18 - Critical Speeds and Vibrations

Abstract

This chapter discusses the effect of the radial loadings, hydraulic, and electrical, on the critical speed of a combined motor pump. The armature of the electrical motor rotates in a magnetic field. The effect of tolerances in measurement of bearing settings and of static deflection of the rotor, involves operation some distance away from the theoretical magnetic center This gives rise to an unbalanced magnetic pull, referred to as UMP, at the rotor, tending to deflect the shaft in a radial direction. The deflection of the shaft at the impeller under radial loading will have no effect on the critical speed of the pump portion of the shaft, which is determined in the usual manner. In the motor armature, however, considerably different conditions prevail, in that the UMP is proportional to the deflection from the magnetic centre. Because centrifugal force is also proportional to the deflection from the center, both these loadings must be added together in the determination of critical speed. The loadings, however, must be added under equal deflection conditions. They can be conveniently added, for example, at unit deflection or, more conveniently in this case, at the static deflection of the rotor under gravity, which is used in the determination of critical speed.