Odd Stator Slot Numbers in Brushless DC Machines—An Aid to Cogging Torque Reduction

Brushless permanent-magnet dc machines often use an integral number of slots per pole (e.g., 3 slots/pole) with fully pitched coils in order to obtain a good trapezoidal back-electromotive-force (emf) waveform. However, this can lead to high cogging torque and load torque ripple. A simple solution is to add one additional slot so that the reluctance slotting that causes the ripple is removed, but the winding pattern is closely retained. This paper illustrates that simple design modification, where one additional slot is used so that the machine does not have an integral number of slots per pole. In this paper, the arrangement is analyzed using simple winding analysis and a finite-element analysis which gives more preciseness to calculations. It is found that there is a substantial reduction in cogging and load torque oscillation, thus proving the principle. However, the stator windings are slightly unbalanced and this can lead to vibration. This is also investigated and the resulting unbalanced magnetic pull under load is found to be present but of a low magnitude.