Pulsating Torques in PWM Multi-Megawatt Drives for Torsional Analysis of Large Shafts

This paper presents new, simple and linear mathematical relationships to predict pulsating torque frequencies in industrial multi-megawatt pulse-width-modulated drives. The proposed relationships provide direct definition of possible drive operating points where the torsional resonance modes of the mechanical shaft assembly might be excited. The calculation accuracy of natural frequencies and the induction motor slip have been taken into account in the definition of a critical excitation band. Variable switching frequency PWM strategies have also been analyzed. Pulsating torque frequencies of complex load-commutated-inverter (LCI) configurations have been proposed as complementary results. Several tests have been performed on an integrated-motor- compressor system operating at high-speed (3.3 kV, 6 MW at 10000 rpm). Intensive simulation results of a higher power PWM drive for compressor applications are shown to validate the suggested approach. This paper is a key tool for electrical engineers involved in the design of multi-megawatt drive systems such as in LNG compression and transportation, mining or cements.