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

7253 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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I'm not sure if it's OK to run this type of motor at different voltages. I know that most (if not all) DC motors run fine at different voltages. A major part of my job is SCR systems for controlling 1000 HP DC motors.

AC motors can't be controlled like this. An AC motor's speed is determined by frequency (60 Hz standard in US) and number of motor windings. Many industrial applications use VFD technology (variable frequency drive) to control the speed of AC motors. But your electric fans in your house just tap power into a different motor winding to slow it down.

If it's OK to do this, then you should check out how the blower motor for your heater is wired. You'll see that it has a resistor in line with the power to the motor. For each fan speed there is a wire coming from the switch that taps the resistor in a different place. For low speed the power comes in through the end of the resistor so it gets full resistance and lowest voltage to the motor. For medium it taps the resistor somewhere in the middle so it only gets partial resistance for somewhat higher voltage. For high speed it bypasses the resistor completely and feeds full voltage to the motor.

You could wire it like this if the motor will accept it. No diodes required.

Personally, I prefer manual control of my electric fan. I don't have to worry about a thermostat turning the fan on while I'm working on something. When the temp starts to rise while in traffic I turn it on. I always leave it off while cruising on the highway.

My Elky Page Updated 7-21-00
"Think for yourself. Don't let popular opinion make your decisions for you."
Chad Landry
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'68 El Camino

[This message has been edited by cjlandry (edited 02-05-2001).]
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