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Can I reduce voltage to electric cooling fan to reduce noise?e

7252 Views 4 Replies 4 Participants Last post by  Joel Koontz
I am planning to install an electric fan, problably either a Mark VIII or a Windstar, before spring. I have not heard the fans in operation but I have heard some electric fans that make a lot of noise. If the Mark VIII pulls 5000 CFM I would assume it makes quite a bit of noise. I will try it wired direct but if it seems to be too loud I thought I would try to rig it up so it runs at two speeds or variable speeds if that is reasonably feasible.

I assume I can do this by running two thermostatic switches, and setting one 10-15 degrees cooler that the other. I would then use the the T/S that comes on first to send reduced voltage to the fan. The reduced voltage should slow the fan down and reduce fan noise. If this does not provide enough cooling the engine will warm up enough to activate the second T/S, it would provide full voltage to the fan and maximum cooling.

Will reducing voltage to the fan motor damage it?

What can I use to reduce the voltage?

How much voltage reduction would you suggest?

Is there an easy way I can make the voltage adjustable?

How should I wire it?

Do I need to put a diode in each power supply wire? If so, do I need Heavy Duty Diodes, what size and where would I get them?

I think a two speed is all I will need but might be nice to have it variable speed. Any suggestions on how to do that?

Thanks, Joel
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It's possible to do either 2 speeds or a variable speed thing. DC motors will run fine at lower voltages. It actually is easier on them than full voltage.

The air a fan is basically a squared function of the speed. This means that at 1/2 speed you would get about 1/4 of the airflow. So, you would probably want to drop the speed maybe 15 to 25%.

The cheap way would be to find a small value large wattage resistor. Start with something around 3 ohms and around 20 watts. Put this in series with the lower level switch to the fan. Get a ceramic one and mount it to the side and in front of the rad where it will get airflow since it will likely run hot. If you know the running current of the fan, I could give a better guess on the resistor to try.

If the fan runs too fast, get a higher value resistor. Too slow, get a lower value. You can get variable high power resistors, but I don't think the ceramic variable resistors should be used under the hood of a car. You will probably have trouble with the wiper corroding when water hits it. Look in electronic surplus type stores for resistors. You can probably find them really cheap if you scroung around.

Connect the second temp switch across the resistor and first temp switch to provide full voltage to the fan. This also has the second bonus of acting as a back-up temp. switch in case the first one fails.

Search for posts on electric fans. There was one a little while ago that mentioned an electronic controller that did variable speed control of the fan. The controller is expensive, but it will work.

You could also build a circuit with a transistor, thermistor and some other parts to make your own simple variable controller. The transistor would have to be high powered on a fair chunk of heatsink to work though.

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