The motor designer´s viewpoint of an adjustable speed drive specification

Historically, AC motors have been installed on fixed voltage/frequency power supplies. When motors were specified for these power supplies, it was adequate to specify a few things like voltage, frequency, horsepower, service factor, enclosure, NEMA design, and so on. As demands to reduce energy consumption surfaced, some new features began to be specified, including efficiency and power factor. With the advent of the adjustable speed drive (ASD) and its adjustable voltage/frequency power, new features were again added to those specified-type of load, speed range, and ASD type, to name a few. When specifying a motor for use on an ASD, it is not enough to think about what additional features are required. One must also look at relaxing some of the requirements historically specified. Cost is added and benefits are sacrificed if some of these features are specified when not necessary. The author bases his comments on his own experience, which has been with NEMA frame sizes 182 through 5011-though some of what he says here may be applicable for motors in frame sizes outside this scope