# DC motor size??

Registered Users Posts: 8 ✭✭
I need to harness wind energy from a box fan (cheap square box fan from walmart \$20). I need to know the DC motor (size/model number) that will maximize the amount of energy i can collect. The DC motor will sit in front the box fan and collect the wind from the box fan. What kind of fan blades do i need to purchase along with the motor. Any help is greatly appreciated. Thanks.

• Solar Expert Posts: 1,360 ✭✭✭✭
It depends, so let's start with what you intend to accomplish by doing this.

I am available for custom hardware/firmware development

• Registered Users Posts: 8 ✭✭
This DC Motor/generator will have 5 minutes in front of the fan to collect and store energy in a battery that will be used to power a rc car. The battery that it is charging is (2) 2200 mAh 3.7 volt batteries. I need this DC motor to sit in front of a box fan, collect eind, and charge batteries.
Say you want a 50% charge on the battery bank:
• 2 batteries * 2.2 AH * 3.7 nominal * 0.50 state of charge = 8.14 Watt*Hours of stored energy
• 8.14 WH * 60 minutes per hour * 1/5 minute charging cycle = 98 Watts of power
• 98 Watts
If you assume that your coupling to the fan unit is somewhere around 10% to 30%, then the driving fan would need to be:
• 98 Watts * 1/0.30 = 327 Watts from driven fan
• 98 Watts * 1/0.10 = 980 Watts from driving fan
If you assume that the driving fan is ~50% efficient at moving air, then its motor would have to be:
• 327 Watts / 0.50 eff = 654 Watts
• 654 Watts / 745.7 Watts per HP = 0.9 HP fan motor
• 980 Watts / 0.50 eff = 1,960 Watts
• 1,960 Watts / 745.7 Watts per HP = 2.6 HP fan motor
Lots of guesswork here... However, it is close enough to see if your proposed system comes anywhere near what your plans are...

A successful turbine system design needs to pretty much match up and seal with the source fan. And a 100+ Watt turbine motor (DC or AC), plus possibly a voltage converter to match the battery bank charging voltage (i.e., 12-50 VDC or VAC down converted to x.x VDC for battery bank).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Here is a somewhat related set of posts from last year or three... This was talking about placing a "wind turbine" on the output of an Air Conditioner Condenser... In their case ~11% coupling between fan motors and driven turbine (of course, they still need to near full volume of air through the condensing tower/unit for the A/C to work correctly).

https://forum.solar-electric.com/discussion/351987/wind-solar-charge-controler-one-battery-bank/p1

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
I'm really lost now. I have to find a DC motor that i can put fan blades on and stick it in front of a box fan. I'm using this DC motor as a wind turbine. I need to select a size of DC motor that is going to give me the maximum possible output for this situation. I thought a DC motor would be my best bet because if i used an AC motor, id have to convert that to DC so i could charge the battery, I didn't want to lose any energy with that conversion. So thats why i'm looking at DC motors. I need to know what size/model number DC motor thats going to potentially work best. For example i'm not going to get a really big DC motor because that box fan probably couldnt turn it fast enough to help me any. Does that make sense? I'm just not very knowledgeable when it comes to picking out DC motors. Thanks for your help.
• Solar Expert Posts: 8,606 ✭✭✭✭✭
Why are you looking at using a fan ? Just get a simple AC powered charger and no fans. Do you need to air-gap isolation between the Grid and the batteries ? using a 20" box fan and some other fan wired to batteries as a charger is not going to be an easy match. You will need a 20" set of blades off a dead box fan, and wire that to your DC generator.

Or even easier, build your own dyno-motor ..
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 8 ✭✭
This is for a project. I have a wind source (AKA box fan). I have to harness energy from the "wind" from the box fan. My best option would be a DC motor that was properly sized for the situation. If i put a motor on in front of it thats too small i won't generate enough to charge the rc car i'm required to charge. I need to know, what kind of DC motor i need to use. One thats close to the right size. I don't even know how to go about picking one. I keep asking this same question and no one has been able to help me at all. Thanks
• Solar Expert Posts: 1,360 ✭✭✭✭
As Bill calculated, you can use a ~100W generator. But he also shows that your plan won't work (because the box fan is too weak) and you will probably get < ~10W out of it.

I am available for custom hardware/firmware development

• Registered Users Posts: 8 ✭✭
Well what will work? I have to generate power from the airflow of the box fan. I just need to find the right size thats going to maximize my attempt. Any insight would be helpful
You gave the size (AH @ volts) of the battery bank and a time limit (5 minutes?). I guessed that a 50% charge into those cells in 5 minutes (that is a C*6 rate of charge--That is very fast for any battery--You need to check the battery specifications to ensure you do not "blow up" the batteries).

Is that what how you want to charge your batteries? Otherwise, what is your energy requirement? A 5 minute charge is very fast... Generally a "fast charge" battery is something on the order of 60 minutes or so.

You have to tell us more about the energy needs for the RC car. This sounds like a school project of some sort.

For example, a 20 inch fan rated at 120 VAC and 0.8 amps will take around 0.69 amps @ 120 VAC and use ~82 Watts (just tested one of my box fans with a Kill-a-Watt meter). Interestingly, the fan consumed 82 Watts whether the air flow was unrestricted, or if the fan was flat on the floor (zero air flow). Normally, for a "fan", I would expect the fan to consume more Watts when the air is flowing, and less when air flow is blocked. It would seem that not very much of the 82 Watts is transformed into airflow with a box fan.

Anyway... A first pass approximation would be a well designed "wind turbine" would draw around 11% of the 82 Watts, or ~8 Watts charging power (that helps you size the motor).

8 Watts for 5 minutes is not very much energy:
• 82 Watts into box fan * 0.11 efficiency = 9 Watts available energy for wind turbine harvest
• 9 Watts * 5 minutes/60 minutes per hour = 0.75 Watt*Hours
• 0.75 WH / 3.7 volts (if batteries are parallel) = 0.203 AH = 203 mAH @ 3.7 volts
• 203 mAH * 1/2x2,200 mAH = 0.046 = 4.6% of battery capacity
So, if the project is a 5 minute timed charge from a 20 inch box fan, very roughly you would fill the battery bank ~5% of capacity (assuming the rest of the design is "done correctly")... Not very much.

If you look through EBay, you can find small DC permanent magnet motors (old tape drive motors and such)...

Here is a nice write up on building a wind turbine (rated for several hundred watts, you would scale down for what your needs are) (Ebay link was from this project page):

http://www.mdpub.com/Wind_Turbine/

But, back to the questions... What are your energy needs and what can be realistically be harvested from a box fan??? You have to size your DC motor and fan blades to the available/needed energy. It is pretty difficult to design anything in a vacuum (no knowledge/design requirements). And you can only harvest so much energy with a wind turbine (swept area, available wind speed, etc.).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 8 ✭✭
Bill, Thanks for your detailed response. The given problem statement says that we will need to harvest the wind energy produced by the box fan to charge a RC car. The RC car battery is (2) 3.7 V 2200mAh rechargeable batteries. I know that i will not be able to fully charge the batteries in that time period. The goal here is to charge them as much as possible. So i'm trying to figure out which DC motor is going to provide me the best charge in that short window period. What size tape drive motor do you think i'd need?
• Solar Expert Posts: 3,274 ✭✭✭✭✭
@ryanperkins To figure out if your project is going to work, you might want to take a short cut and buy a micro wind turbine generator for less than a couple of dollars might as well order a couple or more to experiment with. Trying to figure out all the details would be time consuming, but if this is to be a self built project you would need a permanent magnet motor rated at slightly higher voltage than what's needed say~5-6V and a current useage slightly higher than what's needed to charge the battery the blade design would be the difficult part to get right.There is tons of information throughout the Web if you take the time to search but starting with something like this will hasten the learning curve dramatically.

Don't be discouraged, a willingness to learn is something to be admired.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Registered Users Posts: 8 ✭✭
Yeah that is a possibility for this project. I just don't think those micro sized wind generators are "big" enough. I'm afraid that wouldn't even charge it any during that 5 min period. Would wiring a few of them together be more beneficial? I'm looking to have a generator that has the same size fan blades as the box fan. Thank you so much for all your help
edited October 2018 #15
For example, McGivor's EBay motor rating is:

Output voltage : DC 0.01v - 5.5v
Output current : 0.01 - 100mA
Rated speed : 100 - 6000 rev/min

Probably the average maximum power you could harvest is around 9 watts:
• 9 Watts / 5 volts charging = 1.8 ampsA
• Motor: 100 mA = 0.1 amps maximum
So, optimally, this motor is too small to meet the maximum expected energy harvest from a box fan (remember, lots of guesses here).

Larger motor/generators are going to require higher torque to spin... Which you simply may not be able to get from a box fan (big pm motor, will simply not spin in light wind from wind turbine).

If 100 mA (0.1 amps * 5 volts = 0.5 Watts) would be "satisfactory, then it would be a good starting point.

More or less, the RPM of the DC generator sets the voltage, and the torque sets the current. So you have to spin the motor fast enough to get >3.7 volts charging (say 5 volts), and >10 volts if the two cells are in series. The output voltage of McGivor's motor is (probably) 5.5 volts at 100 mAmps -- Enough to charge your two cells in parallel, but not enough to do it in series.

And it is possible that the maximum power (Watts @ 5.5 volts and 0.1 amps) is upwards of 6,000 RPM, you are not going to get that RPM from a box fan (wind not fast enough)... Also there is a design parameter called "maximum tip speed" --- You need a fairly small blade diameter to get high RPMs (if you see large wind turbines, they turn very slow because they are limited by tip speed).

As much as I would like to help... This is not really in my area of expertise (quick design that meets your needs). You can try these other links/forums and see if they can help you with this project (you still need to define what is a "successful project in terms of voltage and amperage to the RC battery pack). Of course, they are usually working with much larger wind systems, but they may be able to give you some good guidance for your project.

(updated post with links--See they did not copy over. -Bill B.
www.otherpower.com (good forum for DIY Wind Power)
http://www.otherpower.com/
Hugh Piggott - Scoraig Wind Electric site for tons of info (from mike90045)
http://www.scoraigwind.com/
Scoraig Wind "Recipe Book" for DYI Turbines (from Chris Olson... From his 4/11/2013 post)
http://scoraigwind.com/axialplans/index.htm
www.greenpowertalk.org (added from "russ"--Like here but more wind/less solar)
http://www.greenpowertalk.org/

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 3,274 ✭✭✭✭✭
Multiple small generators may well be better than a single one, for reasons Bill has pointed out, I too am no expert on the subject, experimenting will be rewarding but do accept failure as a possibility, we all learn from mistakes, at least I hope so.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 8,606 ✭✭✭✭✭
simple physics shows it's not going to work well at all
85W fan, loosely coupled via air flow, to another blade & motor/generator assembly, is only going to harvest a couple watts.

You are better off zip tieing the fan blades together (Driver & driven) to get the generator to spin. But it's still not going to give you much of a charge.

Your class instructor is either a genius or an idiot, because the battery will not be charged to do squat with the conditions you've given us. Genius because you are in the school of hard knocks, OR idiot because it does not realize it's impossible to achieve a useful goal
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 1,360 ✭✭✭✭
edited October 2018 #18
That's what I would try. Get a ~10W capable DC generator and another similar box fan (for the blade). Use a belt drive and play with ratios to get maximum power. Use an adjustable, current limited DC-DC converter to adjust the voltage/amps/watts.

Or buy a "High Wind Turbine Generator" by Pacific Sky Power.

I am available for custom hardware/firmware development