Apparatus for applying torsional vibration to a rotating machine is provided. In one exemplary embodiment the apparatus includes a permanent magnet machine (12) connected to the rotating machine (22) and configured to generate a three-phase AC output voltage. A converter (36) may be coupled to receive the three-phase AC output voltage from the permanent magnet machine to supply a DC load current. An electrical load (44) (or exciter) may be coupled to the converter to receive the load current. An oscillator (42) is connected to the converter to provide an oscillation signal for modulating the load current from the converter. Modulated load current causes variable loading of the permanent magnet machine thereby creating an oscillatory torque in the permanent magnet machine (12). The oscillatory torque causes torsional vibration in the rotating machine. Various components of the apparatus for applying torsional vibration may be part of a field-deployed power generating system.