an electric motor includes a stator , at least one rotor , an output shaft engaged with the at least one rotor , and a contact area changing device configured to change an area of contact between the at least one rotor and the output shaft , wherein the area of contact between the at least one rotor and the output shaft affects a characteristic resonant frequency of the at least one rotor . a method to reduce vibrations in an electric motor includes engaging an output shaft into at least one rotor , rotating the output shaft and the at least one rotor with respect to a stator , changing an area of contact between the at least one rotor and a radial protrusion of the output shaft , and shifting the resonant frequency of the at least one rotor with the changed area of contact .