Split ratio optimization of high torque density PM BLDC machines considering copper loss density limitation and stator slot leakage

The paper deals with analytical modeling and design optimization of electrical machines as basic energy conversion elements in power systems. A new method is proposed for designing high torque density PM BLDC (Permanent Magnet Brushless Direct Current) machines with analytically derived optimal split ratio, applicable even to large high pole count machines with considerable impact of the stator slot leakage and with limited value of copper loss density. The method relies on the fact that the product of the electric loading and the current density does not depend on the machine size, while it is primarily determined by the cooling conditions. Moreover, the method also considers the influence of the stator slot leakage on the optimal split ratio. It is shown that the method provides an accurate optimum of the machine design for a wide range of machine sizes and cooling conditions. At the same time, the optimum design can be found for different machine sizes without any need to update the input data. It is shown that the impact of the stator slot leakage is significant for large machines with a high pole count, where an increase of the leakage shifts the optimum towards larger values of the split ratio. The results of the analytical optimization are thoroughly verified by computer simulations using finite element analysis (FEA).