Low switching frequency and high dynamic pulsewidth modulation based on field-orientation for high-power inverter drive

In megawatt-rated inverter-fed induction motor drive systems, the gate-turn-off switching losses account for a considerable amount of the total losses, hence, the switching frequency in such systems must be kept at a low value of only a few hundred hertz. To avoid undesired torque harmonics under such operating conditions, the pulse control of the inverter is made dependent on the orientation of the rotor flux of the drive machine. This method transfers a major portion of the unavoidable current distortions into the field axis where they have no influence on the machine torque. An optimal trajectory-oriented control is presented that achieves low switching frequency at low torque and current harmonics and exhibits a very fast dynamic response. The performance of this method is demonstrated by measured results from a 30-kW model drive