A five/nine-level twelve-switch neutral point clamped inverter for high speed electric drives

A twelve-switch, three phase, five/nine level inverter is presented for applications using a high speed electric machine with a relatively low per-unit leakage reactance. By reducing the voltage blocking requirement of the semiconductors, the neutral point clamped variant of the coupled inductor inverter topology (NPC-CI) is more suited to high DC bus voltages, and features a significant reduction in magnetic material required over six-switch coupled inductor inverters. This paper describes the operational and design techniques for the NPC-CI inverter. A simple continuous PWM technique for operation with a three-limb core is presented and the specific design challenges involved in the NPC-CI inverter are highlighted: designing the three-limb coupled inductor, the natural balancing action of the split-dc link, semiconductor stresses, inverter construction and practical considerations for inverter operation of the drive. Machine performance is then illustrated on an unloaded 18,000 rpm 15HP induction machine to emphasize the harmonic quality improvement, and on a loaded 2HP utility speed induction machine to demonstrate transient performance.