Torsional vibrations in electrical induction motor drives during start up

When electrical induction motors are started direct on-line they generate a considerable pulsating torque. This start up torque can create problems when the motor is connected to mechanical lends such as fans or pumps and there are reported cases of the interconnecting shafts having been sheared on start-up. This paper examines the inter-relationship between the electric motor and the mechanical system which is effectively a multi-mass oscillatory system. It will be shown that large transients of both torque and speed occur immediately on start-up due to excitation of the oscillatory system. It will also be shown that later during the run up process, large vibrations can occur due to the negative damping of the rotor winding. A time domain model of the intercoupled electromechanical system is presented and the effect of ac system impedance, rotor resistance and the size of the motor relative to the driven mass is investigated.