Improving Control of Torsional Vibrations of Motor-Driven Reciprocating Compressors

Abstract

Abstract Torsional vibrations establish a remarkable risk for decreased reliability and availability of motor-driven reciprocating compressors in oil and gas industry. This risk can be significantly reduced by careful analysis and design of torsional drive trains. A drive train is composed of separate parts, as motor, coupling, flywheel and compressor. These components are usually produced by separate manufacturers. Thus, the control of torsional vibrations requires seamless cooperation between the system integrator and component suppliers. Torsional analysis is based on the inertia, stiffness and damping properties of the train components. Usually, the chosen motor manufacturer provides this data to the system integrator. In addition to the mechanical properties, electric motors are characterized by the magnetic stiffness and damping induced by the magnetic fields in the air-gap between the rotor and stator. The main aim of this paper is to describe the modelling of magnetic effects in general terms as part of the drive train torsional analysis. The second aim is to show the significance of the magnetic stiffness and damping particularly in motor-driven reciprocating compressors with a soft coupling. This paper describes a recently introduced approach for the modelling of magnetic effects. This approach combines the accuracy of Finite Element Method (FEM) with the portability of parametric space-vector models. This is enabled by the linearization of the electromagnetic behavior in the operation point of the motor. The significance of portability follows from the typical procedure of torsional analysis and design carried out usually by the system integrator. The calculation example is a motor-driven reciprocating compressor with a soft coupling. It is well-known that in these motor-compressors the effect of magnetic stiffness is significant due to several natural frequencies and excitation orders below the motor supply frequency. In general, the magnetic stiffness increases the natural frequencies, and the magnetic damping decreases the oscillation amplitudes of the lowest torsional modes. The calculation example shows that the accuracy of the analytic formulae can be clearly increased by the presented new method. In addition, the calculation results demonstrate the significance of magnetic stiffness and damping in motor-driven compressors with a soft coupling. Thus, the reliability and availability of these compressor trains can be improved by including the magnetic effects into the analysis.