Okay Bob, where to start.
Now, even your iron core alts with little metal core, has very significant reaction.
A simple procedure to test this out for your self is as follows.
Grab any small car alternator, rip out any internal regulator it has. If it is older and has no internal regulator, it will likely have one brush earthed to chassis. The field terminal will go to the other brush.
Mount to a motor with a step up pulley. Place the armature (rotor) to b+ place a light globe say 5 watts betweem b+ and the field, so that you supply about .5A to the rotor and run it. (b-through light globe to field if it was internal regulator, and check the + brush does in fact go to b+, otherwise it needs a prod (usually done with the ign light on the dash) to get the stator generating, and then the small diodes will take over the b+ supply to the b+ brush.
Unloaded, it will have a voltage curve following the rpm. If you load it with a flattish battery, it will likely pull about 5A or so at idle.
Now if it was a non reactive device, when you doubled the rpm from here it would pull the amps up to try to follow the twice emf that the stator would have had unloaded.....but no..... what you will find is the curent remains steady at about where you started. Rev it up to 6000 rpm, and the current will still be the same....... why?
Now, if it was just the ohms in the stator that caused voltage drop, then the stator would heat up, (EMF in stator -battery V X currrent flowing in stator would be losses) and the current will increase with rpm, but it does not happen.
This is because the fairly weak field set up by the rotor at only .5A will have it's field repelled by the back MMF of the stator. As the stator has current running through it (say 5A) it will create a field in direct opposition to the rotor field. At about 5A, it will stop any more flux from the rotor getting into the stator windings, and no matter how fast you spin it, it will generate no more. This is the runaway we talk of in iron core conversions.
Now if we keep the current at .5A in the rotor, and place a step down tranny on the stator leads before the diodes, we can do interesting things.
If it is a 2:1 step down, we will need to increase rpm to double whatever it started to generate before to charge the batteries..... so it's EMF is now twice. The transformer output will be now at your battery voltage again, and charge at twice the original current. So now from the same stator, we have twice the power. As we increase speed, it will still bog down at about 5A in the rotor..(probably higher if the small diodes are driving the rotor voltage so try to power the rotor from the battery exclusively, or you will have probably more flux in the rotor.)
So we can see a transformer can change the point at which the power out is crippled by the reactance, but the stator current still limits at 5A.
If we increase the rotor current to 5A (about max for a small car alt, our reactance limit will be about 30-40 odd amps, more for a bigger alt. It will still current limit, and so stop it from exploding when we rev it to 18000 rpm on a flat battery or shredding the belts.....(inbuilt protection). Yes reactance protects the alternator from blowing itself and it's diodes and belts into next week.
If we now put 24v direct into the rotor, and 10A, we may find we do little better. Depends on the amount of steel in the claws, as it was probably saturated with the 5amp x turns (ampere turns) at 14v, so for all the extra rotor current we may do little better. If the claws were not saturated before, we will do better until the new current limit point is arrived at, as the claws will have a higher flux than before.
It is the stator field trying to push or fringe the rotor field away that is doing this. A transformer just hides this until the same current that was doing it before turns up....... as well as lowering the I^2 R losses. These loses will be the same when the new current limit turns up, but we now have twice the power.... I remains the same, but V is now twice.
It also happens in your big generator heads. Without this, you'd likely bust a shaft if you shorted the big alternators. It is the reactance that saves us.
To put it another way, the torque required to turn a shaft on a loaded alternator is mostly this reactance. It is the stator trying to repel the rotor as the current in the stator increases. You can feel it, your motor can feel it, and you can now see that if the rotor current remains at max, the stator will eventually push back hard enough that it reaches stalemate.... current limit.
You may see this listed as the synchronous impedance of the system.
Now onwards.
"in testing a prestolite/leece neville 110-555jho 12volt 160amp large frame j180 mount alternator works out to be about 52-54% efficient as built.
the same alternator which has external taps directly to the stator, which btw is delta wound when connected to three of the apc transformers will achieve well into the upper half of the 60's % efficiency. this is pretty good when you consider the transformer losses which are factored into the test."
It is very important to try to get to the guts of what is happening. Is the efficiency due to less torque required to turn the alternator at higher or slightly higher rpm? If so is is more due to the gearbox effect of the transformer as discussed above.
If you let the voltage rise (transformer steps down), then more power for less torque is the result.... but...times RPM...nothings for nothing.... but our stator current is less for the same output current.... So our loses are now stator EMF-effective transformer primary EMF X current. As the current is less it stands to reason the voltage drop is less in the stator, so our losses are less.........it works. We would find the upper limit of the alt will increase as well (from above as well). Note, if we were at reactance limit at X rpm, and introduced the transformer, we may well get twice the power with no change of rpm, but a torque increase...... becuse the current will still be the same in the stator, but the voltage will rise
because of the tranny.
It is important to not only test, but know what those results actually represent. This is not always obvious, and what it was that really changed them, versus what you thought had changed them... not so easy really if some angle you didn't know existed was doing the changing, but some other explanation sort of fitted.
"what i am unclear on, and my bet would be that it is the case, is...
the air core alternator ought to be able to use the transformer scheme to the same result?
basically as you say the transformer is nothing more than an electrical gear set, much like a transmission attached to an engine. multi tap equates to a multispeed transmission etc.
so my position is this, if we can get a gain in output and efficiency by using a transformer connected to an alternator designed and optimized to use the transformer then we have made a significant move in the right direction without the problems associated with buck converter"
Okay lets look at this:
The air cores will have advantage in exactly the same way with the E=IR part, but they get no added benefit from the reactance change. This is not a game killer at all, just a nice bonus (large) for heavily reactive machines.
The transformer can be used effectively with air core once..and once only for long line runs perhaps and high towers etc.
It is a single gear change. It will be stuck in first or second or third. This leads to a ......well skip the tranny, just wind the stator differently..... exactly the same result.... no difference.... except no tranny loses from core etc. (line loss scenario excepted)
So no, you must let go of you motor alternator experiments. Forget anything you learnt from them.... this is entirely different, and you cannot/must not try to think like an engineer, where if you optimise each part, then it will end well............it wont and you will end up building a drag mill with a gearbox instead.... and then give up..
With wind power everything relies on everything else. A change anywhere will upset the whole thing. If you fiddle with the blade you need to fiddle with the furling and then fiddle with the alternator gaps /characteristics/resistance and then ...... the damn wind changes speed...... and everything changes as square functions, cubic functions, and linear functions, and stator losses go up as a square of the current.... yes it's a dogs breakfast for sure.
The rotor (turbine) is trying to match a wildly varying dynamic impedance, to a rock solid battery impedance. If the prop ran at 1000 rpm all day, we can pick the best transformer, and be done with it. The moment the wind changes, we get the cubed verses squared verses linear rpm problem, and all the dynamics change drastically. The power curve is cubic, the alternator curve is squared, the voltage curve is linear, and we need to match the three of them. Can you see why we need a DYNAMIC gearbox. A one gear gearbox gives us 1 good speed.... we have that already
Unless we can change transformers mid stream we achieve nothing. If we multitap, and have a way to access them dynamically on the run, then it's a whole new world..... can you see that difference now?
"all that is needed after the transformer set is a controlled rectifier, which because we are going for DC (and not restructuring AC for a 3 phase drive) it does not require separate and complicated drivers. a single driver can fire all elements via some sort of pwm scheme.
what would then qualify the end result is not the lack of a buck or boost converter, but rather the control unit/micro controller and its code, along with the necessary control handles and feedback.
and of course none of this works without good programming."
No, whats needed after the transformer is not simple a PWM and code. The code is almost irrelevant at this point.
WE NEED an AUTOMATIC TRANSMISSION, not a one cog gear box.
If we stick to the transformer route, we need multi tap transformer, and a simple rpm to tap change algorithm..... or analogue chips to do same... simple enough.
If we go PWM, we need a fully fledged buck converter or bridge or two switch forward converter, or half bridge or pushpull..... but we need a converter, and a simple algorithm or analogue chip to drive it.
"the complexity goes way up when you run at higher hz, design becomes supercritical, ground planes and connections must be carefully designed, unless you want the FCC to fine the living crap out of you."
The complexity does not really go high until after 50 khz. In the 25khz range, just look at your computer power supplies.... notice they are all different. There must be a hundred ways to squash those 300watt converters into that same size box.
If you multi drive as Opera says, the problems become quite small.
However, look at this:
Now those pictures are from a fair size 1.5kw pwm unit. Note the careful ground planes, the careful layout, the special printed circuit boards etc...
Notice there are no electrolytics to fail
It's bigger brother looks identical but has another output transformer, and bigger input filter toroid, and still only 2 fets on the output, and natural cooling for 60A@60V.
They run all night every night charging forklifts, have C ticks (fcc stuff) and all relevant govt nonsense. they are approved for warehouses and are a first choice. They last forever, and I have never seen one fail. With no caps, they should never fail.
So all the general nonsense I hear about black magic converter principals are off the table. All other higher and lower power pwms I have seen all rely on stable inputs via caps. This style actually follows the rectified DC waves, and so has no power factor problems either. A micro is not fast enough to do this. It needs to be real time current mode pushpull chip... or 45 cents worth.
It has about 50 bucks of parts in it, so no Bob, I don't agree with what you stand by, but now you know why. The algorithm to drive this for stepdown is little different to a buck, just different ways of doing the same thing.
The driving "code" is simple jump tables for the coders, or rpm chip for this sort of thing.
True fuzzy logic has been shown to not work for windmills by Gwatpe many many moons ago.... so don't bother with true MPPT, no-one has cracked it, and in truth Chris Olsen has shown that it is desirable to overide the MPPT sometimes anyway when you need some whoa mule medicine.
"there is a large emphasis on continuing to use the lundell claw pole, alternator because of the huge manufacturing bases. the automotive application because of its variable speed prime mover relates better to wind power than does a fixed speed design. this variable speed requirement has led many to adopt and develop the mppt to good result.
"
I would ask them if they really think prime movers (motors variable) relate to wind..... and apart from varying speed.... how?? They are dreaming all day long. Not even remote cousins. Reactance gives them the backstop, so that anything will work. Even driving the field directly from the battery (battery won't be happy for long though)
"
it is my belief that a mppt controller that uses buck converter technology and is capable of delivering ~3 kwatt or more is going to be expensive and difficult to get right. not impossible, just difficult."
It would be cheap......Look at those photo's above and point out the expensive bits?
I don't see any.
Now they are not bucks, they are off line (safe) units.
Thats why I don't see the problems you have seem to see...... they are just not there. It takes time and I don't value mine, I just don't even need the mills any more, never mind an MPPT..... plus I have two 3.6kw MPPT units in the shed. Even they only cost 400 dollars freighted.... and they are approved grid tie.
I think I have a good handle on this, just haven't bothered to build a buck, and for this i would probably go the pushpull off line route, and wire for 200-500vac. Actually that unit pictured would do that with little rework.
Hope I have cleared some of the misunderstanding Bob G.
Happy to answer any question on any aspect if you think I'm wrong. We will go at it until we agree on something.
Research what I have said Re reactance, do the tests... it is how it is.... it is how it works, and transformers are very forgiving of frequency ..... once you establish the projected power and frequency... then it all pans out.
Tapped transformers will work as well or better than buck, but will be heaps heavier.... but more reliable to boot.
Transformers alone will make little difference than a decent rewind..... unless you have steel in the mix. Then you can push it harder.
...................oztules
