Effects of Manufacturing Imperfections on the Circulating Current in Ironless Brushless DC Motors

This paper deals with the effects of manufacturing imperfections on the circulating current characteristics of an ironless brushless dc (BLDC) motor through a two-dimensional finite-element method (2-D FEM). The circulating current caused by the manufacturing imperfections of the windings' spatial location and the permanent magnets' magnetization is analyzed. It is found that the former has a larger impact on the circulating current, and the parallel magnetization is smaller than radial magnetization. Then, the effect of the circulating current generated from the impedance unbalance in the parallel branches on winding losses and disturbance torque is further studied. The results show that the circulating current leads to large winding losses and different torque characteristics. The torque ripple becomes small when the circulating current exists in all the phase windings and the outer radius of the parallel branches has the same spatial location. Finally, the ironless BLDC motor is manufactured and tested. The experimental results are in good agreement with the 2-D FEM. It gives a significant guideline to the manufacturing process of such ironless BLDC motors.