Vibrations of a cup-shaped rotor in a bushless DC motor are investigated. The 4-phase motor has 12 pole pieces in the stator which is surrounded by a cylindrical, cup-shaped rotor with 14 magnets. The strong, pulsating magnetic forces acting on the rotor result in strong vibrations. Finite Element Analysis is used to analyze the vibrational pattern of the rotor. The impact of small changes in the dimensions of the rotor on its vibrational frequencies is also investigated. Details of FEA modeling will be presented and its predictions will be compared with the experimental data.