Loss Minimization Control Strategy for Linear Induction Machine in Urban Transit Considering Normal Force

Linear induction machine (LIM) in urban transit suffers greatly from poor efficiency due to the large air-gap length and the end-effects, including both transversal edge- and longitudinal end-effects caused by the cut-open magnetic circuit and different width between primary and secondary. Besides, the unique normal force existing in LIM that could be as high as four times of the thrust would create undesired additional resistance force and power loss, bending of the guide way, and tire wear. Addressing these issues, this paper proposes a novel loss minimization control (LMC) strategy for LIM to reduce the steady-state loss and normal force simultaneously. First, an improved loss model of LIM is proposed on the basis of the analysis of LIM copper and core loss. Second, the LIM normal force including both attractive and repulsive components is modeled in the same manner as the loss model. Third, a novel loss minimization cost function is set up, based on which the optimal solution is obtained through both analytical and numerical approaches, and thus an improved LMC strategy is carried out to achieve the minimization of steady-state loss and normal force at the same time. The proposed method is comprehensively investigated on one 3-kW arc induction machine (one prototype for actual LIM), and its effectiveness is fully validated through both simulation and experimental results.