Method for evaluating stresses and fatigue life of a coupled multi-shaft rotor supported by active magnetic bearings in dropdown events

Abstract The versatility of high-speed drives can be enhanced by incorporating a flexible coupling that isolates motor shaft and application dynamics, enabling a single motor to operate with a diverse range of applications. However, this isolation increases driveline flexibility, necessitating strength analysis for the flexible rotor assembly, during dropdown events in active magnetic bearing supported systems. It is also necessary to implement the nonlinear models of the bearing elements to acquire realistic dynamics of the system for stress estimation. To that end, this paper extends the traditional dropdown analysis by proposing a method to calculate stresses and fatigue life during dropdown and identify critical driveline components. The study employs a megawatt-class induction machine driveline coupled with a quill-shaft coupling as a test case. A finite element model, based on Timoshenko beam theory, is developed for the rotor system. A nonlinear contact model is implemented in touchdown bearings to obtain the friction forces during contact of rotor and bearings. The system is levitated using a designed controller and allowed to fall freely on touch down bearings due to gravity. Bending and shear stresses are estimated based on vibrational responses under different dynamic conditions. Stress intensity distribution highlights that the quill-shaft coupling experiences significantly higher stresses than other rotor locations. Furthermore, the coupling’s geometric dimensions significantly influence stress intensity, a critical factor in integrated driveline design.