Current Harmonics in Surface-Mounted PMSM Electrical Drive for Monitoring Torsional Vibrations

Electromagnetic torque harmonics commonly exist in electrical drives due to inverter modulation, current harmonic propagation, etc. They excite torsional vibrations on the shaft which are reflected in the electrical quantities, such as stator line currents. In this paper, the existence of current harmonics originating from both voltage harmonics due to inverter modulation and mechanical oscillations in a surface-mounted permanent magnet synchronous machine (SPMSM) drive is analysed in depth. An analytical model which can accurately predict current harmonic amplitudes is presented. The theoretical development is also useful for detecting incipient torsional resonance or for identifying the natural frequencies of the electrical drive, both analytically and experimentally. An experimental setup consisting of a 6.9 kW SPMSM coupled to a dc generator is used to validate the theoretical studies.