To keep phase balance and improve output voltage quality, a novel multiple-loop control system in 400-Hz inverter power supply based on unipolar sinusoidal pulse width modulation (SPWM) is proposed. The causes of phase imbalance in a threephase combined inverter power system are analyzed, and a new multiple-loop control strategy based on phase control is presented to eliminate it. For reducing the total harmonic distortion (THD) of high-power inverter output, unipolar SPWM is added with this new control strategy. The designs of drive circuit and snubber circuit are discussed to eliminate dead time effects and voltage oscillation. The stability of the control system is analyzed and the power system and the control strategies are realized with digital signal processing. Contrastive simulation results indicate the advantages and the correctness of this novel controller for inverter. A three-phase combined inverter system based on a multiple-loop control strategy is constructed, and experimental results show that the inverter system can make a phase difference between each phase less than 2 • , and the THD of output voltage can be controlled within 2%.Index Terms-Medium frequency, multiple-loop control, phase feedback, three-phase combined inverter, unipolar SPWM.

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Ladle slag carry-over detection technology (SCDT) is of important practical significance to steel continuous casting production (CCP), which can effectively improve the casting blank quality, increase molten steel yield ratio, and protract the service life of tundish. The current SCDT realization methods and their application circumstance were summarized, and their main problems during the course of factual production were pointed out. The difficult technical points of detection principle, digital signal processing for vibration style SCDT development were described. To aim at the problems of vibration style SCDT, such as low recognition stability and long applied adjustment time, its key technologies including water model experimental platform establishment, two-phase sink vortex entrapment mechanism, forced vibration response of shroud nozzle and steel stream shock vibration signal processing optimization were analyzed deeply, and the corresponding research route and advices were given. ladle slag carry-over, sink vortex, slag entrapment, shock vibration Citation: Tan D P, Ji S M, Li P Y, et al. Development of vibration style ladle slag detection methods and the key technologies.