The Influence of Axial Magnetic Centering Forces on Sleeve Bearing Induction Motors

All motor rotors have an axial position called a magnetic center, which is the location where the motor's axial magnetic forces are balanced. The magnitude of these axial magnetic forces that hold the rotor in this position can vary greatly depending on the machine size, speed, electromagnetic configuration and mechanical geometry. Very little research has focused on this phenomenon and other issues that can cause weak magnetic centers, floating magnetic centers and multiple magnetic centers. This paper presents comparisons of calculated and tested data to describe the axial magnetic forces and their effects. It also suggests methods to strengthen weak magnetic forces. This condition exists in induction motors, but is most serious in sleeve bearing configurations where mechanical rotor endplay can permit violent and damaging rotor motion when weak magnetic centers or multiple centers are present.