a traveling wave ultrasonic motor includes a ring - shaped stator and two groups of electromechanical transducers . each group comprises a pair of diametrically opposed longitudinal transducers , polarized in opposite directions , disposed perpendicularly to the stator . the transducers are in permanent contact with the stator and excited by an alternating current with a π / 2 phase shift between the groups so as to produce a traveling wave - like deformation on the surface of the stator . the motor comprises at least one rotor held elastically in contact with the stator for the rotational driving thereof by the traveling wave produced on the stator . the stator is in contact with the rotor only via two or four equal and diametrically opposed segments whose aggregate span is at most equal to a wavelength of the traveling wave produced in the stator .