Cogging Torque Control in Brushless Dc Motors

During the past five years, cogging torque in HDD spindle motors has decreased drastically as designers and manufacturers have developed a better understanding of the cogging phenomenon. This paper examines various methodologies of cogging torque reduction. Timing techniques such as dead zones and tooth notching, smoothing techniques such as sinusoidal magnetization, and geometric techniques such as tooth crowning and slot skewing are identified. Cogging torque waveforms are examined in frequency and time domains. Recommendations for accurate cogging torque prediction and design of magnetizing fixtures for low-cogging motors are forwarded.