A New Method of Accurately Identifying the Location and Movement of Forward and Backward Bending Modes Using Magnetically Suspended Rotor Systems

Abstract

Rotordynamic calculations done on most turbomachinery often discount or even completely ignore the “backward” modes. In fact, many rotating equipment engineers have never observed a backward mode, leading some to doubt their very existence. The author first observed such a mode in 1990, and has since routinely dealt with them, largely due to the ever increasing use of magnetic bearings in cryogenic turboexpanders. This paper will provide a magnetic bearing systems, and present a novel method of measuring the frequencies of both forward and backward bending modes using traditional Campbell diagram obtained by simply generating a cascade plot of the transfer functions during the run up or run down of the machine.