VFD Machinery Vibration Fatigue Life and Multilevel Inverter Effect

This paper documents fatigue-related mechanical failures in variable-frequency drive (VFD) motor machinery due to mechanical vibrations excited by drive torque harmonics which are created by pulse width modulation (PWM) switching. Present effort models the coupled system with a full electrical system (including the rectifier, dc bus, inverter, and motor), and an industrial mechanical system (including flexible couplings, gearboxes, and multiple inertias). The models are formed by a novel combination of a commercial motor code with a general self-written mechanical code. The approach extends failure prediction beyond the simple occurrence of resonance to fatigue life evaluation based on the rain-flow algorithm, which is suitable for both steady state and transient start-up mechanical response. The second contribution is a demonstration that the common use of a multilevel inverter to reduce voltage/current harmonics may actually exacerbate resonance and fatigue failure. This is shown to be caused by a resulting amplitude increase of torque components in proximity to potential resonance frequencies.