Research Of Electromagnetic Exciting Force Of Surface-mounted Permanent Magnet Motors And Its Reduction Methods

Noise is one of the four environmental pollutions. Working in the noise environment for a long term will affect human's acoustic organs and nervous system, or damage one's health seriously. Therefore, noise is one of the most important indexes evaluating machine quality. Over past decade, permanent magnet (PM) motors are widely used in all fields of the national economy with the development of power electronic technology, permanent magnet materials and microcomputer control technology. The design and optimization of low noise PM motors is of important real significance to improve the characteristics of PM motors, enlarge their application scope and increase their competitive ability.There are electromagnetic, mechanical and aerodynamic noises chiefly. It is electromagnetic noise that plays a significant role. The electromagnetic noise of induction motors have been widely studied in China and abroad and lots of useful conclusions have been drawn. However, due to the existence of permanent magnet the conventional method can not be directly applied to the analysis of electromagnetic vibration and noise of PM motors. At present, there has been relatively less research about the electromagnetic vibration and noise of PM motors. Existed researches are mainly focusing on the calculation and measurement of vibration and noise. However, the structure of PM motors are complicated and various, and the presence of frame and ending will lead to the difficulty in predicting the mechanical parameters accurately; the sound power level measured in anechoic chamber is the combination of those three noises which are hardly distinguished. Besides, only small part of energy is converted to sound power, so the analysis from the point of accurate calculation of vibration and noise faces enormous challenge. Electromagnetic vibration and noise origin from exciting forces. Therefore, this thesis commences its research from the aspect of exciting force to analyze the main exciting force which plays a important role in determining the electromagnetic vibration and noise and put forward some reduction approaches. Major works are listed as follows: The analytical expressions of exciting forces of PM motors with load are induced. In PM motors, EMF is the addition of rotational rotor EMF and stator EMF. Based on the superposition principle and ignoring the saturation of magnetic field, the air flux density is the product of EMF and permeability. Then the analytical expressions of exciting forces can be obtained using the Maxwell Tensor equations which indicate that the vibration mode r can be expressed by Ap+BZ; the amplitude is not only related with the configuration parameters of PM motors, but also closely associated with the control method; some special exciting forces with certain frequencies will be generated under loading operation.Most investigations show that the rated load noise is 20% higher than no load noise, so the no-load exciting forces are our emphasis. The influence of stator parameters on exciting forces are studied firstly and the relationships between the amplitude of exciting forces and slot width and slot skewing are obtained. According to the analysis of exciting forces of fractional slot PM motors, a reduction method based on teeth notching is proposed. The air flux density and force density are calculated by FEM software and the spectrums of exciting forces are obtained by space and time Fourier transformation to verify the effectiveness of the presented method.By analyzing the influence of rotor parameters on exciting forces, relevant reduction methods are brought out. The EMF expressions of permanent magnet with radical magnetization and parallel magnetization are induced to predict the amplitude of exciting forces, which demonstrate that by selecting appropriate pole arc coefficients the main exciting forces can be reduced effectively. The expressions of magnetization thickness are also obtained in order to determine the optimum eccentric distance of magnet pole. In practice, the pole arc coefficients of the adjacent magnet poles may differ from each because of the manufacture or dedicated design. The exciting forces under this situation are also studied and the results show that exciting forces with lower mode will be produced and large vibration and noise will be introduced. At last, the influence of rotor eccentricity on no load exciting forces is studied. Based on the equivalent remanence, the analytical method of calculating the eccentric magnetic field is developed. The relationship between no load exciting forces and eccentricity ratio is determined. The results show that rotor eccentricity not only affects the cogging torque but also generates exciting forces with lower mode to make much more noise.