Determination of magnetic coupling and its influence on the electromagnetic tube forming and discharge circuit parameters

Abstract

The coil-workpiece magnetic coupling coefficient K is one of the factors that influence the efficiency of electromagnetic forming (EMF) process. Along with this, an evaluation of the magnetic coupling coefficient is important in simplifying the electrical representation of the process. The first aim of this study is to analyse the effect of K on the EMF process efficiency, and second is to obtain a simplified relation between K and coil-workpiece gap (h) and thereby study the variation of discharge circuit parameters with h.The analysis is carried out with the help of experiments, and the finite element based numerical simulation. Aluminum tube of 100 mm length and 1.5 mm thick is electromagnetically expanded using a 7 turn helical coil. Trials are taken for the different gaps between the coil and tube keeping the discharge energy constant. The coupling coefficient (K) variation with the coil-tube gap (h) is related with curve fitting, and this K-h relation is then used to analyze the discharge circuit parameter dependency on h. Simulation results, as well as K-h relation based analytical study, show a good match with the experimental observations. The EMF process efficiency and tube displacement are found to be improved exponentially with the increase in K. Among the discharge circuit parameters, with the increase in h, exponential rise in inductance and exponential decay in peak current is observed with a marginal decrease in the resistance.