Synchronous Reluctance Motor With an Axially Laminated Anisotropic Rotor in High-Speed Applications
Abramenko, Valerii (2023-04-27)
Väitöskirja
Abramenko, Valerii
27.04.2023
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Sähkötekniikka
Kaikki oikeudet pidätetään.
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-932-1
https://urn.fi/URN:ISBN:978-952-335-932-1
Tiivistelmä
Electrical drives are applied everywhere in the world starting from heavy industries and ending with home appliances. Nowadays, the demand for high-speed machines is constantly growing, whereas their design sets a number of challenges. Difficulties in selection of the motor type and related aspects such as material selection, control questions, and electromagnetic, mechanical, and thermal stresses pose a need for alternative high-speed solutions; instead of, for example, induction motors or permanent magnet motors, the focus of this work is on the Synchronous Reluctance Motor with an electrically conducting Axially Laminated Anisotropic Rotor (ALASynRM) dedicated to high-speed applications.
The ALA rotor can be manufactured with temperature treatment methods, such as hot isostatic pressing (HIP) and vacuum brazing, and with a solid state welding process, such as explosion welding. The magnetic and nonmagnetic materials of the rotor should have a similar thermal expansion coefficient to avoid residual stress after the manufacturing process and during operation, when the rotor is heated up. Then, an ALA rotor should behave as a solid rotor, at least in principle, having a high mechanical robustness. The synchronous rotation ensures the absence of slip-related losses that are present in an induction motor. The ALASynRM also does not have permanent magnets, which eliminates the problems related to the magnet installation, potential irreversible demagnetization, eddy current losses in the magnets, no-load iron losses in the stator, the risk of overvoltage in the field weakening operation, and means of keeping the rotor robust (for example, titanium or fiber sleeves).
However, at present, the lack of studies on the SynRM with a solid ALA rotor prevents this motor from a wide practical application. The goal of the present study is to validate the potential of the ALASynRM as a high-speed motor. This dissertation considers the ALA rotor manufacturing methods, electromagnetic design aspects, and the impact of vector control on the motor performance.
The case under study is a 12 kW ALASynRM, where the stator of a solid rotor IM of the same power was applied. The research was mainly implemented in the finite element analysis (FEA) software Flux 2019.1.1 by Altair Engineering. The verification of the simulated results was implemented with practical experiments.
The results of the FEA simulation showed that the ALASynRM significantly exceeds the electromagnetic performance of the IM motor with a smooth solid rotor and even has a 0.25% higher efficiency than the IM motor with a slitted solid rotor. The number of magnetic and nonmagnetic layers and the insulation ratio (the optimum found is close to 0.5) were identified as important design parameters. They impact all loss components: stator winding, iron core, and what is important, also rotor eddy current losses. The connection type of the stator winding (delta or star) is very important if the rotor geometry distorts the flux “injecting” a third harmonic. In this case, a delta connection is strongly not recommended because of the significant growth in the rotor eddy current losses. The control strategy should also be defined during the design, because at certain current angles the eddy current losses in the rotor may increase significantly as a result of an increase in high-order flux density harmonics at high currents even if the fundamental one decreases (for example, if κ is larger than 45°). An analytical approach to the analysis and minimization of the torque ripple of an ALASynRM during design is also proposed. A torque distortion harmonics (TDH) factor is introduced for the analytical prediction of the torque ripple in relatively different rotor and stator designs.x
Two vacuum-brazed and one HIP-treated ALA rotors were built. One vacuum-brazed rotor underwent static experiments. Torque and inductance as a function of rotor angular position were measured. The results coincide with the simulated ones. Because of the incompatibility of the ALASynRM with commercial frequency converters (ABB and Vacon), which are not designed for operating synchronous reluctance motors with solid rotors with a very high saliency, the dynamic test was implemented only at no load at the speed of 10000 rpm (while the rated speed is 24000 rpm). However, the breaking test of the vacuum-brazed joints showed that the yield strength of the joint is 80% of the magnetic material yield strength, which is relatively close to the case where it can be stated that the rotor (made of metal layers) behaves as a solid piece.
The electromagnetic design of the ALASynRM is recommended to be started with the selection of the air gap and the insulation ratio (in the case of a fixed stator), as they have the most significant impact on the inductances. Then, the number of layers can be adjusted. If the rotor has a third harmonic in the permeability distribution, a delta connection of the stator winding should be avoided. Based on the presented findings, the recommended control logic for the ALASynRM is maximum torque per ampere (MTPA). However, the impact of the vector current angle was studied only for one motor case, and it can have a different influence on another motor with, for example, a different air gap.
The work showed that the ALASynRM is an evident substitution to a solid-rotor IM in high-speed applications. However, the control of the ALASynRM sets challenges when driven by commercial converters. This fact provides a niche for the further study.
The ALA rotor can be manufactured with temperature treatment methods, such as hot isostatic pressing (HIP) and vacuum brazing, and with a solid state welding process, such as explosion welding. The magnetic and nonmagnetic materials of the rotor should have a similar thermal expansion coefficient to avoid residual stress after the manufacturing process and during operation, when the rotor is heated up. Then, an ALA rotor should behave as a solid rotor, at least in principle, having a high mechanical robustness. The synchronous rotation ensures the absence of slip-related losses that are present in an induction motor. The ALASynRM also does not have permanent magnets, which eliminates the problems related to the magnet installation, potential irreversible demagnetization, eddy current losses in the magnets, no-load iron losses in the stator, the risk of overvoltage in the field weakening operation, and means of keeping the rotor robust (for example, titanium or fiber sleeves).
However, at present, the lack of studies on the SynRM with a solid ALA rotor prevents this motor from a wide practical application. The goal of the present study is to validate the potential of the ALASynRM as a high-speed motor. This dissertation considers the ALA rotor manufacturing methods, electromagnetic design aspects, and the impact of vector control on the motor performance.
The case under study is a 12 kW ALASynRM, where the stator of a solid rotor IM of the same power was applied. The research was mainly implemented in the finite element analysis (FEA) software Flux 2019.1.1 by Altair Engineering. The verification of the simulated results was implemented with practical experiments.
The results of the FEA simulation showed that the ALASynRM significantly exceeds the electromagnetic performance of the IM motor with a smooth solid rotor and even has a 0.25% higher efficiency than the IM motor with a slitted solid rotor. The number of magnetic and nonmagnetic layers and the insulation ratio (the optimum found is close to 0.5) were identified as important design parameters. They impact all loss components: stator winding, iron core, and what is important, also rotor eddy current losses. The connection type of the stator winding (delta or star) is very important if the rotor geometry distorts the flux “injecting” a third harmonic. In this case, a delta connection is strongly not recommended because of the significant growth in the rotor eddy current losses. The control strategy should also be defined during the design, because at certain current angles the eddy current losses in the rotor may increase significantly as a result of an increase in high-order flux density harmonics at high currents even if the fundamental one decreases (for example, if κ is larger than 45°). An analytical approach to the analysis and minimization of the torque ripple of an ALASynRM during design is also proposed. A torque distortion harmonics (TDH) factor is introduced for the analytical prediction of the torque ripple in relatively different rotor and stator designs.x
Two vacuum-brazed and one HIP-treated ALA rotors were built. One vacuum-brazed rotor underwent static experiments. Torque and inductance as a function of rotor angular position were measured. The results coincide with the simulated ones. Because of the incompatibility of the ALASynRM with commercial frequency converters (ABB and Vacon), which are not designed for operating synchronous reluctance motors with solid rotors with a very high saliency, the dynamic test was implemented only at no load at the speed of 10000 rpm (while the rated speed is 24000 rpm). However, the breaking test of the vacuum-brazed joints showed that the yield strength of the joint is 80% of the magnetic material yield strength, which is relatively close to the case where it can be stated that the rotor (made of metal layers) behaves as a solid piece.
The electromagnetic design of the ALASynRM is recommended to be started with the selection of the air gap and the insulation ratio (in the case of a fixed stator), as they have the most significant impact on the inductances. Then, the number of layers can be adjusted. If the rotor has a third harmonic in the permeability distribution, a delta connection of the stator winding should be avoided. Based on the presented findings, the recommended control logic for the ALASynRM is maximum torque per ampere (MTPA). However, the impact of the vector current angle was studied only for one motor case, and it can have a different influence on another motor with, for example, a different air gap.
The work showed that the ALASynRM is an evident substitution to a solid-rotor IM in high-speed applications. However, the control of the ALASynRM sets challenges when driven by commercial converters. This fact provides a niche for the further study.
Kokoelmat
- Väitöskirjat [1038]