An Effective Control Technique for Medium-Voltage High-Power Induction Motor Fed by Cascaded Neutral-Point-Clamped Inverter

For a cascaded neutral-point-clamped (NPC) inverter applied to the medium-voltage high-power induction-motor (IM) drives, an effective control technique is proposed in this paper. The novel sinusoidal pulsewidth modulation (PWM) pulse-rotation-control approach, for a wide range of output voltage, provides a simple way to implement vector control for IM when the cascaded NPC inverter is employed. The proposed method presents great benefits to the cascaded NPC inverter. The output voltages and power of all inverter modules and the two series-capacitor dc voltages of each inverter module are perfectly balanced. Moreover, a low switch frequency of all inverter modules supports a synthesized high-frequency PWM phase voltage. The internal voltage drop of the inverter, due to the cascade structure of many insulated-gate bipolar transistor-diode modules´ series connection, is analyzed, which causes the distorted phase voltages and currents at low speeds when the frequency and the output voltage are low. The current closed-loop control compensates the distortion of phase voltages and currents. A rotor-flux-oriented vector control is combined with back-electromotive-force-based model reference adaptive system speed estimation, which results in a speed closed-loop control. The voltage sensors together with the filters of changeable parameters ensure the precision of speed estimation for the whole frequency range. The experimental tests are carried out through an 800-kW 4160-V IM drive fed by the 1-MVA 6000-V 17-level cascaded NPC inverter. The results verify the proposed scheme.