Proper application of motors operated on adjustable frequency control

Most high-efficiency motors can withstand the rigors of adjustable speed drive (ASD) operation. However, there are significant differences between applications of motors operated on sinewave power and motors operated on adjustable frequency controls (AFCs). This is an ongoing subject, rapidly changing due to advances in the technology of the AFCs. Subsequently, many of the older problems associated with the application of motors on AFCs have been solved, while at the same time, new problems were introduced. In many cases, high-efficiency, low-voltage motors can be applied on AFCs with no or little modification. In order to do this successfully, however, basic motor operating principles and the affect of applying a motor on an AFC need to be understood. The proper application of a motor for use on an ASD is dependent on an understanding of: thermal motor limitation-from AFC harmonics and reduced-speed operation at constant load (i.e., constant torque); and upper-speed limitation (beyond base speed)-constant power electrical limitation and mechanical upper-speed limitation. This article targets the application of motors not specifically designed for AFC operation. Generally, NEMA motors (i.e., motors up to the 440 frame) are more apt to be indiscriminately applied on AFCs than above-NEMA (ANEMA) motors (i.e., motors larger than the NEMA frame sizes). The concepts/guidelines presented here are equally applicable to both NEMA and ANEMA motors