Author Topic: Understanding Stall (Read 2046 times)

Shadow · « **on:** March 09, 2007, 03:03:08 AM »

I'm sure my wind turbine is in stall mode, I've heard where different ones add some resistance in the line to speed it up. I'm just wondering where and what to add to try this?

I'll start at the beginning here, Its a dual rotor, 3 inch by one by half inch magnets, 12/9 110 turns of 15.5 awg. wired in star,14 foot blades built to the Dan's specs. Cut in for 48 volt is around 125 rpm.

I've had this up at 48 feet for about 8 months or so, used it all summer when we were building our basement. Its always had tons of power I often saw 18-20 amps, and it would take the battery voltage up past 62 volts and the inverter would shut down from over voltage. So now were moving into our house I've hooked up an Outback 3648 Inverter, and a Trimetric 2020 so far. And for some reason the wind turbine performance is dismal.For awhile the best I could do was 6-8 amps, I thought maybe bridge rectifiers might be the problem so upgraded from 30 amp to 50 amp. Didnt seem to help. Last few days have been 30 -50 km winds day and night, most amps I've seen was 12. Now thats according to my digital amp meter running through a 75mv-30 amp shunt. But according to the Trimetric that reads everything across a 500 amp shunt, its saying 2 amps is the best I've seen!

So today I kept an eye on it off and on all day and it appears to be badly stalled, fully furled or not blades never change speeds. But when the wind is dieing down and its just a light breeze the blades seem to go fairly fast. As soon as the wind has any power in it the blade speed remains constant. I'd like to resolve this in two days thats how long we have internet for before we move and eventually get onto to satellite service of some kind. Cant figure out what might have changed or why its stalling.

willib · « **Reply #1 on:** March 08, 2007, 08:17:20 PM »

hmmm

could two of the phases be shorted?

your rectifiers are ? at the base of the tower?

tomorrow you could try disconnecting all three phases and see if it speeds up.

assumming the wind is not howling of course.

it was working , then you changed what?

same batteries ?

Shadow · « **Reply #2 on:** March 08, 2007, 09:05:40 PM »

Yep rectifiers are at the base. Same batteries. I guess I'm not real sure when this started, I just started paying attention to it in last couple weeks. I couldnt figure out why the Trimetric was so out of whack compared to the panel meter.

willib · « **Reply #3 on:** March 08, 2007, 09:20:47 PM »

your trimetric is so different from the panel meter

Because your using HUGE shunts , much larger than what the trimetric was designed for.

i would go by the panel meter, until you get the right shunts

IIRC they wernt that costly?

Shadow · « **Reply #4 on:** March 08, 2007, 09:47:06 PM »

The Trimetric calls for a 500 amp shunt, its just that I stacked a 100 and 2-200's together to get there. I'll be ordering a 500 amp shunt in the future.

willib · « **Reply #5 on:** March 08, 2007, 09:49:37 PM »

just a stab in the dark here ,

is the trimetric controllong anything ?

Flux · « **Reply #6 on:** March 09, 2007, 01:07:06 AM »

I have had a look at your figures and it seems fairly unlikely that you would be badly stalled.

What have you changed, the first suspect is line resistance, have you altered the cables?

The second thought is that you say you were going up to 60 something volts, now you are holding the volts down, can you remember if it ever produced significant current when the volts were down to about 48?

Assuming that you have not altered the line resistance and it used to perform at 48v, then something has changed.

If it is not very windy you may be able to break the dc output of the rectifier ( be careful at 48v, it's not fun so don't do it unless you can do it safely and be sure you can get it back easily, don't touch the wires.) The speed should instantly shoot up and you should see the dc volts rise instantly. Don't risk it if there is a lot of wind, the speed would rocket and the volts go crazy.

If the thing still doesn't want to speed up freely they you have a winding fault.

You say you have changed the rectifiers so that seems an unlikely problem but still possible.

Try disconnecting each ac connection in turn and see what happens. The effect should be the same in each case and you should see a speed rise. If the trouble is stall then you will almost certainly see an increase in current.

If indications are that you are stalled then you can add some resistance to the line. Simplest thing is to add to the dc line, but you can add to each ac line if it suits your resistors better.

Try 2 ohms in the dc line. If your resistors are not rated big enough try 1 ohm in each ac lead. If the speed doesn't pick up and current increase then you have a fault other than stall.

I suspect you may have a winding or mechanical fault, but that all depends on what you have changed, knowingly or otherwise.

Flux

Opera House · « **Reply #7 on:** March 09, 2007, 06:56:33 AM »

If you have AC available, you can disconnect the rectifiers and feed about 12V AC into each pair of windings through a lamp. Comparing the brightness between phases would give some indication of shorted turns. Lamp would be brighter on a shorted coil. The mill would have to be stopped when you do this.

Shadow · « **Reply #8 on:** March 09, 2007, 07:04:31 AM »

Its way to windy again today to try much, when you say break the dc at the rectifier do you mean one at a time or all 3 at once? I'm not sure how much current it was producing at 48 volts as it was rarely down to 48 volts. More common to be 50-57 thats when I would see such high amp numbers too.The lower the battery voltage the lower the amps you'll see going in? Is that the rule? I have the negative dc input going through the 500 amp shunt, maybe I'll change that and go directly into the battery,see if that changes anything.

Flux · « **Reply #9 on:** March 09, 2007, 08:42:25 AM »

When I said break the dc, I meant that. There are only 2 dc leads pos and neg, it wouldn't matter which you break.

If it is too windy you can try removing one AC lead from the rectifier and see if it speeds up. Try each one at a time and the effect should be the same.

I suspect you have always had stall and not noticed it with the high battery volts.

You will need to wait for little wind to try resistors on the dc side, but if you have 3 resistors about 1 ohm each that will stand the current you can try them in the ac leads. If you do one at a time things should not go wild.

If you don't have suitable resistors, long pieces of cable may do the trick but it needs a fair length to have much effect at 48v.

Flux

Shadow · « **Reply #10 on:** March 10, 2007, 08:09:00 AM »

Checked again yesterday in 30 km wind, appears the Trimetric is correct 3 amps was the best I could do. That was using a different amp meter. All this time with 2 turbines I've been using a digital amp gauge thats been reading at least ten amps to high.I'm getting very discouraged with these wind turbines trying get more than 2 or three amps out of both a 12 foot and a 14 foot.

I added a 100 foot piece of #16 wire for resistance, but the wind went down so couldnt test it. It really takes off though when you unhook the DC lead.But just dosnt seem to want to move off that 2-3 amps even with wind gusts.

Flux · « **Reply #11 on:** March 10, 2007, 08:29:57 AM »

Seems as though everything is ok with the alternator and rectifier if it takes off on no load.

Your bit of wire has only added about 1/4 of an ohm, That will not make a big difference but see if you can see any improvement. If so add more.

From your figures I can't imagine why you are stalling that badly but I have no idea how you made your blades.

Reading 3 A on a 500A shunt is not likely to be very accurate. Do you by any chance have a frequency meter to measure the cut in speed, I suspect it is much lower than you need.

Just keep adding resistance for now until you get the speed up and see how things go. When you get it out of stall then it would be an idea to get some idea of speed to decide if you would be better off with a much larger air gap.

Flux

Flux · « **Reply #12 on:** March 10, 2007, 08:33:47 AM »

Try it single phase by pulling one ac lead off the rectifier. That should get it out of stall. It will be far less efficient so don't expect any dramatic increase in output but if there is an increase then it points to bad stall.

Flux

Shadow · « **Reply #13 on:** March 10, 2007, 08:47:03 AM »

Thanks Flux, I'm glad you have so much patience with me and everyone on this board! I'll keep messing with resistance over the next few days. The blades I use are pretty much DanB's plans to a T.They are one of the few things I am fairly confident about that are built correct.We have no wind today but forecast for 30 k tomorrow so hopefully. Thanks again

Shadow · « **Reply #14 on:** March 10, 2007, 12:07:18 PM »

Will Nichrome wire make a good resistor?I could cut diff lengths and try.

Flux · « **Reply #15 on:** March 10, 2007, 12:24:11 PM »

Yes, Nichrome will work perfectly well. You may have to parallel sections unless you have some really heavy stuff that will take 20A direct.

Flux

Shadow · « **Reply #16 on:** March 10, 2007, 04:54:09 PM »

I've tried about a 4 inch piece, appears about 15 AWG, it gets warm to the touch but not real great winds today maybe 20km at most.

willib · « **Reply #17 on:** March 10, 2007, 05:01:27 PM »

did the blades seem to go faster?

Flux · « **Reply #18 on:** March 11, 2007, 12:52:37 AM »

4" of 15 AWG Nichrome doesn't give you much resistance, it is somewhere about .4 ohms per yard giving you about .05 ohms. Assuming your 3A, you would have about .45W in that wire so I wouldn't expect much of a temperature rise.

I am still not sure about your current measurements, I wouldn't trust a 500A shunt and digital meter at 3A. Don't you have something analogue that you can trust good for 10 to 20A to be sure what is happening.

If you are sure that wire is 15AWG you could measure the volt drop across it to find the current, but don't take my figure for the resistance as exact, it is off my chart.

Flux

Shadow · « **Reply #19 on:** March 12, 2007, 08:30:39 AM »

WooHoo! I think I'm onto something, I used a length of Nochrome wire about 14 inches long (all coiled up) Wind turbine took off and I saw 10 amps!, So now I've shortened that to about 8 inches or so. My OHM meter reads about 1.6 ohms cold, so gonna try that today. Big question now is how fast is it gonna go when approaching furl? Another thing I noticed last night was I was getting small shocks while hooking up alligator clips to this Nichrome wire, is this normal or a sign of leaking rectifiers? I put the resistance wire on the negative side.

Flux · « **Reply #20 on:** March 12, 2007, 09:21:27 AM »

Good, now it is just a case of finding the best resistance.

This freeing from stall may influence the furling wind speed so make sure you have enough strands of resistance wire to comfortably handle well over the 10A.

At 48v you are at the point when you start to feel shocks, if your hands are damp it may become significant. If you actually break the circuit while messing about with the clips the alternator will produce well over 50V.

Not a bad idea to install a length that is too long, decently so that it can't go open circuit, then you can use your clips to short out part of this without risk of breaking the circuit.

I am not sure what things you have connected but it may be better to add the resistor in the positive line as a final thing. Other devices may have earth connections referenced to the negative line. If your alternator and the lines is totally isolated then really it doesn't matter.

Flux

Shadow · « **Reply #21 on:** March 12, 2007, 03:10:52 PM »

Thanks Flux

I have testing to do, we saw some high winds today up to 90 k so I shut it down. Problem I'm having is a resistor gets it out of stall but then it seems to really spin once up to speed. This morning was a runaway for awhile until I got it loaded down with heat elements to slow it down. So seems to be a thin line between breaking stall and going to fast. I'll keep testing. Can this Nichrome wire be mounted as a permanant fix? Or should I look for actual 1-2 ohm resistors?

Flux · « **Reply #22 on:** March 13, 2007, 12:46:07 AM »

Probably wasn't runaway, you should still have had a good load on it.

If you have been used to the slow leisurely stall, normal operation will seem like runaway.

The value of resistance to just avoid stall and still keep a high load at full speed is fairly critical, for low speed and comfort you need the absolute minimum that avoids bad stall. It may well be that in normal operation when you clear the stall your furling speed is too high, If fact you may have always been stall limited and it may have not furled before. Don't regard it as furling when the tail lifts off the stop, when the tail is round at at least 45 deg it is probably furling and any increase in wind shouldn't give a further increase in power.

You would have done better to have wound with less turns of thicker wire, you may still find it worth increasing the air gap to raise cut in speed so that you can avoid the stall with less resistance in the line.

The nichrome wire will be ok permanently if you use enough strands in parallel to keep the temperature reasonable. The lower you can keep the temperature the more reliably you can make the connections. If you use stainless bolts or braze to stainless strips of metal as tags it will be ok. If you can use the heat and you can find tank heaters of the correct resistance you have the best of both worlds but finding correct values will probably defeat you.

Operating at the top of the power curve would imply that if you cut in at 6 mph, you would be running at 4 times cut in speed at 24 mph. This is fast and noisy.

Normally with resistive matching you get the best balance when you are running at about twice cut in speed at about 20 mph.

Raising the cut in speed and letting the prop run away on winds just above cut in gives you a broader operating band before you come into bad stall.

Flux

Shadow · « **Reply #23 on:** March 13, 2007, 10:31:48 AM »

I tried a piece about 2.6 ohm I think, in fairly high winds (30k), it seemed awful fast and my coil of Nichrome turned very red! I'll try a shorter piece today maybe about 1-1.5 ohm see what that does. Thanks Flux

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Author Topic: Understanding Stall (Read 2046 times)