A power conversion system for converting between electrical power at different frequencies, including a rotary electrical machine including a rotor, a stator, first and second independently controllable field windings, and at least one armature winding for each phase; a switching circuit including a plurality of switching devices and having first terminal means interconnected with the armature winding which carries a high frequency signal established by the machine, and a second terminal for interconnection with an impedance establishing a lower frequency signal; the first field control circuit for monitoring the machine to sense phase difference between the optimum zero crossings and actual zero crossings of the higher frequency signal for driving the first field winding to adjust the phase of the higher frequency signal to minimize the phase difference; a second field control circuit for modulating the higher frequency signal carried by the armature winding with the lower frequency signal and for monitoring the second terminal means to sense amplitude difference between a selected one of the voltage and current parameters of the lower frequency signal and a reference level for driving the second field winding to adjust that parameter towards the reference level; and a switch firing circuit responsive to the machine voltage and one of the voltage and current parameters of the lower frequency signal at the second terminal means for selectively triggering to the on state and self-commutating to the off state the switching devices synchronously with the zero crossings of the higher frequency signal for transferring power between the higher and lower frequency signals through the switching circuit.