Variable-Speed Control
for Universal Motors
M
ANY WOODWORKERS HESITATE TOuse variable-speed controls
with their tools because they have heard that these devices
work fine with some motors but may damage other motors. Here’s a
simple primer.
The variable-speed controls offered by several manufacturers,
including MLCS, Penn State Industries, and Lutron, are really adjust-
able voltage controls and work very much like transformers used for
electric trains. One exception is that the electronics in these devices
can sense motor load that regulates the speed of the power tool to
maintain a nearly constant speed, during both light- and heavy-duty
work loads. This is highly desirable on tools that must handle variable
loading, such as portable drills and routers.
Speed control units always carry the disclaimer that they may only
be used with tools powered by universal AC/DC motors. The feature
that distinguishes a universal from other motor types is the use of
brushes. Motors with brushes are easy to spot: Two removable caps
that cover the brush assemblies can usually be seen on either side of
the motor housing. With the exception of battery-powered tools,
almost any portable power tool (and some small stationary machines)
equipped with brushes has a universal motor and can be used with a
variable-speed control. AC motors used on most stationary power
tools sport induction motors. Their speed cannot be controlled by
voltage but is instead fixed by the frequency of the power.
CHAPTER ONE