Comprehensive Design of Slotless Permanent Magnet Synchronous Motors: Effects of Series and Parallel Turns

A comprehensive design methodology of a slotless permanent magnet synchronous machine (PMSM) based on a grid search and elimination method is presented in this study. The design process aims to investigate the effects of various design choices on machine cost and efficiency but not to obtain an optimal design. A high precision long cylinder type actuator with 1 kW rated power, 12,000 rpm rated speed, 50 mm diameter, and 135 mm maximum axial length is selected as the target design. The performance of design candidates is evaluated using an analytical model that includes the calculation of magnetic flux density, induced voltage, torque, and phase conductor losses. As the phase conductors are directly exposed to air-gap flux, conductor losses are calculated considering the the air-gap magnetic field. Due to this effect, the conductor design in slotless PMSMs should be done carefully. Therefore, in the next step, first the designs that satisfy the application requirements are distinguished, and then the series turn numbers as well as wire diameters and parallel turn numbers are assigned to each of them. Those assignments are made based on assumptions that assure the manufacturability of the winding. All the output designs are depicted with respect to their magnet volume and winding conductor losses to obtain a Pareto-optimal front curve. Thus, the effects of different design decisions on these two performance variables were observed. As expected, the designs with a high number of parallel turns have higher efficiencies. Furthermore, when the designs with the same number of parallel turns are compared, the ones with smaller permanent magnet volumes are found to be preferable since the total cost of their raw materials is around 20% cheaper with almost no difference in their efficiencies.