A six-phase two-motor drive system with series connection of stator windings of a six-phase and a three-phase machine and decoupled dynamic control, has been introduced recently. The electric power to the two-motor drive system is supplied from a single six-phase Voltage Source Inverter (VSI) and controlled using vector control scheme. If current control is to be implemented in the rotating reference frame, the Pulse Width Modulation (PWM) method for the six-phase VSI is required to generate the required voltages. This paper proposes a scheme to generate inverter output voltages using space vector PWM in such a way that independent control of two series-connected machines is achieved with a minimum interaction between the two machines. The method generates output voltages that contain only two low-frequency fundamentals at the operating frequencies required by the two machines. The application area of the proposed configuration is identified. The required six-phase inverter rating under steady state is obtained analytically and the concept is verified by simulation.

A novel concept for multi-motor drive systems, based on utilization of multi-phase machines and multi-phase inverters, has been proposed recently (Levi et al., 2003 and 2004). Since vector control of any multi-phase machine requires only two stator current components, the additional stator current components are used to control other machines. It has been shown that by connecting multi-phase stator windings in series with an appropriate phase transposition, it is possible to control independently all the machines, with supply coming from a single multi-phase inverter. One specific drive system covered by this general concept is the six-phase series-connected two-motor drive, consisting of a six-phase machine and a three-phase machine. This has been analyzed in considerable depth in Iqbal and Levi (2004) and Jones et al. (2005), and experimental verification of the existence of control decoupling in this two-motor drive has been provided in Iqbal and Levi (2004), Jones et al. (2005), and Levi et al. (2005). The studies of Iqbal and Levi (2004), Jones et al. (2005), and Levi et al. (2005) have been based on inverter current control in the stationary reference frame, using phase current control in conjunction with ramp-comparison current controllers.

Implementation of current control in the rotating reference frame requires an appropriate Pulse Width Modulation (PWM) method in order to impose generated voltage references. Although ramp-comparison method can be used for this purpose, the trend in digital control of AC drives has been, for a long time, to use space vector PWM. A space vector PWM scheme that would be applicable in conjunction with the series-connected six-phase two-motor drive system does not exist at present.

Electrical and Electronics Engineering Journal, Voltage Source Inverter, VSI, Pulse Width Modulation, PWM, Vector Control Scheme, Space Vector Modulation Scheme, Space Vectors, Magneto Motive Force, Vector Controlled Drive, Harmonic Components, International Conference on Electrical Machines, ICEM.