low RPM rumble

[TheCasualTraveler]

     Since adding a second steel disc to my turbine I get a low rumble sound when it is  spinning a little above cut in. The rumble starts at about .75 amps and continues till just under 2 amps. 2 amps and up it is quiet. The noise seems not to be mechanical. My guess would be it is just a vibration from the coils.

     When I was testing coils I remember being surprised the first time I held a shorted coil over the spinning magnets and felt it vibrate in my fingers. I can imagine the effect amplified by many coils.

     Now my question, first, is that what the noise is? And second, what can be done to lessen it? Aside from raising cut in are there any tricks in how the alternator is built such as 5 phases instead of 3 that helps to lessen whatever causes the rumble? Would different gauge wire affect it? Perhaps it is that my steel discs are only bolted on at one point through the center instead of 4 or five bolts like on a brake disc machine.

     Anyway, if I know the cause of it I think I can handle any mechanical changes but I need to understand what causes it. Any help would be appreciated.

Andy

[wooferhound]

The voltage that the turbine produces is Alternating Current (AC). The vibration comes from the moment that the coil voltage reaches battery voltage it suddenly has magnetic drag. Since the voltage is AC its voltage will reduce and cross the Zero voltage point. When the voltage goes below Battery Volts then there is no magnetic drag. repeat as necessary and you have vibration

You can't help a single phase machine because all the magnets and coils line up at the same time.

A 3 phase machine is almost vibration free because not all of the magnets are crossing coils at the same time.

[TheCasualTraveler]

     The machine is 3 phase. 12 pole, 9 coil, second disc with no mags. Sounds like my suspicions are right. Will more phases make for less vibration possibility?

[wooferhound]

Yes

but I don't think you can ever get away from the vibration / sound completely.

That is why it's not recommended that wind generators be mounted to buildings and especially don't mount them to the place you sleep or there will be no more sleep.

[wpowokal]

What Woof said, and my 5 phase duel rotor still has vibration, thankfully I'm deaf enough not to hear it unless I go out to the tower.

It is a fact of AC life, now you are aware of it try listening to an AC motor or a small (or any size actually) transformer etc. Place a screw driver on the motor with the plastic end on your ear Hmmmmmmm.

allan down under

[ghurd]

The 3 phase to DC output still has ripples, but a lot less ripple than single phase because single phase goes to 0V.

My bet is the ripple between 0.75A and 2A is when the peaks of the ripples are above battery voltage, but the voltage valleys are not above battery voltage.

More phases would help, but I don't know if it would completely go away, without too much money and probably a loss of efficiency.

G-

[DamonHD]

I wonder if you could decouple the genny from the battery in some way, such as with a big power capacitor or a small in-line inductor or even something a little more elaborate such as an MPPT tracker.

That might take the sting out of the 'edges' as you move into (and out of) conduction and thus dampen the vibration.

Just thinking out loud and almost certainly rubbish...

Rgds

Damon

[Flux]

All electrical machines have the potential to make a noise. Every coil is going to have a pulsating force acting on it and there will be forces reflected on the magnets. Even dc machines ( dynamos) make a noise.

With a single phase winding the torque pulsation is additive from all coils . Any polyphase machine will balance most of the total torque variation and be much smoother. Beyond 3 phase there will be virtually no improvement in total torque variation ( even 2 phase is almost there).

What sometimes happens is that a mechanical resonance picks up and amplifies a certain frequency and that is almost certainly what you have in your case. Finding the resonance is a more difficult problem. Usually the noise can be lived with, I have always got noise from my machines at some speed. If you had a fast commercial machine with noisy blades you would be unlikely to even hear the alternator noise.

If you have potted the magnets then my immediate suspicion would be bell mode resonance on the return disc with no magnets although I have never experienced significant noise with this type of construction.Casting a layer of epoxy or vinyl ester on the back of the blank disc may be enough to damp it ( it would at least eliminate that as a possible source). Stator mountings may not be rigid enough and if you beef them up and shift the noise to another speed then you have a clue that that is causing a problem.

I suspect you have a strong resonance if you are exciting it near cut in where the magnetic forces are small. You may even be able to excite it mechanically by tapping things with a small hammer and looking for the same note. Even bearing tightness may be a factor is you have taper rollers.

No point in doing any electrical redesign with different number of phases, you may accidentally get an improvement but the potential to excite resonance will still be there and you could hit things even worse at another speed.

If you have such a thing try a large inductance (choke) in the dc line, it works wonders with single phase, I have never seen any advantage for polyphase but lengthening the conduction period may reduce the exciting forces. Unfortunately a decent very low resistance choke will cost the earth so if you don't have one dismiss this option.

Flux

[Capt Slog]

Yup Ghurd, I hear those ripples.  I have a homemade ammeter in my set-up, it's mounted on the wall in the shed above my batteries.  The coil in this is a few turns of thick copper wire, and the whole thing buzzes when the needle moves and I'm making power, and the plate the thing is built on acts as a sounding board for it.

Personally, I love the sound, but I can guess that it must be worrying to hear it come it from the turbine.

.

[Flux]

"Perhaps it is that my steel discs are only bolted on at one point through the center instead of 4 or five bolts like on a brake disc machine."

Didn't spot that bit before. That is likely to be very relevant , you will have very little damping on the plain disc and it could be excited by small vibrations at a resonance.

Flux

[TheCasualTraveler]

OK, thanks people.

     Flux mentioned if the magnet disc was potted,   well, it's not. Magnets are epoxied and the disc painted. Only my test machine so it's fine but now I know for the new machine I'm working on. The machine I'm building now has four bolts in addition to the center bolt. I need this to make sure that with a second magnet disc the two stay aligned. With only a blank steel disc on my test machine there was no need to have more than the center bolt as the discs did not need to be aligned at a certain point.

     So for my  machine in progress I will have a thicker, stronger stator, potted magnet discs, four bolts between discs and I think I will also try a large plastic bushing on the center shaft between discs for dampening any noise. I won't expect silence but I will try to eliminate unnecessary noise.

     On a side note I have finished carving the wooden blades for this machine. They came out great. I used Ed's plan, here

http://www.windstuffnow.com/Blade-layout.pdf

     About the time that my arm was getting tired from patting myself on the back I had the humbling realization that the instructions were so good anyone following them would have the same results I had. I asked Ed about the TSR of these, (I scaled them down to 36" for 6 foot from his 39") He said they will run about 6 maybe a little higher since I made them shorter. My greatest motivation was the fear that responses from Flux would have a line something like, "I forget what your doing, are you still playing with PVC blades?" Here's a couple pictures.

Mr Tom, sorry if I screwed up on picture size but I really tried to get them legal.

[TheCasualTraveler]

Flux wrote,

""If you have such a thing try a large inductance (choke) in the dc line, it works wonders with single phase, I have never seen any advantage for polyphase but lengthening the conduction period may reduce the exciting forces. Unfortunately a decent very low resistance choke will cost the earth so if you don't have one dismiss this option.""

     My electronics knowledge is just enough to enable me to ask silly questions, but here goes. I have an old blown inverter that contains a nice heavy transformer. Is it at all conceivable to use half the transformer as an inductor?

My apology in advance, I am

[joestue]

Yes and no. You can keep the coil and core, but you are going to have to relaminate the core with all the E's on one side, and a piece of plastic about 1/16th inch thick separating all the I's, forming an air gap. Otherwise, the core will saturate, and have relatively no inductance.

[dbcollen]

When I first put up my 48 volt 10 footer I had 3 35 amp single phase bridges to rectify the ac to dc. It was really noisy, loud hum above cut in. I replaced the single phase bridges with a 100 amp 1600 volt 3 phase bridge and the noise was reduced quite a bit, not gone though.

Dustin

[TheCasualTraveler]

     You know Dustin, come to think of it I changed the rectifiers on this machine also, from six individual rectifiers to three bridges. It didn't make noise before but then again that was without the second disc. I have room in the middle of the stator to add a 2" bushing of some sort between discs. If that doesn't dampen it maybe changing the bridges will. Thanks.