Nine level T-type neutral point clamped voltage source inverter for induction motor drive

The paper proposes a three phase nine level T-type neutral point clamped (9L-T-NPC) inverter for medium voltage induction motor drive. The primary objective of the proposed inverter is to improve the output voltage quality with reduced number of devices and passive components. The generation of nine levels in the output pole voltage is achieved using only ten switches per phase. Although the similar structure of the inverter is well-suited for seven level operation, by using a suitable modulation technique, nine levels can be generated in the output pole voltage without increasing the active and passive devices. In this proposed inverter, the flying capacitors in the inverter are properly controlled at the required voltage levels using redundant switching states. The performance of the inverter fed induction motor drive is analyzed using simulation results under varying modulation index and load conditions. Vector control is implemented for the speed control of the induction motor and tested under different transient conditions. A new PWM technique is proposed to balance the flying capacitors during common-mode voltage injection so that the inverter can deliver full active power up to modulation index of 1.15.