A Study on the Dynamics of an AC Induction Motor Rotor Based on the Analysis of Free Response at Impulse Excitation

This paper focuses on a topic frequently involved in the dynamics of transient phenomena on rotary machines driven by an AC induction motor supplied by a three-phase four-wire electrical system: the free response of the dynamic system mirrored in the torsional vibrations of its rotor. This dynamic system is reducible at a single degree of freedom system, equivalent of a spring-mass-damper but as a torsional vibratory system composed of the motor rotor (having a moment of inertia) the rotating magnetic field (having a torsional stiffness) and a hypothetical damper (generating a viscous damping mainly due to the air friction). This free response is useful mostly to characterize the transmissibility related to the conversion of information from mechanical to electrical power through this dynamic system. A simple computer-assisted setup is used to describe indirectly and to analyze the free response (generated when the motor is switched-on or after star to delta commutation sequence), using the active electrical power absorbed by the motor.