Running cooling fans off a rheostat - safe?

Question

My entertainment center has an enclosed area underneath the TV that holds my A/V receiver, bluray player, Xbox 360, etc.  This area gets extremely hot when even one of these devices is turned on for any length of time.

I have two 110V fans that were originally used for cooling an industrial computer system.  They are Muffin model # MX2B3, which lead me to this spec sheet: http://parts.digikey.com/1/parts/437723-fan-115vac-119mm-x-39-mm-mx2b3-028422.html

I wired the two fans up in series with a standard household rheostat like you'd find at home depot for dimming lights.  I used a standard lamp cord with the male plug still on it to plug in to a household receptacle.  This works as far as the fans turn on and the speed varies based on how I set the rheostat.

Before I install this permanently in my entertainment center to help vent the hot air, I want to make sure the way I have them wired is safe and is not a fire risk.  The fans are each rated at approx 1A and both the wire and the rheostat should be able to handle that load with no issues, my primary concern is if using a rheostat with something not designed to work with one is unsafe in any way.

Any advice is appreciated, and any links to reference material with useful information is also appreciated.

? · Accepted Answer

The fan you linked to is not a true variable speed fan.  Using a rheostat on that fan will cause rotor slip when the voltage is varied and will cause the motor to burn out prematurely.  You should never use a rheostat with a fixed speed fan.  To prevent rotor slip, the voltage must be stepped at exact intervals in relation to the number of poles on the rotor and/or stator.  The stepping of voltage cannot be adequately accomplished with a light dimmer.

? · Answer

There are such things as fan speed controls which will do this job much more effectively than the rheostat.  Rheostats dissipate as much energy as the load they're controlling, so if you've got your fans turned way down, the rheostat can get pretty warm.

Or consider getting a couple of muffin fans that are smaller, slower, and quieter.  Then you won't need the speed control.  Run them inside a duct with some noise baffling and you won't be able to hear them.

BEINLICH · Answer

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Running cooling fans off a rheostat - safe?

3 Answers