[ The Influence of Eddy Current Losses of Magnets on Rotor Temperature Rise of Permanent Magnet Motors ] Rare-earth permanent magnet synchronous motors (REPMSMs) are characterized by their small size, light weight, and high efficiency. Theoretically, the rotor should have no fundamental wave loss and therefore a low temperature rise. However, this is not the case in reality. Taking an increased safety type rare-earth permanent magnet synchronous motor developed by the author as an example, the rotor temperature rose to 125°C during the experiment. Excessive rotor temperature can cause demagnetization of the neodymium iron boron permanent magnets, affecting the normal operation of the motor. This article analyzes the possible causes of excessive rotor temperature rise and proposes measures to reduce it.
1. Rotor structure:
The stator of a REPMSM (Remote Starting Module) is taken from that of an asynchronous motor, and its structure generally refers to the rotor structure. The rotor of an asynchronous starting REPMSM consists of squirrel cage bars, a shaft, a rotor core, and permanent magnets. The rotor core is made of laminated laminations, and neodymium iron boron permanent magnets are filled inside the rotor core. Simultaneously, aluminum is cast to form the squirrel cage, as shown in Figure 1. Its starting process is the same as an asynchronous motor. When a three-phase symmetrical alternating current is applied to the stator armature winding, a circular rotating magnetic field is formed. At this time, the rotor is stationary. The rotor squirrel cage cuts the magnetic lines of force, inducing alternating current to form an alternating magnetic field, which interacts with the stator magnetic field, and the rotor begins to rotate. When the rotor speed approaches the synchronous speed, no more induced current is generated in the squirrel cage bars. Instead, the constant magnetic field formed by the permanent magnets rotates synchronously with the stator magnetic field, and normal operation begins.
Figure 1 Schematic diagram of rotor structure
2. Causes of rotor temperature rise:
The heat generated during motor operation all comes from motor losses. When REPMSM operates synchronously, rotor losses include permanent magnet losses and harmonic losses.
2.1 Permanent Magnet Losses: Neodymium iron boron (NdFeB) magnets have a resistivity of (1.44 × 10⁻⁶) Ω·m and a certain degree of conductivity, which leads to eddy current losses in alternating magnetic fields. NdFeB magnets also have a low thermal conductivity of 7.7 cal/m·h·°C, indicating poor heat transfer. Furthermore, NdFeB magnets are prone to rusting and oxidation, making it difficult for heat to dissipate and exacerbating the temperature rise of the rotor.
2.2 Harmonic Losses: Due to factors such as cogging effect and 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 relative to the rotor at different speeds, inducing currents in the rotor core and squirrel cage bars, thereby generating harmonic losses and causing the rotor temperature to rise.
3. Measures to reduce temperature rise:
Based on the above analysis, the following solutions are proposed.
3.1 ) Permanent magnet segmentation and layering: The permanent magnet is no longer placed as a whole piece of material, but is divided into multiple small segments or multiple layers. The surface of the permanent magnet segments (layers) is electrophoretically treated to reduce eddy current losses and reduce rotor temperature rise.
3.2 ) Increasing the air gap: For asynchronous motors, increasing the air gap will increase leakage flux, which will increase the excitation current and reduce efficiency. However, for rare earth permanent magnet synchronous motors, increasing the air gap can increase the magnetic reluctance and leakage reactance of the higher harmonic air gap magnetic field, reduce the linkage degree of its magnetic flux, weaken the harmonic current, and reduce the surface losses and harmonic losses of the stator and rotor, thereby reducing the temperature rise.
3) The rotor adopts semi-closed or closed slots: This can reduce the surface loss of the rotor core and the pulsation loss inside the teeth, and reduce the effective air gap length, improve the power factor, and at the same time reduce the pulsation amplitude of the air gap magnetic permeation harmonics, thereby reducing the harmonic loss caused by the magnetic permeation 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 stator to rotor slots is close to 1, the loss is minimal, so select close slot matching as much as possible.
5) Stator winding double-layer short-pitch distributed winding: The double-layer short-pitch distributed winding can be selected with different spans as needed, which can reduce higher harmonics and make the fundamental electromotive force not much reduced, thereby effectively improving the waveform of the air gap magnetic field, reducing harmonic losses and lowering the temperature rise.
6) Use high-quality NdFeB permanent magnets: In practical applications, it has been found that the performance of NdFeB permanent magnets of the same grade produced by different manufacturers varies significantly. Different NdFeB grades produce different levels of eddy current losses and also have different thermal conductivity. Selecting high-performance NdFeB permanent magnet materials with relatively high thermal conductivity is beneficial for heat conduction on the magnets, thereby reducing rotor temperature rise.
4. Measures and effects of improving rotor temperature rise in prototype machines:
Based on the above analysis, the neodymium iron boron permanent magnet used in the prototype was changed from 40SH to 33UH. A new temperature rise test was conducted. The results showed that the stator core temperature was 80℃ with a temperature rise of 51℃, and the rotor core temperature was 140℃ with a temperature rise of 110℃. The rotor core temperature rise decreased by 10℃ after replacing the permanent magnets, demonstrating that eddy current losses in the permanent magnets have a significant impact on the rotor temperature rise.
5. Conclusion:
This paper explores the causes of excessive rotor temperature rise in rare-earth permanent magnet synchronous motors and proposes methods to reduce it. Experiments conducted on a prototype after changing the grade of permanent magnet showed that eddy current losses in the permanent magnet have a significant impact on rotor temperature. Therefore, if measures such as segmenting or layering the permanent magnet during motor manufacturing are adopted, rotor temperature rise can be reduced .
