slowing a 12v dc fan motor

Discussion in 'The Projects Forum' started by scobar, Apr 8, 2011.

  1. scobar

    Thread Starter New Member

    Jul 6, 2010
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    this DC fan motor pulls 19amps at full speed. It's too fast. I have a switch with on/off/on and would like one position to slow the fan say to half speed. I would I do this?
     
  2. mbxs3

    Active Member

    Oct 14, 2009
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    How do you know it is pulling 19 amps? Was this measured with a DMM?
     
  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    A 19Amp DC fan.....that is not possible.

    is it 19mA?
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    A fan in a car cooling system can draw that current or more. Scobar What kind of fan is it. Do you have any details like a type or serial number. Is 19 ampere the fuse size. Or have you data saying 19 ampere is the current it draw.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ah yes..A car fan could draw that amount.. I forgot.
     
  6. RFactor

    Active Member

    May 1, 2009
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    That's still a lot of amps for a fan!
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    Good grief, what automotive fan draws 19Amps? I'm not saying it isn't so. I just want to know so I never buy a vehicle that has one! :eek:
     
  8. russpatterson

    Member

    Feb 1, 2010
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    Last edited: Apr 9, 2011
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    It's quite likely your car already has one. ;)

    Just pop the hood, look in front of the motor and see if it has an electric radiator fan or a fan driven by a vbelt. With modern cars it's probably a 50/50 chance of either type.
     
  10. RFactor

    Active Member

    May 1, 2009
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    Like Russpatterson says, you could use an N-Channel MOSFET and pulse width modulate it with a 555. The IRF530 shown in the diagram is only good for 14A so you might need one that will handle more current.

    The RB: I know that many radiator fans are electric motor powered, I was surprised at the 19 amps but I guess that is not too far out of line. However, my 2.4 liter Toyota engine's electric radiator fan is rated at 3.4 amps. Perhaps a larger vehicle could have a 19 amp current draw. But...we don't even know if this is a radiator cooling fan, Scobar didn't mention the motor's application.
     
  11. ifixit

    Distinguished Member

    Nov 20, 2008
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    The fan speed in many car heaters is controlled by putting a power resistor in series with the motor to slow it down. Mount the resistor, which is going to get very hot, in the air flow to keep it cool. A 60W, 0.6Ω resistor would be a good place to start.

    Regards,
    Ifixit
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    Alright, you've got us kicking conjecture all over the place. Are you going to tell us what the application of this fan is?
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Wow. I'm as surprised at how low that amps rating is as people were surprised at the high 19 A figure!
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    I have a 120VAC box fan in my wood shop. I draws 1.3 Amps and moves a heck of a lot of air. If you scale that to 14V you get about 11 Amps and I've never seen a passenger car electric radiator fan that moved this much air..

    I still find 19 Amps a bit much;... maybe a Kenworth! :D
     
  15. scobar

    Thread Starter New Member

    Jul 6, 2010
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    it is out of a race car that crashed. I am using the fan in my farm tractor. It is a Flex-a-lite 580 dual motor fan. When it's on (had only one speed) it is moving some air. I have the on/off/on switch so that is why I want twop speeds.....sorry for the delay in returning, you guys are quick. I am hoping for a simple fix.
     
  16. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U will need a MOSFET to do this....I guess our Sgt will be up for the task
     
  17. scobar

    Thread Starter New Member

    Jul 6, 2010
    10
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    please do help with this MOSFET switch. I was doing some google but still need help.

    Could I also use the resistor, high wattage that simply goes in series from sw to fan? There was metion of a 60W .6 ohm. Is the a radio shack purchase for either?
     
  18. PerunaPete

    New Member

    Mar 17, 2011
    22
    1
    A MOSFET is basically an electric switch. Putting a voltage between the gate and source allows current to flow through the transistor (it's more complicated than that, but it should give you an idea).

    A great way to control a motor is by using PWM, which basically turns the transistor on for a certain amount of time, then turns it off, then back on..repeatedly (and quickly) to vary the amount of current going to the motor. This is what the 555 timer is for.

    You can get the 555 timer, and most components for the circuit from radioshack, but you'll need to get the transistor online (probably).

    Using a resistor is very inefficient, as all of the energy that isn't going to the motor is just being dissipated as heat.
     
  19. scobar

    Thread Starter New Member

    Jul 6, 2010
    10
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    So this looks like it's a field fabricated switch. I was hoping to buy it "pre-built" where all I have to do is install it. Remember this is a farm tractor, outside 100%, so I am hoping for a weather tight setup.
     
  20. PerunaPete

    New Member

    Mar 17, 2011
    22
    1
    Well, then it's not much of a circuits project.

    You might try checking an automotive forum, I don't know of many fan controllers that can handle 19A off the top of my head.
     
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