DC motor advice

Hey, so I’m looking for some advice on this DC treadmill motor I bought in hopes of using it as a wind turbine. Its specs are 1.5 hp (1118 watts) with a voltage of 120V current of 11A. The problem is the 4800rpm needed. That means I need 40 rotations for 1 volt, thus 560 rotations in one minute to gain 14 volts which after losses and what not is my goal if I want to charge a 12V battery bank. Will this motor work as a wind turbine? If so how what kind of wind speed and how long should my blades be to get that 560 rpm?

[Gotwind Ben]

Hi William.
560 rpm is a bit too fast really, it could be achieved with a fast 3 blade 1 meter diameter prop.
It depends how free spinning the shaft is as to if this can be obtained.

Need a good breeze also.

William,

What you are looking for is the relationship between the tip-speed (TSR), the wind speed, and the RPM. For instance, if the wind-speed is 10m/s and your TSR is 8, then your RPM is as follows:

Blade Diam * Pi = m/rev
Tip Speed = 8 * wind-speed ==> 80m/s * 60sec/min = 4800m/min

RPM = 4800m/min / (Blade Diam * Pi)

For a 1M Diam blade set this would reduce to:

RPM = 4800/Pi ==> 1528RPM

Sounds Great doesn't it? Sadly this is just part of the story :-(

A more typical TSR for small blades might be 3, and this TSR would lead us to 573RPM. Still sounds good :-), but then we run into the next problem:

P = 1/2 * Rho * V^3 * Swept Area * CP

Where:

Rho = Air Density (~1.225kg/m^2)
CP = The sum of losses, NEVER to exceed ~ .59, and typically less than 0.3

So, P < 1/2 * 1.225 * 10^3 * (.5^2 * 3.141) = 481W

481W is the energy available in the wind, assuming you could convert 30% of it, your output would only be 481 * .3 = 144W, and this is in a 10m/s wind (roughly 23mph). If the wind-speed were only 5m/s (11mph) your output would drop to a mere 22W. Achieving a CP of 0.30 in a small-scale HAWT built from "off-the-shelf" parts is highly improbable; I think you would be lucky to achieve a CP of 0.10, meaning outputs more like 50W @ 10m/s and 7W @ 5m/s.

To achieve anywhere near the 1.2kW rating of your motor, you would need to increase the swept area tremendously, but this in turn lowers your RPM.

If your goal is to play with a wind turbine for educational purposes, then by all means continue with this motor, but if your goal is to actually produce usable electricity then you need to consider spending considerably more time in the design phase of your project. Consider the axial flux approach, it is by far the best DIY approach to the low RPM, high torque demands of DIY HAWTs.

If you want to maintain the ease of build the treadmill motor offers, give serious consideration to re-winding it, replacing the magnets etc, but remember, it was designed as a motor to be driven by 120VAC, any other configuration is a compromise. This does not mean that it can not be configured to produce usable electricity, it simply means that it is not likely ever going to be the most efficient way of producing that electricity.

Cheers,

Fish

Hey thanks a lot for the help I appreciate it.

~William

The real problem with output is that 1118W and 11A.
Really can not go much past 11A without danger of burning out the motor,
so it is 150W on a 12V system.

Heat = burned windings = Watts = IxIxR(coils) = 11A max, meaning 150W.

Might pull it off with 3' dia blades, but it's still awful fast for reasonable output in reasonable wind.
G-

I am going to post this in this thread because I think it belongs here. If William objects I will edit his portion out, but I see no reason he should. He asked me the following in PM:

hey you definitely seem to know a great deal about wind turbines and all of the math behind it. You were saying the dc motor i bought was good for educational purposes but not so much for usable energy. Luckily I am looking for the educational experience but would like to produce some "usable" energy, not to power a house or anything but at least a 12V battery. But in order to charge 12V battery i need to at least attain 12V output. From reading your post it seems i might be able to attain this but there would need to be a good wind and my blades should be between 2 -3 feet long each?

I responded to him with:

Hey William,

As I posted, you are working against two things:

1) Available Energy

2) RPM at a given wind speed.

Assuming a loaded TSR of 3, roughly 1m diameter blades should give you the RPM you need. There are numerous factors that are extremely difficult to predict in a system like this. Your TSR is actually a function of generator loading, blade design and wind speed and is all but impossible to calculate w/o some empirical data. As ghurd pointed out, the windings in your treadmill motor are designed to handle ~11 Amps, clamping your voltage to ~14V (charging a 12V battery) implies the max output will be ~150W. Honestly I would not consider 150W MAX output as being "usable power". Assuming storage and inverter losses combine to ~50% (a realistic figure) you would need one full hour of 150W charging to run your 1500W microwave for 3min. Put another way, if you made 150W for 24 hours, you could "save" 3.6kWh / 2 = 1.8kWh of energy; this represents ~$0.20 worth of electricity from the grid. If you live in an area with fairly constant 10m/s (23mph) winds, then the system you are contemplating could produce $72.27 worth of energy a year. (This assumes 10m/s 24-7-365.) If you live in an area with constant 5m/s (11mph) winds your "savings" would be roughly $9.03 per year. If you live in a more typical area with variable winds you will likely "save" considerably less than the $9.03. If you opt to "grid tie" your HAWT then you can multiply these numbers by 1.25.

With the above considerations in mind, I would consider a treadmill motor based windmill purely educational, providing no "usable energy". I mean no disrespect with this assessment, I think building a small scale HAWT to become familiar with them a viable educational pursuit, but it does not, IMHO offer any potential for "usable energy". If you have a remote cabin for weekend use that has no access to the grid, such an HAWT could provide useful LVL (low voltage lighting), or perhaps run a radio or other low power device, but it is unlikely to ever pay for itself in an environment where the grid is readily available.

My advice? If you want to learn about HAWTs, order a set of blades and set it up. Power some LED lighting in your garage/basement/workshop. Go into the experience with eyes wide-open looking to learn, not save money or power anything meaningful. I might even suggest that instead of buying a lead-acid deep cycle battery you consider purchasing a Lithium Ion cordless tool battery and a "12V charger" for it. This won't be any cheaper than a lead acid battery (likely more expensive), but this type of battery can handle deep discharge w/o damage, and you can avoid the entire charge controller expense. My goal is not to discourage you, but rather make certain you have a firm grasp on what to realistically expect from your system. All the ads make it sound like the world can be powered from the wind, and perhaps it can, but in the engineering world there are some very real obstacles to overcome to just power a DVD player.

In the future, it is best to keep these types of exchanges in-forum. In PM there is no chance anyone else will have a chance to read this response prior to asking the exact same questions. I do not mind helping, in fact I enjoy it, but the idea is to help everyone who wants to learn. To that end I try to do more than reply with, "No", "Yes", "Try 1m Blades"....I try to explain the math behind the answers so that others can make informed decisions for themselves.

Just for the record, everything I know about wind turbines is math and physics, I am still in the design phase of my first build. I am about 18 months into the design. My only goal is to pump 50% of my domestic water from a 45ft well into a cistern, and to add a "point-of-interest" to my landscape. You can reference my thread in the "Wind Power" portion of this forum. Ghurd and others have more "hands on" experience with HAWTs; but to date, nobody has "beaten the math"; it simply is what it is; I didn't come up with it, I just understand it.

Cheers,

Fish

I see no reason this should not be part of this thread.

Fish

wow really impressed with gotwind.org right now, definitely a great deal of knowledge from its members. Thanks again fish for your responses, very in depth which always helps.... Hey Ghurd I remember that name when I was doing my initial research on charge controllers, are you the same "Ghurd" from the fieldlines website?

[Gotwind Ben]

William,
Yes it is the very same Ghurd, from your side of the pond.
I am based as is this form in Great Britain, there are the occasional differences such as wire gauges and thread sizes (Metric and English) but we manage to get by.

Would anybody choose a screen name "Ghurd" on purpose?

(edit- who would pick that when they could choose anything at all?
Evidently a lot of people. It is NOT me on ebay, photobucket, dot-com, etc)

I am basically a 'hippy WASP' if that narrows down the search options.
ghurd1 may help. That is the #1.
G-

I see that Ghurd1 is even on facebook - although it must be someone else as a picture of a black and white cat comes up...

Getting back to Wills DC motor - I guess this is yet another example of just how hard it is to generate power. We have been spoilt with cheap energy from fossil fuels, trying to capture energy yourself makes you realise just how much we use.

For me the fact that its not easy is part of the fun and the challenge. I'm pretty much like Wills, I dont have the resources (space) to ever make 'useful' energy, but I will have fun trying - and if it knocks a few pennies off the electric bill and saves a few grams of carbon then everyone is a winner :)