Effect of rotor eccentricity on spindle vibration in magnetically symmetric and asymmetric BLDC motors

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. Ecentricity between stator and rotor is inevitably introduced during the manufacturing process, such as 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 EMF, phase current and torque ripple. Based on the results of the unbalanced radial forces, the radial runout of the spindle motor is analyzed using finite element transfer matrices. An asymmetric motor, mostly chosen to reduce the cogging torque, shows a worse effect on cogging torque, unbalanced force and radial runout 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.