AN INSTABILITY ANALYSIS PROCEDURE FOR 3-LEVEL MUTI-BEARING ROTOR-FOUNDATION SYSTEMS

A procedure for the instability analysis of three-level multi-span rot or systems is described This procedure is based on a distributed mass- elastic representation of the rotor system in several eight-coefficien t bearings. Each bearing is supported from an elastic foundation an da mped, elastic pedestals. The foundation is represented as a general di stributed mass-elastic structure on discrete supports, which may have different stiffnesses and damping properties in the horizontal and ver tical directions. This system model is suited to studies of instabilit y threshold conditions for multi-rotor turbomachines on either massive or flexible foundations. The instability condition is found by obtain ing the eigenvalues of the system determinant which is obtained by the transfer matrix method from the three-level system model. The stabili ty determinant is solved for the lowest rotational speed at which the system damping becomes zero in the complex eigenvalue, and for the whi rl frequency corresponding to the natural frequency of the unstable mo de. An efficient algorithm for achieving this is described. Applicatio n of this procedure to a rigid rotor in two damped-elastic bearings an d flexible supports is described. Application of this procedure to a r igid rotor in two damped-elastic bearings and flexible supports is des cribed. A second example discusses a flexible rotor with four damped-e lastic bearings. The third case compares the stability of a six-bearin g 300 Mw turbine generator unit, using two different bearing types. Th ese applications validate the computer program and various aspects of the analysis.