Author Topic: Axial Flux Alternator (Read 5786 times)

dna11207 · « **on:** February 09, 2009, 02:07:52 AM »

I was wondering if you could give me some helpful information. I am in the process of building a wind turbine alternator here but I seem to be running into some difficulty. I use 24 N45 2" x 1" x 1/2" on two rotor discs. I also have 18 coils, 6 connected in series for each phase. I made the first stator with #20 wire and used 80 turns for each coil. This produced some very high voltage but little in the way of current. Any load of more than 2 amps or so would make it almost impossible to turn but turns and produces voltage freely without load. Well, I figured way too many turns and too thin of a wire. So I made a second stator with #13 wire, 21 turns each. In delta, I was able to produce 10 amps into 12v batteries but still, it was extremly hard to turn, hard enough that wind would not be able to. Any Ideas what may be going on here? You can see some pictures of what I have done here http://www.instructables.com/community/Axial_Flux_Wind_Turbine/

Thanks,

Dave

electronbaby · « **Reply #1 on:** February 08, 2009, 07:26:06 PM »

sounds like everything is going as it should. You just need to sort out your voltage and cut in speed. You made no mention of what prop diameter you are flying, so it is a little hard to know what rpm you should be running at, therefore, a little hard to know what your alternator RPM will be. Once you know this, you will know if you need to add or subtract the number of turns in your coils from what you previously had.

dna11207 · « **Reply #2 on:** February 08, 2009, 07:41:15 PM »

This will be a 10 foot diameter. But like I said, as the load increases to just a couple of amps, it gets almost impossible to turn at least for turbine blades.

Dave

dbcollen · « **Reply #3 on:** February 08, 2009, 07:52:47 PM »

This is why matching the blades to the alternator is so vital, the alternator will run unloaded until it reaches cut in rpm, then the alternator should be a little bit more powerful than the blades to keep them slightly in stall. If you make your cut in too low, or your alternator too big then you will stall the blades badly and see poor power output. Also wiring the alernator in Delta is generally a bad idea. unless the geometry of your coils is perfect and you have the same length of wire and # of turns in your coils, then you will have circulating currents in you stator that eat all your power. Star or Jerry rigged is better.

Dustin

electronbaby · « **Reply #4 on:** February 08, 2009, 08:03:03 PM »

yes, this is correct....as long as when you are UNLOADED, it turns very freely. Your battery load will change as the state of charge changes, this has a lot to do with the temperature and the chemistry of the battery, (for lack of better terms, you cannot control this)

Like I said before, you need to have your cut in (nominal voltage vs. operating RPM) worked out. For a 10' prop, you should to be somewhere around 140RPM. This means that if you are running a 12v system, then you want to start charging around 12v (the batteries will start to pull current around 11.5v). You will want to make your alternator make approx 12v at 140RPM (we also usually account for rectifier and feed line loss).

Under normal battery charging circumstances, VERY LITTLE load will be placed on the machine till it reaches cut in speed. We call this freewheeling. BUT, once the machine reaches cut in, the batteries will start to charge and pull the voltage down. This will lead to an increase in current. If you multiply the current times the voltage, you will get the wattage. For all practical purposes, the voltage is always held steady by the batteries. Its the rising and falling of the current (from wind induced speed and torque on the spinning blades) that makes the wattage fluctuate...."in a nutshell".

dna11207 · « **Reply #5 on:** February 08, 2009, 08:06:19 PM »

Ok well, I think I have the coils on the second stator just about perfect. Perfect in wire length and placement. I was able to produce 10 amps in star and 20 amps in delta. Delta required more speed but about the same torque requirement as star. I don't think it should be that hard turn to produce such little power.

Dave

dna11207 · « **Reply #6 on:** February 08, 2009, 08:15:52 PM »

I understand what you are saying but I don't think it should take so much power to produce 10 amps at say 15 volts. It takes a tremendous amount of energy to turn this alternator. I am not talking about blades here, just the performance of the alternator and force required to turn it vs output. If you look at my chart, I am getting about 12v at 120 rpms.

electronbaby · « **Reply #7 on:** February 08, 2009, 08:23:24 PM »

At 120RPM, you are a little low on the cut in speed,...you will most likely be running in stall early.

12v systems are the least efficient because you will have the highest I^R losses. If you make your feed line thinner, it might prevent you from running in stall as often.

And I also seem to disagree. You think you have a problem with drawing 10A. I think your alternator is fine. You just have a good appreciation now for how much power is in the wind.

dbcollen · « **Reply #8 on:** February 08, 2009, 10:07:09 PM »

Also if you are calculating cut in from AC Voltage you need to multiply AC X 1.4 to get the peak voltage that the rectifier will begin putting into the batteries, so about 8.6VAC equals 12VDC, this can cause some major problems calculating cut in.

Dustin

Flux · « **Reply #9 on:** February 09, 2009, 01:11:44 AM »

You can't get something for nothing. Electrical output depends on mechanical input. Cranking something by hand you are limited to a small part of the available man power. At best a man can only maintain an output of a few hundred watts for short periods so that is the limit on your electrical output ( and you have lost nearly 40% in inefficiency). Your 10ft wind turbine will have a mechanical input of up to 3 hp or 2.5kW in a decent wind at furling point so even with 50% efficiency you have potentially over 1kW out.

Your delta winding will have lower efficiency than star but if it matches the prop better it will still give you more output.

Finally power is torque x speed and with delta having close on twice the speed for the same input you will get the same power with close on the same torque. All is working according to the laws of physics, that's the best you can hope for, no perpetual motion here.

Flux

TheCasualTraveler · « **Reply #10 on:** February 09, 2009, 04:35:24 AM »

Just wondering, I read through your post twice but didn't see,

Whats the size of your rotors? You say 2 rotors, is the second one a blank disc?

Looks to me as if magnets and coils are pretty crammed in, not much space between mags at the bottom. And the coils, what's the size of the inside hole of the coils?

Also, when wiring the stator did you check the voltage of 1 coil and make sure they were wired right by measuring 6 times the voltage of 1 coil per phase?

How about some more pics of the finished alternator?

I agree, the bits would seem to add up to more than 150 watts.

CmeBREW · « **Reply #11 on:** February 09, 2009, 08:45:29 AM »

I think you are in stall. The alternator is to strong for the 10' blades. Looks better suited to a 12 footer or more. If the alternator really has 48 mags total (24 pole) then thats twice as much magnet as the typical 10' mills here.

Then you could try STAR hook up with the 13ga stator.

I usually only do 150watts on my big pedal generator for ONE minute. It is quite a little workout. It really helps me appreciate the awesome torque and power in a 10' windmill.

CmeBREW · « **Reply #12 on:** February 09, 2009, 05:06:24 PM »

This certainly is confusing to me. I think many want to try and help, but they need even more information.

I looked at everything and see you are testing on a strong lathe. So you must have not yet made or tested the blades yet.

What you are making sure looks to be at least TWICE as powerful of an alternator than the typical 10' mill alternators here. You have twice the magnets AND twice the poles. So it needs to be much bigger than just a 10' mill. I am guessing 14 footer.

Your 24 pole/18 coil layout is correct except it is tight with the magnets and coils and a little cancelling is going on from the bottom corners of the magnets. But I doubt that is the problem.

I am not as experienced as many here, but it looks like your coils are not thick enough. You mentioned only 21 turns of #13 per coil. The voltage (12v) you mentioned you are getting at 120rpm doesn't seem anywhere near high enough to me for that thin of a coil and that many poles. I would think you would be getting twice the volts at that rpm or more.

I suspect for a 12v system and that big of an alternator you are talking about BIG AMPS which means you also need a LOT bigger gauge (or equivalent) coils-- Like '3 in hand' #13 perhaps??. But the voltage you are getting from only one strand of #13 at 120rpm seems way to low.

Can you tell us what the thickness of your stator is? And more importantly what the Mag to Mag total airgap is? (from one magnet rotor to the other)

Are you absolutely sure all the magnet's polarities alternate, N,S,N,S,N,S,ect.?

Have you measured the AC voltage of each phase at a given rpm, to make sure they are the same. If one phase is lower, then a coil may be backwards causing bad cancelling and vibration, and lower voltage.

I'm afraid if you make only 10' blades they will stall very badly on that bigger 24 pole altenator.

-

-Keep giving as much info as you can think of and they can help you get to the bottom of your problem. -Sorry I'm not more experienced with BIG coils.

dna11207 · « **Reply #13 on:** February 09, 2009, 05:45:45 PM »

There are 24 magnets on each rotor. 2" x 1" x 1/2" N45 Mags. The air gap between mags at present is 3/4". I can go about a 1/4" closer together on the rotors before they hit the stator, but I have to make a new spacer to keep the rotors from sticking together and crushing the stator. The 1/4" will probably make an impressive improvment. The mags do alternate and are attracting each other through the coils. Each phase produces identical output and I was very meticulous about how I wound them and length of wire used for each coil. The outside diameter of the rotors is 12". Looking at the coils, in the 12:00 position, the vertical measurement inside the coil is 2.1 ". My goal was to try and keep the overall alternator as small as possible. I don't think that is presenting a problem, but I could be wrong.

dna11207 · « **Reply #14 on:** February 09, 2009, 05:49:00 PM »

Just testing alternators, not using blades yet, they will be determined after I find best way to go with the alternator.

dna11207 · « **Reply #15 on:** February 09, 2009, 05:50:18 PM »

Outside dia. is 12" on the rotors.

TheCasualTraveler · « **Reply #16 on:** February 09, 2009, 06:09:12 PM »

Good,

Can you tell us a little more.

What was the voltage you measured per phase and at what rpm?

Can you give us more stats on what amps & voltage the alternator put into batteries at various speeds on the lathe?

What I was thinking was using a lathe to test it the 150 watts sounds way wrong. If all else checks out maybe something in the delta / rectification wiring?

dna11207 · « **Reply #17 on:** February 09, 2009, 07:00:41 PM »

It seems like it is working out to be 1 volt for every ten rpms.

= 2.3v ac
= 3.9v ac
= 6.2v ac
= 9.5 v ac
= 12.56 v ac
= 15.10 v ac
= 19.10 v ac
= 24.90 v ac
= 30.50 v ac
= 37.50 v ac
= 49.50 v ac
= 59.20 v ac
= 74.00 v ac

These measurements were taken across 2 leads of the 3 phase output and it was wired in a star configuration.

The battery voltage was reading 19 volts when I could spin it up to 10 amps. The battery is on it's way out and I don't want to smoke a good one testing. It does provide a load that can be measured. When not charging, the battery drops to 11.6 volts after a while and holds there. Thats all I have right now. When I get more time to test, I will post some more info here. Thanks for your help,

Dave

CmeBREW · « **Reply #18 on:** February 09, 2009, 07:37:34 PM »

Ok I see. The voltage after the 3 bridge rectifers would be higher. Probably 12v around 100rpm. I had a bridge rectifier once that unknowingly shorted and caused me quite a bit of frustration and confusion for some time with my 3-phase alternator testing. You might want to make certain your bridge rectifiers are perfect.

Also, I have doubts about that 12v testing battery. You even said it was going bad. If it is shorting inside it, that will bear down (by shorting) on the mill too. Almost 'bad' batteries can do really weird things.

The batt being 11.6v is near dead,,, but when you start charging into the battery it acts as if it is WAY over filled?? Can you hear the batt de-gasing at such a time?

I would any other 12v car battery that you know is still good and test into it just to make sure. If you already have done so, then....never mind.

-Keep trying and testing.

dna11207 · « **Reply #19 on:** February 09, 2009, 07:45:20 PM »

Yes, I can hear the battery hissing and bubbling. I don't have a good one to try right now. Also starts getting hot on the sides near the top.

TheCasualTraveler · « **Reply #20 on:** February 09, 2009, 07:49:50 PM »

A little more clarification,

In your post at the start of this thread you posted a link to the Instructables website. In that thread there are pictures of a rotor that looks like it had most of the magnets removed. Counting the spots were magnets are or were glued it looks like 24 spaces on one rotor. Do you have 24 magnets all on one 12" rotor or are all those marks where magnets were and then removed? If so does that mean you have 12 magnets on each 12" rotor?

TomW · « **Reply #21 on:** February 10, 2009, 05:52:16 AM »

Been kind of just watching this thread as it unfolded. I keep thinking that a wiring goof could have gotten just these results. Especially if it is hard to turn with no electrical load. In my years of troubleshooting electronics I have found some very silly / dumb mistakes on wiring can and will produce really bizarre and unexpected results.

That said, sometimes it pays to back way up and in this case if possible I would be looking at the coil wiring just to satisfy myself it was as it should be. It is very easy to make a goof then not recognize it when checking later especially with starts, ends and one phase with reversed polarity. Just a general bit of advice.

For instance 2 phases mostly canceling and one outputting would give near expected voltage, like 1/3rd power and bigger load than expected for the output. It would perhaps exhibit these symptoms?

Good luck with it. You have some good feedback going. Just wanted to toss this on the pile.

Do not mistake me for a builder, I just have some experience with electronics over a few decades in the industry.

Tom

oztules · « **Reply #22 on:** February 10, 2009, 12:09:01 PM »

Tom,

He has even phase to phase voltage, free spinning under no load, and is outputting about what I would expect with this configuration. I'm not sure he has done anything untoward.

I think he just underestimates what the wind can and cannot turn. People seem to forget that most of the power is coming from out near the tip area, and for a 10 footer, it has a 4'- 5' leverage on that "hard to turn when shorted" alternator you have sitting on the bench.

I think a lot if us have lathe tested alternators, only to find the 3-4hp lathe struggles and then ultimately fails to get it up to any where near full power. But the wind has no trouble driving mine (4 meter).. which I could only turn by hand with great difficulty when shorted on the bench.

In fact the wind has pushed mine up to 5kw and beyond ( and frightened the hell out of me in the process) says a lot about wind, blade length, torque and tip speed.

Just my take on it.

........oztules

TomW · « **Reply #23 on:** February 10, 2009, 12:28:27 PM »

Oz;

Good points and why I usually try to stay out of "issues with a build" threads. A 5 foot lever arm is pretty significant, although perhaps not obvious at first glance.

Some problems can be pinpointed with a quick look see, yet require acres of text to explain how to recognize it.

All good fun.

Tom

dna11207 · « **Reply #24 on:** February 10, 2009, 05:14:21 PM »

Those pictures just show what happened to the magnets after I was careless while building the rotors. There is 24 mags on each rotor. There is 2 rotors that face each other, 48 mags total. 24 poles.

dna11207 · « **Reply #25 on:** February 10, 2009, 05:34:48 PM »

And yes, it could be just that. I have no experience using blades to tun these a of yet, so I probably am under estimating what they can do. I do think I mentioned earlier on, that my higher voltage stator required alot less effort to turn to produce the same amperage as the lower voltage one. I am using a transformer to reduce the voltage back down. If I produce 480 volts and transform it down to 24 I use 1 amp at 480 to produce 20 amps at 24 or less. I am using thinner wire for the coils allowing me to wind larger coils to get the high voltage. The stator showed no signs of producing any heat while this was running under load for about 5-7 minutes. It seems like the high voltage may be a more efficient way to go, even with the loss of having to run through a transformer and the 60 hz tranformer issue. It still works well at a lower frequency and as I get closer to 300 rpm (60hz) the better it gets. Just my thoughts, trying a bunch of different ideas just to hopefully find us something that will work and be very efficient at the same time. Maybe I am beating my head against a rock, but I have to satisfy the curiosities. I am hopeful that I am going in the right direction with this. But your feedback is very much helpful and appreciated. Thanks,

Dave

TomW · « **Reply #26 on:** February 10, 2009, 06:54:31 PM »

First I noticed a transformer mentioned being connected to the thing??

That is an immediate load all the time so that bit of info is something that changes everything drastically.

Woot, I love troubleshooting via forums.

I'm done on this one.

Tom

TheCasualTraveler · « **Reply #27 on:** February 10, 2009, 08:05:36 PM »

Well, I would have thought that is way too many magnets for a 12" rotor, possibly better suited to 16" rotors. If I cipher right the bottoms of your magnets are touching each other or very close to it (less that 1/16").

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Author Topic: Axial Flux Alternator (Read 5786 times)