Searching for Optimal Solutions for Motor Performance Design 2020-01-0460
Because strength and the torque/rotational speed characteristic play a considerable role in determining the maximum speed and the acceleration force of a vehicle, they have been taken up as optimization issues. By contrast, loss and torque ripple have not necessarily been focused on in the initial search for optimal solutions. They are both linked to efficiency, comfortability, and the cost of development of the cooling system and supporting structures, however, it is necessary to reduce both parameters. For this reason, it is important to search for the Pareto front for strength, the torque/rotational speed characteristic, loss, and torque ripple at the initial stage of design.
The strength constraint was set that von Mises stress in the rotor core in relation to the centrifugal force load at maximum speed would not exceed the breaking strength of a standard electromagnetic steel sheet material. The torque/rotational speed characteristic employed the maximum torque for each rotational speed when maximum torque per ampere control and field weakening control are applied, with consideration of maximum input voltage and current. Maximum torque at low engine speeds and maximum power at the highest engine speed were set as the constraint conditions. For loss, the total of DC copper loss occurring in the coils and iron loss occurring in the stator and rotor was employed. The 6th-order torque ripple in electric angle was evaluated. The objective function was to minimize each of these parameters. The size of the interior permanent-magnet synchronous motor (IPMSM), the arrangement of the magnets, and the magnetic flux paths were defined by parameters. Conducting the analysis described above automatically enables quantitative visualization of the tradeoff relationship between the results. The developed method has made it possible to select the highest-performance motor from a group of motors that meet performance targets.