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Can I make a single speed e-fan run slower with reduced voltage ?

Discussion in 'A/C & Heater systems - HVAC' started by Carguy3J, July 12, 2014.

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  1. Carguy3J

    Carguy3J Well-Known Member

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    Ok, so I did an e-fan on my '93 Ranger. I used a Volvo fan. It was, I think, out of a V70/V90 or something. It pulls a ton of air. Unfortunately, it also pulls a ton of amps. Too much for the the alternator to keep up with while sitting in traffic at idle. The fan is a single speed fan, in that there is only a single power wire, and a single ground wire going into the motor.

    My truck doesn't currently have a/c, but I just bought a parts truck with all the parts I need. Typically, when a/c is turned on, it triggers a relay to power the e-fan, on vehicles factory equipped, in order to ensure airflow over the condenser. However, I don't think I need the full speed this fan is capable of, for that purpose. I also don't think the electrical/charging system could handle it running all the time I would want the a/c on. I know that there are expensive fan controllers capable of regulating speed via PWM, but I don't want to spend a bunch of money on one.

    The question is, can I safely run the e-fan at a slower speed, by reducing the supply voltage too it? I'm thinking the A/C clutch wire would trigger a relay supplying a secondary power supply to the fan, with a voltage reducer inline. Then, when the regular temp-based controller turns on, it can just send normal full voltage to the fan.

    Will this work?

    A.) Will reduced voltage also equal slower fan speed and reduced current draw (amperage)?

    B.) Will this harm the fan motor?

    D.) Do I need to worry about or protect against "backfeeding" full 12-14v, from the temp controlled circuit, into the reduced voltage circuit, when the full voltage side activates?

    E.) What is the approximate relationship between voltage reduction and fan speed reduction, as well as current draw reduction?

    It appears that the fan currently draws about 40amps.
     
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  3. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    I suggest investing in a good controller, reliability is top priority for a cooling fan. There has been a thread before about making a fan controller yourself, I don't recall if it was a PWM type.

    I'm not advocating the DCC unit, but as it works I like it a bunch. The terminals are fragile and many people don't like the DCC customer service.

    That being said, I have a Volvo fan in my 99 Explorer, SOHC, and it turned on very slowly every time, with little noise. It never ran fast at all, the radiator cools very well. I believe the Volvo fan is reliable, but I didn't know at full power it would pull that kind of juice.

    Good luck with your system, but do concentrate on reliability. The fan must not ever fail to run.
     

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  4. Carguy3J

    Carguy3J Well-Known Member

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    Almost a 100 thread views and nobody knows? I thought there were some electrical "gurus" on here?

    I did some Googling, and it looks like it might be ok to undervolt a DC motor, but definitely not an AC (alternating current-not "air conditioning") one.

    However, I was hoping some experts would offer opinions.
     
  5. 2000StreetRod

    2000StreetRod Staff Member Moderator Elite Explorer

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    2 speed fan

    My Volvo turbo wagon has a relay controlled two speed electric fan but there are two windings (3 wires) for the different speeds. As I recall a D.C. motor can be speed controlled by applied voltage but the method is not very efficient when using a resistor to drop the battery voltage. If you measure the resistance of the winding (the two wires) and add a resistor of the same value in series you will halve the voltage to the fan and approximately halve the current flow (i.e. 40 amps to 20 amps). However, the resistor will have to dissipate the same power as the motor consumes. Power = voltage * current or 13.6 * 20 = 272 watts. Pulse width modulation (turning the total voltage on and off) is much more efficient.
     
  6. CDW6212R

    CDW6212R Hauls the mail. Elite Explorer

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    The thread I found 1-2 years ago about making a controller sounded feasible to do, and not terribly difficult with a little soldering skill. I just found and read that once, I'm not sure where or when it was.
     
  7. pkn

    pkn Active Member

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    >A.) Will reduced voltage also equal slower fan speed and reduced current draw (amperage)?

    Yes... to some extent. DC fans have kinda threshold, a minimal starting voltage - come any lower, and it wouldn't start spinning at all. Can't say for auto fans, but computer 12 volts DC fans typically refuse to start if the voltage is below 6-7 volts - at best.

    Useful article: http://en.wikipedia.org/wiki/Fan_control

    >B.) Will this harm the fan motor?

    It might, if the voltage goes below minimal starting voltage - the fan will try to start spinning, failed and probably overheated.

    >D.) Do I need to worry about or protect against "backfeeding" full 12-14v, from the temp controlled circuit, into the reduced voltage circuit, when the full voltage side activates?

    Er... ahem. I would put a diode in. Where - depends on the actual scheme.

    >E.) What is the approximate relationship between voltage reduction and fan speed reduction, as well as current draw reduction?

    It strongly depends on the fan itself, all I can say it's highly non-linear. Something like "15% voltage drop will reduce fan speed in half".

    >It appears that the fan currently draws about 40amps.

    Wow. That's a lot of amps for a cooling fan. If you plan to reduce voltage with a resistor, you will be wasting a lot of energy and generating a lot of excess heat by it. OEM controllers use PWM scheme for a reason - and I would say, for a very, very good reason.

    Edited: Forgot to say! I'm not a guru! No way! :D
     
    Last edited: August 5, 2014
  8. cougar190

    cougar190 New Member

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    How about wiring in the variable fan voltage control unit found in older Ford products used to control the Heater fan ? Usually found under the hood in the (don't know the correct name?) heater core/air box. My "92 Ranger has one as well as my wife's "79 F-150 so they've been used for years in that capacity. Just a thought! cougar
     
  9. Mbrooks420

    Mbrooks420 High Voltage. Elite Explorer EF Vendor

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    I’d hope he found a reasonable answer in the 5 years since this was asked ;)
     
  10. cougar190

    cougar190 New Member

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    LOLOLOL Didn't notice Posting date! I was scanning for articles on PCM amperage draw with key off and ran across his post !
     

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