28 IEEE POTENTIALS 0278-6648/10/$26.00 © 2010 IEEE R ecently, multilevel converters have attracted attention in medium-voltage and high- power applications due to reduced voltage stress on power semicon- ductor switches, reduced voltage harmon- ics, and electromagnetic interferences. For higher voltage and high-power applica- tions, the switching frequency and device ratings are limited. Therefore, it is desir- able to distribute the stress among the number of devices using a multilevel con- verter. There are three important multilevel topologies: the neutral point clamped (NPC) multilevel converter, flying-capaci- tor multilevel converter, and cascaded H-bridge multilevel converter. Research in the area of multilevel bridge converters has mostly concentrated on three-level NPC converters for medium voltage and high power drive applications. Theoreti- cally, diode clamped topology with any number of levels can be visualized. But some of the problems like unbalanced voltages across capacitors, voltage clamp- ing requirements, complexity of switching algorithm, circuit layouts, and packaging constraints have limited the number of levels in a practical multilevel converter to seven. The main focus of this article is to develop an experimental setup for gradu- ate level research for a three-level NPC front-end ac-dc converter and a three-level NPC inverter that feeds the induction motor drive. By using this topology, the front end ac-dc converter can operate in utility friendly mode drawing sinusoidal input current at unity power factor while the induction motor drive is operated with closed-loop speed control having four- quadrant capability. Time for a brief intro Å three-level NPC inverter-fed induc- tion motor drive ensures more flexible control, but the complexity of voltage selection and neutral point voltage balanc- ing becomes another problem. Some switching states generate unbalanced operation of the neutral point voltage. To overcome this problem, a voltage control Digital Object Identifier 10.1109/MPOT.2010.937049 © LUSHPIX AND DIGITAL STOCK Multilevel converter fed induction motor drive for industrial and traction drive Ranjan K. Behera and Shyama P. Das