Effect of magnet retaining sleeve thickness on cogging torque of radial flux permanent magnet brushless DC motor

Torque ripple is undesirable component for smooth operation of permanent magnet motors. Cogging torque is prime creator of torque ripple, which creates noise and vibrations. Torque ripple can be lessened by minimizing cogging torque. By proper design improvement in permanent magnet motor, cogging torque can be reduced. This paper presents design technique to reduce cogging torque of two standard rating motors by selecting proper magnet retention sleeve thickness. Permanent magnets barely bear the high centrifugal force at high speed rotation. The major challenge in high speed permanent magnet motors is to retain magnets properly at high speed. For holding magnet in its place, high strength retention sleeve made of stainless steel is necessary. Initially, motor is designed without magnet retaining sleeve and then after magnet retaining sleeve thickness is varied from 0.1 mm to 0.3 mm. Cogging torque profile is obtained using Finite Element Analysis (FEA). Cogging torque is minimized from 0.588 Nm to 0.234 Nm (i.e. 60.2 %) in 70 W, 350 rpm motor and 5.42 Nm to 3.08 Nm (i.e. 43.1 %) in 2.2 kW, 1450 rpm motor. Hence it's concluded that magnet retaining sleeve of permanent magnet motor is effective in cogging torque reduction and its essential to decide proper thickness of magnet retaining sleeve.