Line start permanent magnet motors - Starting, standards and application guidelines

The advantages of using permanent magnets (PM) in electric motors to increase efficiency are well documented [1]. Due to the synchronous operating nature of a PM motor, most motors of this type are used in combination with a variable frequency drive (VFD). PM motors capable of direct on line (DOL) starting have been manufactured for over 30 years [2], but the application of this technology has historically been limited to low horsepower, niche applications. In an effort to provide motors with higher than premium efficiency levels, this technology is now making its way into much larger motors. This paper discusses the transients associated with DOL starting this type of motor and the potential challenges they pose. These include transient torque, inrush current and corresponding dips in voltage. The same issues exist with induction motors and comparisons of the two technologies are offered. In addition, the ability to synchronize a given load must be considered. Test results from line starts of this type of motor are presented. Various techniques to diminish the effects of the starting process, including application guidelines, are discussed. Ongoing efforts to establish industry standards for this type of motor are reviewed.