A Study on the Efficiency of a Variable Speed Slotless Axial Flux Permanent Magnet Motor

A permanent magnet motor is commonly derived by its variable frequency inverter-based supply. Therefore, it is a big issue for the designers to specify a suitable value for the frequency to run their designed PM motors with high efficiency. It is of greater importance when the machine operates at variable speeds. In this regard, the present paper tries to study the influence of frequency, pole pair numbers, and output power on the efficiency of a variable speed slotless Axial Flux Permanent Magnet (AFPM) motor. This investigation is performed by applying the electromagnetic and thermal models of the machine. The results are concluded as a formula defining the efficiency of the motor versus the supply frequency, the pole pair numbers, and the machine output power. The proposed formula is evaluated by determining efficiencies for many small power rating machines with various speeds using FE analysis and measurements of a few fabricated. Hence, the obtained formula can be included in any design algorithm of a small slotless AFPM machine.