Adjustable speed drives and motor protection

One of the application considerations that must be evaluated when utilizing an adjustable speed drive (ASD) is motor protection. This is not as simple as it may seem. There are many potential motor protection relay (MPR) options in the market; however, these MPRs have typically been designed to provide protection to motors operating on standard sine wave utility power at nominal frequencies assuming some form of direct or reduced online starting. As a result, there are a number of considerations that engineers must take when applying these relays to ASD applications, since the variation in frequency can render certain relay features ineffective.