Effect of starting position of crankshaft on transient body vibrations of reciprocating compressor

Abstract

The maximum reciprocating compressor vibrations occur during the startup and shutdown operations and un-derstanding the factors that influence the transient vibrations can help in developing solutions to reduce the excessive displacements. In this study, the effect of crankshaft starting position on the vibration phenomenon is investigated and the optimal starting position to deliver a smooth start is proposed. The structural forces are analytically modeled to estimate the body displacements by considering the system resonance frequencies and modal vectors. An experimental setup is also constructed to validate the prediction model results, with the integration of an encoder to track the crankshaft angular position and transducers to measure the discharge and suction pressures. The transient responses of three different crankshaft start positions are then compared using the experimental setup and the analytical model. The results indicate that if the crankshaft starts to rotate from positions close to the bottom dead center, a higher amount of startup vibrations is observed, revealing the optimal starting position zone. The magnetization effect can potentially be addressed by the sensorless starting strategy developed by Lee et al. (2008) that implements a phase current controller for a smoother startup. While out of scope for this study, shutdown strategies can also be devised to have the piston consistently land at the favorable SAP range. By separately investigating the instances when the 'stop' command is provided to the steady-state operating compressor, solutions that attempt to replicate the conditions that lead to the favorable stop position can be developed; a limiting factor in mass production will be implementing the encoder for continuous position tracking and an accompanying undesirable cost increase in manufacturing. Nevertheless, the investigation of passive and active strategies is ripe for research in literature.