Control method of PWM inverter for driving LSM to reduce the burden of output transformer

In Japan, a superconducting magnetic levitation (Maglev) system is under development as a means of super high-speed transportation. This system employs a linear synchronous motor (LSM) with the primary side on the ground. The driving power for the LSM is supplied from a power converter installed at a power conversion station on the ground. The power converter should be able to generate power of variable voltage and variable frequency with low harmonics. As a power converter of this type, a multiple pulse width modulation (PWM) inverter is adopted which consists of plural full bridge inverter units with an output transformer. However, when the output frequency is low, it is difficult for the inverter to generate the voltage required, because the inverter is multiplied by the output transformer. Therefore, one unit inverter is switched to a half-bridge inverter without an output transformer when the output frequency is low, and the inverter is controlled by a particular voltage assignment control method, instead of a general voltage control method, to reduce the burden of output transformer as low as possible. Firstly, this paper describes the voltage assignment control method of the inverter to reduce the burden of output transformer. Secondly, the output voltage is analyzed in detail. Thirdly, the effect of the voltage assignment control and characteristics of output voltage are investigated. Lastly, these results are verified by simulation.