I'm not familiar with the FEIN, but all 9 vacuums (2 Craftsman, 4 Shop-Vac,
3 Kenmore) in our house have universal, not induction, motors, and control
quite nicely with SCR speed controls (essentially lamp dimmers).
Universal motors have brushes and can run on either DC or AC. Most
hand-held power tools (drill motors, routers, etc), vacuums, and even
portable planers and cheaper table saws use universal motors because they
can provide high speed and power in a small package. To control them I use
a couple of $20 speed controllers, like Harbor Fright has at:
In my larger Craftsman vac I built in an SCR speed controller. Unless I'm
cleaning up a big mess, I usually cut down the speed a bit so I don't feel I
need to don my hearing protectors. The vac does lose power when you slow it
down significantly, but often it's still enough.
Actual "lamp dimmers" might not do the job, because motors are inductive
loads and the SCRs in the dimmers have trouble shutting off (to slow speed)
unless a couple of extra components are used in the dimmer. You'd have to
try it, if you have one lying around. Otherwise, I'd think HomeDespot, etc,
might have a controller like the HarborFright unit in their tool dept.
Reminder - this is only if your FEIN has a "universal" motor. Induction
motors, like in most table saws, jointers, drill presses, furnace fans, etc,
do NOT control well with such controllers.