Applying API and NEMA Specifications to Limit Electrical Current Pulsation and Torsional Vibration of Synchronous Motors Driving Reciprocating Compressors

Unsteady torque from a reciprocating compressor can cause angular oscillation of the motor rotor. The rotational system is electromagnetically coupled to the motor stator through the air gap flux. Proper sizing of a flywheel and the motor inertia are necessary to limit speed fluctuation and current pulsation.Some current pulsation is to be expected; however, excessive levels can cause problems such as unstable current and power readings, failed motor synchronization during startup, nuisance trips during operation due to high current, increased stator temperatures, and flickering of lights.This paper discusses how to evaluate current pulsation and compare them to allowable limits. The paper also gives background information on synchronous motors, exciter control, and V-curves, as well as modelling the synchronizing torque coefficient. Case studies are provided with recommendations for reducing speed fluctuation and current pulsation to acceptable levels.