controlling field current with frequency to voltage?

[Tess]

am attempting to improve efficiency of car alternator for wind, i know they are not the best but suppose you had nothing else....

A speedometer converts the rpm to eddy current to turn the speedo dial, could this be turned into a current for the field current in the alternator, like having a mini alternator for the excitation? If not how about the newer digital ones? I want to achieve a low field current at low wind speeds but then to increase proportionally as the wind picks up, making the best of a bad gen for wind.

Also would replacing the voltage regulator with one from a lower rpm vehicle (tractor, 4x4) be good or would this just limit the field current at all wind speeds and you would lose out if the wind was blowing hell for leather??

Am trying to come up with scrap/cheap/pragmatic approaches to building a turbine from old car.....any advice, experience welcome.

[bob g]

well that blows big time

i wrote up a very detailed response to your question, several paragraphs

and posted it,, only to later find it didn't post for some reason!

boy that is frustrating!

anyway, i don't have the time or energy to rewrite and repost the complete work, but

here is the long and short of it.

1. using automotive alternators is generally a bad idea for various reasons

   
   

2. the use of units modified with pm magnets such as typical of at least two vendors
   
   
   

is a waste of money and time, unless you live in a very high wind area (greater than

25 mph with some consistency)

3. if you want to work with automotive units, step away from working with small cheap units, and toward large frame units typical of hd trucks

   
   

4. be able to engineer the needed components to put it all together.

   
   

5. realize that it is a bold step working against what most folks have already come to accept, and also realize it is far easier to adopt a popular design that has proven to not only work but make good power.

   
   
   

bob g

[Flux]

Basically the things are too small to do what you need properly. They are intended to run at high speed. The cut in speed is already far too high for wind so you need all the excitation you can squeeze into the field to get cut in down. I suspect that in most cases you could get away with 10v excitation in low winds and this is not going to save you a lot in terms of lost power in the field.

Further up the range you are still needing more speed than you can reasonably get so again the requirement is maximum field. The end result is that you can never gain by using much less than full field. using a pilot exciter to supply the field does nothing more than increase the inefficiency. If you must control field the way is with a switching regulator.

Replacing the wound field with a permanent magnet gains you about 40W all the way up the range but you still have a winding that is far too fast. Rewinding with more turns reduces the rating even more but does get you something at least for small fast blades.

If you must chase the dead duck then my suggestion is to try exciting the field at about half current to cut the field loss to 1/4 and try a mixed bridge diode/mosfet rectifier and switch the mosfets with a pwm driver. Complicated electronics but I suspect you will gain quite a few watts and turn a disaster into something much better and only about useless. My advice if you have nothing else don't bother unless you have average winds above 15mph. In very high winds you can obviously get something but way worse than an old car dynamo.

Flux

[joestue]

"I want to achieve a low field current at low wind speeds but then to increase proportionally as the wind picks up, making the best of a bad gen for wind."

This is the exact opposite of what you want to do.

Iron loss is sinsignificant below 30 hz, when the laminations are less than 15 mills thick, and when the magnets have laminated pole pieces. Most car alternators, their laminations are 50 mills thick. and the pole pieces are solid blocks of cast iron.

That's the primary reason why they are inefficient, the second reason is that they need to be able to produce 12 volts at 1500 RPM, if you were to run full field at 6000 rpm and buck the 60 volts down to 12, they would be nearly 70% efficient like any other small motor/generator, and generate around 2-3 times as much power.

My advice if you want to experiment is take a three phase induction motor and make your own rotor up a 2 or 4 pole rotor, laminated, and bury the neodynium magnets in the rotor, then add a field coil and experiment with field weakening at high rpm. (by high rpm i mean 6000 rpm.)

if under that 0-500 rpm wind turbine direct drive, then there is no point, either buck the high voltage or boost the low end, 90% of your losses are copper, or poor magnet design.

[Tess]

ok so...i was under the impression that with a high field current the resistance of the induced emf produced a braking force on the rotor making the cut in higher, therefore needing a lower field current to get the thing spinning and then as the rpm picks up increasing the field would increase the induced emf. By using a buck chopper at the other end to charge at 12V.

My thinking was a variable resistor for the field current but one controlled by the rpm somehow

I understand that this is a dead duck but this is for my dissertation and i want to make a scrap car into something useful. I have been through the mosfet idea with my old electronics teacher but is there anything in modern cars that can do this without complicated electronics.

also there is a sensing wire for the warning lamp could this not be used to sense a voltage difference and adjust the field current accordingly? Another electronics geek suggested the following..

By  controlling  the field current or the generator speed, you will be able to control the generator voltage output. The easiest way as we previously discussed is to control the field current by using a variable resistor if your generator speed kept constant. You can also change the speed and fixed it to different constant  value and control the generator voltage by changing the resistance. This mwthod is known as a step change method. However, you can contineously control the field current by a voltage regulator based on a closed loop control :Proportional and Integral control .

I know it is hard for people to understand why i want to do this but i like to utilise bits of 'rubbish'. This is my project whether i like it or not now and my options so far are

to replace voltage regulator with plain resistors (between 1-2 ohms)

increase resistance of voltage regulator (from alternator secrets)

replace voltage reg with tractor regulator (from word of mouth)

but i'd like to something more.....

I will be bench testing on a pillar drill and the aim is to improve the performance, if only a little, and to explore the possibilities if there were no other choice. Even though i am studying renewable energy i am a mature student and new to electronics so please explain things in laymens terms! thank you.

Oh also wondering about 'jerry rigging' I understand how star and delta affect the power output but how would jerry??

[Opera House]

If you have had some electronics, replace resistors with a switching regulator.  The field is a great power waster, a resistor is just wasting more power.  Your farm tractor regulator idea has absolutely no technical merit.

[scoraigwind]

bob wrote: "The use of units modified with pm magnets such as typical of at least two vendors is a waste of money and time, unless you live in a very high wind area (greater than 25 mph with some consistency)"

I'd be interested to learn more about this if you have patience to try again, Bob.  I see these alternators advertised with aluminium blades and no furling, and I really wonder what the story is.  Have you tried them?  They seem quite popular and claim to be reliable.  I would expect the output to be quite low due to iron and copper losses.  And it's limited by the alternator at speed.  Any comments?  

What amazes me most is that the metal blades don't crack and fly off (or at least they don't report this happening).  There are several suppliers and it seems like quite a little industry going on, with several suppliers like Tlg windpower, windy nation, hurrican windpower, and wind blue.

[Flux]

If you really want to give this a try and you want to keep away from the booster idea this is the best I can offer you.

Do a bench test on your alternator and plot field current against output volts at fixed speed. You may find that the field will saturate the stator core before you reach full field current. If the graph turns over then work with the field current that gives you near full volts for a modest current. I suspect it is not far below the nominal field current when supplied at 12v but you may get away with something less.

If you supply this field current via a  switching regulator you will use the minimum power possible on the field for a practical cut in speed ( using a resistor will increase losses and defeat you).

Choose a set of blades that will get you to cut in speed in a 10 mph wind and that probably means very fast blades about 5ft to 5ft 6" diameter ( pvc or metal blades won't do it).

Make your regulator take it to full field when you are producing over 5A and accept that this is the best you can do.

Jerry connection will raise your cut in speed even more and make things worse but if you are really desperate you could separate the three phases and rectify each separately, connect a monster capacitor across each rectifier and connect all 3 supplies in series. This will lower your cut in speed but will come at a cost to high wind output but may still be better. The capacitors will have to be 50,000uF plus to have much effect.

You can improve things if you are prepared to rewind with more turns but this is not worth the effort while you are wasting so much power in the field winding. If you change to permanent magnets a rewind will get you something more useful.

Have fun, you will learn a lot and if you start with the assumption that it will be hopeless you may even be pleasantly surprised.

The internal regulator or one from a tractor or a horse will not help in the least so remove any regulator you may have and just keep the rectifier.

Flux

[bob g]

i may have been overly broad with my comments, in that

i am probably overly critical of some of the claims i have seen from some

of these companies.

when you see claims of a kwatt or more from a baby delco altenator modified

with neo's and driven by a set of blades i am automatically suspect.

me and automotive alternators go way, way back, having rewound them for various voltages from 12volts at lower rpm, to as much as 480 volts at 600rpm and everything between, i just don't see them as a viable windpower unit, most especially those anemic little delco's, unless

ones need is realistically somewhere around 50 to maybe 100watts?  then maybe

however, for the amount of money and time to get one up and flying safely the same money could have been spent on a good solar panel and one would likely get far more power with far less work.

now if someone comes up with a wind unit that can provide 4krpm to an alternator

with perhaps 3.5kwatts of mechanical power, i can provide you with alternators that will make nearly 3 kwatts for a 24volt system charging at 28.8vdc and 100amps. and do this without neo magnets.

or one can scale it anyway he wants to, the key here is one needs the higher rpm in order to get into a useful power range and get the efficiency needed to make it worthwhile.

so far i have not seen many diy'er capable of building a transmission of suitable ratio that is relatively high in efficiency, low in maintenance and rugged enough to do the job reliably.  that is so far  

getting back to the little pm modified alternators, one of the sellers of these units was going on about how his unit could make some insane amount of amperage, i don't recall how much because i was put off by his claims.  quick math in my head clearly indicated that one might not get 1/8 the claimed output in reality, and that is just no where near honest in my books.

bob g

[scoraigwind]

thanks Bob,

That all tallies with what I know of car alternators, and I gave up on them in the 1970s for direct drive wind energy.  

I was just wondering if anyone had actually flown one of these little pm conversion alternators with alu blades and if so how long it lasted and was it any good in less than a gale of wind?  The fact that they describe the charge controller as sort of an optional extra idea makes me think they don't produce a lot of serious energy.  They do mostly offer graphs that show current leveling off at about 15 amps over a wide range of rather high speeds.

But how do those metal blades not fly off if there is no furling?  (or maybe they do?)

[(unknown)]

I bought the "windblue" car alt as one of my first wind generator attempts. It has the doughnut neo rotor. Ive ran the cyclone AL blades as well as 12-15 sets of blades I made. The problem I had was hitting enough rpm to make it useful in direct drive with wind. After a friend and I tried every possible way we could think of to get it to charge batteries including jerry rigging stator and multiple stators with different gauge wire I gave up on it. Since then I mounted the same alternator to a gas engine by way of belt drive. Now I have a gas generator I literally had to tame down to put out 30a max at 30v. It makes a great gas generator capable of 10-30a for my 24v battery bank. This is the only use I found for my "windblew" but it works good when you can spin it at the rpm it needs.

Good luck!

Fused

[bj]

   Tess--most of the negative comments on car alts are quite correct.

Messed with them for 2 1/2 years. Not sorry I did, but the biggest value

was in learning.

   In my case, to get anything worthwhile out of it, it took rewinding the stator, replacing the wound rotor core with a big neo disk, and a five foot six blade, blade set. I saw somewhere around some of the advertized outputs, but only in ridiculous winds.  It was however, in my case bullet-proof.

It survived 90 kph winds with no furling.  Along the way, it ate a tower,tail assembly, and a piece of siding from the house, in similar winds.

  My advice then, from having done it, is go ahead, you will learn lots.

  Then, like me, you will want to build something better.

  Just my opinion, and good luck.

[Tess]

Thanks alot for advice, thats basically where i'd got to. It IS a  learning exercise rather than a practical one. I am going to build a mosfet regulator and will let you know how it goes!!

Thanks again

Tess