Virtual Vector - Direct Torque Control for a Nine-phase Induction Machine Drive System With Eight Active Voltage Vectors Emulating Virtual Voltage Vectors

This paper discusses a Virtual Vector - Direct Torque Control (VV-DTC) technique that employs eight active voltage vectors to emulate the virtual voltage vectors used to control the electromagnetic torque and stator flux magnitudes of a three-neutral nine-phase induction machine drive system. In this configuration, the DTC strategy must take into account the harmonic planes in order to mitigate or even nullify the low-frequency current harmonic components. It will be shown that the previous solutions discussed in the literature (that proposed the use of two and four active voltage vectors to emulate the virtual voltage vectors) are not capable of nullifying the most significant 5th and 7th harmonic components simultaneously, and only by applying eight active vectors this is possible. By means of experimental results, this paper presents a comparative analysis between the conventional DTC and VV-DTC strategies for steady- and transient-state operation. It is shown that the proposed technique reduces the phase current total harmonic distortion (THD) and torque ripple at the expense of an increase of the switching frequency, and presents similar dynamic response than the considered counterparts.