【作者】 周凤争；

【导师】 林瑞光； 沈建新；

【作者基本信息】 浙江大学 ， 电机与电器， 2008， 博士

【摘要】 高速电机由于转速高、体积小、功率密度高,在涡轮发电机、涡轮增压器、高速加工中心、飞轮储能、电动工具、空气压缩机、分子泵等许多领域得到了广泛的应用。永磁无刷直流电机由于效率高、气隙大、转子结构简单,因此特别适合高速运行。高速永磁无刷直流电机是目前国内外研究的热点,其主要问题在于:(1)转子机械强度和转子动力学;(2)转子损耗和温升。本文针对高速永磁无刷直流电机主要问题之一的转子涡流损耗进行了深入分析。转子涡流损耗是由定子电流的时间和空间谐波以及定子槽开口引起的气隙磁导变化所产生的。首先通过优化定子结构、槽开口和气隙长度的大小来降低电流空间谐波和气隙磁导变化所产生的转子涡流损耗;通过合理地增加绕组电感以及采用铜屏蔽环的方法来减小电流时间谐波引起的转子涡流损耗。其次对转子充磁方式和转子动力学进行了分析。最后制作了高速永磁无刷直流电机样机和控制系统,进行了空载和负载实验研究。论文主要工作包括:一、采用解析计算和有限元仿真的方法研究了不同的定子结构、槽开口大小、以及气隙长度对高速永磁无刷直流电机转子涡流损耗的影响。对于2极3槽集中绕组、2极6槽分布叠绕组和2极6槽集中绕组的三台电机的定子结构进行了对比,利用傅里叶变换,得到了分布于定子槽开口处的等效电流片的空间谐波分量,然后采用计及转子集肤深度和涡流磁场影响的解析模型计算了转子涡流损耗,通过有限元仿真对解析计算结果加以验证。结果表明:3槽集中绕组结构的电机中含有2次、4次等偶数次空间谐波分量,该谐波分量在转子中产生大量的涡流损耗。采用有限元仿真的方法研究了槽开口和气隙长度对转子涡流损耗的影响,在空载和负载状态下的研究结果均表明:随着槽开口的增加或者气隙长度的减小,转子损耗随之增加。因此从减小高速永磁无刷电机转子涡流损耗的角度考虑,2极6槽的定子结构优于2极3槽结构。二、高速永磁无刷直流电机额定运行时的电流波形中含有大量的时间谐波分量,其中5次和7次时间谐波分量合成的电枢磁场以6倍转子角速度相对转子旋转,11次和13次时间谐波分量合成的电枢磁场以12倍转子角速度相对转子旋转,这些谐波分量与转子异步,在转子保护环、永磁体和转轴中产生大量的涡流损耗,是转子涡流损耗的主要部分。首先研究了永磁体分块对转子涡流损耗的影响,分析表明:永磁体的分块数和透入深度有关,对于本文设计的高速永磁无刷直流电机,当永磁体分块数大于12时,永磁体分块才能有效地减小永磁体中的涡流损耗;反之,永磁体分块会使永磁体中的涡流损耗增加。为了提高转子的机械强度,在永磁体表面通常包裹一层高强度的非磁性材料如钛合金或者碳素纤维等。分析了不同电导率的包裹材料对转子涡流损耗的影响。然后利用涡流磁场的屏蔽作用,在转子保护环和永磁体之间增加一层电导率高的铜环。有限元分析表明:尽管铜环中会产生涡流损耗,但正是由于铜环良好的导电性,其产生的涡流磁场抵消了气隙磁场的谐波分量,使永磁体、转轴以及保护环中的损耗显著下降,整体上降低了转子涡流损耗。分析了不同的铜环厚度对转子涡流损耗的影响,研究表明转子各部分的涡流损耗随着铜屏蔽环厚度的增加而减小,当铜环的厚度达到6次时间谐波的透入深度时,转子损耗减小到最小。三、对于给定的电机尺寸,设计了两台电感值不同的高速永磁无刷直流电机,通过研究表明:电感越大,电流变化越平缓,电流的谐波分量越低,转子涡流损耗越小,因此通过合理地增加绕组电感能有效的降低转子涡流损耗。四、研究了高速永磁无刷直流电机的电磁设计和转子动力学问题。对比分析了平行充磁和径向充磁对高速永磁无刷直流电机性能的影响,结果表明:平行充磁优于径向充磁。设计并制作了两种不同结构的转子:单端式轴承支撑结构和两端式轴承支撑结构。对两种结构进行了转子动力学分析,实验研究表明:由于转子设计不合理,单端式轴承支撑结构的转子转速达到40,000rpm以上时,保护环和定子齿部发生了摩擦,破坏了转子动平衡,导致电机运行失败,而两端式轴承支撑结构的转子成功运行到100,000rpm以上。五、最后制作了平行充磁的高速永磁无刷直流电机样机和控制系统,进行了空载和负载实验研究。对比研究了PWM电流调制和铜屏蔽环对转子损耗的影响,研究表明:铜屏蔽环能有效的降低转子涡流损耗,使转子损耗减小到不加铜屏蔽环时的1/2;斩波控制会引入高频电流谐波分量,使得转子涡流损耗增加。通过计算绕组反电势系数的方法,得到了不同控制方式下带铜屏蔽环和不带铜屏蔽环转子永磁体温度。采用简化的暂态温度场有限元模型分析了转子温升,有限元分析和实验计算结果基本吻合,验证了铜屏蔽环的有效性。更多还原

【Abstract】 Due to small volume, high power density, high-speed motors are extensively used in industry applications such as compressors, centrifuges, vacuum pumps, turbo generators, flywheel applications, tooling machines and so on. Because of high efficiency, large airgap and simple rotor structure, PM BLDC motors are very suitable for high-speed operation. The main problems of high-speed PM BLDC motors are: (1) rotor mechanical design and rotor dynamics, (2) rotor loss and rotor temperature.Rotor loss is a major problem in high-speed PM BLDC motors, as it reduces the power efficiency and may demagnetize the magnets. The rotor eddy current is mainly caused by the space harmonics and time harmonics of armature current, as well as the airgap permeance variation due to slot-openings. This dissertation focuses mainly on the analysis and reduction of rotor eddy-current loss in high-speed PM BLDC motors including optimizing stator structure, slot opening, airgap length and utilizing of a conductive copper shield between the sleeve and magnets.Firstly, the influence of various stator structures (2p-3s non, 2p-6s over and 2p-6s non), slot opening width and airgap length on the rotor loss was studied, with both analytical calculation and FE simulation. Space harmonics of armature mmf are calculated with current-sheet method and Fourier analysis. The rotor loss is then derived with an improved model which takes into account the skin depth and the eddy-current reaction field. The results show that 2p-3s non exhibits high even-order space harmonics of stator mmf, and consequently causes much rotor loss. Bigger airgap length and smaller slot openings are preferred from the point view of reducing the rotor eddy-current loss.Secondly, the effect of segmenting magnets on rotor loss is analyzed. Only when the number of the magnet segments is bigger than 12, the eddy-current loss in magnets can be reduced. The influence of the conductivity of retaining sleeve on the rotor eddy-current loss in high-speed PM BLDC motors is analyzed with FEM. The loss in the Titanium sleeve is much higher than that in the carbon fiber sleeve. However, the eddy current in the Titanium sleeve smoothens the field in the magnets, hence, reduces the loss in the magnets. Furthermore, the influence of the copper shield between the retaining sleeve and magnets is inspected. Although eddy-current loss occurs in the shield, the losses in the other rotor parts are dramatically reduced, resulting in a much lower overall rotor loss. The relationship between the thickness of copper shield and the rotor eddy-current loss is also examined. Thirdly, for a given motor envelope, two high-speed PM BLDC motors with different winding inductances are designed. The research shows that big winding inductance can smooth the winding current and reduce rotor eddy-current loss.Fourthly, influence of parallel and radial magnetization for surface mounted magnets on the performance of a high-speed PM BLDC motor is comparatively studied. It shows that parallel magnetization is better than radial magnetization. Two kinds of mechanical systems with different stator cooling and bearing lubrication are designed. Rotor dynamics of two kinds of bearing supporting systems are comparatively studied and it shows that bearings introduce low-frequency cylindrical and conical vibration modes and the rotor natural frequencies can be improved by changing the bearing supporting from single side to double sides.At the last, a control system based on MC33035 for high-speed PM BLDC is developed to provide a reliable experiment environment for no-load and load operations. The effects of PWM current control and copper shield on the rotor eddy-current loss are investigated. It shows that the rotor eddy-current loss can be dramatically reduced by using copper shield and the PWM current control method will introduce high order time harmonics causing additional rotor eddy-current loss. The experiment results are validated by transient temperature finite element simulation.更多还原