Drives, Motors and Circuit Protection

Variable frequency drives, soft starters, and other power electronic devices are becoming increasingly more common in motor circuits. These power electronic devices are much more sensitive to the damaging effects of shortcircuit currents and therefore require a level of protection that may not be provided by circuit breakers or conventional fuses. In the past, manufacturers of these devices provided internal protection in the form of high speed fuses, which are much more current-limiting than conventional branch circuit fuses. However, as drives and soft-starters have grown smaller and smaller, the internal fuses have been omitted by starter manufacturers in favor of short circuit testing to UL standards with external protection. Now, in many cases, drives are shipped without fuses, and it is the responsibility of the installer or owner to provide this protection. During the design and installation stages, it is important to check the data sheets, label, or manual of the power electronic device to understand the short-circuit protection options. With the proper fuse selection, a safer installation may result, with better power electronic device protection. This can result in more productive operation and higher short-circuit current ratings