Direct torque control of IPMSM driven by a three level NPC inverter with torque and capacitor voltage ripple reduction

The amalgamation of direct torque control (DTC) and three-level neutral point clamped (3L-NPC) inverter is an exciting prospect for the control of high power interior permanent magnet synchronous machines (IPMSMs). The use of 3L-NPC inverter in DTC schemes in general, is hampered by issues such as neutral point voltage fluctuations and smooth voltage vector switching. The major disadvantages of conventional DTC are variable switching frequency and high torque and flux ripples, especially at low speeds. A novel DTC scheme incorporating a 3L-NPC inverter, utilizing all of its active voltage vectors, is proposed in this paper to overcome the abovementioned shortcomings. Torque and flux ripple reduction is achieved through appropriate application of two separate vectors during each switching cycle. Furthermore, neutral point voltage fluctuations are minimized through the proper selection of redundant short voltage vectors. Simulation results are provided to prove the feasibility and effectiveness of the proposed method.