what to expect from a low RPM turbine with a kick ass alternator?

[brandnewb]

Please let's assume (it's still under construction but I believe I have cracked the code) I have an alternator that can generate 54V (to charge a 48V battery array) at like 6 RPM.

Crazy yes? I also need to see it before I believe it but I think it's rather trivial to get there after having learned about Lenz's law.

Why 6 RPM? well because that is roughly the speed at which my drag type VAWT spins on my lawn on a typical day.

The turbine arms have a diameter of 5.2m so nothing to sneeze at and have an incredible torque.

when using
https://www.omnicalculator.com/ecology/wind-turbine
I get around 54 watts at 3 m/s which is at the very least something to hold on to as that is what we have here rather often.

The question is, will the alternator be able to also trickle charge a battery array or is there something in the laws of physics hindering that?

[hiker]

As long as you get your desired charging voltage,,with your added load,,just my opinion,,

[brandnewb]

well, I am suggesting that it is rather trivial to get any voltage at any rpm.

But my stance does need to be confirmed though. I have not done so yet but I have this gut feeling telling me it is.

[JW]

Depends on the capacitance of the load.

[brandnewb]

in this instance, by virtue of the seller, we can assume that the battery array has a 1C (280A) discharge rating and a .5C(140A) charge rating.

Or were you referring to something else? I am more than willing to dive into specifics if I can find more details.

[JW]

Or were you referring to something else?

Ya I was referring to a voltage regulator circuit.

https://www.google.com/search?q=capacitance&rlz=1C1CHBD_enUS910US910&oq=capaci&aqs=chrome.2.69i57j0i433i512l4j0i131i433i512j0i433i512l2j0i131i433i512j0i433i512.6178j0j15&sourceid=chrome&ie=UTF-8

[brandnewb]

I am a bit confused JW. I did arive at exactly the same search results earlier and also now I come to the same conclusion. The ability to store energy.

In this hypothetical scenario there is no voltage regulatory circuit.

Just a rather brutal turbine/alternator, wind and a 48V Lifepo4 battery array.

Please help me help you help me. What should I find out next?

[JW]

Do you have a schematic ov what your working with.

[Ungrounded Lightning Rod]

Why 6 RPM? well because that is roughly the speed at which my drag type VAWT spins on my lawn on a typical day.

If that's the freewheeling speed, don't forget that it will spin a bunch slower under load, so design for that.  (Ballpark from using a ruler on the usual TSR vs. efficiency graph for various rotor types:  A Savonius at max power point would be spinnig about 55% of freewheeling speed, a darrius about 62%.)

The question is, will the alternator be able to also trickle charge a battery array or is there something in the laws of physics hindering that?

The laws of physics will be happy with you.  For a given magnet/coil/geometry/turncount/etc. you get less power as you spin slower, but you can compensate, given a particular amount of magnetic material, by increasing the radius and using a higher count of smaller poles.  (Remember the pictures of those early motors/generators with giant wheels and a whole bunch of little coils on spools around them?)

What counts is how much flux crosses how much copper (for power at a given current density), how many turns (for voltage) in a given time.  More poles means it crosses more times per second and each crossing is faster, so more voltage.  Larger radius means more room for more poles (assuming a given wire gauge) and more speed to get the magnetic field across the wires, combined with less force.  This trades away the torque for speed but the horsepower remains the same, letting you match your slow, strong rotor to the multipole magnet/coil arrangement's needs.

[JW]

https://www.fieldlines.com/index.php/topic,130316.msg848338.html#msg8483https://www.fieldlines.com/index.php/topic,130316.msg848338.html#msg848338

All these machines work with distributed windings so the best way to think of things is in terms of flux linkage. Any turn that is bigger than the magnet will link all the flux at some point. Turns smaller than the magnet will not link the total flux but can still contribute some voltage. These turns are very short and have little resistance so you can make the hole smaller than the magnet and still gain but not to the point where you are reducing the loop area to silly levels. If you use rectangular magnets then making the holes trapezoidal with the large outer dimension the width of the magnet and the inner dimension smaller than the magnet seems to be a good compromise.<p>
All turns bigger than the magnet contribute properly to the voltage but changing the magnet spacing does alter the voltage waveform and the peak to mean voltage ratio does change. Normally it has little effect.<p>
Increasing the magnet spacing lets you have the same ( or similar) number of turns but lets you use thicker wire, this is where you gain from ( reduction in resistance). To benefit you must use the available space and wind your coils full until they touch.<p>
Eventually if you went silly you would reach the point where the large increase in turn length would probably be worse than the gain in wire size as far as resistance is concerned and I have little doubt that there is an ideal spacing but certainly you can make modest increases in magnet spacing to advantage. With rectangular magnets, spacing of magnet width at the inner radius seems to be fine . You usually benefit by squeezing the coils at the centre by creeping under magnet size for the inner part of the hole. If you don't do this the coils tend to touch at the centre and leave wasted space round the outside.<p>
This is all a compromise and as far as I know there has not been a detailed analysis done to find the exact optimum. Not that it really matters as we have to take available magnets which are probably not ideal shape anyway.<p>
Flux

like I've said before the database is huge...

[brandnewb]

for real, your database is massive. For newcomers like my self it is easy to get lost in it. No disrespect meant but the sheer size of it can be intimidating.

So I'll try and give an overview of what I am planning.

Here is an older version of the turbine with a diam of 4m. The one under construction has 5.2m diam.



The alternator is going to be something along these lines.

It might be the Dunning Kreuger effect at play here in where the less one knows the more one thinks one knows but I still think I know how to build alternators by now. I hope I do not disappoint anyone including myself.

I went rather deep with a self conjured lenz's law spread sheet and decided that the alternator above is not optimal.
I will make some changes to the alternator as to reach the target voltage at the target RPM.
So hang on to your buttocks everyone. More to follow on this in the not so distant future.

After that I had planned this charge controller and BMS.

[brandnewb]

So assuming I am able to get the intended voltage when the turbine is under the load of the battery. May I assume I can at least trickle charge the battery array when the turbine is spinning?

If so then I will jump for joy and all well become right with the world later down the line.

Or is there something that makes this endeavor basically without hope due to physics?

[Adriaan Kragten]

The rotor which you are using is a pure drag machine. Drag machines are described in my public report KD 416. The optimum tip speed ratio and the maximum Cp are very low, especially if the area of the buckets is small with respect to the swept are of the whole rotor. The Cp-lambda curve is about a parabola and the Cq-lambda curve is about a straight line (see KD 416 figure 2 and 3). In these figures it can be seen that the optimum tip speed ratio is about 0.15 and that the maximum Cp is about 0.05 for the chosen geometry. The optimum tip speed ratio is about half the unloaded tip speed ratio.

So if your rotor turns unloaded at 6 rpm at a certain wind speed, the load of the generator must be that strong that the loaded rotational speed is about 3 rpm to get the maximum Cp. This is extremely low for a PM-generator and even if the generator has many poles, the frequency will be very low. The generator must be that strong that it can produce the rather high torque at this low rpm. The maximum torque of a radial flux PM-generator is proportional to the armature volume for a certain flux density in the air gap. So a large torque level means a large and therefore expensive generator. Everything becomes much cheaper if you use a normal 3-bladed HAWT with a maximum Cp of about 0.4 and a design tip speed ratio of about 6. If you still want to use this drag machine, I would use an accelerating gearing with a 1-step multi V-belt transmission with a gear ratio of about 10 as used in some washing machines. Only the small wheel on the generator shaft has grooves. The big wheel on the rotor shaft can be flat.

[topspeed]

Very interesting looking device.

[brandnewb]

yes thank you topspeed.

The more I think about it the more I am starting to believe it.

Especially since recently learning about MPPT controllers suitable for wind turbines. Then it seems a bit less difficult to designing a low rpm high output turbine and still be able to charge a battery array.

But i'll keep one updated about my progress.

Would you mind sharing what it is you find interesting about this device? other than the way it looks of course