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quick update on blades plus question regarding induction conversions
By cslarson, Section Wind Posted on Tue Jan 22nd, 2008 at 11:03:43 AM MST
update on blades with pictures and question regarding predicting output voltage from induction conversion

Hi all,
It is bitterly cold here in Kabul. I have included some photos of the first fiberglass blade (black) that was pulled from the fiberglass molds (made from the wood example piece to the left). This was done while I was on vacation in England (where I got engaged!) and I was very pleased with how it turned out. (the rotor diameter is 4m)
We are currently experimenting with induction motors. We have two motors, a 3kw unit and a 5.5kw unit. Both were initially 4-pole, but the 3kw unit has been rewound to be 12-pole and 380V. I have read many posts and Zubbly's conversion guides, and I apologize if I should have gleaned from them the answer to my following question. How do I predict the final output voltage? I am looking for high-voltage (roughly 100V-300V dependent on wind speed) ac current to flow from the final design.
From what I have read recently it seems that for the blades we designed the larger 5.5kw unit would be more appropriate.
Ok, now I see that there is a section in Zubbly's conversion guide that I should look at more carefully! Anyway, if anyone has any additional tips to help me get my head around this I would be grateful.
Sincerely,
Carl

fixed your pics next time name them something without spaces in the filenames you won't have this trouble.
Kurt

quick update on blades plus question regarding induction conversions | 11 comments (11 topical, 0 editorial)

Re: question regarding induction conversions (3.00 / 0) (#1)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Thu Jan 24th, 2008 at 01:03:45 PM MST
(User Info)

Hello Mr. Larson,
I remember your posting from many months ago. Glad you're still well.

If you have access to several different types of motors, perhaps you can mount a motor side-by-side your generator. With a pulley on each and a belt between the two, you can drive the generator at constant speeds, speeds you will know quite exactly. Select pulley ratios that are representative of the speed the windmill will turn.
Run the set with no load on the generator to see what voltage is achieved at that speed. The Volts/RPM is a straight line proportion, so you can predict what you have at any other speed from this data.
Then apply loads, of the kind you will use in the final installation. I see what I think are capacitors in your pictures. They may not be useful in these tests - they may indeed be dangerous if you don't enclose or cover them. If your generator will charge a battery system, then attempt to charge the battery during your generator test.
Interested in seeing what you come up with. Keep us posted!
Steven Fahey

Re: question regarding induction conversions (3.00 / 0) (#2)
by Ungrounded Lightning Rod on Fri Jan 25th, 2008 at 07:05:21 PM MST
(User Info)

any other speed from this data. ... I see what I think are capacitors in your pictures. ...

I suspect the capacitors are for running an induction motor as a self-excited alternator, rather than converting the motor to a magneto. The text also talks about a "conversion" that involves rewinding, but has no mention of replacing the rotor with a permanent magnet assembly or modifying it to mount permanent magnets.

Such alternators are resonant devices. They do not produce a voltage that is proportional to speed. Instead they produce virtually nothing until they exceed synchronous speed, after which the voltage takes off in a positive feedback loop as the resonant circuit starts to ring. Load will also steal excitation, so the generation will collapse again if the load is too high or the speed is allowed to deviate significantly from just above the resonance. (And if the machine is allowed to stop while under load it may act as a demagnitizer, removing the residual magnetization from the rotor, after which it will not self-excite the next time it spins up and thus will allow the turbine to run away.

Using a resonant alternator on a wind turbine is a very problematic thing. Unlike a fuel-driven engine, the wind has no particular preference for spinning the rotor at the "right speed".

[ Parent ]

Re: question regarding induction conversions (3.00 / 0) (#3)
by cslarson on Sat Jan 26th, 2008 at 04:40:21 AM MST
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Yes, I should have clarified that. We had been experimenting somewhat with the capacitors. After the rewind of the 3kW unit we could not get excitation and produced no voltage even after boosting capacitance quite a bit. The following paper prompted the experimentation: href=http://www.nrel.gov/docs/fy00osti/26713.pdf.

I am talking about the conversions that modify the rotor assembly to turn the induction motor into a permanent magnet alternator. I believe that these may still yield cost savings and a simplified frame design. I realize that my original post was far from clear, but what I was hoping to learn was how to predict the final voltage of the generator. I would like to modify the induction motors as little as possible while still achieving the desired voltage range.

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#4)
by cslarson on Sat Jan 26th, 2008 at 10:03:06 AM MST
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Following is a mockup how the magnets for a conversion might be arranged for the 5.5kW motor if it was not rewound (stays 4-pole). There are 24 magnets of size 10x20x59mm.

I would welcome any comments and suggestions. Should I use larger (thicker, at least) magnets? I have noticed a number of different methods being used to affix the magnets to the rotor. I think Zubbly had a nice method of filling around the magnets with resin, but he used circular magnets. If I employed that method I might have to use smaller magnets and leave some space between them.

I am expecting the rpm range of my set of blades to be between 100 and 400 or so. Without a rewind could anyone hazard a guess at what volts I might obtain from this setup?

Re: quick update on blades plus question regarding (3.00 / 0) (#5)
by ghurd on Sat Jan 26th, 2008 at 05:24:35 PM MST
(User Info)

It looks like nobody wants to guess... So I'll give it guess.
I think the volts will be lower than the target.

Is it wired in Delta now?
Changing it to Star will help, and might get it up to what you want.

Bar magnets will have a pretty large effective gap. Might try magnets less than half as long, and put them in like Zubbly would stagger his if he was using 2 per pole.

It is going to be a real treat getting them on there. I don't know how anyone would get that many magnets of that size next to each other.
Be careful.
G-

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#6)
by cslarson on Sat Jan 26th, 2008 at 10:47:04 PM MST
(User Info)

Thanks very much for your response. Could you explain what you mean when you say bar magnets will have a pretty large effective gap? I think I understand you to mean that they can be spaced out somewhat, but could you explain? Also, in that mockup I have made the airgap (gap between magnets and stator) as small as possible thinking that I want to take advantage of as much magnetic flux as possible. Increasing the airgap would leave some room to wrap fiberglass around the magnets to hold them in place.

If I have this right, according to Faraday's law of induction, the voltage produced is proportional to both the number of turns in the coil and the magnetic flux. Increasing either will help me reach my target voltage range. If possible I would like to leave it in a delta configuration to take advantage of the increased efficiency (less resistance). Also, I think star-delta switching might not work with the water pump controller we will be using which will hiccup if the V/f ratio changes.

The following mockup allows for more space between the magnets (that would presumably be filled with resin). There is also about a third more magnetic volume. Clearly, to answer my question about predicting the final voltage I need to spend a little time with Faraday's Law! Regardless, comments are much appreciated.

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#7)
by ghurd on Sun Jan 27th, 2008 at 12:23:44 AM MST
(User Info)

I think you are past what Faraday intended!
Zubbly put the magnets on the rotor, then tried a test winding. He was good at guessing, but he did a test winding.

The smaller the air gap the better. Yes.
I am not so good with software.
Cut the top and bottom off a soup can to be the laminations. Use a pencil as the magnet.
Skew the magnet (pencil) inside the laminations (can).
The middle of the magnet is far from the lanimations.

Use a ruler as the magnet, and the edges make it worse.

I have a few concerns...

Do you have neo magnets?
It will be VERY difficult to get them that close together with the proper pole up.
The size of the motors or magnets is not clear to me, but I would not try it with bar magnets because it sounds somewhere between impossible and simply dangerous.

The air gap is important. However, if the magnets are that thick, I have concerns about completely or over saturating the laminations. It causes problems I can not explain. Too much flux causes problems.

Simply replacing the armature into the stator will be a dangerous task, requiring block and tackle.

Is this the first conversion you have done. This is not a good size to start with.

I believe you may be seriously under estimating the forces and violence involved.
G-

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#8)
by cslarson on Sun Jan 27th, 2008 at 05:05:48 AM MST
(User Info)

Thanks for the warning. Since the idea is to come up with something that will be easier to make I will certainly heed it! I looks like I am back to the original Zubbly design. The mockup below uses 18(dia)x15mm disk magnets. These would be used in the 5.5kW induction motor pictured in my first post. Modeling it makes it easier for me to line everything up and make templates and such. Yes, I will be using neodymium magnets.

So you are saying that I cannot simply use thicker disk magnets to increase the flux and thus increase the final voltage? Does the magnet configuration shown below look sensible?

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#9)
by ghurd on Mon Jan 28th, 2008 at 03:44:27 AM MST
(User Info)

It looks a lot more sensible to me.

Increasing the magnet thickness (some people call it 'length') will increase the voltage.

The big thick bar neos looked like it might have saturated the laminations, causing problems. (if you could have got them on there!)
I had the problem a couple times. I can't explain it, but here is where it was explained to me...
http://www.fieldlines.com/story/2006/8/23/144336/344
G-

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#10)
by cslarson on Mon Jan 28th, 2008 at 08:08:09 AM MST
(User Info)

Thanks for the link, though I'm not sure how much of it I understood. Is the gist that the laminations in the stator can only usefully take a certain amount of flux going through them? I came across these magnets which are the same cost (by volume) as the 18x15mm ones in the image above, but are 3/4" x 3/4" (19.05mm x 19.05mm), so thicker. Could using thicker/longer magnets like these cause over-saturation problems?

Just one more question if I may. Zubbly's method is to machine down the rotor, and with the help of a pre-drilled fiberglass cylinder, set the magnets in place with resin. Could the holes be drilled straight in to the rotor (though problem might arise getting them all exactly the same depth)? I was also thinking it might be possible to make something that has the same initial function as the fiberglass cylinder that Zubbly uses, but that could be removed to function in the same way for the next conversion, eliminating the need for drilling all those holes again. The problem with the second idea is if the fiberglass is needed for added strength.

Thanks very much for your help. I realize I have less experience with these, but believe there is knowledge here that could help encourage a better small wind power industry here in Afghanistan.

[ Parent ]

Re: quick update on blades plus question regarding (3.00 / 0) (#11)
by ghurd on Mon Jan 28th, 2008 at 08:31:42 AM MST
(User Info)

I do not know when saturation will happen. It just looked like if those big bar neos were installed, it could have been a problem.

I don't think it would be a problem with round neos.

Setting the magnets into the steel rotor will short out the flux.
They could go in a small percentage, but not much.
Many people have machined the rotor, then used a pressed on aluminum cage dilled to hold the magnets. It's something I can't do.

This would work with a lathe, and a lot of time with a file...
http://www.fieldlines.com/story/2007/2/2/55123/75010

[ Parent ]

quick update on blades plus question regarding induction conversions | 11 comments (11 topical, 0 editorial)

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