In this paper, a fault severity estimation method is presented for high-resistance connection (HRC) in a permanent magnet synchronous machine drive system, being dependent upon the zero sequence voltage component (ZSVC) and stator currents. According to the relationship between the stator current, ZSVC and resistance deviations, a bivariate quadratic equation group is established with regard to the resistance deviations caused by HRC. The resistance deviations are obtained by solving the equation group. Based on the obtained resistance deviations, not only the fault is diagnosed and the faulty phase is identified, but also the HRC fault severity is estimated. Both the simulation and experiment are studied to validate the fault severity estimation method. The results verify the effectiveness and robustness of the proposed fault severity estimation method. INDEX TERMS High-resistance connection, permanent magnet synchronous machine, fault severity estimation, zero sequence voltage component. NOMENCLATURE u a , u b , u c Three-phase stator voltages u 0,m ZSVC between the artificial neutral point and the neutral center of the stator windings i a , i b , i c Three-phase stator currents R a , R b , R c Three-phase stator resistances R s Stator resistance as the PMSM is healthy R m Mean value of three-phase stator resistances. R n Resistance in three-phase balanced resistor network. R Resistance deviation δR j Processed resistance deviation L Stator phase self-inductance M Mutual-inductance between the stator phases d x Angle differences θ Rotor electrical position θ v Angle between the fundamental component and vth harmonic component The associate editor coordinating the review of this manuscript and approving it for publication was Baoping Cai.