I scavenged an old vacuum motor and I was wondering, how do I slow it down? I want it to run about 100-150 cfm but since it's an ac motor, I have no idea how to do it. I thought about running a dimmer switch to an outlet to which I could plug it in but I read that I shouldn't since it's against code. It wouldn't be mounted in my house, just on a piece of wood.

I would use an inlet gate and an air dump. Dump could be a tee with a slide gate between the vacuum and the forge. Inlet gate could be a plate pivoting on a screw to partialy block the air inlet to the vacuum. You may get away with just the inlet restriction.

Do not use a dimmer switch, it will burn up your motor. Get a speed control switch for a ceiling fan, they are made for motors just check the load rating first.
Rob

I scavenged an old vacuum motor and I was wondering, how do I slow it down? I want it to run about 100-150 cfm but since it's an ac motor, I have no idea how to do it. I thought about running a dimmer switch to an outlet to which I could plug it in but I read that I shouldn't since it's against code. It wouldn't be mounted in my house, just on a piece of wood.

You don't have to slow down the motor; you can put a valve on either the intake or output to control the airflow. I used to control one blower I had by putting a piece of cardboard on the intake, the suction would hold it in place and I could slide it back and forth to get just the right amount of air.

Eventually you'll want to look for a blower with big pressure creating vanes instead of a little velocity producing squirrel-cage, but by then you won't have any plumbing questions.

Most light dimmers are not built to handle motor speed control.
You can try something like this.
http://www.harborfreight.com/router-speed-control-43060.html

If it's an old style motor with brushes it's probably a universal motor and so can be slowed down with a simple rheostat.

If it's a modern motor, just dump the excess air!

Thomas is absolutely correct. If you see sparks when the motor is running, it is a universal motor and can be slowed down by lowering the voltage or putting a resistance in series. The permanent magnet motors work really well with the lowered voltage approach. I just use a junk battery charger for my leaf blower powered forge. No modification required to the leaf blower! If you don't have a battery charger, you can use a cheap resistor (incandescent light bulb in series: you can often get these free when people are ditching them for CFL's).

I've heard of hair dryer's burning up because of the lack of air flow in one direction; it has nowhere to go but back into the fan. If this will not harm my vac motor using a valve type system, then I will do so. Will this happen to the motor?

Maybe get one of the motor speed controllers as stated above and also put a gate on it also. Just make sure the gate has a hole in it to let some air always move. I have a very old forge with an original electric blower that has no gate just a speed control for the motor. I would do the motorcontroller first and add the gate later if you needed more adjustment of the air.
Rob

To confirm what's already been said, I got started with an old style vacuum cleaner and used a cheap dimmer switch to slow it down.

Used it for a couple years 'til I scored my Canedy Otto hand crank, but it was still running when I took it down.

It served me well.

Don

The over-head fan controllers cost about the same as the dimmers. Vacuum motors run way too fast and noisy. I'd use a controller to set the max output and a gate for the off-on of regular use. It's nice to just throw it open or slam it shut rather than playing with a controller.

I use a squirrel cage blower for most of my forging, but it's not strong enough for welding, so I have a shop-vac motor for that. One thing about vac motors is the noise. They are much louder than the squirrel cage. So I can see the need to slow it down over gating. For my vac blower, I tried my router speed control, but that didn't do much. The motor didn't turn at all. The nice thing is this vac has a resistor-based speed control, it just doesn't slow it down enough. I need to play with that resistor when I get that round tuit.

Thanks for everyones input. I came across one problem though, it sucks regardless of wire position. I couldn't figure it out so I just bought a cheap $10 hair dryer and it works great.

it sucks regardless of wire position.

A centrifugal fan throws air from the center outward regardless of which way it turns.

A centrifugal fan throws air from the center outward regardless of which way it turns.

To add to what Grant said, most vacuum blowers that come out of vacuum cleaners do not have an exhaust per se. The air is drawn into the front of the blower through a filter of some sort creating the suction of the vacuum cleaner. But the exhaust is blown out through the motor to cool it. So the blower would have to be placed into some sort of housing to direct the exhaust.

I'd recommend using a hairdryer.

I'd take a ride out to your local auto wreckers and pull the air con (hot/cold air blower) fan out of a car. Run it from a battery or battery charger. If you're lucky or patient enough you can take the switch gear from inside the car wiring loom to have additional control (three or more speed settings) and you can take some of the ducting too.
I've used these 12v blowers before and they work well. Just my 2 cents worth

Thanks for everyones input. I came across one problem though, it sucks regardless of wire position. I couldn't figure it out so I just bought a cheap $10 hair dryer and it works great.

If you can, disconect the heating element - the "cheap" hair dryers have a tendency to overheat and shut down pretty quickly.