Idea for a simple high-current brake circuit

[Beaufort]

One of the things that seems to plague home-built wind turbines is the variability of furling and therefore stator heating.  It would be nice to have a simple additional way to protect these things that we spend so much time building, so there needs to be some method of limiting output when the stator current gets above a certain point.  I've seen some mention of adding temperature sensing to the stator but this would seem to present problems with packaging and variable heat transfer rates. 

What about putting a thermal switch on one of the rectifier heat sinks and having it throw a relay to short the DC output?  A dead short may not be advisable on all machines, but alternatives could include switching in braking resistors.  Here is a list of possible benefits of using this method:

-  Thermal switches are very cheap (just a few dollars).  50A, SPDT automotive relays are also very cheap.
-  Case temperature for rectifiers varies as a linear function with current.  One of the bridges I use goes up 25 deg C for every 10 amps...doesn't seem too hard to dial this down to a few amps.
-  The heat transfer across the rectifier rail/heat sink can be easily calculated with basic thermodynamics.  There would be a time delay as the temperature climbs, and this could be easily changed by varying the gap between the thermal switch and the rectifier. 
-  The time delay is very useful to allow the turbine to pump a lot of amps through gusts, but not continuous which is where it seems the temperature rating of the stator is exceeded.
-  There is also a time delay as the rectifier cools down and the thermal switch is actuated, allowing the stator to cool down too.  This hysteresis effect would seem to prevent rapid run/brake cycles.
-  The components are small enough to easily fit onto machines with diodes/rectifiers up on the turbine.
-  Since the relay needs battery voltage to hold the short open or closed, the system would be fail-safe with the relay wired up "normally braked".  In a compliant system, there has to be a circuit breaker between the wind turbine and battery.  If the breaker trips, it goes open and would normally result in a runaway machine.  With this braking relay, the machine would stop in the event of overcurrent at the battery.


Has anyone tried this, or see any problems with the logic?

[DanG]

Any metal exposed to the swarm of changing flux near the stator will be trying to generate current and/or heat from eddy currents.

[tanner0441]

Hi

I for one would not like to drop a dead short across the output of an already over delivering turbine after the diodes.  The junction temperature can go to destructive levels while the heat sink is still cold, add the fact you are monitoring the heat sink and looking for it to be getting into its upper limits before you short it out.  Reading on here it seems stators can burn out with sudden gusts of wind as much as a sustained high wind.

Furling still seems to be the best option and the simplest.  There are other ways, variable pitch on the blades, heat sensing over centre spring loaded brakes, centrifugal brakes. At the end of the day it has to be simple and reliable if the turbine is unattended. A big switch at the bottom of the tower used when high winds are expected is still the most reliable with the option of furling if you miss that.

A lot of turbines are reluctant to start up into a dead short, but run on quite happily once they are running.

Brian.

[Andrew Cates]

Why not  set up a voltage divider ( two resistors ) across the alternator. Connect them to the gate of a SCR. Also Connect the anode and cathode of the SCR across the alternator. When the the alternator spins slow, there is not enough current to turn on the SCR, but as the wind picks up the alternator puts out more current- the voltage developed across the gate of the SCR will also increase. When there is enough voltage across the gate (which is set by the values of the two resistors ), the SCR turns on and the anode to cathode resistance drops to near 0 ohms. Remember the anode and cathode are connected across the alternator- which shorts it out. The blades will slow down and therefore the alternators current will start decreasing. When the current has dropped enough the SCR will turn off ( Opens ) and the blades are free to turn again. By using variable resistors the SCR turn on point can be set and adjusted.
 I have not tried this method of overspeed protection. I just though to throw it in here. I am working on the method and not sure if it will work. So, please suggestions and comments will be greatly appreciated.
AC

[tanner0441]

Hi

Read on here there are many accounts of people shorting alternators out for braking and it doesn't always work and the blades just keep turning.  When it is spinning the only guaranteed way to stop the thing turning is with friction.  If you are lucky and have one that stops that is good, another thing is that a shorted stator that keeps turning, is turning all that kinetic energy into heat in your stator.

I have been playing with wind turbines for a year or two, I only have a Chinese copy, so I know very little, but there are people on this site been doing it for many years and they have written plenty of reports on their successes and failures I am happy to listen to them and act accordingly.

Brian

[SparWeb]

Throwing the WT shorting switch is like driving on the highway and slamming on the brakes.  It gets you stopped all right!  But most of the time you only need to brake to a gradual stop, either gently or firmly.  The analogy is just as applicable to those poor brake rotors that must dissipate a huge amount of heat very suddenly, or much more gradually if you don't brake as hard.  This is what causes your car brake disks to warp.  So it goes for an axial WT stator...

Althought I haven't made a diagram to make it concrete, I've always considered a progressive resistance brake would be worth building, if I ever needed additional braking power or thought I had trouble with furling.  I also believe that it would be easier to design the system if it was based on RPM only, using an independent pickup to measure the speed.  It would NOT work if it was just trying to read across AC phases.  A separate and somewhat protected circuit monitoring the turbine speed could then activate a resistance brake if a critical speed (Hz) is exceeded.  The resistance load to be switched in would be a trio of resistors across the AC lines.  The set of resistors would be sized to divert approx 1/2 of the current from the rectifier circuit.  Now the resistance load is soaking up the heat energy, instead of forcing it to be dissipated by the stator.  This situation will add only a modicum of control and braking, but a further progression of the system switches in additional resistors in parallel, causing yet more current to be "shorted", ending up with a full-short switched in once the circuit detected the resistors' inability to finally control the RPM.

This system is ineffective for under-sized generators.  With a very "stiff" generator like a properly sized axial, it could work well.

Has anyone tried this before?  (Zubbly experimented with this didn't he?)

[ghurd]

"Thermal switches"
"hysteresis"
"Case temperature for rectifiers varies as a linear function with current.  One of the bridges I use goes up 25 deg C for every 10 amps"
"basic thermodynamics"

My imagination can envision an issue or 2.

Lets say a turbine in Calgary, if someone was actually crazy enough to live in those temperature conditions.
The ambient temps can go from below -40C (-40F) to 44C (110F)?
The coils are insulated, but (hopefully) the rectifier heat sinks are not.
I can imagine the coils frying without the heat sink reaching 44C (110F).

And case temperature for rectifiers varies as a linear function with current, when the air flow is a fixed and given quantity and given ambient temperature.
A moderate temperature and moderate wind cool the moderately loaded rectifiers, less than a gale at -40 degrees.
But because the coils are insulated, and making a lot of heat, the inner coil turns are a lot hotter than the rectifier would indicate.

I think a thermistor in between the coils would be more accurate.

And I think the rectifiers will get the hottest when the wind is the strongest.
Shorting could cause run-away in some machines, hard to imagine though I have seen it happen in small machines, which will mess with the furling, resulting in a worst case scenario.

I can imagine, in a Star connected 3-ph PMA, if heat was used as the baseline, it would be best to implement a 3 stage resistance on the AC side of the rectifiers?
G-

[Beaufort]

I did a little bit of bench testing today by checking rectifier and stator temperature, going up to 10 amps and I'm not so fired up about using the rectifier rail anymore.  Granted, this is on the bench with little airflow but it seems like the stator might respond quicker to changes in power than the rectifier (in terms of heat).  And as far as specifying a proper thermal sensor, you really want to know what the stator is doing given the limits of your casting materials and wire.  I completely agree with the comments about direct shorting being deadly for many machines...that's the caveat stated in the beginning.  I tend to build them to be tight in Star, and small machines under 6' where it doesn't take much to pull the machine down.  I wouldn't even dare to think about this method on some 20' torque monster.  And again, this sort of idea would only be used as a last resort when furling hasn't been dialed in or it gets out of furl for some reason in high winds. 

[fabricator]

Air brakes, guaranteed stop first time every time, easily controllable for a nice smooth stop.

[ChrisOlson]

One of the things that seems to plague home-built wind turbines is the variability of furling and therefore stator heating.  It would be nice to have a simple additional way to protect these things that we spend so much time building, so there needs to be some method of limiting output when the stator current gets above a certain point

Hi Beaufort,

I came up with a wild idea the other day when I was walleye fishing.  I decided to build a new delta generator for my 13 foot 12 volt and incorporate an automatic star/delta switch in it.  I decided to go with delta because I only want three wires coming down the tower instead of six for IRP.  I left all six wires in it for right now because I have a manual star/delta switch at the base of the tower so I can test it before I built my automatic thing that's going to mount on the turbine.

I didn't really take many pictures of the deal because other people have done this before and it's not really all that ground-breaking.  About the only picture I have is one I took when I was winding the stator and laying it out in my new steel stator mold that I built:



I wound the new stator so it fits under 10" generator rotors instead of the 14's that were on it, and I used 1/2" wedge mags in it instead of 1 x 2 x 1/2" bars.  And I wound the stator with a single strand of AWG 14 wire.  It's wound for proper cut-in speed in delta.

When I put the thing up it works good in star in low winds.  It start putting out 1 amp at about 5 mph and it will get up to about 14 amps and that's as far as it will go - it's only turning at ~100 rpm.  I throw the switch to delta, the rotor takes off into the wild blue yonder, and away she goes.  I also turned the furling way up on it because I wanted to test this AWG 14 to see what it would take in delta (according to my guesstimates, single strand AWG 14 in delta is about the same as two-in-hand AWG 14 in star).

Yesterday we had 25-35 mph wind, sustained, pretty much all day.  I found out the new generator will get to 20 amps in star in pretty strong wind but it just won't go above that.  The rotor is going so slow you can count the blades going by.  I thew the thing into delta and it was like, holy ^&%$, it started pushing 80 amps on my Fluke meter, and was still climbing.  I looked up and it wasn't even attempting to furl yet.  So in a panic I threw the switch back to star.  I don't know exactly how big the power surge was, but I know I saw 160-something on my Fluke meter.  But it was very brief and then it died down to 18-20 amps and the rotor was turning real slow again.  I threw the shorting switch and it brought the turbine to a crawl, even in the strong wind.

I didn't see any smoke come out of the generator so I decided I had to try this again.  If the gen goes I'll just clear the blast zone and hope for the best.

I actually ended up running that same test at least a half dozen more times, then decided to leave it in star until I get the furling turned down on it to more reasonable levels.  I dropped the tower this morning and adjusted the furling on it and inspected the generator.  There's no apparent damage to it from pushing 80+ with surges over 150 several times, for brief periods.  Then I forgot about until I read your post because I'm busy building my automatic switching outfit.

Here's my wild idea - why can't a person build a delta generator like this that starts in star, switches to delta, then switches back to star for braking at a pre-set point?  It appears to me that it worked just as good as shorting it except that the turbine continues to put out a low amount of power after it spools down.  I don't know what sort of wind it would take to get it to come out of stall in star, but even if it did I suspect if the furling was set properly it would furl anyway.

This would all take some sort of electronic controller that I don't have the slightest clue how to start building.  But, based on what I saw in my experiment, I think it would work.
--
Chris

[Beaufort]

Here's my wild idea - why can't a person build a delta generator like this that starts in star, switches to delta, then switches back to star for braking at a pre-set point? 
--

Until last week this is exactly the scheme I was going for on my latest creation.  I really liked the low resistance in Delta and the good starting in Star.  I even built a wind sensor to trip a series of relays to go from Star to Delta around 15 mph, and then back to Star around 30 mph.  After a bunch of vehicle testing, I just didn't like what I was getting from Delta and decided to add a few blades and let it run up in Star.  But the result is a very "tight" machine that slogs along in stall, which is fine with me...it makes controlling high-wind stuff a bit easier.  So now, I've got a very hot machine to get decent high-wind output...hence the idea for a heat circuit.  My latest testing tonight shows the stator getting up to 185 deg F at 18 A (12 VDC) for only 10 minutes, so I'll continue to tinker with some sort of a heat limiter.

I know the thrill of the Delta speed-up after switching.  I was also surprised that going back and forth wasn't more violent on the system. You've got a nice system going there.

OK, now I'm jealous of Fabricator's air brakes (and the sweet welds on it!).

[Flux]

I have to go out in a few minutes and don't have time now but there are some interesting ideas here.

It's not possible to design a controller from star to delta that keeps the thing permanently in the correct mode in variable winds but it is a step forward from running bogged down in star all the time.

The delta connection and stall braking in star will work but the controller will be tricky and may need prop speed and wind speed input. Once you go to mppt you gain far more than this single step change hints at and you can stall the thing at constant power by phasing the converter back.

Again what Chris has done makes me suspect even more that most of these machines are stall regulated and at best only partially furl, if they get through the stall phase it is burn out.

Running in stall mode makes easy control as long as you don't pull out of stall but it leads to miserable high wind performance and all the associated problems of stator heating.

There is no reason why machines can't furl and furl reliably in this stall mode but you have to make them do so. If you accept this then you can forget all this trickery and the worries about heating burn out braking and all the other stuff but ad Dave b said you have to be very modest with your expectations and accept a miserable power out and expect to be fully furled by 25mph or less.

If you want things to perform you need a different approach to load matching.

Flux

[ChrisOlson]

I know the thrill of the Delta speed-up after switching.  I was also surprised that going back and forth wasn't more violent on the system. You've got a nice system going there.

How big is your rotor on this machine?  I can't figure out why your stator would be running that hot.

I've been tinkering with this 13 foot 12 volt for awhile now.  When I built it, I built it with the idea that I wanted reliable power in low winds.  That's why I got the big blades on it.  The generator I had in it before was actually wound for a 10 foot set of blades.  But I wasn't getting the low wind power I wanted in IRP.  So I built this new generator for it, which has 77 turns in it, with the hope that in star I'd get good power in low winds.  It does do that now - 14 amps at 12 mph wind I thought was pretty good.  But I was a little disappointed when I found out it wouldn't do more than that on the first day I tested it.  But come to find out, it will get to 20 amps if the wind is really blowing.

I really don't want more than 50 amps from it (about 700 watts) on the top end.  It's easily capable of doing twice that, but not without damaging something long term (rectifiers, batteries, dump load, stator).

When I built the last generator for it I had too much space between the coils and the confounded thing vibrated and buzzed all the time (typical with 9 coil generators).  That vibrating/buzzing is caused by the mags "letting go" of one coil leg then "grabbing on" to the next one that's too far away.  So I decided to fix that with this one.  I scrunched everything down to 10" with .2" between the coil legs and used the wedge mags.  The wedges give a longer coil saturation time and more width at the outside of the coils where it really matters - so it now runs smooth as silk no matter if it's delta or star.

But as I also found out yesterday, the power it (can) put out in delta is a little scary.  I have to come up with a way to control that a little better.  I have a feeling that even running almost fully furled it's going to push 70-80 amps, which is a little too much for AWG 14 wire.  So I'm continuing to fiddle with ideas to brake it with the stator long before it ever gets there.  Fab's air brake would be sort of nice, but I got no air compressor at the lake where I'm running this turbine, and I don't want it to sound like an 18-wheeler pulling up to the stop sign every time the wind blows either.
--
Chris

[Beaufort]

There is no reason why machines can't furl and furl reliably in this stall mode but you have to make them do so. If you accept this then you can forget all this trickery and the worries about heating burn out braking and all the other stuff but ad Dave b said you have to be very modest with your expectations and accept a miserable power out and expect to be fully furled by 25mph or less.

This is a good summary of the situation for many of these machines, and I have a hard time accepting it.  With typical furling, the machine will not stop the instant the wire rating is exceeded or the potting material temperature is exceeded.  So instead, we must accept furling to gradually "round off" the top end of your power curve and sometimes that begins happening at a very useful wind speed (not gale-force winds at all).  Granted side-furling is relatively simple and passive compared to other methods, but it still carries risks as we've all read here over the years.  So this original idea was to have a simple, cheap way to directly control a wind turbine without accepting the power limitations of a conservative furling setup.  I suppose another assumption here is that this would not work well for high wind locations since it would be braking all the time, getting no power at all above the top end limit.

I think the bottom line is that this approach 'may' work for a narrow set of conditions for smaller machines.

[ChrisOlson]

So instead, we must accept furling to gradually "round off" the top end of your power curve

That pretty much sums it up.  Everybody I've seen here that defends the method of turning the rotor out of the wind to control power output points out that Bergey has done it for years and it works.  But they seem to forget that a Bergey Excel's rated wind speed is 31 mph, it doesn't start furling until the wind is blowing 36 mph, and it doesn't go into full furl until 44 mph.  And they also seem to forget that a Bergey Excel will run facing dead into the wind with a blown generator at 100 mph and survive it.  If you buy the optional blades for it it'll run in free-spin to 150 mph wind speed and survive it.

Of course, the Excel has a real generator in it that can handle what Bergey calls in their operator's manual "Occasional Higher Peak Outputs", and it has pultruded PowerFlex fiberglass blades - not wood.

So only part of that system got copied.  The rest of it - the most important parts of it - got bypassed in the homebrew business.  The generators in these homebrew turbines are made of materials that don't dissipate heat very well and actually insulate the coils to save the heat in the stator, they use primarily wood blades that will fly apart in free-spin at anything over 60 mph, and then the assumption is made that simply turning it out of the wind is going to keep it in stall in high winds.

It's no wonder people burn their homebrew turbines up all the time.  And they keep getting bigger - 17's and 20's - and the bigger they get, the more dangerous they are.  I know that I wouldn't want to be anywhere in the vicinity of a 17 or 20 foot homebrew turbine that only relies on turning the rotor out of the wind to control it in a thunderstorm with gusts over 80 mph.  And we get one or two of those storms here in the Midwest every year.

If you're flying a machine that big and don't have a Fabricator-style air brake, or similar on it, to make sure it's parked and stays parked in strong winds, you're nuts.
--
Chris

[ChrisOlson]

And they also seem to forget that a Bergey Excel will run facing dead into the wind with a blown generator at 100 mph and survive it.  If you buy the optional blades for it it'll run in free-spin to 150 mph wind speed and survive it.

I can't find what I was looking for, but I had a pamphlet or similar from when the NREL tested the Bergey Excel.  Going from memory, when the Excel goes into its unloaded mode (inverter shuts down) the highest observed rotor speed was something like 600 rpm and it never went over that due to the PowerFlex blades limiting the top rotation speed, even though the wind speed increased to 100 mph.

Even so, 600 rpm on a 23 foot rotor is singin'.  I wonder how many wood blades will take that?
--
Chris

[BrianSmith]

Well here's my 2 bit idea...

Make a circuit that can measure the current the wind gen is delivering to the batteries / charger.  When the current exceeds a high current setpoint , the circuit opens the connection between the rectifier and the battery / charger like a breaker, and closes a dump load across the wind generator.  The dump load would need to be chosen carefully to provide a suitable current load that would keep the wind gen from over revving (ideally stopping it) without allowing too much current to damage the stator. 

The other thing is if you can measure the current fast enough, you can probably make a nifty RPM meter as well.... 

Could this work?
 

[tanner0441]

Hi

I have a Chinese controller on my unit does just that..............  Battery voltage goes up above set point, contactor goes over and connects the alt into a big encased resistor, Ghurd has a device that is similar, I think they are called charge controllers. As for RPM if you are producing AC then all you have to do is measure the frequency  of the pulses across any two phases and you can work it out or display it on an Auto DVM with RPM on the scale.

I have a Snap On meter I set to 6 Cyl 2 Str and read the speed of my prop from the meter.

Brian

[SparWeb]

Even so, 600 rpm on a 23 foot rotor is singin'.  I wonder how many wood blades will take that?
--
Chris

Just fine, Chris.  Provided they were designed for it, of course.  (Just like they would on an airplane, huh? )

[Ungrounded Lightning Rod]

Re: Delta-star switch.   A jerry-star switch:
 - does the same voltage/rpm/load shift,
 - without the circulating current losses of delta,
 - uses two single-throw poles rather than three poles with at least two double-throw with a coil's output voltage across the opposite poles,
 - no switch failure modes that short a coil.

Downside: extra diodes.

[ChrisOlson]

Re: Delta-star switch.   A jerry-star switch:
 - does the same voltage/rpm/load shift,
 - without the circulating current losses of delta,
 - uses two single-throw poles rather than three poles with at least two double-throw with a coil's output voltage across the opposite poles,
 - no switch failure modes that short a coil.

Downside: extra diodes.

I've played pretty extensively with all three configurations and have found the circulating current losses of delta are overrated.  The problems people have with delta stem from sloppy design and construction of the generator.  If you pay attention to winding proper coils (i.e. not oval shaped with rectangular mags, giving you a square sine wave, or other such things I've seen in homebrew designs) and make sure the generator is perfectly symmetrical, there is no circulating losses in delta.  Comparing delta to IRP (jerrry rig) on a generator that was built with attention to detail yields no advantages for either.  If you have a sloppy built generator IRP will perform ever so slightly better than delta, but not much.

Regardless, IRP requires two wires per phase be run from the generator to the rectifiers.  A delta/star system requires two wires per phase to the switching system, and one wire per phase thereafter.

I've run both IRP and and delta machines and there's pros and cons either way.  In the end it's a wash, in my experimentation, as to which you use.  But the important thing is that you do.  It appears to me that hardwiring in star configuration has been accepted as the "norm" in the homebrew world to cover for poorly designed and built generators.  The result is hot running, inefficient generators at high outputs.  And I don't care who wants to defend using star - I'll take another of that same generator rewired delta, put it up against the star wired unit and put the smoke test to 'em in high winds and I can pretty accurately predict which one is going to go up in smoke.
--
Chris

[Flux]

For change over during running there is probably not a lot to choose between delta and IRP, the IRP switching is easier but you have the lead issue unless the rectifier is up the tower ( I wouldn't want the switching up there either from my attempts)

The difference between delta and IRP is greater if you use that connection for low wind. If you have very good waveform then there may be little difference but with IRP the harmonics can't circulate below cut in so you get the same start up results as for star. Even with IRP the harmonics circulates soon as the rectifier conducts properly but you may be a bit better off in the early cut in period where the losses count more.

On the last point the criticism is not really of star but of matching the whole speed range with a single connection, if you wind for the same cut in speed with delta or IRP you will still have the heat problem.

Apart from the effect on start up there is no real point in worrying about a bit of circulating harmonic loss in delta when you have adopted a concept that needs an alternator of under 50% efficiency to get the prop to work even remotely on its power curve.

Flux

[ChrisOlson]

On the last point the criticism is not really of star but of matching the whole speed range with a single connection, if you wind for the same cut in speed with delta or IRP you will still have the heat problem.

Hi Flux,

At first glance this would appear to be the case but it isn't.  When I built my first generator designed for delta, the star version of that generator was wound with 44 turns of AWG 14 wire.  The delta version of it was wound with 77 turns of AWG 15.  Both stators cut in at the same speed with the same air gap.  The star stator measured .60 ohms, the delta one measured .27 ohms.

I tested both stators with my hydraulic motor setup, loading them with my Sun AVR's carbon pile.  I use a chart from the manufacturer for my hydraulic motor's output that shows it's shaft power based on pressure and flow, use the measured output on the AVR and calculate the difference in heat losses in the stator.  The star generator came in at 52% efficient, the delta was 64% efficient.  The delta generator ran cooler, even for 10 minutes 5% over the rating of the wire, than the star one did at the max rating of the wire.  I measured the stator temp with my infrared pyrometer that I use on diesel engines to check exhaust port temperature.

That's when I decided delta is definitely the way to go.

To build a successful delta generator you have to pay attention to two things - make sure on any coil that the N pole mag and S pole mag wipe their respective coil legs at precisely the same time.  And make sure that the coil legs are perfectly parallel with the magnets - no weird shaped coils where the flux starts wiping part of the coil leg while the rest hasn't been touched yet.  And if you want it to run smooth you can't have any more than half the width of the magnets as space between adjacent coil legs.  This requires smaller diameter generators than what most people build.

Pay attention to those details and you can build a delta generator that cuts in just as nicely as a star unit, runs cooler, and puts out more power to boot.  If you can't build a precise generator then you're better off to stick with star because it'll "drag" pre cut-in and won't come up to speed like it should.  But if you have the resources to calculate your coil spacings, shape and generator layout, and build it as precise as your calculations, there's some definite advantages to the delta unit.
--
Chris

[Flux]

I don't want to get bogged down with this argument but this is not a true comparison.

Your 44 to 77 turns is close to the root 3 value so i agree your cut in speed will be very close.

There is something wrong with the resistances, if you take #15 as the delta wire then  you should be able in theory to use a wire with 1.73 times the csa for star. I don't have American wire tables and I can't be bothered to convert it all to metric but I think you will find that the equivalent star wire would be thicker than #14. Things are complicated by stacking factor and you may have to use 2 strands in hand to get the equivalent size copper in for the star case.

If  you reconnect from star to delta the resistance drops by a factor of 3 for the same winding. If the wire sizes are equivalent then the resistance for a given cut in speed ought to be the same.

Obviously if your star winding effectively has higher resistance it will heat more so on the basis of your wire sizes then I agree the star version will be poor.

I find the thick wires to be tricky to wind and they waste a lot of space with poor stacking factor, in this respect delta should have some advantage with the thinner wire and better space factor if you change nothing else. It seems very obvious that the higher voltage machines with thinner wire do come out a lot better and we can't blame it all on diode drop.

Flux

[Beaufort]

To build a successful delta generator you have to pay attention to two things - make sure on any coil that the N pole mag and S pole mag wipe their respective coil legs at precisely the same time.  And make sure that the coil legs are perfectly parallel with the magnets - no weird shaped coils where the flux starts wiping part of the coil leg while the rest hasn't been touched yet.  And if you want it to run smooth you can't have any more than half the width of the magnets as space between adjacent coil legs.  This requires smaller diameter generators than what most people build.
--

These are good tidbits to follow, did you do some experiments to come up with them?  The only thing that makes sense for me is to build it as symmetrical as possible, given the rotating phasing of Delta.  The other parts about coil shape....I don't know about those but I guess the idea is to keep the waveform good.  I've been working with Delta setups for the past 9 months and have been confounded by how sensitive they can be.  I'd get some results with one setup and then try another and get something completely different.  The payoff is great if you can get the input power curve as you've done, and then figure out where it crosses with Star to give the best results.  I don't see many homebuilders measuring what they build, so making a "good" Delta machine is probably Level 2 stuff (I'm still at Level 1..

[ChrisOlson]

There is something wrong with the resistances, if you take #15 as the delta wire then  you should be able in theory to use a wire with 1.73 times the csa for star. I don't have American wire tables and I can't be bothered to convert it all to metric but I think you will find that the equivalent star wire would be thicker than #14.

Yes, you are correct here.  This generator was a hot runner to begin with because it should've been wound with AWG 13.  I placed a max continuous rating on it of 840 watts when I built it (24 volt system).

The delta unit was closer to "ideal" and had a rating of 890 watts continuous with smaller wire.  I ran that delta generator at 940 watts for 10 minutes without any significant temperature rise in the stator, and still within acceptable limits.  Attempting to run the star unit at anything over 850 watts would get it very hot, very fast.

Like you, I don't want to get bogged down here, but a generator designed for delta configuration is more efficient, runs cooler, can use more turns of smaller wire, for the same application.  The stator heating, based on my testing, is less with delta.  That difference is obvious in the difference noted in efficiency of the two units I compared.  That difference in efficiency is converted to extra heat in the star-wired generator.

YMMV but I spent three evenings testing these generators and that's the data I came up with.  Since then I have replaced two of my previous generators with my latest design delta units (one was a IRP gen, the other a star gen) on my turbines and have gotten the same results on the tower that I got in the shop.
--
Chris

[ChrisOlson]

These are good tidbits to follow, did you do some experiments to come up with them?

Lots of experimenting.  I'm not one to accept things at face value.  When I saw the homebrew community primarily using star-wired units, but I look around me and the vast majority of commercial three-phase generators and motors are delta-wound, I put 2 and 2 together added 12 and carried 5 and decided something didn't add up here.

To make a long story short, I've sent a couple of the forum members here some CAD drawings of my latest delta generator layout and if you're interested in building one feel free to email me and I'll send a copy off to you.  I'm too busy to email a copy to everybody who just wants to look at it because it takes too much time to explain it.  So don't bother unless you're serious about building one because we're in the peak of fishing season here and it might be several weeks before I get back to you.
--
Chris

[Ungrounded Lightning Rod]

I've played pretty extensively with all three configurations and have found the circulating current losses of delta are overrated.  The problems people have with delta stem from sloppy design and construction of the generator.  If you pay attention to winding proper coils (i.e. not oval shaped with rectangular mags, giving you a square sine wave, or other such things I've seen in homebrew designs) and make sure the generator is perfectly symmetrical, there is no circulating losses in delta.  Comparing delta to IRP (jerrry rig) on a generator that was built with attention to detail yields no advantages for either.  If you have a sloppy built generator IRP will perform ever so slightly better than delta, but not much.

I agree that if the waveform is a nice sine (or a number of others where the sum is always zero) the circulating currents in Delta are small.  But there are two problems with just saying "so build it to make a good sine wave and do it accurately.":

 1) For battery charging a rectangle wave (for Jerry/IRP) or a rectangle or trapezoid (for Y) is even better than a sine.

 2) Part of the POINT of these designs is that accuracy is not required, which makes them easy to construct.  Tolerances can be wildly off.  Coils and poles can be whatever shape is convenient for available magnets and coil shapes optimized to minimize resistance rather than waveform.  Phases don't have to be at accurate angles.  And so on.

Jerry/IRP preserves these design and construction advantages.  The main cost is sacrificing the

Regardless, IRP requires two wires per phase be run from the generator to the rectifiers.  A delta/star system requires two wires per phase to the switching system, and one wire per phase thereafter.

Sure.  But the relay location is probably also a good spot for the rectifiers, so that doesn't seem like a big deal.

On the other hand a Jerry/IRP - Y switch has an additional advantage over a delta-Y:  You easily add one or more intermediate steps where the Y connection is resistive (or reactive) to achieve a three (or more) speed "transmission and clutch" during the changeover.  Dump more of the energy from slowing the blades outside the alternator's coils.

[ChrisOlson]

I agree that if the waveform is a nice sine (or a number of others where the sum is always zero) the circulating currents in Delta are small.  But there are two problems with just saying "so build it to make a good sine wave and do it accurately."

Well, the thing is, what circulating currents in delta?  I asked the fellow (who's been in the business for over 40 years) at the electric motor/generator shop where I get my magnet wire about these circulating currents in a delta generator.  He looked at me over the top of his bifocals and said, "What?  Circulating what?"

Then he said, and these are pretty much his exact words, "Some dimwit looked at a picture of the classic delta connection and decided that since it's all connected in a circle there must be circulating currents in it."

Then he pulled out a piece of paper and scribbled down the KVL calcs as to why there's not.  I didn't know what KVL was but I found a little ditty on it on the internet when I got home.  If you scroll down about 2/3's of the way on this page where it says At first glance it seems as though three voltage sources like this would create a short-circuit, electrons flowing around the triangle with nothing but the internal impedance of the windings to hold them back. Due to the phase angles of these three voltage sources, however, this is not the case.
http://www.allaboutcircuits.com/vol_2/chpt_10/5.html

And this is pretty much what I've found too - no "drag" pre cut-in, I've tried measuring these "circulating currents" with my milliamp meter - nothing.  Only once, when I tested an older generator that I had built that had the phase timing off did I get the milliamp meter to detect 1 or 2 milliamps of current flowing in an unloaded delta wired generator.  And that's how I arrived at the conclusion that the whole thing is a myth.
--
Chris

[Beaufort]

Well, the thing is, what circulating currents in delta?

Hmmm....WWFS?  (What Would Flux Say?)

So if we wanted to put this Delta circulating currents thing to bed, what kind of experiment could be done to prove or disprove circulating currents?  Or, did you already perform the experiments to show this?  Ideally, it should be something that others could follow in order to build a machine that will perform well in Delta (make sure all coil tails are +/-0.5", coil positions are all +/-0.125" from theoretical, etc.).  I feel the payoff is worth the effort to address this...just look at the increased slope of the theoretical Delta curve compared to Star, with very low resistance and wasted heat.  I remain skeptical from my work in Delta...there is an "X" factor in all configurations that makes your efficiency not match theoretical by a large degree at the top end.  It would be great to have some objective stuff come out because it's very simple to make a Star/Delta switch to help these machines perform much better and safer.   It certainly doesn't merit some of the opinions on Delta I've read here.....things like "just don't use Delta, it's terrible".   

[DanB]

Chris - I find myself arguing with you more and more lately (don't take offense) 

It's not been my experience.  I know lots of axial flux machines that furl and behave well even in extremely high winds and shutting down by even shorting 1 phase on most of these systems has never been a problem (even in high winds while the machine is producing lots of current).

Not just Bergey - a *lot* of good machines rely on furling.  SW windpower (that may or may not be good), fortis, hummingbird (they build a fairly bomb proof 10kW turbine with a good track record that relies on furling only - it has no brake and you cannot fold up the tail with a winch), ARE...  and more, and most old water pumpers.  I really believe that if you get it right it can work just fine and be quite simple.  However...  there is surely room for improved efficiency and life can be as complicated as you like but it does seem to me that the more complex stuff becomes the more stuff fails.

My own 20' turbine ran reliably for 8 months right after I put it up and furled beautifully and would sustain output over 4kw with occasional peaks of over 9 - it did fail after that in freak winds that nearby were recorded to be over 100mph (the blades struck the tower).  I've been more gentle with it since and have shut it down in very high winds.  Last fall I put some old Jake blades on it and now it's very much stall regulated and I've not bothered to shut it down at all since August.

Also - I may be wrong, but I am pretty sure the Bergy 10kW machine is rated at 11 meters/second (just under 25mph) - not 31 as you have said.

[ChrisOlson]

So if we wanted to put this Delta circulating currents thing to bed, what kind of experiment could be done to prove or disprove circulating currents?  Or, did you already perform the experiments to show this? 

I've both experimented with it (quite a bit, in fact) and asked experts in the field.  The experts tell me it's a myth passed on down thru the generations by people who don't know what they're talking about.  I DID get an amp reading on one generator that I tested wired up in delta.  That was one I built a long time ago and it had 18 coils and 12 poles with phase timing all screwed up because of it.  I ran it in IRP.  But when I started testing this circulating current theory I dragged that old stator out and tested it and I got 2 milliamps reading turning the generator by hand on the delta connecting wires with no load on the generator.  But if I hooked that generator up to the scope I had two phases at 180 degrees and then in the next 180 degrees I got the other phase at peak.  It was all screwed up.

But that's the only one I find any circulating currents in with no load on the generator and that wasn't a true three-phase with 120 degree phase separation.
--
Chris

[ChrisOlson]

Also - I may be wrong, but I am pretty sure the Bergy 10kW machine is rated at 11 meters/second (just under 25mph) - not 31 as you have said.

Maybe they changed it or something but I got a propaganda pamphlet for the Bergey Excel-S with the PowerSync II inverter laying right in front of me on my desk and it says reference rated power is 10 kW @ 31 mph with a furling speed of 36 mph.  It says the AWEA rated power is 8.2 kW @ 25 mph.  It says peak power is 12.5 kW but it doesn't give a wind speed.  It says cut in speed is 5 mph.  Cut out speed: NONE.

What rating do you want to use?
--
Chris