Squirrel cage alternating current (AC) induction motors, the usual subject of Motor Doctor articles, are one of the world’s great “evergreen” technologies. The basic principles and fundamental designs have been known and applied for approximately 100 years. (And no, I was not around when they were first introduced.)

Most of the changes over the years have been in the area of materials — primarily metallurgical changes associated with new alloys and chemicals related to insulating materials.

Direct current (DC) motors became of less importance when DC distribution systems fell into disfavor. But as you will read later in this article, they may be one of the industry’s “comeback kids.”


AC motors found their preeminent place as the result of a drawn-out and very public battle between two titans of American commerce: Thomas Edison and George Westinghouse.

The Westinghouse camp advocated the distribution and use of alternating current, backed by the enigmatic innovator, Nicolai Tesla.

Edison’s adherents were steadfast in the use of DC distribution and devices.

The argument became public, political, and often personal. Spectacular “hair-raising” demonstrations were stages to promote or disparage one technology over the other. In some ways, the rivalry resembled some of the worst political campaigns of the era, complete with name calling, innuendo, and (occasionally) outright slander.

It is fairly clear which technology won. Virtually no public distribution systems that use DC power are left in the world today. For a long time, DC motors were relegated to applications separate from the electric power grid, or in applications requiring specialized, highly defined characteristics.


In spite of the relatively high cost of providing a DC power source, DC motors have some advantages over their AC rivals.

The most noticeable advantage of DC motors as a class (there is a wide variety of DC motor designs) is that they lend themselves to speed and torque control to a far greater degree than a standard squirrel-cage, AC induction motor. DC motors also are inherently more efficient than AC motors, when you don’t consider the relative efficiency of the power source.

As a result of the emergence and widespread use of electronics, DC motors have been experiencing a bit of a resurgence lately, with a growing number of applications in industrial and commercial markets.

Less costly semiconductors have made the conversion of AC into DC more reliable and more economically feasible. The continually decreasing cost of semiconductors and logic circuitry is making it more cost effective to take advantage of the unique capabilities offered by DC motors.

DC motors are now finding their way into new applications that were previously the exclusive domain of their AC brethren. This includes exercise equipment and residential garage door openers.

In future articles, the Motor Doctor plans to look at specific DC motor designs, their uses in the field, and the unique service issues that relate to this “other” motor technology. I do not plan, however, to resurrect the Edison/Westinghouse argument.

Simon, The News’ “Motor Doctor,” is with A.O. Smith Corp., P.O. Box 245010, Milwaukee, WI 53224-9510.

Publication date: 06/10/2002