Torsional shaker apparatus for inspecting rotatable power generation machinery

Apparatus and method 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 and to supply a DC output voltage. An oscillator ( 42 ) is connected to the converter to provide an oscillation signal for modulating the DC output voltage from the converter. The modulating voltage induces an oscillatory torque in the permanent magnet machine ( 12 ) that 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.