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Squirrel cage alternating current induction motors, the usual
subject of Motor Doctor articles, are one of the world’s most “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 associated with
new alloys and chemical related to insulating materials.
A/C 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 direct current
distribution and devices.
The argument became public, political, and often personal. Spectacular
“hair raising” demonstrations were staged 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 the debate. There are virtually
no public distribution systems that use D/C power left in the world today.
For a long time, D/C 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 direct current power
source, D/C motors have some advantages over their A/C rivals. The most
noticeable advantage of D/C motors as a class (there is a wide variety
of D/C motor designs) is that they lend themselves to speed and torque
control to a far greater degree than a standard squirrel cage A/C induction
motor. D/C motors also are inherently more efficient than A/C motors,
when you don’t consider the relative efficiency of the power source.
As a result of the emergence and widespread use of electronics, D/C 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 alternating current into direct current 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 D/C motors.
D/C motors are now finding their way into new applications that were previously
the exclusive domain of their A/C brethren. This includes exercise equipment
and residential garage door openers.
In future articles, the Motor Doctor plans to look at specific D/C motor
designs, their uses in the field, and the unique service issues that relate
to this “other” motor technology. I don’t plan, however,
to resurrect the Edison/Westinghouse argument.
by Neil Simon, aka the Motor Doctor - regional sales
manager for A. O. Smith Electrical Products Company.
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