Question of the day 5/20/2013, Aeromotor Fan

A question from Jeff H.

Subject: windmill retrofit

Message Body:

I was curious about retrofitting old Aeromotor windmills with a belt reduction to a commonly available truck alternator. With the advent of serpentine belts, it sounds that much more possible. My old 2003 Chevy duramax diesel truck had a serpentine belt perimeter of appx 11.5 ‘. My question is; could the 20″spacer ring of the old Dayton rear truck wheels be machined and modified and attached to the fan for that purpose. It’s otherwise impossible to find a serpentine pulley that large. Any thoughts on that would be appreciated.



George’s Answer:

It’s rather amazing what Engineers of the past were able to do with so little wind energy.  I think that’s the case with the old Aeromotors…. lifting the cover off that gear rack, and studying the simple but elegant method to drive the sucker rod is a delight.

I think your first goal should be to identify how much electrical power you would need to generate in order to be happy with your effort, and then another important study might be what the folks of the day did to make electrical power.

How many brands of electrical wind chargers were there? RCA, Zenith, and Winco for Three. If you study their design, they’re all direct drive units that operate at higher RPM, and hopefully deliver enough energy to power a radio.

My first thought is IF there was an easy way for Aeromotor to have gone after that business of selling wind chargers with their existing fans, they would have done so,  but I would imagine their fan was optimized to drive that sucker rod via a reduction box to start work in lower winds with low energy, they knew a Rancher needed water not bragging rights that his outfit pumped the most during a storm..

Consider that the Aeromotor fan was designed to drive through a reduction box and driving a sucker rod whose mass was often ‘countered’ with a weight set up by the Erector to assure that every grain of energy went into lifting water. You’d be taking a fan tuned for a specific chore and adding a speed increaser to drive your Generator.  I have seen such things, but why did all the popular wind chargers of the day go direct drive?

But here’s a more specific reality for you.. a typical truck alternator is quite inefficient, some are able to convert about half the mechanical energy that is put into the generator shaft into electrical energy, others are a bit better, but as I see it.. you are attempting to mate a tiny source of power with a device that was engineered to survive under the hood of a truck, in high heat, produce energy required over a wide range of RPMs, and to be made quickly and cheaply. Perhaps one of the last priorities of the design is efficiency.

Fabricating your very large pulley is a job for sure, some experimenters have used a bicycle rim in the past, and some have even used tiny cable instead of belts.

I think if you define your ‘power’ expectations, and then attempt to calculate the power potential of the beautiful and simple Aeromotor fan, you’ll find a miss match, and decide to invest your time with the more popular designs.

Here’s my advice: The Aeromotor fan is a thing of beauty, leave it unmolested, maybe put it up for sale? Look up Hugh Piggott, (a must read)… visit the website “”  and study some of their easy to build wind turbine designs. You’ll likely have fun, and the learning never ends.

So.. you now have my unprofessional advice, but don’t forget to study your property and determine if there’s ever enough wind potential to merit your efforts to capture a bit of it..

Oh yes.. you knew I’d get this in.. didn’t you? And Jeff, I don’t claim you are afflicted.. only a warning to you.  The Gang Green Mind has no patience for figuring.. if they see a breeze come through that causes the leaves in a tree to move, their minds often envision a whirly gig making power equal to what the power company delivers….. everything is simple in their minds.  For the rest of us, it’s best we learn how to measure before we build, and our Carpenter Friends warn us.. measure twice before you cut!

All the best….




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18 Responses to Question of the day 5/20/2013, Aeromotor Fan

  1. Russell says:

    I have a 1935 Southern Cross Z windmill ( a cheeky knock-off of an Aermotor 304- parts interchange!) pumping water.

    If you work out power by lift x flow the output is tiny. Robustness and reliability were the design goals. The energy is free and (fairly) continuous so squeezing the last bit out doesn’t matter so much.

    Fact is that a power windmill worth building needs to be a BIG sucker.

    Another thing to consider with the use of a water- pumping windmill for power is how to get the power across the axis of rotation- I don’t really like slip rings all the way up there away from convenient maintenance.

    A bevel- gear set up will turn the mill against the torque of the generator- like a helicopter with no tail rotor. I have imagined modifying a differential to address that problem.

    I do think that a large correctly designed wind turbine in the right spot seems good but once you work out $ per kwh- you’d be better with the lister.

    The Victorian government has published some case studies for small wind power and none add up from an economic perspective. AND that was gang green writing propaganda!

  2. Richard says:

    From my studies a windmill for generating electricity needs to be either 2 or 3 blade. the multi-blade is good for lots of torque but is not good for electricity. Work out the amount of power this type of blades and you will be surprised how little power it can make when it comes to electric power. Remember the diameter of the blades decides how much power can be generated. I memory tells me that maximum efficiency can be no more than 48%. You need at least a 10′ diameter to generate enough power to make it worth taking the time and money to put it together. If you want me to find the math of all of this I will. Just ask. Here is a pdf file help.

    • George B. says:

      Richard, the math is uncomplicated.. but so many refuse to open the book and learn how easy it is! Just a few minutes of study could keep a DIYer from investing money and time in something that might never make a watt!

      It reminds me of that irrigation ditch. The Sharks tempting you to invest in the idea of harvesting energy out of a canal WHEN one of the primary engineering goals of the canal was to assure there was the least amount of energy possible in the water at any given point.

      And.. grade school math is all you need to figure the truth.

  3. Jetguy says:

    Just a thought, but I’m not buying the 50% efficiency rating for alternators. Here’s why, if that was the case, then the entire body would be smoking red hot. Energy in = energy out, I mean that is what we are taught. So the problem is what part of the system does get hot? The answer is friction in the belt drive. The issue is that those alternators need to spin very fast, even in the intended application. The alternator itself is not so much the issue, it’s the mismatch between the RPMs required for efficient operation and the ability of our power source to do that. The other thing is that any speed increaser is where we see the highest losses. It’s very easy to minimize losses in a reduction. Increasers though will always be a problem. Speed in itself is friction, that’s why and where the losses come from. It’s the belt, the pulleys and even the very air flowing though the fast spinning alternator.
    To each is his own on the thought of how efficient the alternator is. I would agree the combined system, to get the alternator into the RPMs required is the primary loss, followed by a secondary loss in air friction alone. It’s the system that is a problem, not any given component.
    The part we do agree on is direct drive is the only way to go. This then requires a custom alternator with a extremely high number of poles so that it works at the required direct drive input speeds. Now, let me at least share some commercially available parts that meet that number
    Keep in mind, some specs don’t mean what you guys in the Eletrical world think. For example, the Kv rating is not kilovolts! That is the RPM per volt of input on these brushless motors. The thing here is this is a 3 phase motor, so simply putting a 3 phase rectifier on the 3 leads and it becomes a highly efficient permanent magnet generator/alternator. In theory, due to the pole count here, this occurs at very low RPM (390 per volt). Thus, this $30 “motor” rated at 300Watts might very well keep up with any car alternator and the required speed increaser in a side by side test.
    It would in theory provide 11-15A at around 12-20 volts. Those motors are designed to direct drive big props at low speeds for high lift in multi bladed helicopter applications. Designers wanted to maximise lift and efficiency, and in turn, the system is then optimized to turn low speed wind energy back into power. Thus, go with a BIG prop on one of these and it’s likely to impress. Given the low cost, certainly worth the try.

  4. George B. says:


    I have all day to argue about Amonix, or the ill deeds of the DOE, but car alternator efficiency (or the lack of it) is fairly well documented, and I have no time to argue the point.

    The claw pole design is a big factor as per poor performance, as I say.. efficiency of the unit has been a lower priority in the design for years, far better to assure it’s cheap to make, and that it continues to make juice under adverse conditions. Autos are expensive enough as it is.

  5. Bill knighton says:

    I buy that car alternators are 50% efficient for a couple reasons. One is that when I bought a listeroid from utterpower about 2006 I did not yet have an alternator. I went to napa and looked through their big truck alternator brochures. They had in the catalog a 2 or 2.5k alternator from bosche that was bragged as setting a new standard in high efficiency using new tech. Was 50% efficient. This was a $2000 alternator. I ended up getting an st-5, also from utterpower.
    As far as the alternator glowing red if it wasted 50% I don’t believe that because they have a fan and blow massive volumes of air through them. A much smaller hair dryer made of plastic can take 1800 watts and make nothing but heat yet still not melt because of air flow.

  6. Bill knighton says:

    I don’t understand the hobby motor application. I thought you were taking about the need for direct drive. If it’s 390 rpm per volt that would require 4500 rpm for 12 volts. How could any prop be driven 4500 rpm by wind?

  7. bob g says:

    study the “betz” limit…
    then study and understand the “square” and “cube” laws of windpower

    once you get a handle on those three laws, it is much easier to sort out the “wheat from the chaff” so to speak.

    betz tells us we can harvest no more than ~57% of the available wind power, and in practice we will never even come close to that.

    the square law tells us that when you double the rotor diameter you square the power, or in other words, double the rotor diameter you get 4 times the power. the inverse is also true, halve the diameter and you will get 1/4 of the power.

    the cube law dictates that when you double the windspeed you get 8 times the power, (cubed)… the inverse is also true, halve the windspeed you get 1/8th the power.

    so from this we can quickly tell a few things, we need a large rotor to make useful amounts of power and we need a mean windspeed that is probably for most folks going to be over 12mph,,, anything less is just not worth harvesting. we also need to be aware of “betz” and the fact that we need extremely efficient airfoils to get as much of the Betz limit of 57% as possible, going with a multiblade pumper is likely only going to get you maybe 10% of the power available, and that is a problem with multiblades as they create huge amounts of turbulence for the following blades as the windspeed goes up, drastically limiting their ability to make power just when we need to make it most!

    2 blade props are very efficient, but they have problems with tower shading causing vibration, yawing/direction changes also cause vibration and as a result they don’t last as long as 3 blade units

    3 blades are about the best compromise and have been very popular for small to very large mills.

    lastly one doesn’t want to admit failure points such as belt drives in my opinion, if for whatever reason you lose the belt, therefore the load, you will have a machine that is going to runaway to destruction in higher winds… such events can not only cause loss of investment but also threaten the life of anyone or anything around them, out to in some cases a couple hundred yards.

    i second going to hugh piggots site as well as and do a bunch of reading.

    don’t try to reinvent the wheel, it is not likely that anyone in their first effort is going to come up with something new and revolutionary, and much more likely to have real problems and not make any useful amounts of power.

    bob g

  8. bob g says:

    as for hd alternator efficiencies

    i have built/tested and written reams about this topic, and will refrain from getting into the weeds on this topic, however in short i offer the following for consideration

    most all clawpole alternators will never see over about 50-60% efficiency, as built and as used

    the problem is they are built to be the “jack of all trades and master of none” in order to do this compromises must be made.

    certain large frame alternators can achieve 80% efficiency (even with factoring in losses due to rectification), however the alternator has to be turning at a higher rpm range, it must be coupled to a 24volt or higher battery bank, and it must be controlled by something a bit more advanced than the 20 dollar oem regulator.

    and no this does not mean you can simply go out and buy a 24volt alternator and think you will see 80% efficiency, it doesn’t work that way because those same compromises that are made for 12volt alternators also apply for 24 volt units.

    its very difficult to get better than 60% efficiency from a clawpole/lundell alternator and do so while expecting it to produce relatively high output from engine idle all the way up to engine cruising speed… no alternator is going to be efficient given that parameter.

    for the record a 110-555jho leece/neville-prestolite alternator (12volt version) will be right at 80% efficient if you turn it at 4500-5000rpm, and use it to charge a 24volt battery bank at 28.8volt @100amps, if you use the proper controller and connect things up right. yes that means 12volt excitation and 28.8volt output.

    that puts it within a percent or two of the best alternators in efficiency if you factor in the rectifier losses too. some of the other alternator manufactures claim 85-88% efficiency but fail to factor in the rectifier losses, which will drag down the efficiency of any alternator to near 80-82% efficiency.

    again, fwiw

    having said all this, what does this mean for windpower? it means even the best clawpole/lundell alternator is not going to see higher efficiencies because we can’t drive the darn thing fast enough with a windgenerator without some sort of transmission, and with that we enter more losses and risk to the equations.

    bob g

    • George B. says:

      A Great Conversation, and we need remember, the best of all environments for windpower is inside the Gang Green Mind. In there you’ll find far less loses, and in many cases gains! But.. there is energy in that wind, just far harder to turn a profit than many know. Building your own is key, keeping the cost low and learning that harvesting 300 watts often is far better than collecting 2500 watts during a blue moon.

  9. Mike says:

    Small car alternators use what, 300-500 watts for the fields alone? 60Ax 13.8V= 828W ?? I’ve done some study and several publications indicate 3 bladed props are best for various reasons, those being efficiency and balance. Odd numbered blades eliminate some nasty gyroscopic effects. This being said, I’ve seen 6 and 8 bladed mills that were designed for low wind areas that churn out fair numbers but you’ll most likely need a huge one to do any good if you want to run anything besides a few high efficiency light bulbs. Just the rate of return on batteries alone is enough to make a person pull their hair out. Otherpower has a ton of great info. Piggott is nobodies fool.

    • George B. says:

      Mike.. Good point, that’s a lot of field power, and there are a lot of tricks inside the alternator to assure it survives.. some not so well understood. The Auto Engineer knows the primary goal is for his Employer to make a profit, to make the alternator inexpensive, AND to keep it working. Efficiency? How many priorities are ahead of that?

    • George B. says:

      Piggott? Mike.. just another example of my Sermon. Find the man with hands on experience.. a lot of it.. We add to that, Hugh’s neck of the woods offers challenges. DanB and DanF, they see some weather less than kind to the machine no so well designed. The machine needs to stay up on the mast to make a watt.

    • bob g says:

      the typical hd truck alternator such as the 110-555jho as outlined in my previous post will require no more than 35-40watts of excitation current (typical 3.5amps at 10vdc)

      bob g

  10. Randall says:

    I bought the Dan’s book (Otherpower) a few years back, and gave it a read. then I read it again and underlined and highlighted. Good reading, and still is. I bought a small Kubota.

  11. CrazyJerry says:

    Hi Folks – great topic!
    I am off-grid meaning I generate all of my own electricity less that 30 ft from and existing utility line. A number of years ago, I went down the same road of wind generation the original poster is considering. I’ve built and flown several units ranging from geared-up alternators, treadmill motors, and axial flux diy. (Some of those can still be seen here: ) Wind is tricky in that what you feel on the ground may or may not exist 20-30-60ft (etc) as you go up. I used crepe paper on my tower one year and it was amazing to see some of the tufts going right, left, and then an occasional downward wind sheer. All of these forces should be considered – sometimes the wind can be violent.
    What I found where I am in Northern, NY is that a couple of times a year the wind is really useful. Typically the transitions from winter to spring and fall to winter; this is when the fronts come through. When the fronts come through, the clouds shade the solar and the wind helps to minimize the losses. I also found that on a realistic basis with a decent size battery bank (1700ah) a minimum 10ft blade is needed. Anything smaller is not significant unless you have monsoon winds or multiple units.
    The car alternator is perhaps the one of the easier ways to regulate the incoming power – however – a 3 phase axial flux with tail furling run into a simple rectifier is super simple and a dump load at the battery bank effectively will take care of excess power if you are so lucky to have that problem.
    Good luck in your quest and should you go the Aeromotor blade route please keep us posted on your creation. Wind-Ho!

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