Slowing a Fantastic Vent fan

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Gary RV_Wizard

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Our bnew rig has three of the standard model Fantastic Vents, the model with manual three speed controls. We had the fancy thermoostat model in the Dolphon and really miss the continuously ariable speeds, or at least the ability to run at very low, whisper quiet speeds.  Can one of you electronics mavens suggest a way I can modify these fans to get lower speeds than the fairly high RPMs of speed #1?  Conceptually I would like to install something that would cut all three speeds by 30-50%, so I have Real Slow, Slow and Half-fast instead of Fast, Faster & Fastest!  ;D  I'm thinking if I could insert a "slow down device" on the lead to the fan motor, it would slow the output of the existing three speed switch.

Is is as simple as adding an in-line  resistor or do I need a PWM type motor controller or something different altogether? What size and type component should I be looking for?
 

Jim Godward

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Gary,

I would call Fantastic Fan and see what they recommend.  I wouldn't be surprised f they have a kit to do that and the price might be right, read free.

Just a thought,
 

edubb

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http://www.en.wikipedia.org/wiki/Electric_motor#Speed_control
Someone with electric knowledge please check me on this:
Existing resistance is 6.3 ohms on low (1.9 amps) and 4 ohms on high (3 amps).
If you added 2 ohms, resistance would be ~30% higher on low and 50% higher on high.
But can you get a 2 ohm resistor that can handle 24 watts?  I don't know.

Or here's an item for 12V fans, continuously variable speed:
http://www.sailorsams.com/mall/dimmers-ilvpc.asp

I'll be interested to hear what you come up with.

 

Bob Buchanan

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RV Roamer said:
Can one of you electronics mavens suggest a way I can modify these fans to get lower speeds than the fairly high RPMs of speed #1?

. . .Slow and Half-fast

In my first rig some time ago, the fast speed on the fantastic fan was very loud. I took it back to the dealer and they moved the fan to a different source -- telling me that some of the DC lines are filtered and some aren't. Anyway, it ran very quietly after that. It would seem that having the ability to slow down the fan would be OK, but I would not want to eliminate the top speed altogether. On some hotter days, I like that exhaust ability to be as high as possible. But then I guess having three would solve that even at lower speeds.

But I would be careful doing "anything" to it that is Half-fast . . . ;)
 

edubb

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RV Roamer said:
Phew! $85 plus shipping is a pricey solution for a rheostat. I believe I've seen pulse-width-modulated 12V motor controllers for around $35.

You know those boat people, they're made of money :)
 

Gary RV_Wizard

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I had already looked at that one too - it's the catalog of stuff OEMS can get to build into a new coach. I'm familiar with those controls - we had the fancy t-stat and speed control model  in our previous Dolphin.  As I read it, the wall mount speed controls cannot be used with the standard 3 speed fans - it says the control boards are different.  And I cannot reasonably add a wall control anyway - too much stuff in the way to run wires.
 

Gary RV_Wizard

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Karl,

That would be great and I would be happy to pay for the parts and labor!  Variable would be good, but all I would need to do is to splice that into the circuit between the existing speed controls and the fan motor and adjust it once, to get a satisfactory new range of three speeds. I would not expect to re-adjust thereafter, at least not very often.  I assume you are familiar with the layout of a Fantastic Vent - the little controller would have to mount in the open vent area ad space is limited.  Hopefully it wouldn't get wet very often cause the fan has the rain sensor that auto-closes the cover, but it would be subject to whatever blows through the fan's  air stream.

The fan spec says 3A at high speed, 2.29A on medium and 1.86A on Low.  I'd need three - maybe four to have a spare, since they are somewhat exposed to the elements.  :p
 

Jeff

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Jim:

I can see him now sitting out in the desert wiring circuits. ;D ;D
 

John From Detroit

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Been thinking on this since it was first posted.  The fan is a DC motor and is voltage senestive, if you lower the voltage, (One way is to put that variable resistor/power transistor in line that someone else suggested, another way is a good old fashion rheostat fan control from as used in cars way back when) it will slow

BTW, I used that variable resistor/power transistor thing on windshield wipers once  Easy circuit to build
 

Gary RV_Wizard

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A basic rheostat will certainly work, within limits. At some point the voltage drops enough that the motor stalls while still drawing current, which is basically wasted as heat in the rheostat and motor windings. Pulse width modulation is the preferred way to go with a motor.

Karl has quickly worked up several simple designs and we've been discussing pros and cons. I think replacing the existing control with a continuously variable one is the best and most straight-forward solution.
 

Karl

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One thing I mentioned to Gary in a PM that you may get a kick out of was that the initial design was tested using an auto headlamp as the load. Everything worked as expected, but when I hooked it up to my fan, it caused the motor to 'sing'! Had to go back and modify the frequency of the PWM. ;D

James,
Careful Karl, you may wind up with a business!  VBG
I'm glad you used the term 'business' and not 'work' :D
 
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