Electromechanical oscillations of common-shaft cascaded induction motors driving a large mechanical load

Although several papers investigating the transient performance of induction motors can be found in the literature, a little attention was given to investigate the electromechanical oscillations of a multi-motor set connected on a common mechanical shaft which is widely used in oil and mining industry. This paper investigates the torsional oscillations phenomena occurring in a cascaded set of induction motors driving inertia load through elastic shafts. The electrical and mechanical mathematical models of the investigated induction motors are developed in the ABC reference frame and the system model is implemented using Matlab/Simulink. The electrical and mechanical transients during startup are studied and attention is given to the transmitted torque on the mechanical shaft sections. Different case studies are presented with different loading conditions in order to accurately study the transient performance of the electrical and mechanical components of the system under study. Results indicate that severe torsional stresses will be induced on the mechanical shaft during the motor startup especially when the system starts against load. These high torsional stresses may lead to fatigue failure or fracture and affect the life time of the mechanical shaft if no adequate countermeasure is considered.