Shunt not halting alternator

[Boss]

Pushing ahead on wind turbine

This is sort of a mishmash of images, but it pretty well represents what we are doing these last few days

 Glen Hurd http://ghurd.info sent us one of his diversion load controllers last year and we are just now building it.  Unfortunately we are having some difficulties with it, possibly because I tried to use a variable autotransformer and a rectifier to supply varying DC voltage to it at first and we think it was hard on the P-Fet. The first N-Fet was switching when we adjusted the potentiometer as it is supposed to, but the next stage of the circuit a P-Fet isn't changing states. Glen is helping and we bought two more of these kits from him on Friday. It  is our intention to use one controller to run a 3 double throw three pole contactor which is like a relay (24 VDC coil) as an auxiliary shunting kill switch foe our 10 foot axial flux horizontal 24 volt wind turbine. The other two controllers will be for dump loads, to discharge storage batteries when nearing fully charged.

 After brushing up on electronics skills long unused and scouring the various out-buildings for test equipment and tools we set up a bread board to try different things in our trouble shooting the controller.

 Above is the variable transformer controlling the power input to a power supply with outputs of 10,  18, 48 VAC plus two  16 VDC outputs on the back, handy little supply I built back in the days of my service shop Outland Tech.

 Above Ghurd's diversion load controller, before we unsoldered and resoldered components. It ain't too pretty no more.

Ah, the work bench at one stage before it got really crazy with all the added test gear.

 We took a break from electronics to work on the tower. We are co0nstantly working on the anchor and base foundation holes,  in between the snow, the cold and a constant 30 mile an hour wind which reminds us to be humble in the face of nature.

Above is one of the many inventions we are coming up with to solve problems as we build our first tower. Constantly reminded by the howling wind outside that everything we design will soon be put to a grueling test  The adapter above will slide down inside the 3 inch square tubing which the first 40 feet of tower are made from. This bushing will be welded to a ten foot long two inch diameter schedule 40 steel pipe. Another bushing which is three times thicker will be closer to the top  and welded in place near the guy cable hooks. These bushings are mainly to keep the round pipe from flopping around inside the square tubing.

We actually made the bushings before making the bracket that holds the hinging tower base  because we are almost out of 1/4 thick steel and we needed to make sure we had enough.

In the background of the above image you can see our Granberg Alaskan chainsaw mill which we just used last weekend to cut two 3 inch thick 20 inch wide and 12 foot long slabs which will be the next workbench in our new shop.

Finally, today, April 4th 2009 we built the brackets for the tower base hinging system. The triangles fit around the 3 inch square tubing of the tower and 20 foot gin pole. The square bracket on the right is one of two which we will be welding 5/8 inch rebar which is buried in the foundation. Tomorrow if it doesn't come a Spring blizzard we set the tabs in place in the foundation holes and measure the rebar and mark each piece for a custom fit. We borrowed a builders level, which is sort of like a surveyors transit to get the anchors and the base all exactly the same height.

 Kevin uses the portable band saw to cut out the 1/2 thick tower bushing. Handy tool that bandsaw, especially now that we bought the proper blade for it (more teeth per inch)

And finally, we are trying to figure out why our turbine doesn't come to a complete stop when we shunt the three phases of coils together. Everything is sloppy so the air gap seems too wide. today we pulled it apart and removed the washers we had to install between the outer rotor to get it to not rub on the stator. We installed washers that are 1/3 as thick and put it all back together, but with the ten foot blade system we can still move the turbine.

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[TomW]

Boss;

Your turbine will not stay still when shorted. Mine rotates lazily around at a few RPM in stiff wind even shorted. There is no braking without motion so it moves slowly. If thats what you see then it is pretty normal.

Tired of winter and spring teasers myself.

Looking good.

Tom

[brokengun]

Also, in theory the magnetic braking works more with more induced current. So basically at the beginning, when not much is being produced, the magnetic field induced in the coils isn't as much as at a higher torque. You should be able to feel it with your hand the harder to spin, the harder it is to turn. I've noticed this during my bench testing this week too!

Best of luck with your tests, I know how frustrating it is to have shotty test equipment. This week I found out that the ammeter I'm using shifts it's reading with a slight nudge on the wire connection.... ahhhh!

[Boss]

Thank you guys, that is very reassuring. I am anxious on so many levels I can't see straight, then the wind kicks it up a notch, take a deep breath, Brian

[BruceDownunder]

Hi Boss,

I've noticed that my 11 foot mill when "braked" slowly turns also.

But, if I brake it(3 phase electrical breaker,shorted)  while it's spinning fairly fast , it keeps going as if not braked at all . I have to wait for a lull in the wind till it gets down rpm ,then switch the breaker on .

She just turns around slowly at 1-2 seconds per rev.

Nice pictures and info,good to see you back..

Bruce

[oztules]

Bruce,

"But, if I brake it(3 phase electrical breaker,shorted)  while it's spinning fairly fast , it keeps going as if not braked at all"

This means your running the F@P or some other iron cored machine. The AWP over here has the same antics. It is due to the ampere turns being sufficiently high, that the armature reactance so messes up the field from the permanent magnets as to make the inductive reactance  go very high. (magnetic flux not linking the turns very well any more)

If the synchronous impedance gets high enough, then not enough power will flow to stall the blades.... so it keeps on running.

If you want to stop it without waiting for that lull that may never come in time, use a fairly heavy load  of about the size of your max output (ie 1kw for your 1kw mill etc), and it will help quicken the slow down considerably.

This works much better than a short, as it keeps the ampere turns down low enough to allow the genny to utilise as much power as it can. and hopefully stall out the blades.... faster than manually waiting for it to happen. The amp turns are not watt turns.... so shorting will give you high current at lowish volts (depending on sync impedance)....low watts, which won't stall the blades. The load resistor will load the mill up much more... maybe a little less amps, but much higher volts... so higher watt drain, and as soon as possible will overpower the blades.

Thats what we have found over here... we found 1kw load was enough to tip the scales in our favour, a dead short just allowed it to run at least as fast as before, possibly faster (at least it appears too when your desperate to stop it!!).

...........oztules

[SparWeb]

I've seen my motor conversion do the same thing, too.  Thanks Oztules for the explanation!

It's very embarassing when your neighbour comes by on a windy day to see the windmill working, but when you try to demonstrate how the OFF switch shuts it down - it keeps going!

[Boss]

Yes thanks

I don't know where I got the idea that shunting the 3 phases (simultaneously opening the rectified side) would halt the alternator dead, but I was worried that it didn't. Is there a method for determining the difference in alternator heat buildup between loading the alternator through the storage system to stall it and shunting? Or are there too many variables?

Brian Rodgers

[Ungrounded Lightning Rod]

I don't know where I got the idea that shunting the 3 phases (simultaneously opening the rectified side) would halt the alternator dead, but I was worried that it didn't.

You don't need to open the rectified side.  The diodes already handle this for you.  (Just don't SHORT it on the rectified side or you'll fry your diodes from the high current.)

Is there a method for determining the difference in alternator heat buildup between loading the alternator through the storage system to stall it and shunting?

Figuring the heat INPUT is easy in a coreless alternator:  It's the square of the current times the resistance of the windings.  (The resistance doesn't change a whole lot between operating and melting temperature so you can approximate it as constant.)

With a core you also have heating from eddy currents and hysteresis losses, which are mainly related to the RPM rather than the output current.  (To the extent that raising the output current by loading affects eddy current and hysteresis losses in a core it causes them to DROP by inhibiting the penetration of the field in-and-out-of the core.  So ignore them and stick with the coil heating.)

Figuring the heat BUILDUP is another issue entirely, involving heat capacity, thermal resistance, air cooling, etc.  But because you generally shut it down in high wind (good cooling) and already have an estimate of max sustained current you can ignore that.  Just remember that square-of-current law and figure that if you appreciably exceed your intended max current for more than tens of seconds you risk a damaging temperature excursion.