AC fan speed

[sbisdabomb]

I just bought a 2 in one fan/light speed controller switch from our local home depot style thrift store for $4. It has two knobs, one for the fan, one for the light, but the only one that works on my AC fans is the switch specificaly made for the fan, not the light.

[Doc B.]

Hi Guys,

I just built a projector with a closet mounted external box that houses a variac for controlling the speed of three 120VAC 120MM fans and also houses the ballast and cap for a 400W MH lamp. 25 ft. long 16ga. umbilical cables feed the fan voltage and lamp voltage to the projector. Obviously my projector setup is not what one would call compact or portable...

A variac (variable autoformer) is a very nice way to control the fans, as you can dial whatever speed, and hence noise level, that you can get away with temp-wise. The 120V fans typically draw only 200mA, so the Variac can be a small one, even with multiple fans. I used a 2A rated variac for two fans in the projector box and one fan in the external control box itself, to cool the ballast. I haven't measured the voltage I ended up running, but it looks to be about 60V judging by the knob position.

Different fans will behave differently at a given voltage. Some may run freely at 40V and others may need 60V to get spinning. A simpler way to drop the speed of two AC fans that are of the same type is just wire them in series. That worked well for my test runs on the lab bench. I had the fans and variac in my parts stash, so I took the more complex route for the final installation.

This is getting OT, but I also had some good luck mounting the ballast on foam damped springs to cut the transmission of the 60Hz buzz into the cabinet.

[mengalva]

Doc B.

What is a variac (variable autoformer)? can you tell me how to build it?

Greetings

[BobEllis]

It may not be the most efficient or linear way of doing it but you CAN use resistors to slow an AC fan. My Hafler DH-500 uses two 300 ohm reistors to provide three speeds. If the temp gets too high, a switch closes to short out one resitstor, repeat if it gets warmer still.

My fan is only a 7W one, so you may want to start with something less than 300R. RatShak 10W 100R are cheap enough. Mine will still start with 800R in series, so you have some flexibility.

[mastertech]

using a resistor is the cheapest opption an otherway would be to use some of the
other inductive methods suggested by other members, you could also use
a lower voltage ac transformer this will also lower the wattage consumption of the fan
cheers

king fanatic of fanatics

only joking!