Single Fan Keeping Dual Speeds? (Hopefully there truly are no stupid questions...)
06-06-2013, 06:18 PM
#1
Drifting
Thread Starter
Single Fan Keeping Dual Speeds? (Hopefully there truly are no stupid questions...)
Hopefully, there's an electrical engineer who will take pity on me in this post, haha.
I just purchased a 16” fan to run as a single behind my Dewitts Radiator.
My thought behind this is whole post is tricking the factory relay setup into running low and high speed on the fan. Really, this is only because I still do street driving of the car and I like air conditioning; I don’t see a need to have the fan cranking full tilt when I kick the AC on. So, I’m wondering if putting a (beefy enough) resistor in line where the second fan would be could allow the relays to trigger like they are suppose to and run in series for low speed and parallel for high speed mode.
Assuming this idea is even sound, I’m going to use the values of my new fan to do the calculations to try and figure out what resistor I would need, and it’s been years since I used electrical formulas in college, so hopefully I haven't botched this up too badly.
Fan (Flex-a-lite p/n 396) draws 13.5 amps if rated at 12 volts the fan motor has a resistance value of .888 ohms (V=IR)
So, two fans like this running in series (adding two resistance value equals 1.777 ohms) in a 12 volt system drops the amperage to around 6.75 amps (12=I*1.777) which should be the low speed draw per fan.
Taking the max system voltage up to 14.4 with the same resistance value in the motor the amperage draw rises to 16.2 ohms. Now I’m guessing the resistance might go up in the motor as voltage and current draw increase, so this is probably a wild card on top of everything else here (the current draw may be rated at 14.4v in the first place)
On to picking a resistor to use; Matching the calculated .888 resistance of the fan I have isn’t likely, so, lets round up to 1 ohm for finding a resistor. Now onto what power rating. P=I^2*R at the highest calculated current value of 16.2 amp and .888 ohm resistance power is equivalent to 233.3 watts.
So, if my math is right I could wire in a 1 ohm 250 watt resistor and drop the current draw at 12 volts to around 6.35 amps; very close to my initial calculation for current draw with dual fans, just spin a little slower on the low side.
note: This is actually a pretty expensive and fairly large resistor, which could make this a bad idea as well.
If the answer is shut up and just wire in the one fan and run it on high, I'll accept that.
I just purchased a 16” fan to run as a single behind my Dewitts Radiator.
My thought behind this is whole post is tricking the factory relay setup into running low and high speed on the fan. Really, this is only because I still do street driving of the car and I like air conditioning; I don’t see a need to have the fan cranking full tilt when I kick the AC on. So, I’m wondering if putting a (beefy enough) resistor in line where the second fan would be could allow the relays to trigger like they are suppose to and run in series for low speed and parallel for high speed mode.
Assuming this idea is even sound, I’m going to use the values of my new fan to do the calculations to try and figure out what resistor I would need, and it’s been years since I used electrical formulas in college, so hopefully I haven't botched this up too badly.
Fan (Flex-a-lite p/n 396) draws 13.5 amps if rated at 12 volts the fan motor has a resistance value of .888 ohms (V=IR)
So, two fans like this running in series (adding two resistance value equals 1.777 ohms) in a 12 volt system drops the amperage to around 6.75 amps (12=I*1.777) which should be the low speed draw per fan.
Taking the max system voltage up to 14.4 with the same resistance value in the motor the amperage draw rises to 16.2 ohms. Now I’m guessing the resistance might go up in the motor as voltage and current draw increase, so this is probably a wild card on top of everything else here (the current draw may be rated at 14.4v in the first place)
On to picking a resistor to use; Matching the calculated .888 resistance of the fan I have isn’t likely, so, lets round up to 1 ohm for finding a resistor. Now onto what power rating. P=I^2*R at the highest calculated current value of 16.2 amp and .888 ohm resistance power is equivalent to 233.3 watts.
So, if my math is right I could wire in a 1 ohm 250 watt resistor and drop the current draw at 12 volts to around 6.35 amps; very close to my initial calculation for current draw with dual fans, just spin a little slower on the low side.
note: This is actually a pretty expensive and fairly large resistor, which could make this a bad idea as well.
If the answer is shut up and just wire in the one fan and run it on high, I'll accept that.
06-07-2013, 12:48 AM
#2
There really is no need to have dual speeds. The only possible reason I could imagine is if the fan you are using is loud. In that case it would be easier to purchase a quieter fan. Keep the following in mind. With a single, high speed, the fan will operate for a shorter amount of time because it will cool faster and then turn off so it will not even be running.
06-07-2013, 02:49 PM
#4
Drifting
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At a glance your math and reasoning is good except a motor is impedance not resistance.
Using a 250 watt resistor for your load is crazy
Don't do that!
And it probably won't work for long.
Not to mention you will have to find somewhere to mount it that won't catch your car on fire or preheat the air going through the radiator and negate the effects of the fan!
Get the Dual Speed setup for a single fan or just use the motor from one of the old fans.
Not a great solution but smaller, and less heat than a 250 watt resistor.
Using a 250 watt resistor for your load is crazy
Don't do that!And it probably won't work for long.
Not to mention you will have to find somewhere to mount it that won't catch your car on fire or preheat the air going through the radiator and negate the effects of the fan!
Get the Dual Speed setup for a single fan or just use the motor from one of the old fans.
Not a great solution but smaller, and less heat than a 250 watt resistor.