An extended field balancing procedure for flexible rotors fully levitated by active magnetic bearings

Abstract

An extended influence coefficient matrix (EICM) is presented that enables the identification of inherent unbalances in a flexible rotor levitated by active magnetic bearings (AMBs). The traditional influence coefficient procedure for flexible rotor balancing uses measured unbalance responses to obtain residual unbalances at predefined balancing planes with the help of trial sets of unbalances. Due to partial attenuation of unbalance responses by active magnetic bearings in rotors mounted on active magnetic bearings, difficulty arises in estimating these influence coefficients since responses do not reflect the real effects of trial unbalances. The presented EICM correlates magnetic forces with unbalance forces in the form of trial sets of unbalances. This matrix is identified initially and helps in finding residual unbalances in the rotor system. The finite element method is used to model a flexible five-disc rotor system mounted on two active magnetic bearings for the numerical simulation of an experimental test rig. AMBs uses PID controller. The proposed procedure is applicable for both, discrete (constant) speeds and for run up/down measurements of magnetic forces. The developed procedure is benchmarked experimentally.