Some aspects of electrical machines with open magnetic circuits

Every electrical machine consists of electric and magnetic circuits interlinked. Good machines have circuits of low resistance and reluctance, the latter generally achieved by the use of a steel structure. If magnetic flux is made to thread a relatively long air path, compensation for the higher reluctance otherwise introduced can be sought by making the area of the flux path large. The paper shows how useful shapes of machine can be developed in which a large part or all of the magnetic circuit consists of air. Machines included are open-bar transformers, single-sided linear induction motors, and homopolar d.c. machines with superconducting field windings. Vertical forces in open-sided linear motors are calculated. The paper concludes with a brief discussion of the possibilities of electromagnetic current collection for traction systems employing a moving transformer