The Impact of inverter dead time on performance of n-motor drives supplied from (2n+1)-leg VSI

Industrial applications often require two or more variable-speed electric drives, which have to be controlled independently. Over the past decade research efforts have been made to reduce the number of power electronic devices required in multi-motor drive systems in order to reduce the complexity and hence cost of the drive. One such drive system comprises n three-phase motors supplied by a (2n+1)-leg voltage source inverter (VSI). In such a configuration one inverter leg is common for all machines. While the complexity of the converter and its associated control electronics are reduced, the common inverter leg imposes certain restrictions that make such drives unsuitable for general purpose applications. Moreover, even when there is a niche application to which a particular reduced switch count drive system is well suited, the problems arise in actual implementation. The paper examines in detail behavior of these drive systems for a couple of configurations, using detailed inverter models in simulations. It is argued that (2n+1)-leg VSI supplied n-motor drives are especially sensitive to the inverter dead-time effect. Using both experimental and simulation results, the paper shows that the non-ideal nature of the inverter causes poor performance of the drive in open-loop V/f mode of operation. The solution to the problem is found by using closed-loop current control and it is shown, using experimental results, that such an operating mode fully alleviates the problems caused by the dead-time effect.