F&P Smartdrive vibrations?? Alu prop by meter?

[domwild]

Hi,

In a smartdrive the magnets cover three poles each and that means all three phases at the same time. Loading initially nothing to get around the cogging, then loading one phase, then two, then three should not result in vibrations or am I wrong here or are you right? The load of a single phase is distributed evenly around the circumference as every third coil is activated?

There is a company around that sells an NACA profile by the metre in aluminium. Problem is, no taper and no twist, same width and same pitch over the whole length.

What sort of a loss in efficiency is one looking at compared to taper and twist?

Thanks,

Dominic

 

[jacquesm]

Hey there Dominic,

Cogging is the result of the interaction between the permanent magnets and the electromagnets formed by the coils, the more such interactions the more they 'smoothe out'. In ascii: __^__^__^__^ feels bumpier than ^^^^^^^^^^^^. The cogging caused by the interaction of the stator laminates (pole pieces in the F&P) and the magnets is a constant. How many coils are covered by one magnet does not matter, if only one of them is 'active' then it is as if the other two do not exist and in essense you now have a single phase generator.

There was a note about decogging F&P's by rounding off the pole pieces a little bit ago, that probably is the best way to go (little bit of power loss, better startup). I don't have any experience with these motors, or with converting them but that method makes most sense to me).

[Flux]

Cogging is the torque variation due to the pull of the magnets on the iron circuit and is the thing that the de-cogging process can reduce. Vibration on load is due to the interaction between the magnet field and the field produced in the windings by the generated current.

All single phase generators produce this pulsating torque as the current falls to zero twice per cycle. In polyphase machines these zeros are distributed round so they never occur together and there is no significant torque variation. There is usually a small amount at harmonic frequencies, but nothing to worry about.

Regarding the blade, I would not be too worried about the efficiency of using a parallel un twisted blade except that the starting torque will not be as good and as cogging is considered a problem with these motors, you may have trouble at start up.

What does worry me is the fact that a blade of that shape is not strong at the root

and that aluminium has a problem with fatigue.  These things are probably intended for some sort of aircraft use, where fatigue life has been calculated and the thing is intended to be scrapped before this life is reached.

I personally would not use aluminium in any form whatsoever in any part of a propeller of a wind generator, it is the perfect fatigue testing machine and one day  sooner or later I think you will loose a blade. Just my view, others may not share it. I believe Wincharger got away with it, but the Lucas Freelite changed to wood.

Flux

[Gary D]

Hi flux, I'm not sure if Windchargers had anything but wood blades. I owned a 250 watter- wood blader in the early 80's. And the 500 watter I currently have is also wood. Perhaps larger versions could have used some other blade material tho... just know what I've seen (and the air breaks are noisy!) Gary D.

[Dan M]

If you're talking about the aluminum extrusion sold by mtm scientific, they're small enough you can put one end in a vise and twist each blade by hand.

Aluminum will die quickly in low cycle fatigue (stresses in excess of yield, fully reversing) so twist one blade, then "sneak up" the same amount of twist on the other blade(s).  You don't want to have to twist them the other direction.

The difference between twisted and non-twisted blades is HUGE.  It's like the diference between a ceiling fan and an airplane propeller.  The twist keeps each portion of the blade at the same angle of attack with respect to the relative wind for your designed wind speed.  The calculated twist is not linear along the blade, but a simple twist is far better than none at all.

Taper is not as important, as long as the root of the blade can handle the stresses.  If blades aren't tapered, the load on the blade perpendicular to the plane of rotation becomes much higher, due to the small amount of twist (relative to the plane of rotation) at the tip.

One other important point.  Aluminum has no range of "infinite life fatigue stress" like steel does.  For example, in general, if cyclic stresses are below 20% - 30% of yield, steel will never fail in fatigue.  Any cyclic stress on aluminum will cause fatigue failure.  Very low stresses might be on the order of decades, while higher stresses might be on the order of minutes or hours.  But just because they ran fine for a week, doesn't necessarily tell you anything about tomorrow.

-Dan M