It is an important goal to get larger contact area between armature and rails in the design and operation of railguns. However, oversize contact area may not be so necessary. Most of the current in the railgun will only flow in part of the contact area even the whole flank surface of the armature contacts the rails. So if part of the armature flank surface does not contact the rails, the maximum value of the current density remains almost the same as that with the whole surface contact. It is addressed in this paper what size is large enough from the view of current distribution and the maximum current density. Calculations are also done to analyze the factors affecting the size. Generally, 20% of the armature flank surface can be regarded as the large enough size of the contact area for the certain structure of railgun analyzed in this paper.
The shrinking contact resistance is due to the current shrinking near the so called "a-spot" contact area between conductors. In the numerical simulation of armature and rails, e.g., in the finite element simulation, such "a-spot" model can not be dealt with, since the gaps between conductors are too narrow to be meshed. It is suggested in this paper that a circular truncated cone can be employed to simulate the "a-spot". The truncated cones with suitable dimensions is able to enlarge the gaps wide enough to be meshed, and simulate the contact resistance in a desired accuracy. The method of determining the suitable dimensions of truncated cone is given based on the calculation and analysis of the resistance of truncated cone with different dimensions. With the truncated cone models, the finite element method can be implemented to deal with the partly contact between the armature and the rails, and acceptable simulation accuracy can be gained.
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