Temperature-induced vibration instability in a non-spinning vertical rotor supported by active magnetic bearings—Theory and experiment

Abstract

This work gives an original theoretical and experimental contribution to the problem of axial vibration instabilities of vertical rotors levitated by active magnetic thrust bearings (AMB) due to thermal effects until the complete loss of axial load capacity. A mathematical model combining rotordynamics, electromagnetism, thermal effects, and control theory is derived and simultaneously solved. The rotor equation of motion is obtained using Newton’s second law, the AMB coils are modeled by Ohm’s, Ampere’s, and Faraday’s laws. The temperature distribution within the AMBs is obtained accounting for heat convection and conduction and the temperature-dependence of AMB coils resistance. The set of non-linear equations is linearized around the operational point and used to predict the characteristics of the rotor’s axial vibration instability induced by thermal effects, and the instant it occurs.

A test rig consisting of a vertical non spinning rotor supported axially by one AMB and radially by two passive magnetic bearings is designed to experimentally investigate the thermal effects without achieving extremely high temperatures, at the expense of relatively large axial clearances compared to industrial applications. For large clearances and moderate temperatures, component expansion and variation in magnetic permeability turn negligible. The AMB is heated with hot air blown by heat guns under a well-controlled process monitored by temperature sensors.

Theoretical and experimental results indicate that when the upper electromagnet temperature reaches approximately 50 °C, the original PID controller is not capable of supporting the rotor, leading to axial vibration instability. Solutions for re-levitation and stabilization are addressed.