1. Multiphysics Analysis Method for Motors
In harsh aerospace environments such as a vacuum, the electromagnetic parameters of permanent magnet motors vary greatly across a wide temperature range from low to high temperatures, and the materials undergo nonlinear changes. The coupling relationships between various physical fields, such as electromagnetic field, temperature field, fluid field, and stress field, become more complex. The multi-physics coupling relationships that can be ignored under normal conditions become non-negligible, posing a key technical challenge.
The core loss, windage loss, and temperature rise of a motor are not only closely related to ambient temperature and pressure, but also mutually influential. In a vacuum environment, heat dissipation conditions are unique and related to the shape and surface properties of adjacent components; thermal radiation and surface temperature exhibit a nonlinear relationship. The change from vacuum to high pressure affects stress and material properties, increasing the difficulty of multiphysics modeling for the motor. Therefore, the coupling relationships of various physical fields within a permanent magnet motor under harsh environments are extremely complex, making it very difficult to study the coupling relationships and dynamic changes of various physical quantities and fields.
Multiphysics analysis methods for permanent magnet motors mainly employ numerical analytical methods and finite element analysis. In numerical analytical methods, common modeling approaches include the traditional matrix method, bond graph method, connection method, and network method. Academician Zhong Jue and others have proposed fundamental theories for global coupled analysis and coupled parallel design of complex electromechanical systems.
Professor He Shanghong and others proposed a modeling matrix method for complex network topologies and established a unified model for mechanical, electrical, and hydraulic transfer matrices. The literature uses a generalized control system to establish a unified mathematical model for multi-field coupling numerical simulation of an engine, solving the variable-domain difference problem of aerodynamic, thermal, and elastic coupling. A node mapping method for multi-field coupling is introduced, and load transfer within the field is discussed.
However, numerical analytical methods still have many problems in coupled modeling and solving. Excessive assumptions and neglected factors lead to insufficient computational accuracy. In finite element analysis, numerous CAD/CAE software companies, such as Ansys, Flux, SIMULIA, and UGS, have developed multiphysics coupled calculation tools, which have been applied in fields such as aeroacoustics, magnetohydrodynamics, and dynamic fluid-structure interaction, gradually improving the accuracy and efficiency of electromagnetic calculations. The *International Journal of Multiphysics*, founded in the UK in 2007, holds multiple coupling conferences annually, focusing on numerical models, model calculations, and experimental investigations, including multiphysics analysis of motors.
In traditional multiphysics coupling analysis, alternating iterative methods can effectively solve weakly coupled and periodic steady-state strongly coupled field problems, while direct coupling methods are the best approach for analyzing transient strongly coupled field problems. Early multiphysics coupling calculations used sequential single-step coupling iterations, which required less computation but suffered from poor accuracy due to the lack of consideration for multiphysics coupling. To address the shortcomings of single-step sequential coupling, a sequential coupling calculation method using the same model was proposed, eliminating the need for two modeling steps. However, this method requires consistent and reasonable partitioning of the multiphysics coupling model; otherwise, the calculation results will differ significantly, and the computational cost will be substantial.
Meanwhile, when analyzing brushless DC motors with external circuits, it is also necessary to combine field-circuit coupling analysis to properly handle the contradiction between simulation step size and computational load in nonlinear circuit analysis. Therefore, it is evident that due to the numerous coupled physical fields, complex coupling relationships, and complex environmental boundaries within high-temperature motors, existing coupled field modeling and decoupling calculation methods need further improvement.
2. Variation law of motor material and component properties
Materials used in conventional motors, such as permanent magnets, electromagnetic wires, and insulating materials, can experience performance degradation, failure, and reduced reliability when used in harsh environments such as high and low temperatures. Furthermore, the properties of permanent magnet motor materials change complexly under high-temperature conditions. At temperatures approaching 300°C, the properties of silicon steel sheets change significantly, the conductivity of electromagnetic wires changes by nearly three times, the properties of samarium-cobalt permanent magnets change by 30%, the viscosity of fluids can change by more than ten times, and the conductivity and dielectric strength of insulating materials also change.
High-temperature resistant permanent magnet motors often use samarium cobalt (Sm2Co17) permanent magnet materials, with operating temperatures reaching up to 350℃. For even higher operating temperatures, AlNiCo materials are considered, with a maximum operating temperature of 520℃ and a temperature coefficient of -0.2%/℃. However, their coercivity is low, typically less than 160 kA/m, requiring verification of their demagnetization point during magnetic circuit design. New rare-earth permanent magnet materials, such as neodymium iron nitrogen (NdFeB) and samarium iron nitrogen (SMM), have been developed, with maximum magnetic energy products reaching 40 MGOe, nearly three times that of NdFeB magnetic powder, while their raw material costs are only one-third of NdFeB. However, these are still in the laboratory research stage.
The magnetization and loss characteristic curves of silicon steel sheets are crucial for calculating motor losses and overload capacity. The thermal stability of the adhesive used to laminate silicon steel sheets directly affects the safety and stability of the motor under high temperature and high speed operation. Japanese scholars Takahashi et al. used a network model with 700 nodes to analyze the temperature distribution in the stator coil strands of a rotating motor with a single-turn coil; they analyzed the effect of mechanical stress caused by high-temperature expansion on the magnetic properties of silicon steel sheets. The results showed that with increasing compressive stress, the permeability of the silicon steel sheets decreased significantly, while the specific total loss increased significantly. The insulation performance of insulating materials affects the safe operation, reliability, and lifespan of the motor.
DuPont manufactures polyimide films and tapes for insulation of motor solenoid wires and motor slots, with a maximum temperature resistance of 400°C. If the heat generated by the motor causes the temperature to exceed 500°C, ceramic insulation can be used.
In high-temperature environments, the characteristics of electronic devices not only change significantly, but also exhibit special phenomena such as thermal noise. For example, the parameters and linearity of analog devices vary greatly; the anti-interference ability of digital circuits deteriorates, and special phenomena such as thermal noise appear; the output characteristics of power devices change, and the parameters of capacitors and resistors drift significantly.
Developed countries have developed electronic devices that can withstand harsh environments; however, due to technological secrecy, very few documents are available for reference. Since material and device properties are the foundation of motor and drive control circuit design, obtaining the variation patterns of motor material and electronic device properties and establishing accurate models under harsh environments such as high and low temperatures are key technical challenges for high-temperature resistant permanent magnet motors.
3. Analysis of permanent magnet motor losses, temperature rise, and cooling
In high-temperature environments, the material properties of permanent magnet motors change, causing significant changes in core losses, winding copper losses, and rotor losses. Regarding heat transfer, the heat transfer methods differ depending on whether the motor is in a vacuum or filled with oil, resulting in a complex internal temperature distribution. In terms of heat dissipation, the cooling environment and conditions for aerospace motors are constrained, making it difficult to design water-cooling, air-cooling, or other cooling measures, thus hindering heat dissipation.
When a motor operates under extreme conditions such as high temperature, high speed, and high power density, its temperature rise becomes more severe. Excessive temperature rise can cause irreversible demagnetization of the permanent magnet, damage to the insulation layer of the enameled wire, and even burnout of the motor windings. Therefore, accurate calculation of losses and temperature rise is one of the key technologies in the design and analysis of high-temperature resistant permanent magnet motors, and the temperature rise of the motor is also the most important factor affecting the reliability and lifespan of the motor.
Currently, research on the thermal problems of permanent magnet motors mainly focuses on the study of thermal calculation methods. There are five main thermal calculation methods: formula method, equivalent thermal circuit method, thermal grid method, temperature field method, and parameter identification method, among which the temperature field method is the most commonly used method.
In temperature field calculations, the calculation of heat sources (motor losses) is fundamental. Copper loss calculations should primarily consider the influence of external environmental factors (such as humidity and temperature) on winding resistance, as well as the skin effect of conductors within the slots. For motor core losses, the most accurate current method is based on a separated core loss model. This model categorizes core losses into hysteresis losses, eddy current losses, and stray losses according to their causes, and calculates them separately considering rotating magnetization and alternating magnetization within the motor.
In the calculation, determining the core loss coefficient and correction coefficient is crucial. Under high-temperature conditions, the motor load varies widely, which not only causes changes in the current within the motor windings affecting copper losses but also leads to the non-sinusoidal nature of the air gap magnetic flux density waveform, thus affecting iron losses. Therefore, the calculation of losses in permanent magnet motors operating in high-temperature environments requires comprehensive consideration of various influencing factors, including ambient temperature, motor performance limits, and operating conditions.
Taking losses as the heat source and considering the heat transfer and heat dissipation path of the motor, the temperature field of the motor is established in order to obtain the temperature and temperature rise law at various points of the motor. Usually, the thermal coefficient of the motor material is a constant quantity in the motor temperature field model. However, in high temperature environment, not only are the motor losses time-varying, but the thermal parameters such as the thermal conductivity of the motor material are also affected by changes in environmental pressure, temperature and other factors.
Therefore, it is necessary to fully consider the factors of harsh environments, and to study the thermal problems of permanent magnet motors by combining numerical calculation and finite element analysis. Furthermore, testing and verification should be carried out through simulated experimental environments. This is an important guarantee for expanding the safe operation of permanent magnet motor systems under high-temperature conditions.
4. Motor failure mechanism and life prediction method
In high-temperature environments, the heat generated by permanent magnet motors and electronic circuits is more likely to lead to performance degradation or even failure of the motor and its drive controller. Research on motor failure mechanisms mainly focuses on insulation failure and permanent magnet demagnetization. Due to the lack of accurate mathematical models for aging and the difficulty in quantitatively describing insulation failure mechanisms, the study of motor insulation has always been a challenge in motor insulation diagnostic technology. Current methods primarily rely on non-destructive parameters to predict residual breakdown voltage, thereby assessing the insulation status of the motor.
The main reason for permanent magnet demagnetization is the loss and temperature rise caused by eddy current field in high temperature or alternating high and low temperature environments. Therefore, the research mainly focuses on the calculation of eddy current field and the prediction of motor life by evaluating the main insulation performance.
Currently, domestic research on motor lifespan mainly focuses on large motors. This is because large motors operate under complex and harsh conditions, and their insulation gradually ages and their breakdown voltage decreases over long-term operation. Research on the lifespan of small and medium-sized motors is relatively limited, especially on the failure mechanisms and lifespan prediction of permanent magnet motors operating in high-temperature environments. In reality, for small and medium-sized motors operating under extreme performance conditions or high-temperature environments, due to their extreme applications, permanent magnet motors have high electromagnetic load designs, leading to faster insulation aging than conventional motors. This can result in problems such as winding insulation aging and breakdown failure, causing motor burnout.
Furthermore, conventional motors typically have relatively low electromagnetic load designs, and their design life is often extended to ensure reliability. However, the design of high-temperature permanent magnet motors aims for environmental adaptability and extreme application performance. Only by understanding the motor's failure mechanisms and accurately predicting its lifespan can this goal be truly achieved in motor design and application. Therefore, research on the failure mechanisms and lifespan prediction of high-temperature permanent magnet motors is another key technical challenge.
5. Permanent magnet motor drive control technology for high and low temperature environments
The characteristics and performance of motor systems vary greatly under high and low temperature environments. Motor models and parameters are complex, nonlinearity and coupling increase, and power device losses vary greatly. Not only are the loss analysis and temperature rise control strategies of the driver complex, but four-quadrant operation control is also more important. Conventional drive controller design and motor system control strategies cannot meet the requirements of high temperature environments.
Conventional drive controllers operate under relatively stable ambient temperatures, with little consideration given to factors such as mass and size. However, under extreme conditions, where ambient temperatures vary across a wide range from -70°C to 180°C, most power devices cannot start at these low temperatures, leading to driver malfunction. Furthermore, the overall mass of the motor system necessitates a significant reduction in the drive controller's heat dissipation performance, which in turn affects its performance and reliability.
Under ultra-high temperature conditions, mature methods such as SPWM, SVPWM, and vector control suffer significant switching losses, limiting their application. With the development of control theory and all-digital control technology, various advanced algorithms, including speed feedforward, artificial intelligence, fuzzy control, neural networks, sliding mode variable structure control, and chaotic control, have been successfully applied in modern permanent magnet motor servo control.
Calogero Cavallo proposed a dynamic model of a permanent magnet synchronous motor (PMSM) incorporating iron losses, and based on this model, he proposed a loss-minimizing control algorithm for the PMSM with an embedded core. However, various control strategies have their own inherent drawbacks, especially the parameter problems, coupling problems, loss problems, and model complexity caused by environmental changes, which limit the current methods.
For high-temperature resistant motor drive control systems, it is essential to base them on physical field calculations, closely integrate the changing characteristics of materials and devices, establish an integrated motor-converter model, and conduct field-circuit coupling analysis to fully consider the impact of the environment on the motor system characteristics. Only by fully utilizing modern and intelligent control technologies can the overall control quality of the motor be improved. Furthermore, permanent magnet motors operating in harsh environments are difficult to replace, operate under prolonged conditions, and experience complex variations in external environmental parameters (including temperature, pressure, airflow velocity, and direction), leading to inconsistent motor system operating conditions. Therefore, it is crucial to study the design technology of highly robust drive controllers for permanent magnet motors under parameter perturbations and external disturbances.
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