Selecting Dc Brush and Brushless Motors

A fundamental understanding of the speed-torque curve it the key to specifying a dc motor that will do everything it needs to do. Much has been written about the process of selecting motors for applications in motion control. Motor selection can be complicated. The decisions to be made include whether to employ brushless or brushed dc, or stepper motors. Stepper motors are generally considered specialpurpose devices because they can't handle large inertial loads very well. And operation near their maximum torque ratings can take some finesse. • Shaft-bearing systems, commutation, and motor windings all factor into whether a specific motor will handle an intended application. • It is important to understand where the intended operating point lies on the possible torque-versus-speed curves. In contrast, dc motors are usually considered more general-purpose motioncontrol devices. They are a frequent choice for supplying variable speeds when precise speed regulation is not critical. They can handle brief overloads as well as frequent stops and starts. Dc motors are usually rated at a single speed and torque, and they can operate at rated conditions continuously. There are basic calculations that can point the way to using the right dc motor in the right situation. Parameters that define the best motor type for an application include the mechanical output power, the shaft bearing system, the commutation system, and the possible combinations with gearheads and sensors. The most important criteria include the required speed and torque and the commutation system. Of course, equations define these relationships. In this article, variables on the motor shaft (output) are identified with the subscripts Mot. For example, n gives the required motor speed. Parameters that describe the qualities of the motor have no special additional subscript. For example, stands for the motor's no-load speed. We'll discuss speed and torque requirements first. The