Control scheme and characteristics analysis of three-phase series resonant converter suitable for contactless slipring system

Compared with the loosely coupling transformer, the air gap of the rotating transformer in the contactless slipring system (CSS) is relatively much smaller, resulting in a higher coupling coefficient. Higher coupling coefficient could reduce the sensitivity to the parameter variations. But at the same time, it will also increase the harmonic content in the resonant tank and the analyzed error with fundamental harmonic approximation (FHA). This paper presents the accurate expressions of the primary and secondary current for the S/S compensated resonant converter, which is beneficial for the implementation of the control strategies requiring precise computation. However, due to the existence of the air gap, the power transfer capability is limited. So in higher power range, the three-phase system approaches are an encouraging alternative to single-phase systems. Ignoring the effect of the cross coupling, this paper introduces a three-phase S/S compensated system which is star-connected with an open neutral point. The three-phase inductive power transfer (IPT) CSS is operated under symmetrical and asymmetrical control to regulate the output voltage against the line and load variation. From the comparative study, we find that the asymmetrical control is better than the symmetrical control in the realization of soft switching. Finally, experimental results on a 60W prototype are presented to evaluate the effectiveness of theoretical analysis.