A commutator employs superconducting switches to commutate currents in the field stator of a dc machine. The switches couple the machine's stator windings to a DC link. The switches are exposed to magnetic fields of alternating strengths, turning on and off in a sequence that maintains a constant torque angle between the stator's mmf vector and the flux vector of the machine's rotor or that rectifies a current induced into the stator by the machine's rotor.