reducing a 12volt fan speed

Question

I bought a 12volt, 80 watt aftermarket fan for a car. What I'm making is a portable swamp cooler for my tent when I move to AZ this summer. I plan to charge a 30 amp, deep discharge trolling motor by both a 400 watt power inverter in my car, as well as off a 24" square solar panel when the car is stationary. I needed the 12" size fan but I doubt I need the full 80 watts of power. I want the fan to run slower than it was built to run so it's not loud in order to allow me to sleep, and the 14" square wood-chip mat which water trickles down does not need a high volume fan to cool my tent efficiently.

What electronics are required to make this fan adjustable until I figure out just the right speed for coolness and decibels? Won't slowing down the fan also allow me to use less battery power every night? A rheostat may be easy to dial in, but I see that they are costly for a 80 watt load. Am I wrong in this regard?

I don't mind spending $20-$30 for the electronics, more if necessary for longevity since I plan to use this swamp cooler every night for the next 6 months. What gauge wire should I use and should it be stranded? I plan to hook it up directly to the deep discharge battery every time I use it. Thank you for your insight and help. Keith A.

Answer 1

Speed Control:
There are numerous ways to accomplish your task, it is a very common problem. To list a few solutions:

1. You could buy an off the shelf motor controller, there are very cheap ones all over the place like eBay and Amazon. Here is one that should work fine with your motor. Link Here

2. If you like electronics, you could create your own PWM based speed controller using an Arduino or Analog 555 timer. This can be googled, as there are millions of examples of this, with tutorials far better then I can re-write here.

I would recommend option 1, and buy two control units, in-case one dies. Your fan is rated for 80W@12V, so any speed controller that supports at-least 12V, and at-least 150W (15A) should do the trick.

Wire:
As for the wire, the max power that fan will draw is 10A. if you are running it much slower, i would estimate ~3-5A, but always plan for the max amount of power to prevent fires etc.

for a 12V circuit running 10A, wire size should not be much of a problem. If your run is less then 10ft I would recommend a 18AWG or larger wire. Keep in mind, for AWG sizes, smaller number is thicker wire. Really anything in the range of 20AWG-14AWG would work fine, larger wire then 14AWG is just going to get heavy and hard to route. Also I would recommend stranded wire, since it is more flexible and will handle setting up and taking down better over time.

Finally, for this low voltage wire, you can commonly find it at auto shops for cars, it is more flexible and thinner insulation then equivalent wire used for 120/240V houses.

Answer 2

Your best bet is to try out a 12V PWM motor speed control device. Whilst you could possibly build one of these yourself from basic parts it will end up cheaper getting a low cost one from eBay.

There are devices that claim current ratings of 40A and even 60A. Control is via a simple rotary resistor (i.e. potentiometer).

Here are two representative devices:

http://www.ebay.com/itm/12V-24V-48V-2000W-MAX-10-50V-40A-DC-Motor-Speed-Control-PWM-HHO-RC-Controller-/111851538617?hash=item1a0adf08b9:g:IVcAAOSwXeJXfNDa

http://www.ebay.com/itm/NEW-10-50V-60A-3000W-DC-Motor-Speed-Control-PWM-HHO-RC-Controller-With-Case-/301605051423?hash=item46390fdc1f:g:pgwAAOSwymxVOJce

The low price of these will indicate that there are limited features and it may be a good idea if you also include some fusing and a good on/off switch between your battery and the motor speed control.

Since you will have a quick setup feature each day you will want to use a reliable but easy to use connection to the battery itself. I would recommend that you use battery clamps similar to these:

Clamps like these can also be found on eBay for a small price.