Superconducting three element synchronous A.C. machine

There is a growing interest in a.c. superconducting machines. Several new concepts have been proposed for these machines in the last years. One of the most promising seems to be the "three elements" concept of Pr. Smith, allowing to cancel the torque acting on the superconducting field winding and thus overcoming some of the major contraints. This concept leads to a device of induction-type generator. In the present paper a synchronous three elements superconducting a.c. machine is described, in which a room temperature, d.c. fed rotating winding is inserted between the superconducting field winding and the a.c. armature. The steady-state machine theory is developed. After having the flux linkages established, the torque expressions are derived. The condition for zero torque on the field winding, as well as the resulting electrical equations of the machine are given. In the last part of the paper, the theoretical behaviour of the machine is studied, using phasor diagrams and assuming for the superconducting field winding either a constant current or a constant flux condition.