PWM method to eliminate power sources in a non-redundant 27-level inverter for machine drive applications

A non-redundant, three-stage, 27-level inverter using "H" converters, is analysed for machine drive applications. The main advantage of this converter is the optimisation of levels with a minimum number of semiconductors. However, the system needs six bi-directional and isolated power supplies, and three more unidirectional if the machine is not using regenerative braking. In this paper these nine power supplies are reduced to only four, all of them unidirectional, with three strategies: 1) the utilization of independent and isolated windings for each phase of the motor, 2) the utilization of independent input transformers, and 3), the most important of them, the application of special PWM strategies to keep positive average power at the medium power converters, and zero average power at the low power converters. The generation of this PWM and control of this multi-converter, is being implemented using DSP controllers, which give flexibility to the system.