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Discussion Starter · #1 ·
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.

Peter
 

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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.

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Chad Landry
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'68 El Camino

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

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I have had an electric fan on my 69 for about 2 years, and have been all through the ropes with different sorts of thermostatic switches. The best thing to do is put a relay controlled manually by a switch. I have a single wire comming into the passenger compartment that I used to supply ground to the relay, and turn on the fan. Works great

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Discussion Starter · #5 ·
I am going to put in some type of automatic control switch. I don't want to have to pay attention to it all the time and I don't want to ruin a motor buy not paying attention. I may have a switch for manual override but I will have a thermostatically controlled switch of some type, hopefully a two speed or variable speed switch.
 
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