Starting Transients in Induction Motors with Inertia Loads

Direct-on-line starting of the polyphase induction motor coupled to a mechanically resonant load is studied by numerical integration of the nonlinear system equations and is shown to be capable of generating excessively large electromechanical torque peaks during transients. This study is supplemented by the modal approach which identifies the causes to be: (1) the excitation of the line frequency oscillating transient torque component which arises from the interaction of the line currents and the dominant mode currents and, (2) the possibility of mechanical amplification of this oscillating torque component when the resonant frequency of the shaft is close to the line frequency.