Characteristics of BEMF, cogging torque and UMF in a HDD spindle motor due to magnetization of PM

This research mathematically derives the frequency components of back electromotive force (BEMF), cogging torque and unbalanced magnetic force (UMF) of a hard disk drives (HDD) spindle motor with 12 poles and 9 slots (12P9S) due to unevenly magnetized permanent magnet (PM). Only 3-multiple harmonics originated from uneven magnetization of PM introduces same frequency components to BEMF and slot-multiple harmonics to cogging torque, respectively. However, other harmonics of the uneven magnetization of PM except the 3-multiple harmonics introduce slot-multiple ±1 harmonics to UMF. The proposed mathematical equations are experimentally validated by comparing measured BEMF, cogging torque and UMF with measured surface flux density of PM.