1. Motor multiphysics analysis method

In a wide temperature range from low temperature to high temperature, vacuum and other harsh aerospace environments, the electromagnetic parameters of permanent magnet motors vary greatly, materials undergo nonlinear changes, and the coupling relationship between electromagnetic fields, temperature fields, fluid fields, stress fields, and other physical fields More complex, the multi-physics field coupling relationship that can be ignored under normal circumstances becomes non-negligible and becomes a key technical problem.

The core loss, wind friction loss and temperature rise of the motor are not only closely related to the ambient temperature and pressure, but also affect each other. In a vacuum environment, the heat dissipation conditions are special, which are related to the shape and surface properties of adjacent components, and the heat radiation has a nonlinear relationship with the surface temperature. Variations from vacuum to high pressure affect stress and material property changes, making multiphysics modeling of electric machines difficult. Therefore, the coupling relationship of various physical fields in the permanent magnet motor is very complicated in harsh environments, and it is very difficult to study the coupling relationship of various physical quantities and physical fields and their dynamic changes.

The multiphysics analysis methods of permanent magnet motors are mainly based on numerical analysis and finite element analysis. In terms of numerical analysis, common modeling methods include traditional matrix method, bond graph method, connection method, network method, etc. Academician Zhong Jue proposed the basic theory of global coupling analysis and coupling parallel design for complex electromechanical systems.

Professor He Shanghong and others proposed a modeling matrix method to establish a complex network topology, and established a unified model of mechanical, electrical, and hydraulic transfer matrices. The literature uses the generalized control system to establish a unified mathematical model for the multi-field coupling numerical simulation of the engine, and solves the variable domain difference problem of gas, heat, and elastic coupling. The nodal mapping method of multi-field coupling is introduced, and the load transfer in the field is discussed.

However, there are still many problems in the coupled modeling and solution of the numerical analysis method. Due to too many assumptions and neglected factors, the calculation accuracy is not enough. In terms of finite element analysis, many CAD/CAE software companies, such as Ansys, Flux, SIMULIA, UGS, etc., have developed multi-physics coupling calculation tools, which have been applied in the fields of aeroacoustics, magnetohydrodynamics, dynamic fluid-solid coupling, etc. Accuracy and efficiency are gradually improved. The International Journal of Multi Physics, which was founded in the UK in 2007, holds multiple coupling conferences every year, focusing on numerical models, model calculations, and experimental investigations, including multiphysics analysis of electric machines.

In terms of traditional multi-physics field coupling analysis, the alternate iterative method can effectively solve the problem of weak coupling and periodic steady-state strong coupling field, and the direct coupling method is the best way to analyze the problem of transient strong coupling field. The initial calculation of multi-field coupling is a sequential single-coupling iterative method, with less calculation, but the calculation accuracy is poor because multi-field coupling is not considered. Aiming at the shortage of single sequential coupling, a sequential coupling calculation method for the same model is proposed, which saves the two modeling processes, but requires the coupling model of multiple physical fields to be split consistently and reasonably, otherwise the calculation results will be quite different, and the calculation The amount is relatively large.

At the same time, when analyzing brushless DC motors with external circuits, it is necessary to combine field-circuit coupling analysis to properly handle the contradiction between simulation step size and calculation amount in nonlinear circuit analysis. It can be seen that due to the large number of coupled physical fields, complex coupling relationships, and complex environmental boundaries in high-temperature resistant motors, the existing coupling field modeling and decoupling calculation methods need to be further improved.

2. Variation rules of motor materials and device characteristics

The materials used in conventional motors, such as permanent magnets, electromagnetic wires, and insulating materials, will suffer from performance degradation, failure, and reduced reliability when used in harsh environments such as high temperature and low temperature. On the other hand, the characteristics of permanent magnet motor materials change in a high-temperature environment. When the temperature range is close to 300 ° C, the characteristics of silicon steel sheets change significantly, the conductivity of electromagnetic wires changes by nearly 3 times, and the characteristics of samarium cobalt permanent magnet materials change by 30%. , the change of fluid viscosity characteristics may reach more than 10 times, and the conductivity and dielectric strength characteristics of insulating materials will change.

High temperature resistant permanent magnet motors often use samarium cobalt permanent magnet materials, and the working temperature of samarium cobalt Sm2Co17 permanent magnet materials is as high as 350 °C. When the working temperature is higher, consider using AlNiCo material. Its maximum service temperature can reach 520°C and its temperature coefficient is -0.2%/°C, but its coercive force is low, usually less than 160kA/m. The demagnetization operating point must be checked during design. The new rare earth permanent magnet materials that have been developed so far, such as NdFeN, SmFeN, etc., the maximum magnetic energy product of the magnetic powder can reach 40MGOe, which is close to 3 times that of NdFeB magnetic powder, and the cost of raw materials is 1 of NdFeB magnetic powder. /3, but it is still in the stage of laboratory development.

The magnetization curve and loss characteristic curve of the silicon steel sheet are very critical to the calculation of the loss and overload capacity of the motor; the thermal stability of the silicon steel sheet lamination adhesive has a direct impact on the safety and stability of the motor under high temperature and high speed operation. Japanese scholar Takahashi et al. used a network model with 700 nodes to analyze the temperature distribution in the stator coil strands of a rotating electrical machine with a single-turn coil; analyzed the influence of mechanical stress caused by high temperature expansion on the magnetic properties of silicon steel sheets, and the results showed that with As the compressive stress increases, the magnetic permeability of the silicon steel sheet decreases significantly, and the specific total loss increases significantly. The insulation performance of insulating materials affects the safe operation, reliability and life of the motor.

DuPont of the United States produces polyimide film and polyimide tape, which are used for motor magnet wire insulation and motor slot insulation, and the maximum temperature resistance can reach 400 °C. If the heat generated by the motor makes the temperature exceed 500°C, ceramic insulation can be used.

Under high-temperature environment, the characteristics of electronic devices not only change significantly, but also special phenomena such as thermal noise will appear, for example: the parameters and linearity of analog devices vary widely; the anti-interference performance of digital circuits deteriorates, and special phenomena such as thermal noise appear; The output characteristics of the device change, and the parameter drift of the capacitor and resistor is obvious.

Developed countries have developed electronic devices that are resistant to harsh environments. However, due to the confidentiality of the technology, there are very few literatures available for inquiry. Since material properties and device properties are the basis of motor and drive control circuit design, in harsh environments such as high temperature and low temperature, the acquisition of the change law of motor material and electronic device properties and the establishment of accurate models are key technologies for high temperature resistant permanent magnet motors problem.

3. Loss, temperature rise and cooling analysis of permanent magnet motor

In the high temperature environment, the material properties in the permanent magnet motor change, causing significant changes in the core loss, winding copper loss, and rotor loss. In terms of heat transfer, the heat transfer method is different when the motor is vacuumed or filled with oil, and the internal temperature distribution of the motor is complicated; in terms of heat dissipation, the cooling environment and cooling conditions of aerospace motors are restricted, and it is difficult to design measures such as water cooling and air cooling. Make it difficult to dissipate heat.

When the motor works under extreme conditions such as high temperature, high speed, and high power density, its heating and temperature rise will be more serious. Excessive temperature rise of the motor causes irreversible loss of magnetism in the permanent magnet, damage to the insulation layer of the enameled wire, and even the burning of the motor winding. Therefore, accurate calculation of loss and temperature rise is one of the key technologies for 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 life of the motor.

At present, the research on the thermal problems of permanent magnet motors mainly focuses on the research on thermal calculation methods. There are mainly five thermal calculation methods: formula method, equivalent thermal path method, thermal grid method, temperature field method and parameter identification method, among which the temperature field method is the most commonly used method at present.

Calculation of heat source (motor loss) in temperature field calculation is the basis. The calculation of copper loss should mainly consider the influence of the winding resistance value by the external environment (such as humidity, temperature, etc.) and the skin effect of the conductor in the slot. As for the calculation of the core loss of the motor, the current more accurate calculation method of the core loss is based on the separated iron loss model. The alternating magnetization is calculated separately.

In the calculation, it is very important to determine the core loss coefficient and correction coefficient. In a high-temperature environment, the load of the motor varies in a large range, which not only causes the current change in the motor winding to affect the generation of copper loss, but also causes the non-sinusoidal waveform of the air-gap flux density to affect the iron loss. Therefore, the calculation of the loss of a permanent magnet motor in a high temperature environment needs to comprehensively consider the influencing factors of the external environment temperature, the ultimate performance of the motor, and the working state.

Take the loss as the heat source, consider the heat transfer and heat dissipation of the motor, and establish the temperature field of the motor in order to obtain the temperature and temperature rise law of each point of the motor. Usually, the thermal coefficient of the motor material in the motor temperature field model is a constant quantity, but in a high temperature environment In this case, not only the loss of the motor is time-varying, but also the thermal parameters such as the thermal conductivity of the motor material are also affected by changes in the pressure and temperature of the environment.

Therefore, it is necessary to fully consider the factors of the harsh environment. The combination of numerical calculation and finite element analysis is used to study the thermal problems of permanent magnet motors, and the test and verification is carried out through the simulated experimental environment. Important guarantee.

4. Motor failure mechanism and life prediction method

The heating of permanent magnet motors and electronic circuits in high temperature environments is more likely to cause performance degradation or even failure of the motor and its drive controller. In the study of the failure mechanism of the motor, it is mainly the study of the failure of the insulating layer and the loss of magnetism of the permanent magnet. Due to the lack of accurate aging mathematical models and the difficulty in quantitatively describing the insulation failure mechanism, the research on motor insulation has always been a difficult problem in motor insulation diagnosis technology. The current method mainly predicts the remaining breakdown voltage through non-destructive parameters to evaluate the motor. insulation state.

The main cause of permanent magnet demagnetization is the loss and temperature rise caused by the eddy current field under high temperature or high and low temperature alternating environment, so the research mainly focuses on the calculation of the eddy current field, and realizes the life of the motor through the evaluation of the main insulation performance. predict.

At present, the domestic research on motor life mainly lies in the research on large motors. This is because the operating conditions of large motors are complex and harsh. During long-term operation, the insulation gradually ages and the breakdown voltage gradually decreases. There are few studies, especially on the failure mechanism and life prediction of permanent magnet motors in high temperature environments. In fact, for small and medium-sized motors working in extreme performance states or high temperature environments, due to their extreme applications, the electromagnetic load design of permanent magnet motors is high, the aging speed of motor insulation will be faster than that of conventional motors, and there are also winding insulation aging problems. Breakdown failure leads to problems such as motor burnout.

In addition, the electromagnetic load design of conventional motors is usually not very high, and the design life of the motor is often extended to ensure the reliability of the motor. The design of high-temperature permanent magnet motors is aimed at pursuing the environmental adaptability and extreme application of the motor. Only when the failure mechanism of the motor is clearly recognized and the life of the motor is accurately predicted can this goal be truly realized in the design and application of the motor. Therefore, the failure mechanism and life prediction research of high temperature permanent magnet motor is another key technical problem.

5. High and low temperature environment permanent magnet motor drive control technology

In high and low temperature environments, the device characteristics and indicators of the motor system vary greatly, the motor model and parameters are complex, the nonlinearity increases, the coupling degree increases, and the power device loss changes greatly. Not only the loss analysis of the driver and the temperature rise control strategy are complicated, but also the four-quadrant Operation control is more important, and conventional drive controller design and motor system control strategy cannot meet the requirements of high temperature environment.

Conventionally designed drive controllers work under relatively stable ambient temperature conditions, and seldom consider quality, volume and other indicators. However, under extreme working conditions, the ambient temperature varies within a wide temperature range of -70 to 180°C, and most power devices cannot be started at this low temperature, resulting in driver failure. In addition, limited by the total mass of the motor system, the heat dissipation performance of the drive controller must be greatly reduced, which in turn affects the performance and reliability of the drive controller.

 Under ultra-high temperature conditions, mature SPWM, SVPWM, vector control methods and other switching losses are relatively large, and their applications are limited. With the development of control theory and full digital control technology, various advanced algorithms such as speed feedforward, artificial intelligence, fuzzy control, neural network, sliding mode variable structure control and chaos control have been used in modern permanent magnet motor servo control. successful application.

Calogero Cavallaro proposed a dynamic model of permanent magnet synchronous motor including iron loss, and based on this model, he proposed a loss-minimizing control algorithm for built-in permanent magnet synchronous motor. However, various control strategies have their own shortcomings that are difficult to overcome, especially the parameter problems, coupling problems, loss problems, and complex models brought about by environmental changes, which make the current methods have limitations.

For the high temperature environment motor drive control system, based on the calculation of the physical field, closely combined with the characteristics of the material and device characteristics, the integrated model of the motor-converter must be established, and the field-circuit coupling analysis must be carried out to fully consider the environmental impact on the characteristics of the motor system. Only by making full use of modern control technology and intelligent control technology can the comprehensive control quality of the motor be improved. In addition, permanent magnet motors working in harsh environments are not easy to replace, are under long-term operating conditions, and external environmental parameters (including: temperature, pressure, airflow speed and direction, etc.) . Therefore, it is necessary to study the design technology of high robustness drive controller of permanent magnet motor under the condition of parameter perturbation and external disturbance.