The forces acting between the rotor and stator of a cage induction motor, w
hen the rotor is performing cylindrical circular whirling motion, are measu
red. Time-stepping finite-element analysis is used for solving the magnetic
field, and the forces are calculated from the airgap field based on the pr
inciple of virtual work. The forces are measured with a test motor equipped
with active magnetic bearings. The magnetic bearings are used to generate
the whirling motion and to measure the forces. The computed and measured fo
rces show good agreement. The effects of the magnetic forces on rotor dynam
ics are briefly discussed.