A two-pole induction machine with flexible rotor shaft and whirling rotor motion is considered. A 30 kW test machine is built with four-pole damper winding in stator. The construction of the machine and the test-setup are presented. The amplitude of measured vibration oscillates at twice the slip frequency. This is shown to be caused by the unipolar flux and the stator slot harmonics. The magnetic forces are shown to depend greatly on the position of the rotor with respect to the direction of the two-pole flux. The dependence is derived analytically. Finite-element model simulations are presented to provide further analysis and to assess the contribution of the extra stator winding.