12 footer expected top rpm?

[brkwind2]

Assuming the I get the furling right what would you guess the top rpm of a 12 footer to be?  I'd like to turn the rotors on a lathe because I cut them with a torch & ground down the major discrepancies between them & I'm thinking the rpm's shouldn't be high enough for slight variations to cause too much vibration.  I might be able to sneak them in to work & use their lathe but I only have one center hole in one of the plates & it isn't large enough to fit on the chuck.  I'd have to probably bolt the rotors together & weld a shaft to one in order to get them on the lathe.  I'd rather avoid all that if I can.  What do you think?

The current plan is a 24v mill with a 5/8" thick stator, 14" rotors with 2*1*1" N42's (16 per rotor) I'm hoping to squeeze in around 40 turns on each of the coils, 4 per phase, 12 total.

Ok, start pickin on me & maybe I'll eventually make something usable.  thanks.

[jmk]

 Forty turns of what? I used 50 of #13 one wire in hand and could have gone more. Rpms on just the plates will withstand a whole lot more than the 12' rotor max rpms. I've herd of guys getting their mills up to 800 rpm but thats fast for a twelve footer. Mine is fast at 450 rpm.

[jmk]

I would stick to a 1/2" thick stator. You are going to need your gap to be around 3/4".

[jmk]

 Sory I didn't catch the magnets were 1" thick. You should be fine with a 5/8" stator. You will have lots of room for wire.

[brkwind2]

I think Hugh's original plans called for 25 turns of 13Ga.2 in hand for 12v, 1/2" stator.  I'm going for 24v but with the 5/8" stator & 1", N42 magnets I'd planned 14Ga., 2in hand, 40 turns.  I considered 3 in hand of slightly smaller wire hoping for better stacking.  I want to squeeze in as much wire as I can.  I've trimmed 1/16" off each side of the center of my coil winder making it sort of a V hoping for a little more room where the coils may touch.  I'll have to see what I can squeeze in there.

[brkwind2]

Yes, I originally planned 3/4" thick N50's but Flux pointed out a while back I'd be better off with " thick N42's I believe if I remember correctly.  Comparing the advertised pulling strength he's right.  I'm hoping that with stronger magnets I can get away with a thicker stator & larger wire with a few less turns & still achieve about the same cut in.  Think I could sqeeze in 40 turns of 15 ga. 3 in hand?  Should I try for a few more turns?

[brkwind2]

Make that 1" thick N42's

[Flux]

600 rpm is faster for a 12ft prop than I would tolerate, some may take it even higher with motor conversions. For an air gap machine if you intend to connect it directly to a battery with no other means of load matching and you want low wind results you will end up with it in stall at the top end and you will be highly unlikely to see speeds of 400 rpm.

It is nice to have all the mechanical bits true and it simplifies things a great deal.It doesn't really matter whether discs are true or not as long as you can balance the whole thing. With the prop it only needs static balance and you can do that with a weight anywhere convenient.

You can balance each rotor disc statically and that should work out perfectly well. What you can't do is rely on static balance of a 2 disc alternator if each disc hasn't been individually balanced. You will end up with a couple which you will not be able to balance out unless you have a dynamic balancing machine.

If you can't get things machined exactly then balance each disc on its own with a weight that is in the same plane as the disc and all should be fine. A slight final static balance can be done if needed or it can be balanced out with the prop.

I haven't run any figures on your winding but with 16 poles and 1" magnets I suspect 40 turns is on the high side but use it as a starting point for a test coil.

Flux

[jlt]

My 12 footer has 15" rotors 1x1/2x2'mags.stator is 1/2 thick . I was able to get 140 turns of15.5 wire in it . my coils are only 3/8 thick.1/2 wide on the bottom 1" wide on top.It was a tough job getting them to fit.It is wired in a jerry parallel configuration for 48 volt.                                                              

[tecker]

Check out You tube for Green power science They do a circle cut with a table saw . If you just want to true them up that may work with a metal cut of blade I haven't tried it but it looks good .

[Ungrounded Lightning Rod]

The current plan is a 24v mill with a 5/8" thick stator, 14" rotors with 2*1*1" N42's (16 per rotor) I'm hoping to squeeze in around 40 turns on each of the coils, 4 per phase, 12 total.

As I understand it:

 - As you increase the air gap the flux goes down but the room for copper wire goes up.

 - At small gaps you're gaining more from adding more room for coils than you're losing by weakening the field that induces voltage in each turn.

 - At large gaps you're losing more from weakening the field than you're gaining from stuffing in more turns.

 - And the peak of the curve is where the air gap equals the thickness of the magnets.  (That's assuming you didn't need any clearance around the coils for bearing slop and potting.  Fortunately the curve is essentially flat there so a little error is not too significant.)

I'm pretty sure that "thickness of the magnets" in the dual-rotor situation is the sum of the thickness of the magnets on the two sides.  (Flux?  Is that right?)  So you would be getting maximum power with a 2" thick stator and would have lots of room for coils.  (Even if it were the thickness of the magnet on ONE side, 5/8" would still put your stator into the too-thin-for-optimum region.)

If you're going to err, it's cheaper to err on the side of a thinner stator and save on copper.  Also:  As you approach the "optimum" the slope is flattening out so you're adding a lot of wire for a small gain.  But with 2" of magnets total I'd want to go with a thicker stator, a few more turns of thicker wire, and end up with more current or lower losses and lower heating.

[brkwind2]

Hugh's plan originally called for a 1/2" thick stator with 1/2" thick magnets.  I'll be using 1" magnets so if all things were linear I could have a 1" thick stator.  I don't think it is linear so widening the gap probably reduced the flux field more than just 25% which is what i'm adding to the thickness.  Since I'm increasing the magnet thickness also that should make up for it & then some i hope.  I can always increase the gap if I have to or add resistance to the line.

[brkwind2]

Thanks, I'll try that.

[brkwind2]

"What you can't do is rely on static balance of a 2 disc alternator if each disc hasn't been individually balanced."

Ok it finally soaked in what you mean.  Do each one individually before balancing them together.  That makes sense.  thanks.

[brkwind2]

What guage wire did you wind with?  I'm assuming a single strand since you have so many turns.  Approximately how many amps do you see in a 12 mph wind?

[Rover]

Ok I just need to know what I'm supposed to stop

[Ungrounded Lightning Rod]

It's linear if the rest of the scaling is in proportion.  Of particular interest is the spacing between the magnets.  If that doesn't increase there will be disproportionate flux leakage causing the flux in the scaled up gap to fall off slightly - but not enough to be a big deal unless the gap is narrower than it is tall.

[Janne]

Hi, what are you trying to gain with those thicker magnets? Using the bigger magnets with the same 12' rotor(and same configuration otherwise) is a good way to waste magnet, at least in my opinion.

Those thicker magnets would allow you to run a bigger rotor than the 12'.

[brkwind2]

I like to blame the dog for any bad odors in my area.  No reflection on you.

[brkwind2]

My main objective was to get larger wire in the coils.  I want to increase the current capacity because I read somewhere that a few people thought maybe the stator was a little light for a 12' prop.  I went with thicker magnets because I'm increasing the gap.  I also dream of furling later because I plan to run all the coil wires down the tower so I can switch between star & delta.  I'm also considering trying to charge my 12v solar bat. bank to start with if I can limit current & not stall.  Probably just a dream I have but will give me something to play with.  I have a couple of small plc's I got out of the trash that have 1 bad output I may try to work into this scheme.  

[brkwind2]

I guess I can always make a larger set of blades later too if that proves feasible.

Part of what I'm thinking about is this time when I put a 1750 rpm 5 hp motor on an exhaust fan which replaced an 1150 rpm 1 & 1/2 hp motor.  I was surprised when the 5 hp motor was fully loaded.  I was thinking at the time that increasing the rpm's by 1/3 approx. that it would only take about the same increase of hp.  It didn't because unlike a conveyor or something your moving air so the increased speed translates to a much increased load.  It isn't linear.  Applying the same principle to the wind mill, an increase of 2' on the blades over a 10 footer will have a lot more power than what I feel the original spec. on the stator could handle.  Is this making any sense?  I'm tired so hope I'm not rambling too badly.

[Flux]

You should do well with the bigger magnets. The total magnetic circuit length is 2" so you could reasonably work with a 1.5" air gap. This would comfortably let you use a stator up to 1 1/4" thick.

Choose your turns for a slightly lower cut in than for direct connection, it will cut in in very light winds in star. It will get into stall quite quickly and on other than the lightest wind days you will probably be better in delta.

Although it is overpowered in star I doubt that you will stall in delta and you will have something that will really perform in the better winds with it in delta. I think it unlikely that you will need to do anything to avoid stall in delta, shifting the turns from 100% in star to 57% in delta should get your load curve right up where the prop needs it.

Most likely you will find it better in the end to let it change to delta early and not switch back to star unless the wind really drops. The disappointing thing about star/delta changing is that on certain days the thing spends its life in the wrong mode.

If you get consistent low wind days it may stay in star all day, on better days it will probably be better to let it stay in delta rather than try to use the few seconds when the wind drops enough for it to be better in star.

You will find the transition from star to delta virtually unnoticeable but the transition from delta to star will be fairly abrupt as the speed comes down.

Just one other point you might consider, the star delta switching is messy. Delta performance will not be ideal but you will more than gain over leaving it permanently in star. Star Jerry connection is likely to give similar results and the switching is easier. The Jerry connection is more efficient than delta.

In the past I have tried series parallel and star delta, they are a big improvement over not having any matching at all, I haven't tried star/Jerry under wind conditions but I see no problem.

This was my initial starting point with permanent magnet alternators, after coming to this from wound field machines I was never happy with the simple sit in star and stall method. I finally went on to boost converters to replace the star/delta and the last machine has been a buck converter with mppt.

Keep us informed with the results.

Flux

[PaulJ]

   "Part of what I'm thinking about is this time when I put a 1750 rpm 5 hp motor on an exhaust fan which replaced an 1150 rpm 1 & 1/2 hp motor.  I was surprised when the 5 hp motor was fully loaded."

   So was the fan noisy and blowing a gale with the 5 hp motor? An extra 1/3 or so rpm is not insignificant, the power is proportional to the cube of the airspeed.

   Paul.

[Janne]

Hello,

In a fan(or a pump) if you double the rpm the power required is 8x the power required before.

With a turbine, if you double the rotor size you will approximately need 8x more magnet. This is because the the power is proportional to swept area, but the speed is also halved.

So as you used a 10' for a design start, you should be fairly well matched with those magnets. Sorry about my earlier post, I though you just picked some 12' plan i just threw in the bigger magnets for that.

You will need a fairly thick wire to make the machine work at 12V. One possibility would be to wound the coils from thinner wire, and individually rectify each coil. Though that would practically remove the possibility of the star / delta switching.

[brkwind2]

Yes, the more I think about it, it was several years ago, I think it was a 1 hp I replaced instead of a 1.5 hp.motor.  So it took 4 more hp to increase the speed by a third.  It was blowing pretty hard.  We joked about it sucking the roof in.  It was an exhaust fan on the top of a rendering plant so it couldn't blow hard enough.  No matter what, that place would be hot & smelly.  I just put it on because it fit & we didn't have any new motors of the correct frame.  It was in the "good" used pile.  In another instance we had a guy replace a 1 1/2 hp 1140 rpm motor with a 3 hp 1750 rpm motor on a cryovac exhaust blower & it was always overloaded.  He put bigger overloads in the starter to keep it running.  The only reason it lasted any length of time was that it was a big heavy duty cast iron (& heavy to carry accross the roof) motor.  On that one I put on I just lucked out.  I would have never believed it would have fully loaded that 5 hp motor.  Lesson learned.

[brkwind2]

"So as you used a 10' for a design start, you should be fairly well matched with those magnets. Sorry about my earlier post, I though you just picked some 12' plan i just threw in the bigger magnets for that."

Hi, Probably shouldn't admit it but yea, that was kind of what I am doing.  I was comparing the 10 & 12 footers in hughs last book.  I'm building the 12 footer frame etc. but was thinking the stator might not be heavy enough for 12 foot blades.  I'm sure it would be fine as originally specified because many have use the original design successfully, but I'm hoping to delay furling perhaps & boost the current output but still have a little more alternator to tame the blades so to speak.  As I said before I can always make larger blades, increase the gap or add resistance to the line to compensate for the results, I hope.  I'm aiming for 24v but thought maybe through a transformer or something to limit current & prevent stalling I could trickle a little juice into the 12v bank thens switch to 24v star & or delta.  Probably another unworkable hair brained scheme but mulling this stuff over keeps me out of bigger trouble sometimes.

Since I wanted a heavier stator which by necessity must be a wider gap, then figured the magnets better be thicker.  I'd rather have too much than too little since it is easier to compensate for that hopefully.

It isn't too late to take another path.  I just got the frame & tail done.  They were changing out the pipes in the sprinkler system at work so I got 12, 10' lengths of 4" sch. 40 out of the dumpster for free.  Hope I can make some sort of tower out of that.  I've got quite a ways to go but am quite excited about having the frame done.  Lots of fun.

[brkwind2]

Hope I don't post this twice.  Not sure what button I pushed but something seemed to go wrong.  Must get coffee.  

Did you ever try any wireless controls doing switching at the top of the tower?  Or would it maybe be possible to put a hall effect or prox. sensor or something to detect rpm up there & wirelessly transmit the data?

Thanks for the reply, you've given me a lot to think about.

[Flux]

When I did this wireless technology was not reliable ( not sure if it is adequate now)

I did originally switch at the top of the tower but it was a bad move, far better in the end to drop a lot of thin wires rather than have clever things in stupid places just to use 2 wires.

I used speed as the sense signal ( 2917 tacho chip but it takes some taming). Speed is inherently simple and stable, I did use current on one tiny machine but it was not entirely satisfactory. The original scheme with star/delta relay up the tower used a centrifugal speed switch but that was 30 years ago, it worked well enough but I had to build it specially and it was time consuming to alter its settings.

The 2917 tacho chip will drive a speed sensed relay direct but you may need to use it as a pilot to switch a contactor sized device for star delta on a 12 ft machine. I think you could do star Jerry with a relay small enough to run from the chip direct.

Flux