Split ratio is an important parameter for permanent magnet brushless motors (PMBLM) since it have significant influence on torque and loss. The split ratio of low-speed PMBLM has been optimized analytically in existing papers, while the iron loss is neglected. However, the iron loss in high-speed PMBLM can not be neglected. The split ratio and the maximum flux density in the stator iron is relate to both the copper loss and the iron loss closely. The split ratio, togerther with the maximum flux density in the stator iron, is optimized for maximum torque. An analytical model to determine the optimal combination of the split ratio and the maximum flux density in the stator iron for high-speed PMBLM is proposed in this paper. The analytical results are verified by finite element analysis.

High reliability is required for the permanent magnet brushless DC motor (PM-BLDCM) in an electrical pump of hypersonic vehicle. The PM-BLDCM is a short-time duty motor with high-power-density. Since thermal equilibrium is not reached for the PM-BLDCM, the temperature distribution is not uniform and there is a risk of local overheating. The winding is a main heat source and its insulation is thermally sensitive, so reducing the winding temperature rise is the key to the improvement of the reliability. In order to reduce the winding temperature rise, an electromagnetic-thermal integrated design optimization method is proposed. The method is based on electromagnetic analysis and thermal transient analysis. The requirements and constraints of electromagnetic and thermal design are considered in this method. The split ratio and the maximum flux density in stator lamination, which are highly relevant to the windings temperature rise, are optimized analytically. The analytical results are verified by finite element analysis (FEA) and experiments. The maximum error between the analytical and the FEA results is 4%. The errors between the analytical and measured windings temperature rise are less than 8%. It can be proved that the method can obtain the optimal design accurately to reduce the winding temperature rise.

When magnetic saturation occurs during the operation of a permanent magnet (PM) slotless brushless DC motor, the material permeability will no longer be a constant value, and the neglected magnetic saturation model used for motor optimization will no longer be applicable. And considering that the increase in motor torque will lead to a high temperature rise of the winding, therefore, an electromagnetic heat coupling model applicable to the occurrence of magnetic saturation in the motor is established, and the model is utilized in combination with the particle swarm algorithm to enhance the maximum output torque of the motor. Firstly, a 100 W, 16,400 r/min high-speed PM slotless DC brushless motor is taken as the object of study, and its electromagnetic–thermal coupling model is established to derive the analytical equations for the electromagnetic torque with respect to the split ratio, the thickness of the stator yoke, the PM thickness, and the copper loss. Secondly, based on the modeling, the motor was optimized using a particle swarm algorithm to maximize the output torque and minimize the copper loss. Finally, a prototype was fabricated and verified with the prototype through no-load and load experiments. The difference between the theoretical maximum output torque and the experimental maximum output torque is less than 8%. The results show that this method can effectively predict the maximum output torque of the motor in the case of magnetic saturation, and the model is suitable for increasing the maximum output torque of slotless brushless DC motors under space constraints.

The temperature gradient of high-torque-density short-time duty permanent magnet (PM) brushless DC motors (BLDCM) is large. Increasing the torque will cause the copper loss and the winding temperature to rise rapidly, which will lead to the risk of the winding insulation damage. A split ratio optimization method is proposed to raise the torque considering winding temperature limitation in this paper. The analytical relationship between the torque, the losses and the split ratio is derived. The transient temperature of the winding is calculated analytically based on the expressions of this analytical relationship. The maximum allowable temperature rise of the winding is converted to the optimization constraint. The optimal split ratio is calculated and the maximum torque within the temperature limit range is obtained. The method is verified by finite element (FE) simulation and experimental test. The FE predicted optimal split ratio value is equal to the analytically predicted value. The error between the simulated and calculated torque is less than 5.7%. The maximum error between measured and analytically predicted temperature is 7.2%. The verification results show that the proposed method can accurately calculate the optimal split ratio and maximizes the torque within the winding temperature limitation.