A General PWM Method for a (2n+1)-Leg Inverter Supplying n Three-Phase Machines

Control of an ac machine requires only two currents. It is, therefore, possible to achieve independent control of a number of three-phase machines using various inverter topologies with a reduced switch count. One such drive system comprises n three-phase induction machines supplied by a (2n + 1)-leg voltage source inverter (VSI). In such a configuration, one inverter leg is common for all machines. This paper presents a general pulsewidth modulation (PWM) method that is applicable for any number of machines n in the group. It is based on utilization of the standard three-phase modulators in conjunction with additional logic to generate modulation signals for all (2n + 1) legs of the inverter, and it enables an arbitrary distribution of the available dc bus voltage between the n machines. A developed PWM method has been implemented and tested in two-machine, three-machine, and four-machine drives, supplied from five-leg, seven-leg, and nine-leg VSIs, respectively. Experimental results are reported for open-loop V/f control, closed-loop current-controlled V/f mode of operation, and full vector control. It is shown that the nonideal nature of the VSI causes unsatisfactory operation of the drive system in the open-loop V/f mode, and that closed-loop current control is, therefore, preferred in this type of a drive system. It is believed that the four-machine drive has a good potential for industrial applications since, in certain cases, it enables a considerable saving in the required number of switches and installed inverter power.