Hook vacuum to a dimmer switch?

Hi all,
Would it be safe to run a FEIN vac (the small one 920watts) off a dimmer switch so I can further quiet it down when I'm working in stealth mode (3 month old and 3 year old sleeping upstairs). I will be building a vac box in the future, but the dimmer would be a $3 quick solution for now.
I don't want to screw up the motor via trial and error, so am asking anyone's thoughts before I do this. Thanks.
-Kevin
I have a MIELE housevac and sure wish they made a shop vac.
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Kevin -
Unless that's a DC motor, and it is *way* unlikely, I don't think your solution will work. AC motors are fixed speed. Lowering the voltage with a dimmer will increase the amp draw to provide the proper amount of power to the motor and putting a dimmer on it would be very similar to making it run during a "brownout". The motor would overheat.
Others here have FAR more EE experience, and I'm sure they will weigh in. I wouldn't mind having a "variable speed" for my shop vac either. The sticker on the vac says it has more than TWICE the power of my 3 HP tablesaw and I'm STILL trying to figure out how they do that on 110V. Those engineers at Craftsman, is there NOTHING they can't do?
How about buying a longer hose and putting on the far end o' the shop?
HTH,
John Moorhead Lakeport CA
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Using a dimmer switch on most any AC motor or transformer will eventually burn it out. It may start an electrical fire--don't do it.
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On 6 Jan 2004 18:21:25 -0800, snipped-for-privacy@hotmail.com (Kevin) wrote:

I wouldn't do it. Dimmers are not meant for motors, and Feins are expensive.
Alternatives? Scrape instead of sanding. Sweep instead of vacuuming.
Barry
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On Tue, 06 Jan 2004 18:21:25 -0800, Kevin wrote:

Not a wise move. Even if you were able to slow down the motor, the suction would change.
If the noise bothers you, get ear protection, just one alternative to replacing an expensive vacuum.
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Kevin-
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: http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumberC060
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.
-Dave
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"Dave Plumpe" wrote
Snip of some outstanding information ...

If you DO need to control the speed of a single phase induction motor, you can start here: http://www.anaconsystems.com/text/eagle1.html ... but note these VFDs (Variable Frequency Drives) are NOT inexpensive.
Found this by a google search (single phase induction motor speed control).
If you want to roll your own, here's a great start: http://www.cae.wisc.edu/~ece734/project/f00/yaorpt.pdf
and another at http://gdansk.bradley.edu/iecon02/limited/cdrom_backup/html/pdf/SF-003115.pdf
HTH
Rick
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Kevin-
Dave's response is the best one posted so far. I have yet to see a shop vac with a motor other than a universal one.
I just went out in the garage and tried both my shop vac and an upright vac with: 1. Router speed control - worked fine. 2. X10 lamp dimmer module - worked fine. 3. Wall plate light dimmer switch - worked fine.
I see no problem with using any of the above with a shop vac. The slower speed does reduce the noise and suction of the vac. It also slows the airflow over/thru the motor but it also reduces the power into the motor so there shouldn't be any problem with overheating.
The only negative I see is that it appears that the suction falls off faster than the noise level does as the speed drops (I have no quantitative data here, just my ears and hands). So to get to the noise level you want, you may find that the suction is too weak to be of any use.
Art

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Thanks all,
You confirmed my suspicions. In my simplistic thinking, I was thinking the motor would overheat because it was engineered to run at specified fix speed, slowing it down would cause overheating and then a cascade of bad events afterword.
I don't think it's a univeral motor? I'll check the specs. If not I guess I'll be building that vac box sooner rather than later.
Thanks for advice
-Kevin
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