Based on the theory of H-bridge multilevel converter, the structure of a three phase cascaded multilevel inverter (LVCMI) and its control strategy are proposed in this paper. Several standard three-phase bridges can be reconnected into a new single three-phase bridge at ac-side. Simulation results show that such topological structure can increase the output voltage capacity and reduce the THD. It can be used as an inverter or rectifier in three phase system. In theory, it can be cascaded unlimitedly. The more cascaded, the higher output voltage and the lower THD. Experimental results of LVCMI with 3-module verified the feasibility of the proposed inverter.

For the crawling problems of the changing-wheel device,the paper put forward a single-degree-of-freedom spring-quality system of a physical model of crawling motion,and described the model mathematically.Than the paper analyzed the main factors which produced several crawl,and obtained the displacement and velocity response curve of the system model,and coincide with the motion state results of the actual light rail in the changing-wheel library device. The paper built a simplified hydraulic system model of the rotary device,and made simulation analysis for the hydraulic system model through the Matlab,found the main factors which influence the crawling system is driving speed (force),vertical load, system stiffness and system damping,than given some methods which can help restrain the crawling. After the simply analyzed for the characteristics of throttle valve,the paper made some optimization for the original hydraulic system,that is at the hydraulic back oil piping,joining the throttle to the components,which purpose is to increase the effective damping of the system,thus inhibiting system to crawl.This optimization already have been proved through the practical engineering application.