Low-Speed Operation of a New Four-Level Multilevel Inverter fed Medium-Voltage Drive

Multilevel inverters became an integral part of medium-voltage motor drives. The newly developed multilevel inverters are designed with floating capacitors to generate a multilevel voltage waveform, while fulfilling the motor-side requirements. However, the low-speed operation of motor drive leads to a larger ripple in the floating capacitors voltage. These ripples put more stress on floating capacitors and switching devices, and subsequently cause their failure over a long run. The voltage ripples also affect the quality of motor stator currents and electromagnetic torque. Therefore, the minimization of ripple in floating capacitors voltage is a key aspect of developing a high-performance motor drive. First, a new medium-voltage motor drive for high-power applications is developed. The developed motor drive is fed by a new four-level multilevel inverter. Second, a simple voltage balancing method is developed to balance the floating capacitors voltage in the proposed multilevel inverter. Finally, a modified carrier pulse width modulation scheme is proposed to reduce the ripple in floating capacitors voltage at the low-speed operation of a motor drive. The proposed motor drive performance with a modified carrier pulse width modulation is validated through simulations. Also, the performance comparison with in-phase disposition pulse width modulation scheme is presented.