Author Topic: Noisy Generator (Read 4252 times)

ChrisOlson · « **on:** February 07, 2010, 10:32:01 PM »

There's nothing really unconventional about it - it's a simple 12 pole, 9 coil generator wound with 44 turns of AWG 14 wire, two-in-hand. At lower outputs, under 15 mph wind speed it groans so loud you can hear it from a considerable distance and it makes the tower "buzz" - you can feel the vibration in the tower.

As it picks up speed it quiets down some but the groan changes to more of a whistle or a high-pitched moan. I used to have an 18 coil generator in this turbine with 12 poles and I uploaded a picture of that stator taken when I was winding it:

The 18 coil stator, although the phase timing was off with 12 poles, was one of the smoothest running generators I've ever seen. But wound with only a single strand of AWG 14 the smoke escaped out of it one day in high wind. That smoke was what made it work, because after that leaked out it was done.

Back to the noisy nine coil - this thing vibrates so bad I had problems with the stator bolts coming loose. I finally used stainless steel locknuts and those have stayed tight. The stator makes tremendous power - I had it hit 2.0 kW once when I discovered my furling calculations were bogus, and it didn't hurt the stator. I took a picture of the turbine when it was spinning at almost 900 rpm that afternoon (which I think I posted here once) and I should've taken a movie of it so you could understand the noise - at that speed it was actually making a howling noise.

I don't know what I did wrong in that stator but I know I don't have any coils backwards in it and I know all the phases are wired right because the output between legs is dead even between any pair.

I got a 16 pole 12 coil in my 10.8 footer and that's a sweet running generator - it's as smooth as the old 18 coil stator used to be in this one. I just wish I could come up with a way to make this 9 coil run smoother.

--

Chris

Flux · « **Reply #1 on:** February 08, 2010, 12:19:44 AM »

Virtually all alternators make some noise but the 3 phase axial machines are fairly smooth normally. If you short the phase leads they usually turn very smoothly but occasionally you can feel slight cogging at a harmonic frequency. Under running conditions this can result in audible noise especially if resonant with something in the structure but normally it is not bad and there is no mechanical vibration that causes trouble.

What you describe is typical of these machines running single phase, in that case they do vibrate very considerably. If you are confident your windings are correct then I can only suspect the rectifier and possibly an open circuit diode.

Very difficult to do much testing on the alternator itself under wind conditions unless you can somehow connect it to a 3 phase balanced resistive load ( heaters) on a normal wind day. If it runs smoothly into the balanced load then it is ok. if it still vibrates then somehow you have a winding fault.

As a first move try another rectifier and see if that clears the problem. I don't think it is something inherent that you have done wrong in the design, I have varied most things in these designs and never had one that does this with a balanced 3 phase load.

"The 18 coil stator, although the phase timing was off with 12 poles, was one of the smoothest running generators I've ever seen."

That comment worries me, the timing wasn't wrong for 3 phase but I assume you haven't done anything wrong on the 12 magnet 9 coil if you have equal voltages per phase.

Flux

ChrisOlson · « **Reply #2 on:** February 08, 2010, 07:05:36 AM »

"The 18 coil stator, although the phase timing was off with 12 poles, was one of the smoothest running generators I've ever seen."

That comment worries me, the timing wasn't wrong for 3 phase but I assume you haven't done anything wrong on the 12 magnet 9 coil if you have equal voltages per phase.

When I designed the 18 coil stator I had it worked out in my head how it would work with 12 poles. You'll notice from the photo that it was not a conventional stator - it was hand wound on a machined stator plate and winding it was a very tedious job due to the small amount of room I had to work with in that homemade plate.

After I got it wound I did some thinking about it and the story of what I figured out is outlined in this set of photos, starting with this one and looking at each subsequent photo to the right of it.

http://picasaweb.google.com/christopher.w.olson/76FtWindTurbine#5430081286999881970

After I got it wound I realized I should've either overlapped the coils between phases to provide 120° separation, or wound one phase counterclockwise. I didn't think it was going to work but I decided to try it wired star - and it did work only that the phase separation was 180°. I hooked a scope up to it to check it just to make sure. It wouldn't run a three-phase motor but for battery charging the rectifiers didn't care.

The reason it ran so smooth is because of the coil to pole ratio - way more coils than poles which makes maximum use of the magnets and eliminates any hint of cogging under full load. It was actually the smoothest running generator I've ever built.

Part of the problem with my 9 coil is that it is a large diameter generator - 12" - which left a small amount of room between the coils when laid out in the mold. I suspect that could cause some of the vibration because the magnets aren't busy all the time. I should've reduced the diameter a bit (probably) so the magnets would always be crossing a coil leg. When I went to 16 poles and 12 coils in the generator on the 10.8 footer, that genny is the same diameter (12") but because of the greater number of coils the legs on adjacent coils are touching each other so the magnets are always busy.

That's the only explanation I ever really came up with. My rectifiers are all good and the output is too even to conclude that it has a winding fault.

Thanks for your insight.

--

Chris

bob golding · « **Reply #3 on:** February 08, 2010, 12:13:11 PM »

hi Chris, i have the same arrangement 9 coils and 12 magnets on a 14 inch rotor with no vibration problems,so i don't think that is the problem.

RP · « **Reply #4 on:** February 08, 2010, 04:49:35 PM »

The noisy nine coil stator isn't wired up delta is it?

Also, silicon rectifiers can fail in odd ways that are only obvious when passing significant current or holding back significant voltage. You may have a bad rectifier that won't show up with an ohm meter.

One test you may consider trying for this:

Take a cheap pair of headphones and (using and inline resistor) connect one or both speakers across the phases before the rectifiers. The appropriate resistor value will depend on the system voltage. For 14VAC and 8 ohm headphones maybe 500-700 ohms or so to get about 0.3 watt or so.

The purpose is to listen for differences between the phases. If you have a bad rectifier then you likely have only one bad diode so out of three phases something should sound different if you listen to all of them.

Jon Miller · « **Reply #5 on:** February 08, 2010, 04:52:33 PM »

I did this with the mains when i was about eight, not recommended!

Jon Miller · « **Reply #6 on:** February 08, 2010, 04:53:30 PM »

Should just say that listening to the mains in this way is not recommended but for the turbine would be fine

ChrisOlson · « **Reply #7 on:** February 08, 2010, 06:16:20 PM »

I found a resistor and I just tried this procedure about 30 minutes ago. The wind died down so the turbine was only putting out 6-7 volts when I tried it and it makes a sound like a time machine of some sort spooling up (I don't know how to describe it). Except between P2 and P3 it made a terrible screeching sound.

Each leg has it's own single phase 35 amp full-wave bridge rectifier so there's one AC terminal open on every rectifier. I first pulled P2 and plugged it into the open AC terminal on the P1 rectifier and tried it. No change. So I moved P2 back, pulled P3 and plugged it on to the open AC terminal on the P2 rectifier. No more screeching.

So I plugged the P3 wire back on to the P3 rectifier, except this time using the other AC terminal (that was previously open). I sounds just like the others that way.

I'm no electronics wiz so I assume this means that one half of the P3 rectifier has some sort of problem with it?

--

Chris

RP · « **Reply #8 on:** February 08, 2010, 07:00:28 PM »

Sounds like a bad rectifier. Did it change the moaning/vibration from the tower?

ChrisOlson · « **Reply #9 on:** February 08, 2010, 07:31:24 PM »

Well, I don't know at this point. I changed the rectifier with a spare that I had stored in my truck console. But it has to get to cut-in and put a little load on the generator to find out if it fixed it. Right now the turbine is running just short of cut-in, showing about one or two amps every now and then, but not quite enough wind to bring it to life.

The wind is supposed to pick up out of the northeast and blow at 10-15 mph with this snow storm we got coming tonight (I'm in northern Wisconsin). The anemometer on my little home weather station says it's only blowing at 6-7 mph. So perhaps I'll know more when the wind picks up and that turbine starts to fly.

--

Chris

prasadbodas2000 · « **Reply #10 on:** February 08, 2010, 08:17:08 PM »

Is there no picture of ur stator and magnet rotors? Also to eliminate the rectifier issue, can you try putting 3 phase resistive load directly to the three AC terminals coming out of stator? If all things are well, your point of "magnets not busy all time" seems to be the root cuase, and it must be getting in resonance with the overall structure.

ChrisOlson · « **Reply #11 on:** February 09, 2010, 06:26:01 AM »

Unfortunately, I do not have a photo of the stator that's in it - I figured it was nothing out of the ordinary so I never took a picture of it.

The wind picked up last night and it started putting out usable power again and it's silent after changing the rectifier. I don't fully understand why. I checked the "bad" rectifier with my ohmmeter and it checks the same as a new one, i.e. it doesn't seem to have a leaky diode in it. But it definitely fixed the moaning noise. The generator still whines a little at higher speed but it doesn't "buzz" the tower anymore at lower speeds.

Another thing I noticed after replacing the rectifier is that the volt meter in the panel used to fluctuate a tiny bit when it was making that moaning noise. That's also "fixed" as the voltmeter needle now stays perfectly steady.

I'm confused because I never suspected a rectifier could cause such a thing.

--

Chris

TomW · « **Reply #12 on:** February 09, 2010, 07:01:37 AM »

Chris;

Not uncommon for a diode to fail while under load and test fine with an ohmmeter. After awhile you just learn to swap for a fresh one when you see odd behavior like you saw.

Glad you sorted it out and you learned something to boot!

Tom

Flux · « **Reply #13 on:** February 09, 2010, 07:35:36 AM »

Checking rectifiers with an ohmmeter is a bit hit and miss.

If you want to follow this up try checking with a car headlamp bulb or similar and a 12v battery.

You seem to be using half a potted bridge so check both ways round from the ac connection to dc plus and dc minus. You should have a pair of diodes that let current from the ac connection to dc positive and dc negative depending on the applied voltage.

It is almost impossible that you have a shorted diode as that would show really badly and heat the stator but most likely one of the pair is open circuit.

You should find that the other ac connection of the rectifier behaves normally if you have not used it. I don't like those bridge diodes anyway but if I do use them I always parallel the ac connections so you have 2 diodes in parallel .

In the extreme case you may have lost both diodes and that would be a full single phase loading and these alternators are notorious for vibrating under those conditions.

A single diode out will still cause significant vibration and would be enough to cause the meter flickering you mentioned.

Flux

ChrisOlson · « **Reply #14 on:** February 09, 2010, 07:39:02 AM »

Well, many thanks to the guys who provided some input on this situation. I was dead convinced it had to be something in the generator causing it. I've heard bad diodes in car alternators before and they make a howling or chattering noise but I always assumed the noise comes from the rectifier. But maybe it's actually the stator windings that's making that noise.

Like you said, Tom, I learned something new. Which is a good thing.

--

Chris

ChrisOlson · « **Reply #15 on:** February 09, 2010, 07:46:08 AM »

You should find that the other ac connection of the rectifier behaves normally if you have not used it. I don't like those bridge diodes anyway but if I do use them I always parallel the ac connections so you have 2 diodes in parallel.

That's exactly what it did, flux. When I put the AC lead on the other AC terminal of the "bad" rectifier the screeching noise in the headset went away.

What do you mean, "parallel the ac connections so you have 2 diodes in parallel"?

I have one of these rectifiers on each leg of the stator and they're rated at (if I recall correctly) around 600 volts and 35 amps each. The DC sides of all three rectifiers are paralleled, but only one AC lead (from each stator leg) going to one of the AC lugs on each rectifier. The other AC lug is unused, so I'm only actually using half the diodes in the thing.

Should I hook it up differently?

--

Chris

ChrisOlson · « **Reply #16 on:** February 09, 2010, 07:50:16 AM »

I should point out that this thing pretty easily pushes 90 amps in a good wind. So if I hooked two stator legs to one rectifier, one leg to a second one, and paralleled the DC sides of just two of them, it will melt the one with two AC legs going to it.

Been there, done that. That's why I have three of them.

--

Chris

ChrisOlson · « **Reply #17 on:** February 09, 2010, 07:54:22 AM »

And I should ask another question here - I've always wondered if I could jumper the single stator leg that goes to the AC lug on the bridge rectifier. In other words, run the stator leg to one AC terminal, then a jumper from that terminal to the other AC terminal on the same rectifier so it "splits" the load and uses all four diodes in the bridge?

--

Chris

Flux · « **Reply #18 on:** February 09, 2010, 08:11:55 AM »

Yes that was what I meant. You use the 2 halves so that each arm of your 3 phase bridge has 2 diodes in parallel. This gives a slightly increased rating and both diodes have to fail before it becomes an issue.

These things are not capable of anything like 35A despite the makers literature, the rating anyway is for single phase inductive or resistive load. With battery load it is effectively capacitive and there is a significant de rating. For 3 phase the path at any instant is via 2 diode paths as for single phase and the rating is not much more than the single phase case. With 3 phase resistive load the conduction pattern is very different and the rating is higher.

If you use the two ac connections in parallel you need good heat sinking to get much increase in rating but the gain is worth having.

At 90A 3 of those units are pushed even with both sections in parallel and you will need really good heat sinks. As you parallel more blocks you have more de ratings as the sharing between blocks is not as good as sharing in one module produced at the same time.

Flux

ghurd · « **Reply #19 on:** February 09, 2010, 08:12:05 AM »

Yes, jump the AC terminals together.

G-

ChrisOlson · « **Reply #20 on:** February 09, 2010, 08:22:46 AM »

Ok, thanks ghurd and flux - that answers my question. I've gone thru more than my share of these full wave bridge rectifiers - usually it's because one melts and the damage is visible. I should probably give up on them and get a decent three-phase bridge rectifier out of a 200 amp Leece-Neville truck alternator and use that instead. It would probably solve a lot of my problems I've had with this setup.

--

Chris

Ungrounded Lightning Rod · « **Reply #21 on:** February 09, 2010, 06:46:29 PM »

If you have an open diode, on the part of the rotation where that diode should be conducting it isn't. The current in your coils is lower than it should be. That means for parts of a turn the generator puts less "drag" torque on the rotor (and less reaction torque on its mounting) than on the other parts of a turn.

This periodic variation of the load on the turbine constitutes a vibration torque applied to both the turbine and the mount - and from there to the tower. The braking torque is pretty large, so this variation in it is also very large. This variation is what shakes your tower.

Single phase does this all the time: The drag is large at the wave peaks and goes to zero at the crossovers. So a single-phase alternator will buzz horribly when under load.

Polyphase is nice because, while the drag from individual phases cycles just like the single phase case, the total drag from all the phases (when it's working correctly) comes close to being constant. There's still some vibration from the torque passing from one coil to another, harmonics from non-sinusoidal waveforms (especially in a battery-charging rectified arrangment), vibrations of the coil wires in individual phases, slight imperfections in manufacturing, etc. But it's tiny compared to the load variations of a single phase system.

Of course when one of your diodes flaked out you ended up with one phase loafing for part of the cycle. And so you got vibes much like a single-phase system.

ChrisOlson · « **Reply #22 on:** February 09, 2010, 07:07:28 PM »

Thanks to all you guys for the tips. Like I said, I would've never suspected a bad diode was the problem. That generator runs so smooth after I replaced that rectifier that it's almost totally silent except for the sound of the blades. Although the generator does whine a bit when it gets to around 300 rpm, but my 12 coil generator in my 10.8 footer does that too.

Thanks for the explanation - that makes sense as to what causes the vibration when a diode goes south. Next time I'll know what to look for.

--

Chris

Ungrounded Lightning Rod · « **Reply #23 on:** February 10, 2010, 12:41:52 PM »

Is 300 cutin for it?

With a rectified battery-charging system you expect some whine at speeds just above cutin. The coils are only conducting, and loading the shaft, at the peaks of each cycle. So you get a load variation at a fundamental frequency of ((RPM / 60) * phase count * 2) Hz (with lots of harmonics, mainly the odd ones). The whine is small because the current, and thus the load, is small. As it speeds up the conducting portions of the cycle spread out and merge, the load becomes nearly constant, and the whine mostly goes away.

ChrisOlson · « **Reply #24 on:** February 10, 2010, 01:58:14 PM »

No, it starts charging around 160 rpm and from there to about 230-240 rpm is where it really moaned before. That's completely gone. I don't notice any appreciable noise from the generator now until it gets to about the 300 rpm mark and it's more of a "whine" than a moan. As it speeds up above that it seems to quiet down a little and doesn't even really "whine".

And another thing I noticed since I replaced that rectifier - it puts out more power (amps) on the low end (180-250 rpm). Not a lot - maybe 1-2 amps - but it's definitely performing better.

--

Chris

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Author Topic: Noisy Generator (Read 4252 times)