Optimum contact region geometry of solid armatures

Previous theoretical predictions of the transition velocity of solid armatures based on the velocity skin-effect (VSE) usually assume a fixed geometry and that adequate rail/armature contact pressure exists. In this paper the effect of contact region geometry on the transition velocity is investigated and the optimum geometry derived. The critical geometrical parameters are considered to be the contact region angle φ and rear radial depth. These parameters need to be as small as possible to maximise the transition velocity but large enough to maintain sufficient contact pressure to avoid the early initiation of a current wave which would reduce the transition velocity. Comparison of predicted and experimental results for 15 to 90 mm aluminium alloy armatures shows reasonable agreement. It is considered that the VSE is the dominant mechanism limiting the transition velocity of solid armatures provided their contact region geometry and structural design is such that adequate contact pressure is maintained