Analytical Determination of DC-Bus Utilization Limits in Multiphase VSI Supplied AC Drives

Two-level multiphase voltage source inverters (VSIs) are typically used as the supply for multiphase machines. Such machines are often with concentrated winding in which case, certain low-order stator current harmonics are injected to provide torque enhancement. Also, a multimotor drive system can be realized by connecting a number of multiphase machines in series, using phase transposition while supplying the whole system from a single multiphase VSI. In both situations, it is important to know the limits of the inverter operation in the linear modulation region. This paper develops a simple method that enables analytical determination of the boundaries of the linear modulation region for all multiphase inverters with a prime number of phases. The limits are independent of the actual pulse width modulation (PWM) method utilized, and are equally applicable to both carrier-based and space vector PWM techniques. Theoretical considerations are verified experimentally using the five-phase and seven-phase VSI rigs.