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 is also a frequent source of vibration and acoustic noise. Eccentricity between the stator and rotor is inevitably introduced during the manufacturing process, and includes mass unbalance, shaft bow and bearing tolerances. This paper analytically discusses the effects of rotor eccentricity on motor performance for symmetric and asymmetric motors, such 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 affects the flux linkage and introduces variation of the phase back EMF depending on winding and rotating eccentricity, thus yielding increased mutual torque ripple.