Author Topic: Appropriate generator characteristics for small wind turbine  (Read 1113 times)

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leheim

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Appropriate generator characteristics for small wind turbine
« on: February 05, 2024, 04:01:05 AM »
Hello guys. I am working on a project that aims to implement an MPPT controller for a small wind turbine and evaluate the performance of the controller. I have built a wind turbine prototype using PVC pipes for rotor blades, a 3D-printed rotor hub and nacelle, and a metal pole tower. It is probably not the best wind turbine out there, but I think it will be sufficient for testing the controller. I have a PMSG that I want to use, but I am unsure if it fits the application. I am studying digital and embedded electronics, so I am a bit unfamiliar with electrical machines. Here is my thought process:

The prototype has a rotor radius R=0.6 m, and I will assume an efficiency Cp= 0.25 (while operating at the optimal tip speed ratio). For a wind speed Vw=10 m/s, the output power is P=Cp*(1/2)*ρ*π*R^2*Vw^3=173 W where ρ is the air density (1.225 kg/m^3). If the optimal tip speed ratio is TSR=7, it means that the rotor tip speed is Vr=70 m/s, which leads to an angular velocity of ω=116 rad/s and RPM=1114. The voltage constant of my PMSG is Ke=8.18 V/kRPM, which means that the output voltage is 9.11 V. Since the output power is 173 W, the output current will be 19 A.

It seems to me that it would be beneficial to operate at a higher voltage and RPM to avoid high currents so that the power electronics will be easier and cheaper. I'm thinking of using a three-phase uncontrolled rectifier, and maybe a boost converter to implement MPPT by controlling the load with the PWM signal. Ideally, I will be able to find modules for these so that I can focus on the control logic (sensors and MCU programming). Should I have a different generator? What should I look for? A PMSM with a higher Ke? Or are such current values normal/acceptable? Any help here is very welcomed :)

15818-015819-1

bigrockcandymountain

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Re: Appropriate generator characteristics for small wind turbine
« Reply #1 on: February 05, 2024, 07:34:29 AM »
Is 19a within the rated output of the generator? If it doesn't have a rated output, what is the output wire size?

I would probably just go for it.  Worst case just make some shorter blades that spin higher rpm and make less output. 

I'm interested to see how you do with the testing.  I don't know of any controllers that use true mppt for wind. 

Bruce S

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Re: Appropriate generator characteristics for small wind turbine
« Reply #2 on: February 05, 2024, 08:56:16 AM »
leheim;
Welcome to the forum.
I gotta say that is one fine printed covering.
Traditional ways of controlling the output are of course having the windmill held to a specific output that does not allow it to over spin that can cause blade or generator failure.
I too am curious about how your idea of MPPT will help with this. You could of course use a rotor spin sensor that starts turning the windmill out of the wind once max revs are met.

Do send a few more specs on the generator and wiring so we can assist further.

Cheers
Bruce S
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Adriaan Kragten

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Re: Appropriate generator characteristics for small wind turbine
« Reply #3 on: February 05, 2024, 02:43:47 PM »
I have had bad wind tunnel experiencies with rotor blades made out of PVC because these blades are very sensible to flutter at high wind speeds if the the pipe thickness isn't very large. This is mainly because of the low torsion stiffness. Flutter is a combined bending and torsion oscillation. But if you use very thick pipe material, you should round the nose and sharpen the tailing edge otherwise the airfoil will have a too high drag/lift ratio for a rotor with a high design tip speed ratio. I have used 2 mm thick stainless steel for a rotor with cambered tapered blades as described in my public reports KD 616 and KD 617.

Matching in between rotor and generator is explained in chapter 8 of my public report KD 35. Any windmill rotor has a certain optimum cubic line. The optimum cubic line is given by formula 8.1 of KD 35. If the rotor is loaded such that the optimum cubic line is followed, it will run at the tip speed ratio for which the Cp is maximal. The generator winding and the generator load should be chosen such that the Pmech-n curve for the wanted load has two points of intersection with the optimum cubic line of the rotor which are lying not far apart. The Pmech-n curve for a certain winding depends on the voltage. The higher the voltage, the more the Pmech-n curve shifts to the right. Measured curves for different voltages are given in my public report KD 78 for a PM-generator made from an asynchronous motor using the standard 230/400 V, 3-phase winding.

Matching in between rotor and generator can only be checked if you have measured generator characteristics for the wanted load. So you need a test rig with which it is possible to measure the torque, the rotational speed and the generated electrical power. An example of such a test rig is given in my public report KD 595. All public KD-reports can be copied for free from my website: www.kdwindturbines.nl at the menu KD-reports.

In the photo's, I see no provision which turns the rotor out of the wind at high wind speeds. It is impossible to limit the maximum rotational speed only by the generator load because there is always a wind speed for which the rotor torque is higher than the peak torque of the generator. If this wind speed is reached, the rotor will speed up and the generator winding may burn. Wind turbines with no safety system are very dangerous. Five different safety systems which turn the rotor out of the wind at high wind speeds are described in my public report KD 485.
« Last Edit: February 06, 2024, 02:42:14 AM by Adriaan Kragten »

JW

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Re: Appropriate generator characteristics for small wind turbine
« Reply #4 on: February 05, 2024, 04:29:23 PM »
In the past that used "PVC blades" I remember a safety issue with those blades.

https://www.fieldlines.com/index.php?topic=140024.0

JW

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Re: Appropriate generator characteristics for small wind turbine
« Reply #5 on: February 05, 2024, 05:02:42 PM »

MattM

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Re: Appropriate generator characteristics for small wind turbine
« Reply #6 on: February 06, 2024, 06:10:51 AM »
I'm guessing that's a downwind style.  There must be a hidden bar behind those buckets because the poly bucket material would deform in no time.

Bruce S

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Re: Appropriate generator characteristics for small wind turbine
« Reply #7 on: February 06, 2024, 08:43:06 AM »
IF memory serves me correctly the bucket blades were only done to prove that they could be done and used in just a quick test mode.

The PVC blades that were put into "production" were CPVC so they would stand up to the damaging UV and they were much thicker.

Even then they were for merely testing the gen to see how well it would work. S-style mills used the blue water buckets the same way. For a lark put a few of the larger "Folger's" coffee containers onto an ECM motor as a proof of concept. Should still be a few picks for back to the early 2000s of several.

I'm still curious about the gen.

 Cheers
Bruce S
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taylorp035

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Re: Appropriate generator characteristics for small wind turbine
« Reply #8 on: February 07, 2024, 09:59:42 PM »
I think a few members here know a bit more about electronics and load curves, but I think the general concept for your application is you need the alternator to be able to load the blades more than they need and then you back it off by using PWM.

Your shaft power from the blades is going to be both a cubic factor of the wind speed, but also have a non-linear multiplier of Coefficient of performance depending on your TSR and a smaller factor of Reynolds number.  The Cp number can be quite dramatic for a fixed pitch blade and you will see the windmill "take off" as it greatly increases efficiency as TSR goes from ~1 to something in the 3-5 range.

Your alternator will have a different efficiency curve and also load curve vs. RPM.

You can do a lot of testing + calculations and maybe if you get it all correct, you might come to a conclusion on what to do.  I would instead save the effort and just test it and manually tune in the PWM to target your desired TSR (7 -ish, though I'm not sure if the design of your blades might make that number a bit lower).  If you can, track RPM and wind speed and start to graph all of this after your first round of testing.  If you have a more stable wind source (large fan, blower, vacuum cleaner or leaf blower), maybe you can more easily tune it at the start.

You might consider having it "shut down" before you exceed the capacity of the alternator at high speeds, else it will over speed and break your plastic blades.  You probably want to work up to the maximum speed in small increments.  Most people use a folding tail design to avoid spinning the blades to fast and it's surprisingly effective and relatively easy to do.

SparWeb

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Re: Appropriate generator characteristics for small wind turbine
« Reply #9 on: February 09, 2024, 08:38:31 PM »
Hello Leheim,
Welcome to Fieldlines.  That's a fine looking project.
I have some follow up questions.  I hope you come back and acknowledge what's been suggested so far.
Depending on what you have chosen as the generator, I think I can point you to some "rules of thumb" to simplify your test plans and how you will recognize progress, as you develop your MPPT controller.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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leheim

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Re: Appropriate generator characteristics for small wind turbine
« Reply #10 on: February 14, 2024, 04:32:31 AM »
Hello, again.

Thank you for the great response! And sorry for my late follow-up, it's been very busy at the university. An unnecessarily large number of meetings and emails with professors and academic departments to get access to test equipment and facilities, as well as thesis supervision. On top of it, attending career fairs, doing interviews etc. to land a job after the thesis, just the normal student life. I will try to answer your questions.

More specs for the generator (Alva X60-Kv120) can be found in the attached datasheet, or at the manufacturer's website (I can't link it, but it's on alvaindustries.com). It is designed to be used as a motor for large industrial drones. I can't find the rated current output listed in the datasheet, but I think it says 29.3 A (continuous) and 51.4 A (transient). The green/yellow output wires you see in the pictures are 2.5mm2 (best I could find at the workshop), and the small blue wires are from the thermistors in the generator.

I plan to acquire smaller rotor blades, and let the turbine rotate at a higher RPM which will generate more volts, like you suggest bigrockcandymountain. X60-Kv120 also has a higher efficiency at higher RPMs. I can maybe get my hands on the 28'' diameter "Alva & Mejzlik propellers", which you can see on the linked website. Then my turbine will be similar to the portable Shine Turbine (shineturbine.com), which by the way incorporates MPPT according the the user manual. I find it interesting that bigrockcandymountain says that you don't know of any wind controllers that use "true MPPT". If that is the case, my thesis may be more significant than I have thought. I have been trying to "angle" my thesis such that I don't just do something that many others have done already.

As I stated in the original post, the wind turbine design and functionality itself is not the main focus here, but rather the implementation and performance of the MPPT control strategy. However, I guess the wind turbine should be decent enough to be able to test the controller. My idea is that the controller should be able to optimize the power output for any given turbine. Considering safety features like overspeed protection, I think the solution will be to simply not set it up during strong winds (in addition to a stop switch which shorts the output wires). That is also the approach of the Shine Turbine.

I don't quite understand Bruce S's statement about "holding to a specific output that does not allow it to over spin". Are you referring to fixed-speed wind turbines? I have not chosen a specific MPPT implementation yet, but I have been looking at the simple "Perturb and Observe", where the duty cycle of the DC-DC converter is altered (perturbed), effectively changing the load, then observing the change of output power. If the output power is increasing, the controller will continue to alter the duty cycle in the same direction until the maximum power point is found.

My plan onwards is to do more research on different MPPT control strategies, and how to set up a good test scheme. I plan to to tests in the lab with a "back-to-back" setup with a motor that drives my generator. With the motor, I guess I can for example sweep RPM and wind speed/power with different constant loads, while measuring the output power. Then at a later stage, hopefully I will have time to to field tests (the most fun part).

Greetings,
Lars Erik

DamonHD

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Re: Appropriate generator characteristics for small wind turbine
« Reply #11 on: February 14, 2024, 06:35:33 AM »
My inner glass-half-empty wonders whether there are local maxima that "Perturb and Observe" would get stuck in.  It seems sometimes to be a problem for solar PV for example.

Rgds

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Mary B

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Re: Appropriate generator characteristics for small wind turbine
« Reply #12 on: February 14, 2024, 10:47:15 AM »
While we can help  wit generic questions we CAN NOT do your work for you on a college thesis!

With 28" diameter rotor blades you will get very little output, capture area rules when it comes to amps!

MagnetJuice

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Re: Appropriate generator characteristics for small wind turbine
« Reply #13 on: February 14, 2024, 07:04:08 PM »
Hi leheim,

The motor that you have mounted on your turbine is too powerful and the RPM is too high. The specifications for that motor says that it will output 44 Volts at 5400 RPM. It will output 12 Volts at 1500 RPM.

Try to find a motor with maximum RPM of 500 to 1000.

I searched and the closest that I found that should work is this:

https://www.surpluscenter.com/Electrical/DC-Motors/DC-Motors-Face-Mount/12-Volt-DC-Salt-Spreader-Motor-Arrowhead-SAB0191-Arrowhead-SAB0191-10-3108.axd

That motor produces 12 Volts at 1050 RPM and should be good for about 200 Watts.

You should be able to find something similar to that in the EU.

That sounds like an interesting project.

EDIT:

I would not get those 28'' diameter Alva & Mejzlik propellers. They would spin faster, but they don’t have the torque to turn that big motor that you have now.

I looked at the little Shine Turbine and the specifications. It needs 10 m/s (22 MPH) to make 20 watts. IMHO that is a very expensive toy.

Ed
« Last Edit: February 14, 2024, 10:09:25 PM by MagnetJuice »
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taylorp035

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Re: Appropriate generator characteristics for small wind turbine
« Reply #14 on: February 14, 2024, 10:10:48 PM »
I second MagnetJuice's comments.   Radius of 0.6m = ~4 foot diameter.  Your 121 RPM/volt spec on the data sheet is about 5x too high to hit a useful voltage.  There are better BLDC motors out there with lower KV ratings, though getting down in the 15-30 kv range will be challenging without coging too much for start up and not being too expensive.

For example, 20 RPM/volt * 12 volts = 240 RPM cut in speed.


JW

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Re: Appropriate generator characteristics for small wind turbine
« Reply #15 on: February 14, 2024, 11:23:27 PM »
thinking about these motors. Check this out- I have a GE electric riding lawn mower.

I think my motors (3) on the mower deck. Meet the specification you guys are talking about.

Im having a problem posting pictures, I have to fix something. Ok will get some pictures up.

JW

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Re: Appropriate generator characteristics for small wind turbine
« Reply #16 on: February 15, 2024, 12:45:38 AM »

leheim

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Re: Appropriate generator characteristics for small wind turbine
« Reply #17 on: February 16, 2024, 02:18:32 AM »
Isn't the RPM area for a given turbine only determined by the rotor size? I understand that a wind turbine harvests the most energy from the wind if it operates at the optimum tip speed ratio (typically around 7 for a three-blade HAWT). If the wind speed is 10 m/s, the tip speed should ideally be 70 m/s, which corresponds to a specific RPM for a given rotor size. A small turbine would have to spin faster to operate at TSR=7. This logic is further elaborated in my original post. I think this is the core of my problem, so please tell me if I'm missing something here.

Thank you for the generator suggestion MagnetJuice. I have also been looking at this one:
ebay.co.uk/itm/235042332008?chn=ps&_ul=GB&var=534977374051&norover=1&mkevt=1&mkrid=710-169684-097358-0&mkcid=2&itemid=534977374051_235042332008&targetid=293946777986&device=c&mktype=pla&googleloc=1010976&poi=&campaignid=20649918451&mkgroupid=151788312862&rlsatarget=pla-293946777986&abcId=&merchantid=6995734&gclid=Cj0KCQiAw6yuBhDrARIsACf94RV3voAhQUyhNRR0LcWsCQCp3uWbPtWYMEVdzHyCNEoHbJthoCFvIxwaAqJVEALw_wcB

I have also another question. Some of you are talking about the cut-in speed where the generator will output 12V. Is there a problem with just using a buck-boost converter?

Greetings,
Lars Erik

Mary B

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Re: Appropriate generator characteristics for small wind turbine
« Reply #18 on: February 16, 2024, 11:14:06 AM »
Isn't the RPM area for a given turbine only determined by the rotor size? I understand that a wind turbine harvests the most energy from the wind if it operates at the optimum tip speed ratio (typically around 7 for a three-blade HAWT). If the wind speed is 10 m/s, the tip speed should ideally be 70 m/s, which corresponds to a specific RPM for a given rotor size. A small turbine would have to spin faster to operate at TSR=7. This logic is further elaborated in my original post. I think this is the core of my problem, so please tell me if I'm missing something here.

Thank you for the generator suggestion MagnetJuice. I have also been looking at this one:
ebay.co.uk/itm/235042332008?chn=ps&_ul=GB&var=534977374051&norover=1&mkevt=1&mkrid=710-169684-097358-0&mkcid=2&itemid=534977374051_235042332008&targetid=293946777986&device=c&mktype=pla&googleloc=1010976&poi=&campaignid=20649918451&mkgroupid=151788312862&rlsatarget=pla-293946777986&abcId=&merchantid=6995734&gclid=Cj0KCQiAw6yuBhDrARIsACf94RV3voAhQUyhNRR0LcWsCQCp3uWbPtWYMEVdzHyCNEoHbJthoCFvIxwaAqJVEALw_wcB

I have also another question. Some of you are talking about the cut-in speed where the generator will output 12V. Is there a problem with just using a buck-boost converter?

Greetings,
Lars Erik

Watts generated is determined by rotor size... that is where you need to start. Just because it says it will do 200 watts doesn't mean it will do it with a 3 foot rotor. Research power available vs rotor size.

You start with determining how much power you need, then start looking for an appropriate alternator that reaches that at the lowest possible RPM because high speed on a wind turbine is SCARY. Rotor blades can explode from the forces... tossing shrapnel for hundreds of feet. That is why the axial flux alternator is so popular on this site, it can be designed with low cut in speeds... I for one would not want a set of blades up there spinning at 750RPM at cut in speed!!!!!

SparWeb

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Re: Appropriate generator characteristics for small wind turbine
« Reply #19 on: February 16, 2024, 11:15:31 PM »
Hello Lars,
Have you set up with wind turbine outdoors in the wind yet?

The explanations you've received so far are excellent, so the only improvement possible is for you to take that thing outside on a windy day and observe it, and take measurements of what happens.  You should run it without the MPPT controller at first. 

Set it up on a rigid post, pole, or structure.  Being a temporary set up for test I won't direct you too much but just make sure it won't flip over with a 100 Newton push against the blades.  Be mindful of your own safety doing this, however, and if you stop reading now, ignore the warnings below, and carry it outside and hurt yourself - it's your fault, not the anonymous stranger on the internet. 

First make sure you have shorted the output wires together and expose it to a strong wind in that condition.  If there is a terrible mis-match between the rotor and the motor you will find out quickly enough.  Shorting the wires permits the maximum current to flow, which usually means too much torque for the blades to turn.  This is a brake.  You want a brake, for safety.  If this brake works, it will help ensure your WT is safe in the future, too.  It should turn very slowly in this condition even in a strong wind.  Any other behaviour is not acceptable.

If it does spin uncomfortably fast with leads shorted, don't try to grab the blades!  Regain control by turning the tail by hand until the rotor is facing  sideways to the wind.  Then bring it back indoors or in the lee of a building to sort out why shorting the leads did not restrict the blades.  If shorted leads can't make the blades turn slowly, then stop now, redesign this WT, or start fresh. 

When you are certain it will not spin out of control with the wires shorted, then you can allow it to turn with resistance or battery charging loads.  I would do this to characterize the behaviour of the WT before installing the MPPT.  You will not be able to demonstrate the effectiveness of your MPPT device if you don't know what the characteristics of the WT were before you installed it!

NEVER RUN A WIND TURBINE UNLOADED.
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Adriaan Kragten

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Re: Appropriate generator characteristics for small wind turbine
« Reply #20 on: February 18, 2024, 03:44:16 AM »
Hello Lars,
Have you set up with wind turbine outdoors in the wind yet?

NEVER RUN A WIND TURBINE UNLOADED.

If it isn't allowed to run a certain wind turbine unloaded, there is something seriously wrong with the safety system. The safety system which limits the rotational speed and the thrust must be that good that even an unloaded rotor won't be destroyed at high wind speeds. Reduction of the rotational speed only by the load is dangerous because the required electronics to steer the load may fail and this results in an unloaded rotor.

I have tested the hinged side vane safety system of my VIRYA-4.2 wind turbine with an unloaded rotor and although an unloaded rotor is rather noisy, the rotor running at the maximum rotational speed gave no problems at very high wind speeds. The VIRYA-4.2 has a 2-bladed rotor with a diameter of 4.2 m with constant chord blades made out of massive mahogany wood. The design tip speed ratio is 8 and the unloaded tip speed ratio is about 12.8. The blade chord was 200 mm and the Gö 623 airfoil was used. The maximum unloaded rotational speed was about 480 rpm, so 8 rev/s. The maximum loaded rotational speed was about 300 rpm so 5 rev/s. The slender blades of this wind turbine are sensible for flutter at very high tip speeds but flutter didn't occur for an unloaded rotor.

kitestrings

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Re: Appropriate generator characteristics for small wind turbine
« Reply #21 on: February 18, 2024, 04:34:59 PM »
Hi Lars, and welcome.

You have a really sleek looking turbine; some nicely printed pieces for sure.  I hope to see more as things develop.  You've probably already figured out that you'll get a wide range of perspectives here on any given topic, and the trick is to wade through them.  In the end though, I believe it helps the design process.  The comments have been good.  I don't have a lot to offer, but for what it is worth...

I think your blade connection at the hub may be something you have to beef up.  I'd be concerned that the reduced width might be a weak point without added thickness or some sort of strut.

A couple of us here have been using MPPT type controllers (Midnite Solar, Classic).  They are not a true "feedback and adjust" as with solar, but rather they allow you to input a power curve that follows the curve of the alternator.  One benefit though, is that the curve can be tweaked, if it is not perfect from the start.

I do agree that it is best to attempt to keep things electrically loaded to the extent possible - I think that was Sparweb's intended message.  Side-furling works well.  It's time proven.  If a turbine runs unloaded, in a good wind site for any amount of time though I would say it is only a matter of time before something fails.  It will spin up to rpms that it is not equipped to handle, furl, slow and repeat.  To touch here on that "wide range of perspectives" I mentioned earlier, here's a prime example where we get deep in the theoretical discussion on the one hand, but then make broad-brushed summations based on very limited, antidotal experiences on the other.

Good luck, and I hope to see more, ~ks
« Last Edit: February 18, 2024, 07:03:42 PM by kitestrings »

JW

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Re: Appropriate generator characteristics for small wind turbine
« Reply #22 on: February 18, 2024, 04:47:23 PM »