Selecting a brushed or brushless DC motor – decisions, decisions

Brushless DC motors are more popular than brushed motors at the moment, but the fundamental question when choosing a motor remains: "What does it need to do?" What speed does your application require? Brushless motors offer higher speeds, typically you can't go above 20,000rpm with a brushed motor, because the brushes begin to bounce. Brushless do not suffer from friction loss and are only limited by the speed of the controller and bearings, so are capable of 200,000rpm and above. A controller is not always needed for a brushed motor therefore reducing the complexity and cost of operation. What power is required? Brushless motors have a higher efficiency and greater power density than their brushed equivalents, typically 8-12% better but you also need to take into account a controller is required which can influence the efficiency and add to the cost. What longevity is required? The lifetime of a brushless DC motor is only limited by the life of the bearing. A brushed motor is limited by the brushes wearing out, subject to speed and load. The brushes can crack with a mechanical or physical impact or shock. Is noise an issue? Less electrical noise is generated by brushless DC motors and therefore, less electromagnetic interference. The brushes in a brushed motor create mechanical and electrical noise due to friction against the commutator. Typical applications that exclusively use DC brushed motors are haptic robots, nuclear radiation environments and automotive. Typical applications that use DC brushless include prosthetics, space satellites, implantable medical systems, downhole drilling and hybrid vehicles. Although the perceived efficiency of brushless DC motors mean they are increasingly used in design, for some applications only a simple brushed motor will do, and of course, there are always exceptions! Karen Whittaker is a marketing executive with maxon motor. Email salesuk@maxonmotor.com or call 01189 733 337 for more info.