections or multiple layers, as shown in Figure 2. And electrophoresis is performed on the surface of the permanent magnet section (layer) to reduce eddy current loss and reduce the temperature rise of the rotor.
3.2) Increasing the air gap: For asynchronous motors, increasing the air gap will increase the magnetic flux leakage, increase the excitation current and reduce the efficiency. For rare earth permanent magnet synchronous motors, adding an air gap can increase the magnetic reluctance and harmonic leakage reactance of the high-order harmonic air gap, reduce the degree of flux linkage, weaken the harmonic current, and reduce the surface of the stator and rotor. Loss and harmonic loss, etc., so as to reduce the temperature rise.
3) The rotor adopts semi-closed El slot or closed slot: This can reduce the rotor core surface loss and tooth pulsation loss, and reduce the effective air gap length, improve the power factor, and reduce the pulse amplitude of the air gap permeability harmonics. Value to reduce the harmonic loss caused by the permeance harmonics.
4) Choose appropriate slot matching: the lower the harmonic order, the more the rotor slot number, the greater the loss; when the stator-rotor slot number ratio is close to 1, the loss is the smallest, so choose the near-slot matching as much as possible.
5) Stator winding double-layer short-distance distributed winding: The double-layer short-distance distributed winding can choose different spans according to the needs, which can reduce high-order harmonics, and the fundamental electromotive force is not greatly reduced, thereby effectively improving the waveform of the air gap magnetic field , Reduce harmonic loss and reduce temperature rise.
6) Use high-quality neodymium iron boron permanent magnets: In practical applications, it is found that the properties of the same grade neodymium iron boron permanent magnets produced by different manufacturers are quite different. Different grades of NdFeB have different eddy current losses and different thermal conductivity. Choosing a high-performance neodymium iron boron permanent magnet material with relatively large 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 temperature rise of the prototype rotor: Based on the above analysis, replace the neodymium iron boron permanent magnet used in the prototype from the previous 40SH to 33UH, and perform the temperature rise test again. The result is that the stator core temperature is 80°C. The temperature rise is 51°C, the rotor core temperature is 140°C, and the temperature rise is 110°C. After the permanent magnet is replaced, the temperature rise of the rotor core drops by 10°C, which shows that the eddy current loss of the permanent magnet has a great influence on the temperature rise of the rotor.
5 Conclusion: This article discusses the reasons for the excessive temperature rise of the rotor of the rare earth permanent magnet synchronous motor, and analyzes and puts forward a method to reduce the temperature rise of the rotor. After replacing the permanent magnet brand on the original prototype, the test shows that the eddy current loss of the permanent magnet has a great influence on the rotor temperature. Therefore, if measures such as segmentation or stratification of permanent magnets can be taken during the manufacturing process of the motor, the temperature rise of the rotor will be reduced.
relevant information
-
What kind of strong magnet is of good quality?At present, the sales competition of powerful magnets in the market is becoming more and more intense. In order to obtain a better competitive advantage in the market, many magnet manufacturers will use inferior raw materials to reduce costs. Therefore, when buying a strong magnet, we must know how to distinguish the quality of the strong magnet, so that we can buy a relatively high-quality strong magnet. - Permanent magnet motor and its characteristics 2021-10-20
- Main properties of permanent magnet materials commonly used in motors 2021-10-20
- What is amorphous soft magnetic? 2021-10-20
- The influence of the eddy current loss of the magnet on the temperature rise of the permanent magnet motor's rotor 2021-10-20
- China's magnetic materials enter the international high-end market 2021-10-20
+86(0)139 2386 8896
Messages 
E-mail 
Huizhou Tongda Ferrite Techology Co.,Ltd
Follow us
Copyright ©Huizhou Tongda Ferrite Techology Co.,Ltd All Rights Reserved 粤ICP备14038032号 Power by:www.300.cn