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 during manufacturing process, such as mass unbalance, shaft bow and bearing tolerances. This paper analytically discusses effects of rotor eccentricity on motor performances for symmetric and asymmetric motors, such as local traction, unbalanced force, cogging torque, back 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