Motor Fusing for Adjustable-Frequency Inverter Systems

The use of static adjustable-frequency inverters for group motor drives is growing rapidly and will continue to do so. Since many applications are for critical loads or processes, it is desirable to provide individual motor or branch circuit fusing, such that a short circuit will be cleared without causing loss of power to the remaining motors on the inverter system. The proper coordination of this motor fuse with the inverter is not generally understood or discussed in the literature. This paper discusses and defines the basic nature of the problem. The results of extensive fuse coordination testing are presented for two different size inverters. Some useful conclusions are drawn as guidelines for proper motor fuse selection in the general case.