Dunlite mill exciters

[phil b]

Hi, I'm still researching my third mill. The pics of the Dunlite mill has raised my

curiosity. It uses an exciter on the same shaft as the main rotor.

This site has a few posts about the controls but little about

the Dunlite mill itself. #of winds,poles,wire size, etc? Pearin's web site also has

the most info but no hard electrical specs. Anyone have more info on any of the models?

It seems exciters are still used to make electromagnetic coils and sync

large generators today. Is the exciter primarily used to produce DC

amperage for the main electromagnetic coils?

Can someone explain the electromechanical advantages of using an exciter vs. magnets to create power?

Why are these type of unit not used on small mills today?

This search has been very educational and at lot of fun!

I have looked everywhere on the web I can thing of.

Any help is greatly apreciated!

Phil

[Flux]

I can't answer the specific detail on the Dunlite alternator but I can answer your basic questions. Before the days of decent permanent magnets and solid state rectifiers, you had to use a dynamo for dc and if you wanted an alternator for anything you needed to supply its field via slip rings. The exciter was a special dynamo with commutator and brushes and the exciter brushes fed the brushes of the alternator so you had a commutator, 2 slip rings and lots of brushes to maintain.

With the invention of the germanium and later silicon diode, rectification became fairly easy.

The brushless alternator you refer to has an alternator as an exciter but it is built with a stationary field and rotating armature. It is wound 3 phase and a rotating 3 phase bridge rectifier produces dc on the shaft. This is directly coupled to the wound field ( rotating) of the main alternator.

You now have a simple mechanical scheme and you supply a very small current to the exciter field to control the main output of the alternator. The advantage is obvious for a power station, but you may well ask why use it for a wind generator.

With a wind generator you need a second larger rectifier to rectify the output of the main alternator for battery charging. What do you gain?. You gain the removal of a pair of slip rings and brushes and hence a longer period without maintenance.

What do you loose, you have significant losses in the exciter and rotating rectifier which has to have its power extracted from the wind power input.

Brushes and slip rings are nowhere near as troublesome as often believed but the reliability gained was considered enough to offset the cost and lower efficiency.

How does it compare with permanent magnets?  Now with the introduction of neodymium magnets it is a costly and heavy way to achieve the same thing but it does have one advantage, you have control of the field strength and you can manage more easily to match the output of the alternator to the wind rotor, although the overall efficiency may be no better from the alternator point of view you gain from the better tracking of prop tsr.

Flux

[oztules]

Excellent description Flux.   Thanks for the explanation.

It assume it would be possible to use mild steel for the main rotor, as it behaves as a fixed magnet, but with the exciter rotor, would that have to be laminated plate?, or is it lowpower enough to put up with the eddy currents and make it non-laminate steel as well. If it can, it may just be simply do-able. If not, then from a backyarders view, the laminated rotor for the exciter may be a stumbling block. Do you have any devious ways around this?

.........oztules

[sahlein]

I was Just Thinking about this topic the other day.... Glad someone brought it up.

At the risk of incurring a "barrage", I humbly submit that maybe it would be an idea to use a PM rotor that also has windings that could be used at higher windspeed to increase the amount of magnetic flux to get a better match/output.

Brushless motor/alternators used in large (100-250) KVA Frequency Converters use exciters and my experience with those got me thinking along these lines.

I have worked with Landert, VanKaick and Kato Engineering machines from about 1983 up until about 8 years ago.  Most of these machines ran 24/7 and even though the Landert used slip-rings I don't Ever remember doing Any maintenence on them during that period other than occaisionally blowing out the dust.

What I envision is a kind of "Zubbly-Modded" motor with a custom armature with both Neo-mags and windings.  I could maybe start with a really large motor and use only part of the stator iron just to get the thing "thin" enough to be practical ().

I know it sounds complicated but it just seems that there is a way to strike a good compromise here.

I belive Polaris uses a similar configuration ().

I realize that this is "biting off quite a large chunk" but, I have the time now to put into this.

Anyone have any comments??.

Joe S.

[phil b]

I want to thank everyone especially Flux, for their responses. Please continue...

It has proven to be very valuable.

This is the response I got from Graig Pearen:

The Dunlites use a 3-phase 8-pole brushless alternator with a 5:1 gear box. The blade rotate at about 175 RPM. A brushless alternator (not a permanent magnet alternator)  is fairly large & complex. The tower-top weight of the 2kw machine is about 400 pounds. See my article on brushless alternators in Home Power magazine issue 97 Oct-Nov 2003

I'm not giving up on the idea just yet. IMHO, it deserves further investigation.

What if.. a Hugh's 12 volt pmg is used as an exciter to give the main generator power? Can I then wind the main mill for 48 or 120 volts and have less heat and thereby produce with higher efficiencies?  Do we get more power for less magnets?   (I'm not trying to cut throats here.) Scientific investigation...and more bang for the buck.  Not something for nothing. Simply exploring different ideas out of an very old box.

Dan's large generators and others come to mind.

Again, a big THANKS for the comments.

phil

[wpowokal]

While I believe the idea is sound, the bigest advantage of a typical duel rotor is simplicity, ie every man and his dog can make one.

The Dunlite machines are still very reliable but to build from scratch, dificult, to say the least.

I believe there is merit in a machine that uses a small PM exciter to a main generator, but once again not a project for everybody, which efectivly a compound wound generator is by utilising residual magnetisum anyway. But worthy of thought.

allan down under

[Flux]

Exciter field can have solid poles, exciter rotor must be laminated, if you want to try, a car dynamo might be the way if they are still around.

The main machine field ought to be laminated at least on the pole faces as there will be a lot of slot induced ripple. It will work with solid poles but with more loss.

Flux

[Flux]

Compounding a field with permanent magnets and field coils is not easy.

Normal wound fields work with very short air gaps and if you make the gap large you need extremely large ampere turns on the field winding to get the flux density up and the leakage fields become serious.

You can manage a higher field with neos alone, If you want to combine both you would need to keep the neos very thin as the wound field would see the thickness of magnet as air gap. You would then not have the full benefit of the neo, but you would have some degree of control of the field but at the expense of a heavier than normal field current. May be a case for using large neos to produce full field in low winds and to buck the neo field in higher winds to prevent stall. At this point the field loss would not matter but you would have the worst condition for start up with maximum cog and iron loss.

The Polaris machine uses neos for 2/3 of the main field and the variation for control is provided by a cunning inductor( unipolar) rotor with stationary field coil. It doesn't buck or boost the main magnet field directly.