Design And Research Of Ironless Rotor Structured High-Speed Permanent Magnet Brushless DC Motor

With the advantages of high power density, small geometric size, fast transient response and high operation efficiency, high-speed permanent magnet brushless DC motor (BLDCM) has broad application prospects. However, inside the high-speed motor, the frequency of alternating magnetic field is so high that the stator and rotor loss are great. Rotor has high demand of rigidity because of high-speed rotating. Molecule pump is the key equipment for obtaining excellent vacuum environment. In order to ensure the quality of vacuum environment, the shaft of the molecule pump requires to rotate at high speed. Therefore, design of high-speed motor is more difficult compared with design of normal motor and the requirement of heat load is more harsh. The topic of this paper comes from " development and application of small compound high-speed molecular pump" subject (No.2013YQ130429), which is financially supported by the National Key Foundation for Exploring Scientific Instrument of China. The main contents are as follows:Firstly, from the point of decreasing eddy current loss, ironless rotor structured high-speed permanent-magnet brushless DC motor is designed. Meanwhile, considering the non-conductive fiber protective sleeve and NdFeB matching with the pole-slot structure, magnetization method, torque ripple and the size of stator and rotor, electromagnetic scheme of high-speed permanent-magnet brushless DC motor is also carried out, which satisfies the vacuum pumps driving and temperature rise requirements to prove the design scheme is correct and reasonable.Secondly, equivalent mathematical model of ironless rotor structured high-speed permanent-magnet brushless DC motor is built based on equivalent surface current method and image method. When influences from stator slot are neglected, the air gap flux density and induced electromotive force are calculated. According to the expression of air gap flux density, amplitudes of induced electromotive force’s fundamental wave and harmonic wave are calculated respectively and the calculation results proves to be correct by comparing finite element method based simulation results with sample motor testing results. Finite element method based simulation results verifies the correctness of air gap radial magnet fields waveform. The analytical method provide a powerful tool for the further optimization design of motor.Thirdly, this thesis mainly analyzes and calculates the stator loss, rotor loss and Unbalanced Magnetic Pull (UMP) of the ironless rotor structured high-speed permanent magnet brushless DC motor. Stator core loss is calculated based on the discrete Bertotti model and high frequency core loss curve. The eddy current loss of rotor can be influenced by the stator slot width, protection ring materials, motor speed and some other factors. These influence factors and eddy current loss of ironless rotor structured motor under the no-load and load conditions are discussed in detail. When the static eccentricity occurs on ironless rotor, the amplitude of UMP and the linear relationship between different static eccentricity and UMP are calculated according to the Finite Element Method (FEM) simulation and the stress-tensor method of Maxwell. The theoretical basis can be provided to determine the mounting tolerances and manufacture of motor.Finally, when the dsPIC30F2010 is used as the main chip, design and development of sensorless control system for the ironless rotor structured high-speed permanent-magnet brushless DC motor is realized by using the method of terminal voltage. And the three-step start method is using to ensure the reliable operation of ironless rotor structured high-speed permanent-magnet brushless DC motor. The real-time control and monitoring of the high-speed motor can be completed by the RS232 serial communication and the host computer. The experiments and tests verify that the high-speed permanent-magnet brushless DC motor and its controller satisfy the requirements of the high-speed and small molecular pump’s electromagnetic design and driving.