Accurate Estimation of Start-Up Pulsating Torque of Direct On Line Synchronous Motors Driving Compressor Trains

Abstract

In a compressor train driven by fixed speed synchronous motor (>17MW) was discovered a potential torsional problem on the input shaft of the hydraulic variable speed gear during the start-up phase when only low speed shaft line is engaged. It was due to high motor excitation torque crossing the 1st torsional critical speed during startup causing a very limited numbers of train startups (1400) versus project requirements (5000). Supported by API 617 (8th edition), the motor excitation air-gap torque during startup has been analyzed considering electrical system characteristics that influence the effective voltage drop at motor terminals. A more realistic analysis of the excitation confirmed the correctness of the shaft line design avoiding any redesign and impacts in the projects execution.