Eccentricity Related Forces in Two-Pole Induction Motor With Four-Pole Stator Damper Winding Analyzed Using Measured Rotor Orbits

A two-pole induction motor has been fitted with an extra three-phase four-pole stator winding to damp the unbalanced magnetic pull caused by eccentricity. In traditional two-pole induction machines, the interaction of two- and four-pole flux produces most of the magnetic forces. That force is greatly reduced if the four-pole stator winding is used to damp the four-pole flux. This causes some of the other magnetic force components to become more significant. This paper presents an estimate of what components comprise the total force on rotor and how they change when the speed, torque, and two-pole flux density of the machine are changed. The rotor is on a long flexible shaft supported by external bearings allowing the forces to move the rotor. Since the magnetic force depends on rotor eccentricity, the comparisons are done to the magnetic stiffness derived from the force. The radial movement of the rotor is measured as well as the two-, four-, and zero-pole (unipolar, homopolar) air-gap magnetic flux densities. All the results are based on measurements with a prototype test machine.