Multiple PM alts stacked

[zowwie438]

Group,

I read in here some time ago about a gentleman that tried using multiple PM's, but the second PM was causing some flux issues with the primary PM.

In everyone's opinion... Would it be best to build a single monster PM, or place three of them back to back on a 3 foot shaft?

I am assuming that my recent find on some bearings and three foot shaft would permit me to build 3 PM's onto this rear shaft assembly.    Or it would at least give me the room to add more as times goes on and needs increase.

View of bearings and shaft: http://www.fieldlines.com/story/2004/8/7/113757/0426

If I have three foot of pipe on the rear of the bearing assembly... I am assuming a spacing of 10"-12" for each PM assembly.    

My questions are?

What are the cog issues with multiple PM's on the same shaft?

Is it best to build on large unit, or stack them?

How far do PM's have to be?   Would 10-12" be enough to combat field issues?

How many magnets/coil asseblies would be best for a rotor assembly that is 20" in diameter?

Thanks all.

--TM

[drdongle]

Several PMA's would have the advantage of redundancy.

Carpa Vigor

Dr.D

[DanB]

"Is it best to build on large unit, or stack them?"

One large one would make much better use of your magnets, and your wire... and it would probably cost less.

Lets say you have 48 magnets to work with...

Ill assume its a dual rotor design but that doesnt really enter into it.  But if it were...

with 48 magnets you could have 24 poles on each rotor.  Any single coil would see 24 poles going by per revolution... and - for the sake of simplicity well say its a single phase machine with 24 coils.  

If you broke that down into two machines, on the same shaft - and also single phse, then youd have 12 poles going round per revolution, and youd have 12 coils in each alternator.  

both cases involve the same amount of magnets and the same amount of wire.  The first case, you have twice the number of magnets going by each coil... so you'd have twice the power if you build 1 big unit as you would from two smaller ones on the same shaft.

The only other consideration in my opinion would be alternator diameter... if for some reason you had a limit of alternator diameter in your design then stacking units would make for a smaller dia machine, but doing so would assure a more expensive, and probably much heavier unit.

Best use of materials is to build one machine, not stack them.

And I'd disagree about the idea that redundancy is good for reliability in this case...  if half your alternator failed, you'd have a serious mis-match beetween the prop, and the alternator, and it might overspeed and blow up - or.. at best, you'd have to take it down and fix it anyhow.

"What are the cog issues with multiple PM's on the same shaft?"

In a machine that has no slots in teh laminates... or no laminates at all (a dual rotor design) cogging is a non-issue.  If you do have slotted laminates, which is the design that perhaps makes best use of the magnets, then... having multiple units on the same shaft could reduce cogging if they are appropriately offset.  Folks do this sometimes with the F&P washing machine motors to reduce cogging.

"How many magnets/coil asseblies would be best for a rotor assembly that is 20" in diameter?"

It would depend on how much power you need at what rpm.  You could build quite a powerful unit from such a large disk if you wanted.

[Flux]

I am sure Dan is right, you should do much better with a larger diameter machine.

I think your idea of stacking several alternators on a shaft several feet long will present many problems.

If you do decide to stack, there is no problem with shareing discs between alternators as long as the discs are thick enough. You should get 3 in a foot long.

If you are only using a 10 ft prop there should be no difficulty making one alternator do the job.  If you are contemplating an alternator big enough to hold a 10 ft prop in a storm without furling, give it some further thoughts as to how you can control it if the load comes off.

Have a look at Hugh Piggott's site and see what he is doing with a single big 4 kW alternator.

If you only want heat and want to take all that nature throws at it, some form of hydraulic pump may be a better bet.

Flux

[Gary D]

Perhaps a link would be in order. Close to 20" (not sure on the metric conversion), Hugh has 56 2x1 magnets (duel disc) putting out 4 kw. Designed for 16 ft blades....way too advanced for me at this time.  Gary D.

http://homepages.enterprise.net/hugh0piggott//nirvana/index.htm

[Flux]

Gary D

Thanks for adding the link, I don't know how to do it.

If you want something more modest as a first step for heating, I should think the alternator for Otherpower's Triplets would be a good bet. Sorry about the lack of a link again.

 With heating you can let the volts rise with speed and get a lot more out and you can track the propeller at optimum TSR much more easily.

Perhaps if you used more turns it would make it easier to match standard heaters.

Flux

[zowwie438]

Thanks to all who have replied.   Learning is half the battle, and with you guys providing input... I feel like I am at the civil war with a fully automatic weapon.   You guys are great!

I will do some more reading and Hugh's books are supposed to be here tonight.   I can't wait to soak up the knowledge.

I don't think I will ever learn it all... But it's nice to know that we have a forum to discuss such adventures.

Best wishes to all!

[SmoggyTurnip]

>>With heating you can let the volts rise with speed and get a lot more out and you >>can track the propeller at optimum TSR much more easily.

Any info on this?