A three-phase, wound-rotor induction machine, with its shaft rigidly coupled to the shaft of a three-phase synchronous motor, is used to change balanced 60-Hz voltages to other frequencies at the wound-rotor terminals brought out through slip rings. Both machines are electrically connected to the same balanced three-phase, 60-Hz source. Let the synchronous motor, which has four poles, drive the interconnecting shaft in the clockwise direction, and let the eight-pole balanced three-phase stator winding of the induction machine produce a counterclockwise rotating field, i.e., opposite that of the synchronous motor. Determine the frequency of the rotor voltages of the induction machine.