Fail safe shutdown

[frackers]

Just got to thinking today about a simple fail safe method of stopping the wind turbine if for some reason the controller fails or some other unforeseen nasty happens.

This little circuit uses a hefty relay with normally closed contacts to short the output of the generator until the 'START' button is pressed. This turns off the first transistor and the second one turns on, energizing the relay and removing the short, so the connection is the electronics side after the rectifiers, the relay contacts before the rectifiers.

There are three ways it will stop

1. - the 'STOP' button is pressed
   
2. - the battery voltage exceeds the voltage of the zener diode. This could be due to over charging, the fuse to the batteries failing (if you have one!!) or a bad battery terminal clamp whilst the turbine is running
   
3. - the battery voltage drops below the hold-in voltage of the relay. This could be due to circumstances as in (2) above but when the generator is already stationary
   
   

I've not actually built is yet - not had a chance to get into town to get the parts - so its time for a circuit inspection folks. I've sized the resistor values for 24volt nominal operation and the transistors are rated to 80volts. I expect I'll use a 1N4751 zener that will set the cutoff at 30volts (plus the Vbe diode drop across the first transistor). The relay I have in mind will require about 80mA but since I'll be running 675AH soon, I think that works out well below the self discharge rate!! I think it will be work $20 to put it in a nice weatherproof box if it ends up saving the batteries.

[elt]

I can't comment on the electronics but I have had similar ideas. I've seen heavy duty (200 amps) relays on ebay for about $45 USD a piece. These had two coils, one for switching and one for hold-in. The hold-in current at 24v was IIRC .8 amps... that seems like a lot of wasted power to me but YMMV. But shorting my windmill doesn't stop it so I stopped pursuing the idea.

Also, if you make one circuit to check another, do make yet another circuit to check the checker? And so on. I did make a data logger so that I could watch what was happening inside; that's as far as I went.

And I wonder how long you can boil batteries before they are damaged. Up to some point, I guess it's an equalizing phase; how far can you go beyond that?

Just some thoughts,

- Ed.

[Flux]

I really think you are worrying about nothing. If you protect for everything then I suppose it makes some sense.

The thing you most need protection for is something that leaves the mill unloaded such as winding failure, tower lead failure etc. Under these conditions you can't stop the machine and it will run away and be left to the elements. This thing won't do a thing for that.

If you have a working alternator and intact line then you can shut it down with a brake switch. If you are never there then I suppose such added complexity may make some sense but batteries are not going to disintegrate if a charge controller dies, you have lots of time to deal with it. You shouldn't be able to blow an input fuse if it is sized properly, it should only be used to protect the line if the rectifier fails or you develop a short and no braking circuit can sort that out.

If you develop a high resistance connection somewhere it may help if you sense at the correct point but are you going to have sensing wires everywhere.

Usually with all these things with watchdogs they are sleeping under normal conditions for years and the day something happens it is often them that are found to have failed and it not be noticed.

I suppose some derive lots of pleasure from designing such things and that is enough reason to do it, but leave it to them that like to play if you just want Power.

I would infinitely prefer a manual shut down but that doesn't seem much of an option here.

Flux

[Dave B]

 

 I've been through the overspeed thing twice. For those who have not the big winds will come. I now have a cable operated manual brake to keep the blades from rotating, period. I will never put another machine up without one. I can always use the "iffy" method of shorting it out too but an open any where can render this useless. (I believe the Dans have experienced this) Nothing better than a direct manual connection to the brake from below, my machine is 85' up also and in the very worst case I can lower it besides. You can think it to death (that can be fun) but kiss I get back to when I focus on what I want to accomplish (no rotation). Just my thoughts, Dave B.

[boB]

My understanding about shorting out wind turbines to stop them, or how well it will accept that short if it is spinning fast, depends on the turbine and its design.  i.e. Some handle it well and some can't (it burns up).

For those turbines that can handle it, I would try a triac(s) across the windings, (before the rectifiers of course), and trigger the triac above a certain voltage, like you are doing in your circuit.  Using a triac will let it turn itself off every cycle when the current drops below the holding current of the triac and would be re-triggered next cyle.  This would just appear as a cliped DC voltage seen by at the battery side of the diodes if it was just connected to a big capacitor.

Using a relay when turning off the short leaves a big chance that it will turn off when the current is high and might damage the relay contacts from inductive flyback.

But, on the other hand, if you can sort of control the timing of the relay turning off, it would be a nice rugged way of shorting in emergencies.

boB

[Ungrounded Lightning Rod]

Triacs across the windings triggered by overvoltage are something you should not do.

This circuit would drop out at the next zero crossing and retrigger at the next overvoltage.  The result is that it tries to regulate the mill speed at the setpoint by dumping the extra energy as heat in the windings.  You never slow down the mill to near-stop, stall the blades, and drop the power collected from the wind.  So you'll burn it out the first time you get a big wind.

Even if it held in until the mill stopped you'd still fry it.  Shorting the mill means the energy from its motion is dumped as heat into the windings as you bring it to a stop (along with anything it's collecting from the wind as it spins down).  This may be successful once, or even a few times in rapid succession.  But by dumping ALL the power in the mill when it's collecting the giant excess from the wind, then letting it spin up and doing it over again repeatedly, you're again guaranteeing a burnout in high wind.

[boB]

Like I said, some turbines can handle this and some can't. Maybe more can not than can.

 The ones that can't will fry of course.  If you can get it stopped to 0 RPM, then there is no current and no I^2R in the windings.

If the trigger voltage is real low, like lowest it can be, then it is essentially

a short, minus the triac drop.

Then again, some turbines can fly freely open.   I like the real brake idea.

Something is needed for those turbines that can not be shorted and can not free-run in high winds.

boB

[boB]

Then again, you could apply a load resistance that is the smallest you could apply to the turbine output that won't raise the winding temperature higher than a safe amount (however high that is for the particular turbine) and it ~might~ slow down enough to be stoppable when shorted after that (?)

   How about the builder add a thermistor to the windings for this very purpose so fail-safe methods can be tried for that particular design ?   The you would know when to just let it fly freely.

boB

[electronbaby]

I have shorted out a 16' diameter machine probably THOUSANDS of times. Maybe more. I built a circuit that did this in response to the ALM settings in the old Trace SW's. It worked great. I used an actuated knife switch to do this. This wasnt a safety, this was the actual control mechanism that literally shut the turbine down if the batteries were fully charged. Not the most high tech, but it worked well. I agree however, that there is a lot of energy in the form of heat that is dissipated in the stator when the machine is shorted and running at a good clip. What makes it worse is if you are running a heavy prop and have a ton of built up inertia also. Its important not to have too "stiff" of a feed line, when doing this. If you had a super low resistance feed line, Im sure you could have your circuit dump the power to a resistor to keep the heat out of the stator, but this is pretty close to what a diversion load controller does, and I think you would be wasting your time.

I guess what Im getting at, is that its really nice to have your control equipment connected in parallel all the time. I absolutely hate the thought of the relay or something going bad and either hanging up in an open circuit condition, or causing the turbine to run unloaded. This scares the hell out of me. This is why I ended up simply shorting before the rectifier,...because there is absolutely no way the batteries could become disconnected.  

I guess its just a matter of time until it fails, BUT, I have run axial flux machines this way for YEARS without a problem. Ive still never had a problem. I dont do it this way anymore, however.

Just my two cents.  

[boB]

Roy, what kind of turbine was that 16 footer ? How many volts nominal ?

Was that home-made, axial flux, radial flux, with or without steel backplate ?

Neodymium magnet ?  I'm gonna need to know more about how different turbines

can and can't be shorted or not.

Thanks,

boB

[Dave B]

  I just have to comment again. When the big winds come (and they will) you may get away with shorting out your machine to slow it's rotation and keep self destruction at bay.

  I may be a bit traumatized after watching my first 12' machine start up and accelerate to over 800 RPM (measured) with it completely shorted. True this was in 60 MPH winds and the furling was set too hot but keep in mind that your blade profile will have a lot to do with the effectiveness of shorting also.

  Higher torque lower TSR blades just might not care that you shorted the alternator and could start up and then fly being more than happy to burn up your stator and start the self destruct mode. I can hear it now from those who have manually tried to turn their alternator when shorted (no way you say) and good luck I say.

  Just another tip to those who frequently short out their alternators to actually slow the rotation : take a good hard look at your stator, stator mounting holes, spider mount, hardware and airgap etc. That huge amount of braking force is your magnets trying to move your coils (turn your stator) and can crack, bend, loosen or break the weakest link.

  This same force is acting on the stator when generating power but is not the same slam, bam all at once happening as when shorted.

  Design a manual brake and use this as well as shorting it out when the high winds are predicted, you will rest easy.  Dave B.

[Flux]

boB

I will let Roy answer your specific question but I may be able to help with some general information.

In the old days with dc machines it was impossible to do this sort of thing. The machine wouldn't stand it even if you could maintain excitation ( which shunt machines wouldn't, they just shed load)

The same is true of industrial machines, nobody expected a dc machine to stand a sudden short. Alternators if built well enough could survive and were generally expected to be able to survive a few sudden shorts but in power generation it is a disaster situation and you would never expect it to happen.

For wind power the early alternators used feeble magnets and had to use slotted iron cores to get the output in a finite size machine. These had so much internal impedance that their current was levelling off at full load and a short may have very little effect on the maximum current and no way would it stop in a high wind, Some produced so little extra current that they wouldn't burn out at full speed when shorted.

Then came neo magnets and the prospects of higher field strengths and the chance of reducing the number of turns in the winding, this significantly reduces the reactance and with thicker wire the resistance is lower so the impedance drops. Many of these machines are below the impedance limiting point on full load and on short circuit they may produce perhaps 3 times full load current. Most of these will stop with a brake switch in most winds. In a gale you may have to dump resistance across it first  to develop enough power to stall the prop but then it will generally stop and will hold stopped in high wind.

The ironless axial machines have much less reactance and the characteristics are largely resistance dominated to the extent that in the working range the short circuit current rises linearly with speed. If these are built with high efficiency ( low internal resistance) they develop a braking torque that will stop them even without adding more resistance. Lightly built high resistance versions will fry the winding without producing a torque great enough to stall the prop and will not stop.

There is a limiting case where some will hold braked once stopped, less efficient ones will break loose at some wind speed and burn out.

With the conventional matching you have to have a fair bit of inefficiency some where and the cheapest place to put it is in the alternator where it reduces the cost and size very significantly. If you take this too far it is ok when running but you won't stop it in a gale with a brake switch without burning it out.

When working in the mode that you are thinking of you need the highest alternator efficiency you can get for a sensible cost and this trope of alternator will be capable of braking under virtually any condition.

Besides the ability to stop and not burn out there is also the mechanical consideration. This is a violent process and in any other application would be considered a disaster situation not an operating one. The shock mechanical load is very great and must severely stress everything. You have to be certain that this long term shock loading is not going to cause mechanical structure failure in the turbine, alternator and blades.

The iron cored machines limit to several times full load current and the severity of the shock is less. A very efficient ironless machine could momentarily exceed 10 times full load current and you may need limit resistors in the braking circuit to keep mechanical forces within limits. That can probably be determined on a commercial basis but for a one off machine you don't want to use it as a fatigue test bed for a year before you know that it will be ok.

As for burning the stator that largely depends on the available braking force, a reactance limited alternator with ceramic magnets won't stop and probably won't burn out. Make it more efficient and you may have enough current to burn it and still not have a reliable brake for all winds.

Ironless machines if lightly built may hold braked but it may not be possible to stop them in a high wind without burn out. If you can't stop it within 30 seconds it is safer to let it fly and let the furling take it.

Highly efficient ironless machines will stop so quickly that burn out will not occur but the mechanical shock may be more than you can tolerate and you may have to reduce speed in the first stage with load resistors before the final short.

Sorry this has gone on a bit but I hope it helps.

All of this goes out the window if you have a winding or cable fault and unless you are Bergey and can guarantee running in a storm on no load then I am with Dave B and like a back up, I prefer to turn it from the wind but a good reliable mechanical brake is ok in climates where it doesn't rust up or freeze up.

Flux

[frackers]

Looks like I stirred it up a bit with this notion of putting a belt and braces on the existing belt and braces!! My web site recorded 362 unique web addresses accessed the graphic for the circuit diagram :-) This is one popular forum...

I must admit that my main reason for looking at this in the first place is my lack of confidence in the controller I'm using - a very (maybe too) simple one at present but the next version will be tracking current in and out of the battery bank as well as monitoring the voltage of all the individual batteries in it (shame I can't monitor each cell really) and using some predictive algorithms to make the most efficient use of the power available to keep my pool clean and warm. Complex enough to be sure to go wrong.

Roll on summer (the 10" of rain over the last 10 days - half our annual rainfall here - have had me checking the guy anchors!) when I can get some real use out of the system.

Until the new batteries arrive the mill is parked (i.e. shorted manually) anyway - its a toss-up on whether the batteries or the new mast happen first.

[Flux]

Yes things need stirring up occasionally. A bit of shared thought on some issues is not a bad thing.

People seem very reluctant to copy someone's alternator design that works, they all want to invent the wheel but other factors such as control seem to be set in stone and few ever challenge it.

If you suspect your controller then a back up will do no harm. Whether stopping the machine is better than using a backup voltage limiter is another matter.

A fail safe once only shutdown is reasonable thing even if it needs a slow down resistor and a final short. You know what your machine behaves like. For a normal brake switch you would choose to stop it at an easy moment with a lull in the wind. An automatic device may choose the worst possible moment so you have to bear this in mind.

Flux

[boB]

This can be an important issue in my opinion, if one would like to make the system as reliable as possible, so it's worth talking about.

One idea as far as braking and not burning up windings that comes to mind is to first wait for that "lull in the wind",  (Flux, you might have a new song title there!), and then short it, while monitoring the AC current during the short.  If the auto-short current sense finds the RPMs either not slowing down quickly, or speeding up after the short has been applied, it could then open the short letting the turbine fly freely.

You could also add some variable electrical loading to try and get the RPMs to be within a certain range I suppose.

This all of course depends on the turbine design.

boB

[electronbaby]

boB, the turbine was an axial flux homebrew. It used the bigger .5" thick wedge magnets. Its magnet rotor diameter was 14" and I built it a while ago when DanB was building a similar machine.

Anyway, if I recall, it was an 18" diameter stator, .5" thick. It was a 3 phase alternator using 12 poles (24 mags) and 9 coils. It had 45 turns per coil, and it was wound 'two in hand' with #14 wire. It was a 24v machine. It was used to charge batteries.

The feed line was three #10 wires coming down the tower (approx 95' run) AND three runs of #2 from the tower base to the shop (another 90' run). Rectifier located at the battery bank.

To answer your question regarding "backplate". Im assuming you are talking about the magnet rotor composition? If this is the case, the rotors were 1/2" steel, and they were 14" in diameter.

The prop on this turbine weighed quite a bit if I recall. I wish I had an exact figure for you but I recall it was aprox 40 - 45 lbs. Maybe a little more. It was made of doug fir. Anyway, ....alot of built up inertia like I was saying..

I hope I answered your questions.

[electronbaby]

Flux,

"Highly efficient ironless machines will stop so quickly that burn out will not occur but the mechanical shock may be more than you can tolerate and you may have to reduce speed in the first stage with load resistors before the final short."

This is a VERY good point. There are not many people that get to watch a relatively large axial flux turbine get shorted in a good wind. The alternator will stop very quickly. This puts severe mechanical stress on different components of the system, and if they are not designed robustly, will fail sooner rather than later.

[boB]

Great !  We will just have to be able to slow the turbine down then before the big stop.   Roy, thanks for the answers.  That's what I wanted to know.

Sounds like a versatile "clipper" could take care of this in a lot of cases for safe stoppage.  Especially if the current pulses are monitored to make sure it actually does slow down and stop as wanted.

Thanks,

boB

[electronbaby]

Bob, I just checked some notes I took a couple years ago, and that machine I was talking about,...  the weight of the prop was 60 pounds.

[Ungrounded Lightning Rod]

The problem with variable loading to hold an RPM is that the braking torque is proportional to the current and the heating of the coils is proportional to the current squared.  If the wind is pushing the turbine twice as hard the mill is heating four times as much - regardless of RPM.

(Interestingly, variable load trying to hold an RPM is what a battery charging load approximates - and what it would achieve if the coils and drop wire were superconductors.  We already have trouble with burnout from that in the nontrivial-resistance non-ideal case.  If the non-ideal case puts burnout-level heating into the stator, adding a controller to INCREASE the current - and thus the heating - in the stator {while letting the mill spin fast enough that the blades aren't stalled} will make the problem worse, not better.)

You might want to consider, instead of trying to hold an RPM, trying to hold some maximum CURRENT and let the mill speed up - but only somewhat, since it's still under load.  This would mean an adjustable series dump load - that could take a lot of power and a lot of overvoltage.

You'd still be risking a tear-apart from the somewhat reduced overspeed.  But there's be less risk than if you let it burn out, maybe catch fire, go unloaded, spin up to some hysterical RPM and either tear apart or strike the mast as it yaws with hysterical gyroscopic effect from high RPM and THEN tear itself apart.