Quantitative Reliability Study of a Motor System and Its Key Components used in Electric Vehicles

本研究考慮馬達動力系統包括定子、轉子、轉軸與軸承四大關鍵組件，據以探討馬達動力系統與組件的量化可靠度。其中，在定子部分，本論文蒐集馬達於不同等級絕緣下之加速試驗資料，再依據阿瑞尼士模型，評估馬達定子在各級絕緣、但正常工作溫度下的壽命分佈與可靠度。在轉子與轉軸部分，本論文探討馬達轉速對可靠度的影響，主要是考慮轉子與轉軸尺寸、材料性質等參數的不確定性，透過有限元素分析，獲得其臨界轉速，而後再結合一階可靠度分析方法，得到轉子與轉軸在不同轉速下之可靠度。在軸承部分，本論文依據文獻資料，探討三種不同軸承的可靠度，並特別聚焦於電動車馬達常用到的滾珠軸承。本論文最後綜合以上分析結果，探討馬達之系統可靠度，並以國內某公司在開發的一顆電動車馬達為計算案例，分析評估它的壽命分佈與可靠度，結果顯示該顆馬達之平均壽命在預期水準以上；本論文同時也透過靈敏度分析發現，在諸多不確定參數中，影響轉子與轉軸臨界轉速最關鍵的參數是嵌入轉子的軸長，因此，若要使馬達轉速安全避開臨界轉速，改變該參數最為有效。A motor system having key components of a stator, a rotor, a shaft and bearings is studied in this thesis. Special attention is paid to the quantitative reliability of the system and its components. With regard to the stator, accelerated life testing data are collected under various accelerated temperatures and insulation levels. The life distributions and reliabilities of the stator under different insulation levels but normal operating temperature are evaluated based on Arrhenius model. For the rotor and shaft, the influence of rotational speed on the reliability of motor is studied. The reliability issue arises from uncertainties of size and material properties of the rotor and shaft, which results in uncertainty of the critical speed and hence the reliability of the motor. Finite element analysis and the first-order reliability method are employed therein. As for bearings, three different kinds of bearing are discussed but special attention is paid to the reliability evaluation of ball bearings which are frequently used in motors of electric vehicles. After investigating the reliability of each key component, the reliability of the motor is discussed from system point of view. A motor at its design stage and to be used for electric vehicles is considered a role model for the above analyses. The result reveals that the mean time to failure of the motor is acceptable. It is also found from sensitivity analysis that the most important parameter that influences the critical speed of the motor is the part of shaft embedded in the rotor. The adjustment of its length would let the motor avoid its critical speeds and increase its reliability significantly