Stronger or thicker magnets allow thicker stator?

[brkwind2]

I've been looking at making a 10 or 12 foot wind generator similar to the ones in Hugh Piggots book.  If I get stronger magnets, say N50 magnets 1/2" thick or N42 3/4 or 1" thick magnets can I use larger wire & make a thicker stator, therefore capable of carrying larger amounts of current?

I noticed some people increasing the gap to reduce cogging so I thought if a larger gap is sometimes better then why not a thicker stator in the first place.

[Flux]

There is no cogging in this type of alternator so forget that issue.

Yes you can do as you suggest and make a more robust alternator. The output will not be much different as that is determined by your wind speed and the prop diameter.

What you will be able to do is produce the same power with less temperature rise in the stator. This will give you extra overload capability or perhaps you can furl at a slightly higher wind speed and gain a few extra watts in a storm.

You will need to alter the turns to keep cut in speed the same and almost certainly you will need to add series resistance in the line to keep the prop out of stall, otherwise you will get less out for your greater expenditure.

If you live in a very windy area and you want extra robustness then if you are prepared to spend the money then it can only be a good idea. For a normal wind area it may gain you a little extra robustness and make furling less critical.

All depends on your views on value for money, the normal design is chosen to be cost effective and robust enough for those who make it furl properly. If you want to spend extra money for similar results and a bit extra robustness then fine. Others may choose to use the extra magnet cost to produce a machine with larger blades and more output.

Flux

[brkwind2]

Thanks for the reply.  I confused 2 different issues.  Unless I'm wrong again I believe people were adjusting the gap for stall issues rather than cogging.

On the 12 ft. machine in Hugh's book he calls for 25 turns of #13 AWG 2 in hand for a 12 volt stator.  That is a 3 phase stator with 4 coils per phase as I understand it.  Higher voltages seem to be prefered so I thought I'd try 24v.  That would be 50 turns 2 in hand of wire half the size, #16 AWG.?

Now if I make the stator 5/8" thick instead of 1/2" do you think I could fit 50 turns of #14 AWG, 2 in hand?  I'd be increasing the stator thickness by %50 & the wire size by slightly more.  I have mostly 8-12 mph winds but also have days of 25 mph all day long.  Not as often as I'd like but it happens so I'd like to take advantage of it if I can.  I was hoping by buying N50 magnets or thicker N42's,I could still get good results despite the larger gap.  I believe Hughs book calls for N35 magnets 1/2" thick.

[Flux]

N50 will let you increase the gap perhaps 40% over N35. It should let you use slightly thicker wire. You may manage 2 in hand #14 because the stacking factor for the thinner wire will be better than the thick wire used for 24v. This and the increased gap may just get you there. Much depends on your winding capabilities. You could squeeze the coils a bit at the centre like Dan does, sort of make them egg shaped so there is a bit more clearance where they touch when you assemble them.

Even going to 2 in hand #15 would gain you a fair increase in cross sectional area.

Thicker N42 will likely give you more advantage than 1/2" N50.

If you do any drastic changes with the magnets then you would be wise to do a test coil. The original number of turns will hold for the same gap flux density, but you may find you have increased it even in a 50% greater gap. If so then you will need less turns and that should let you use even thicker wire.

You don't want to end up in a situation where your cut in speed is much reduced as you can only correct that by opening the gap and easte a lot of the potential of the improved magnets.

keep the cut in speed as Hugh intended, then go for line resistance to get you out of stall. That way you will see the advantage of the more efficient stator. Overall efficiency will be the same so you will get the same for the same wind speed but you will be able to let the furling go higher and get some of the extra current of the higher winds that would be risky with the original winding.

If you are in a relatively good wind area then it may pay you to aim for a slightly higher cut in speed. It will loose you some of the lighter winds but it will match you better for the higher winds and you will need less line resistance to keep you from stalling. You could achieve this by increasing the gap on the machine when completed if you are not sure of your winds. If you are happy that you are in a good wind area then consider reducing the turns and increasing cut in speed at the start, then you can choose thicker wire to suit.

Be warned that if you do go for a higher cut in speed then you may have some effect on furling. If anything increase the alternator offset, don't think of going less.

Flux

[Flux]

In the first paragraph it should have said " the thick wire used for 12v"

"You don't want to end up in a situation where your cut in speed is much reduced as you can only correct that by opening the gap and easte a lot of the potential of the improved magnets."

That strange word should be waste.

Sorry, bad hair day.

Flux

[Gary D]

 A thought here. If you are considering upping to 24 volts, might as well go to a 48 volt system immediately. If you plan to go for any more production in the future... The amps are less (smaller wire needed system wise), rectifier losses and resultant heat are less, and it becomes cheaper down the road. Just a thought from the very cheap seats. Your mileage may vary...

[brkwind2]

I'm hoping to use what I generate pretty much immediately rather than store anything.  The batteries are just for a buffer & the higher the voltage the more cells I have to buy was my thinking.  Also I hadn't seen many cheap 48v inverters.  I'm not opposed to higher voltages so all input is appreciated.  I'm still trying to figure out just what I want to build.  I see what Flux means about balancing cost with results.  Thicker magnets are quite pricey.  The thicker stator will take more wire.  The higher voltages will require more expensive inverters & controls perhaps.  It all adds up quickly.

[Flux]

24v is fine for that size of mill, it will give more options with inverters. 48v is better for big installations where it is your vital source of power and cost is a lesser factor.

I agree with your take on the alternator mods, yes it will be an improvement but at a price. If you are not totally dependent on this and want to take what is available then I would seriously consider increasing the cut in speed and improving the high end power.

There is little in low winds with this size of mill and it only pays to go for the low winds when you are absolutely dependent on the little that you get on those days.

You can double your high wind output if you do the right compromise. For your type of needs you may not find it worth chasing 20W on low wind days if it costs you 500W on a windy day. You could in fact gain a fair bit of output just by optimising the winding to higher winds even with the original magnets.

Flux

[Gary D]

 I'm not cutting 12 volt systems at all. If you plan on having a 2500 watt inverter for example with a D.C. disconnect the different voltages become apparent. A 250 amp D.C. disconnect/ breaker will handle a Trace Dr 2512 at rated output, but the Trace/xantrex has a surge capability of 5000 watts- or 400 amps. If going to 24 volts the surge stays under the 250 amp rating for example... Dump loads become more manageable also (less amps needed to dump the same watts). Rectifier heat sinks get less heat due to less amps...

 This is off topic tho for your original question. My apologies, just looking at the system as a whole. Flux has covered your questions precisely as usual. I plan/need a duel voltage system also( wife demanded 12 volts). 12 volts for small loads to start, and in a bit (year or 2?)48 for a bigger system...Gary D.

[kitestrings]

Nice post.  I had a number of related questions and I appreciate the time folks take (Flux in particular on this one) to break things down.

I have a related question and a comment:

Flux spoke of a "stacking factor for thinner wire" early in this post.  I assume that the thinner wire allows you to get a denser concentration of copper in the coil winding procees (i.e less voids; more copper), and that inturn allows you to keep the stator thickness to a minimum?

I've wondered, aside from ease of winding and any cost breaks for the smaller ga. wire, if there was any benefit to winding more strands?  If for example, the spec was for 2 in-hand 15#, would one strand of #12 otherwise be the same magnetically/electrically (hopefully I picked a comparable equivalent resistance)?

On the voltage issue, I generally agree with Gary.  The larger more robust inverters also tend to be higher voltage.  It depends some on how much load overall you are serving.  There is, however, a much better selection of 12V - to a lesser degree 24V - end use equipment, lighting, waterpumps, etc. available if the system is modest.  Finding a reasonably priced, good temperature (2700k), single piece 48V CFL is not quite as easy.  Some will say those loads should go thru the inverter, but may not have waited for one to return from cross-country from repairs, no matter how infrequent.

Good luck Brkwind2, and kind regards to contributors.

-kitestrings

[Flux]

2 in hand #15 has the same csa as #12. It is in most respects identical but you may be able to get more turns of 2parallel #15 into a space than a single thicker wire. This depends considerably on the winder's skill and the relative sizes of wire. Wire as thick as #12 is awkward to wind for small coils and needs a fair bit of tension.

Wires much thicker than #10 will start to give eddy losses with the common air gap alternator and you would be well advised to use more strands of thinner.

It becomes more awkward to use more than 2 or 3 in hand and that is best avoided if possible. For more than 3 in hand you may start to see other issues unless you twist or transpose the strands. Some of the low voltage windings do become a challenge when you reach thick wire sections.

In general the smaller the wire the higher the cost but it doesn't matter much for the normal sizes. Below .5mm the price of thin wire rockets up.

Because of the crosses you will not get as many turns of several in hand wire in as the equivalent single winding. ( 75 turns of single #15 will likely take less apace than 25 turns of 3 in hand #15).

Flux

[kitestrings]

Thanks this is helpful.

Okay, let me probe one other thought...

I don't know the paticulars on the magnets here.  I could speculate maybe 2" dia?  Let me use another example that is hopefully illustrative:

Dan B's larger unit (20'er) uses 3 x 1.5 x .75 inch magnets.  It is a 20-pole, 15 coil star/wye alternator.  And, I can see that there is a formidable price hurdle to go to magnets larger than those.  Setting that aside for the moment - if my math is right the flux/ per pole, assuming a telsa of .70, would be about 2.0 mWb?

If he had instead selected 2 x 3 x .75 or 1" (assuming this size were available), and gone to a 16-pole, 12 coil configuration the flux/pole would increase to about 2.7 mWb?  The coil turns would increase from say 45-46 to maybe 53-54, but otherwise would this be good or bad?  Would he perhaps have trouble coming out of stall in lower winds?

-kitestrings

[Flux]

Interesting and complicated.

In your example there would probably be little difference in the result. For a given number of poles then the output does depend on the flux per pole, but if you reduce the number of poles then it changes things ( I suppose we ought to clarify things by saying it depends on flux per pole and number of poles)

I don't think the 16 pole winding with bigger magnets would be much different from the 20 pole, but then things will be affected by other issues. In general it pays to keep the gap flux between 600 & 700mT as a best compromise between flux and winding space so you would have the option with the 16 pole of running the same stator thickness or going thicker. I haven't looked in detail at all the combinations. Most likely the issue would be settled by the availability of magnets, those commonly used seem to be better value for money than others that seem more ideal.

Dan is running pretty well stalled anyway, he may be compromising the low wind performance a little even now by reaching stall quite early, but the better natured operation in higher winds makes it a safer proposition with the larger mills,