Dual Voltage machine

[Tritium]

The idea of a dual stator machine comes up from time to time and it is fairly common knowledge that this is a waste of magnet and effort when in the form of rotor-stator -rotor-stator-rotor.

What about a machine with 2 rotors with 1" thick magnets and 2 stators roughly 1/2" thick in the form rotor-stator-stator-rotor to produce a dual voltage machine? What would be the problems with such a design?

Thurmond

[Flux]

I don't see any problem with it but why do you want to do it?

[Tritium]

For the ability to use existing 12v components while placing new 48v system without scrapping old system till it reaches end of life. At that point build a new 48v thick stator to replace the dual voltage system with a single.

Thurmond

[Flux]

In that instance it is not a good idea. At best you will have half the power available from each stator. It will certainly work but it is little better than mounting 2 alternators on the machine with one redundant. You waste a lot of material and it would be infinitely better just to build a 48v stator and swap it when the time comes.

If you must make a machine compatible between the 2 voltages then at least use all of the stator. Parallel connections are a bit tricky as you are likely to get circulating currents but it will not be too bad if you are careful. If you choose 16 poles then you have 4 coils per phase which go in series for 48v. They need to be in parallel for 12v.If you are prepared to risk winding 4 wires in hand the chances of circulating currents are a lot less but the 48v connection will be a bit torturous and the stresses between turns will be high. Shouldn't be a problem, the wire is pretty good these days but if you run it open circuit in a high wind or get moisture in the stator you may be pushing your luck.

I think I would make it carefully with accurate coil spacing and careful winding and just connect the coils in parallel for 12v. There will no doubt be a bit of circulating current and a bit of drag at start up but you are not likely to be in serious trouble. I can't see it being worth using 4  3 phase rectifiers in parallel to isolate these currents but if you don't mind rectifiers up the tower then that is easy enough to do. Dual voltage machines are a bit of a compromise and 4:1 range is worse than 2:1

Flux

[Tritium]

Thanks Flux,

It is a 16 pole and I never thought of paralleling the coils in a phase.

Thurmond

[Tritium]

I forgot to mention I was going to wind the thick stator 3 in hand #14  so 4 in hand is probably not that much more of a problem.

Thurmond

[electrak]

seeing that you are winding coils with X in hand, how about winding the coils with say 2 in hand for 48 volts and one wire for 12, or better 2 in hand for 48 and 4 in hand for 12, then when you go all 48 all you need to do is reconfigure the 12 part to 48, must leave all wires hanging out to do it

[Ungrounded Lightning Rod]

If you rectify and then parallel you won't get circulating currents.  Downside is that you need a lot more rectifiers and wires to them from the genny.

You can put the rectifiers up the tower with the genny if you don't need to short the mill for shutdown and are willing to lower it if you need to replace a diode.  Otherwise you need a lot more wires down the tower to do this.

I'd build the genny with the coil terminals coming out and using care in the coil and magnet location and with getting the coils as close to identical as possible.  Then I'd hook 'em up in parallel and measure the circulating currents.  If they're no more than 5% of the expected load current they'll only steal about 10% of your output power and lower the output current where you have to furl to avoid overheating by 5%, so I'd go ahead and fly it wired parallel.

If they're big enough to keep the mill from self-starting (holding the blades in stall above cutin wind speed) it's another matter:  Then you need to do something other than just paralleling the coils to insure low wind startup.

[scottsAI]

Tritium,

Most stators are wound with several coils in series.

Why not tap off 12v and 48 from series set?

Based on the different currents each may need adjust wire thickness for each tap, assuming thinner wire for 48v tap. 48v wire needs to be sized to supply full power for later.

Coil size is standardized, thicker wire will have fewer turns so adjust the tap accordingly. Maybe one coil has both thick and thin wire, with tap coming out with the thicker wire.

The thicker wire is frequently replaced with several thinner wires in parallel, offers better filling of coil space and easier to wind the coil.

The First tap will flatten out the sin wave (rectified DC with clamping load), so connect each series set as Jerry-rigged. No delta or WYE connections.

This situation calls for the rectifiers to be at the top, else requires 9 wires coming down the tower vs 3.

Have fun,

Scott.

[Ungrounded Lightning Rod]

This approach reduces the power available in both the 12V and 48V tap settings - though it's better for 12V than a winding of all one gauge (which would be best for 48V.

The issue is that there is an optimum thickness for the stator on a coreless alternator.  It's about the same as the sum of the thickness of a magnet on each rotor.  Make it thicker and you lose more from opening the gap and dropping the mag field than you gain by the extra windings, make it thinner and you lose more from fewer windings than you gain from increased magnet strength.

In the 12V setting you're using much less than the full volume for windings - the rest of the 48V winding is effectively "open space" - you've opened your gap and weakened your field for no gain in windings compared with just having the 12V winding.  In the 48V setting you have fewer turns than you could have had with a single gauge because the 12V part of the coil takes up extra space due to its extra wire thickness.

You can fix this by putting in extra-thick magnets.  But why not do four sets of windings, being very careful with spacing of coils magnets, and the coil geometry, and hook them in series for 48V and parallel for 12?  (There's another article on that on the front page today.)

[Ungrounded Lightning Rod]

(Actually, it's THIS article.  B-)  )

[scottsAI]

Ungrounded Lightning Rod,

Under some very limiting conditions you may be right.

Here people talk about 0.7 INCHES of air gap between magnets and stator on the larger machines.

To do what I said will cause the stator to be a little thicker, NOT 0.7 inches worth.

The wind gen will supply X power, take it out 12v or 48v, the power taken will be limited to what the wind can supply. This is what matching the blade to the generator is all about, and adjusting the air gap.

Over all less wire is needed if the 12v and 48v share.

Wire thickness ratios will control how much power is delivered to the 12v and 48v sections.

Else the turns ratios can help push more power to one or the other.

Dual voltage solution allows both battery systems to charge at the same time and use standard controllers. Some benefit could be had by modifying the 12v controller (to work like solar controller) and only load dumping with 48v system. (simpler!)

Before I would even consider 4 sets of coils, I would make a transformer to step down the 12v.

The controller to make 4 sets of coils work, would be interesting, must switch configurations to produce 12v or 48v, switching only when the wind is not blowing, else will have problems like star to delta and back systems. Switching systems are not used here for a reason.

Have fun,

Scott.

[Ungrounded Lightning Rod]

You misunderstand "gap".

In a no-core machine the "gap" INCLUDES the stator.  It is the spacing between the magnetic poles (presuming the magnets are mounted on something that provides a good flux return path - if not, add the non-metallic part of the return path to the gap as well).

Making the stator thicker to make room for more coils increases the gap and lowers the magnetic field.  This happens regardless of how much of the gap is mechanical spacing between the poles and the stator and how much is the thickness of the stator.  It's the sum of those that matter.

[scottsAI]

Thank you for clarifying the air gap Ungrounded Lightning Rod.

Always something to learn.

Yet DanB 17' wind turbine produced more power AFTER he opened the gap.

http://www.fieldlines.com/story/2006/3/8/201149/0612

And is believed to be root cause of its destruction.

Many focus on reducing the air gap when opening it up could be more productive.

Blades operating in stall are less efficient, the more stalled the less efficient (could be as bad as 10%), been looking for a plot of power vs stall. Still looking. Going for a low cut in will cause serious stalling at higher wind speeds with a small air gap. As always too much of a good thing is bad.

People have trouble with furling, seriously stalled blades are not resisting the wind allowing it to push the blades into furled position.

Dual voltage system will be a compromise regardless how it's done.

Looking for the best compromise as all engineering decisions are made.

I think dual voltage stator is best solution, based on what I have read here.

Have fun,

Scott.

[Flux]

"Many focus on reducing the air gap when opening it up could be more productive."

Yes I agree that many start with too many turns in the hope that it gives best results in low winds. Others probably go for too many turns knowing that they can get out of trouble with a wider gap.

Increasing the gap effectively weakens the magnets so you need more turns for the same cut in speed. If you get it wrong you have that option.

It is better to get the correct cut in with the correct turns and a smaller gap, that way you can probably use thicker wire. It has to be some sort of a compromise between too high a cut in speed and stall but deliberately using too many turns is not wise. You raise cut in speed with increased gap but you still have a higher stator resistance than necessary.

There are times when with the correct cut in you still reach stall and that occurs when resistance is too low ( alternator too efficient) and the best way to cure that is to add the resistance external to the stator to keep the heat out of it. I wouldn't wind with thinner wire just to avoid stall as the whole loss is dissipated in the stator.

If you devise a way to boost the low speed end then you find that to match the high wind end you only need about 1/2 the turns ( 1/4 resistance) so any attempt at trying to extract something in low winds that is probably not there in the first place really screws up the top end performance. If you need to widen the gap significantly you are wasting magnet and adding to stator heating problems. You get increased performance at a price.

Flux

[kitestrings]

Greetings Tritum,

We started upgrading our system a couple years ago.  Similarly, were moving to 48V, but still had (and have) alot of 12V lighting and light duty loads.  While our PV array is now 48V (actually higher prior to the charge controller) our existing wind turbine is 12V - that will be the last thing to be changed.

We debated at length the best way to transistion our system from 12V to 48V.  What we ended up doing was to pull a 12V tap off the battery bank, and then equalize the batteries using a so called 'autotransformer'.

http://www.solarconverters.com/product_frame.html

It is a bit of a misnomer.  It really is an up or down DC voltage converter, but it has worked very, very well for us.  It is current limited so the battery will supply, or accept higher current of shorter duration.  Here's what it looks like on our system:

Don't know if it is of any help, but perhaps another approach.  Best of luck,

~kitestrings