DC motors: Why are they still used?

Engineers, machine builders and maintenance staffs had and have knowledge of DC. DC converters are simpler in design than AC inverters, lower in cost and easier to repair. DC motors can be repaired repeatedly. If a piece of machinery is powered by a DC converter and motor, and if either one should fail, it’s easier (and cheaper) to replace the failed item then to convert the machine to AC. If a plant has 10 machines using DC and wants to order an 11th, there’ll be a strong bias to purchase what has worked before.

During past several years, DC motor manufacturers’ ongoing R&D has concentrated on redesigning the most maintenance-intensive section of the DC motor, which is the commutator and brushes.

As design engineers continue to increase the power density for a given frame size, the motor’s commutator gets smaller. As the circumference of the commutator shrinks, there’s less brush wear with every turn of the rotor. Reduced brush wear results in extended intervals between brush changes. Engineers also have redesigned brush blocks, pressure fingers and springs to allow for longer brushes. With longer brushes, the interval between brush changes extends further, providing for longer periods of operation without a maintenance shutdown. DC motors can be purchased with brush wear sensors, which warn that a brush is worn down to its lowest level and requires changing. Brush wear sensors often prevent commutator damage from a worn brush being left in too long and resulting in costly repairs.

Active research and development

With the DC motor being one of the oldest technologies, you’d think R&D has ended. Many motor companies continue to offer their older designs and there are some that have dropped the product completely. But, there are motor companies that continue to invest in developing the technology. Using software modeling tools, engineers can get a better understanding of both the magnetic flux and thermal flows in the motor laminations. Companies with active R&D programs are incorporating developments in insulating materials into their designs. Slight changes in lamination geometries, metallurgy, and insulating materials allow for increased power density and smaller motors.

Companies with active R&D also are helping to reduce maintenance costs by extending brush life. This can be done by designing smaller commutators, lengthening the brushes, adding brush wear sensors and making it easier to replace brushes. Studying the brush/commutator junction is a never ending activity. There are groups using the latest sensor and control technology to determine what is the best environment (temperature, humidity, pressures) that leads to optimum junction performance. They’re also asking what can be done to ensure the junction environment is optimum at the locations and ambient environments in which the motor operates.

Everyone has heard the story that in 1899, the head of the U.S. Patent Office sent his resignation to President McKinley because, he said, “Everything that could be invented has been invented.” This turned out to be untrue and so is the tale that DC motors are no longer being used and no one is investing in research and development. The applications available for the DC motor are fewer than in the past. However, the operational characteristics of higher power density, low inertia and higher speed ranges continue to make the DC motor the preferred choice for many machine builders. Also, the magnitudes of the installed and knowledge bases cause users to request DC motors as prime movers even on new equipment.

Bob Simon M.Sc., P.E. is a DC motor specialist at ABB in New Berlin, Wisc. Contact him at bob.simon@us.abb.com and (262) 785-8592