The motor in a Dremel is a universal motor and, as such, has no problems at
all with the usage of a reostat or variable resistor in series with the
motor. I'll note that there are models of the Dremel that have a built in
electronic speed control and you can use that in series with the reostat to
limit the max. speed of the Dremel. The electronic speed control works by
the phase control of the power so it will do its thing while the reostat
will do voltage reduction. There may be a minor loss of control (motor
stops) if the input voltage gets too low for the electronics in the tool.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!