Torsional behaviour of large synchronous machines during asynchronous start-up and system disturbances

A comprehensive study of electromechanical resonances in large drives caused by electric transients such as asynchronous start-up and system disturbances is presented. The electromagnetic and torsional torques on the shaft lines of two identical 5 MW compressors are simulated during electric transients aiming at the demands on the design of couplings, bearings, and gears. The essential difference between the two systems is that one compressor is driven by a synchronous and the other one by an asynchronous machine. Torsional measurement data is used to verify the simulations. The analysis of the torsional torques provides important information for the mechanical design. The comparison of the two machine types shows that, in contrast to the asynchronous machine, the synchronous machine excites electromechanical resonances during each start-up and also during voltage sags. A frequent occurrence of these resonances may cause material fatigue and finally result in destruction of sensitive mechanical system components.