This paper provides a theoretical and experimental discussion on an adjustable-speed motor drive using a modular multilevel cascade converter
based on triple-star bridge cells (MMCC-TSBC) when the motor frequency is close and equal to the supply frequency. The TSBC is one of the most suitable candidates for medium-voltage, high-power motor drives with regenerative braking. However, it suffers from capacitor-voltage fluctuations that become larger as the motor frequency gets closer to the supply frequency. This paper presents a practical solution to overcome it with acceptable circulating currents. This solution makes it possible to achieve an adjustable-speed drive of a line-started motor loaded at the rated torque. Experimental waveforms show good start-up performance from a standstill to the rated speed, keeping each cluster current within an acceptable level.
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