Compressor Response To Synchronous Motor Startup

LecturePg. 95-101.Synchronous machines produce an oscillating torque at twice slip frequency on starting. If the shaft system of a coupled compressor is not adequately designed, or if the motor malfunctions, this torsional excitation will have destructive effect. Recently, mathematical models for this system have been solved by direct integration on both analog and digital computers. Important aspects of the model include: representation of the electrical power feed network for the motor, an equivalent circuit description of the motor itself, and a non-linear characterization of the shafts and couplings connecting the compressors to the motor. In parallel with this analytical effort it was recognized that the instantaneous magnitude of the excitation torque and its resultant shaft stress should be measured during field installation. In order to accomplish this, a torsional acceleration monitor, which is attached to the uncoupled synchronous motor, was developed to gauge the actual torques being applied to the compressor on startup. In addition, a strain measurement system utilizing telemetry was assembled to obtain shaft stress levels. These two instrumentation packages have been used to insure safe operation, detect motor starting malfunctions, and verify the analytical design of the shaft/coupling system. Measurements have been made at 10 installations to date with good agreement between expected and experienced results