coils at tip of blades?

[pmgnewb]

What I had in mind was a turbine that looks like a skinny ducted fan.  A ring or cowling would hold the magnet pairs in position along the circumference with a recessed channel in the cowling to allow the coils in the tips of the blades (connected to each other with a ring for stability) to sweep past in between the magnets along the outer circumference.  The point of this being to increase the relative speed of the flux change by moving the magnets and coils from the center of the rotor and stator to the outer part of the assembly.

Sorry if this has been addressed before, I searched the board but didn't find anything.  I am still in my thought-experiment/what-if stage in thinking about PMGs.

Thanks for your input,

John

[wooferhound]

My instantaneous thought is, the end of the blades flex a lot. How would you control the distance the tips of the blades are from the magnets ?

[wooferhound]

and what about the magnetic circuit that connects all the magnets together to make a much more efficient generator ?

[Flux]

Yes it has been done before by some German company but I have no reference to it now.

The problems are too great  with all the forces on the blades, nearly impossible to keep the air gaps and the effect of mass at that radius puts into the realms of dreams as far as I am concerned.

Flux

[TheCasualTraveler]

Practical, maybe not but the design would be like a large fan with the outer tips of the blades all connected in a loop of steel that held the magnets. I see it like of of those old water pumping mills they used on ranches,or one of those fans in front of a jet engine. Much easier to attach the magnets to the blade than have the coils spinning. The recessed channel arangement you desribed would be reversed. It would be some piece of work to build that prop on a large scale strong enough not to flex and with a perfectly round outer steel loop.

[neilho]

IIRC, Ed Gillette was working on such a thing in Maine in the '70s and early '80s and called it the Zephyr. (Seems to me it was featured on the back page of "Energy Book II or some such thing.) His configuration resembled Andy's suggested version, magnets at the blade tips and coils in the ducting. It worked, but it was more cost effective to get the same output with a larger diameter rotor without a duct. There were problems facing the rotor "out of the wind" since the duct still presented a substantial profile. So most all the structural systems had to be beefier, too, driving the cost up even further.

Neil  

[finnsawyer]

When the blades bend back the distance between the tips of the blades gets less.  A ring connecting the blade tips prevents this, so the blades don't bend back.  The tension along the length of the blades will increase considerably, though.

One can maintain the magnetic circuits like any radial PM design with closed coils.  However, a 12 magnet 9 coil design isn't going to really get you anything with a 4 foot diameter rotor, as it still takes 30 degrees of travel to generate one voltage pulse.  You would get a lot of time with no voltage output unless you went to very many magnets and coils.

I find it interesting that in about two weeks after posting my latest diary two postings have appeared that touch on it.  The diary is here:

     http://www.fieldlines.com/story/2007/12/5/16183/1323

Placing a flat ring around the blades forces the average air speed through the rotor to be constant.  That would give the maximum power out if we had perfect blades.  That is, the pressure gradient through the rotor were also constant.

[jaskiainen]

How about if blades tips would run in some sort of a hole vith ball bearings for exaple? That should keep the airgap regular.

Im aware that there wuold be quite a lot resistance with it like that to spin

so I dont see a point for doing that.

Again my vocabulary is so limited that i cant explain it better...

[TheCasualTraveler]

I'm no expert but I would guess if your going to build a rotor/stator of that large diameter and with all the friction and drag associated with it you'd be better off having it under a VAWT. I'd love to see a VAWT built that way, with a 24" or maybe 36" rotor. I'm thinking it could be done with 40 magnets or so. (single rotor) But again, should it be done?

[pmgnewb]

Thanks for the feedback everybody.  I guess there is no need for me to try to reinvent that wheel

John