Comparison of vibration sources between symmetric and asymmetric HDD spindle motors with rotor eccentricity

The permanent-magnet motor is often the most important element in hard disk drive (HDD) spindles and also a frequent source of vibration and acoustic noise. The eccentricity between stator and rotor is inevitably introduced d uring the manufacturing process, and includes mass unbalance, shaft bow, an d bearing tolerances. This paper analytically discusses the effects of roto r eccentricity on motor performance for symmetric and asymmetric motors, su ch as local traction, unbalanced force, cogging torque, back electromotive force (EMF), phase current, and torque ripple. An asymmetric motor, mostly chosen to reduce the cogging torque, shows a worse effect on cogging torque and unbalanced force when the eccentricity exists. It also adversely affec ts the flux linkage and introduces variation of the phase back EMF dependin g on winding and rotating eccentricity, thus yielding increased mutual torq ue ripple.