The Necessity of a Third Balance Plane for Generator Rotor Field Balancing

Abstract

When balancing a generator rotor in the field, one often assumes that two balance planes at the drive end (DE) and non-driven end (NDE) fan rings would be sufficient to bring 1X vibration down. This paper, however, presents a case where a third balance plane must be utilized in order to reduce 1X vibration level below the alarm limit. Though this type of generator with rated speed of 3600 rpm appears to run just above the second couple mode from Bode and polar plots, couple weights (same weights placed at both DE and NDE with 180-degree out-of phase) have little effect on 1X vibration response. For vibration vectors at DE and NDE bearings presenting certain relative phase, it became impossible to further reduce vibration level at both ends from weights at DE and NDE fan ring planes only as indicated in obtained influence data. A third balance plane at the exciter end was attempted to change their relative phase, which proved to be successful. From the exciter end weights combined with the NDE fan ring weights, the generator rotor was balanced successfully. The purpose of this paper is to show in some cases ineffectiveness of couple weights to couple (pivotal mode) response and success of field balancing with a third balance plane on a generator rotor.