a few questions relating to alternator efficiency

[bob g]

i tried to interject these questions over in the "coil shape" discussion, but it got lost in the
main discussion, so i am asking here.

i have been following that topic with some interest, and now have a question

(or two or three)

lets take for example an ideal alternator, 100% efficient (i know one does not exist but bear with me)

this alternator will consume 1000watts of mechanical  power to produce 1000 watts of electrical power?

if another alternator is only 50% efficient, then for the same 1000watt mechanical input power it will produce
500watts electrical and 500 watts of heat?  (neglecting windage, etc)

so how does a lower efficient alternator take less power from the rotor and allow it to spin up faster and not stall
as quickly?

in example #1 the 100% efficient alternator it produces 1000 watts for the 1000 watts of input

in example #2 the 50% alternator still produces 1000 watts (500 electrical + 500 heat) for the same 1000 watts of input

so both should require the same input power to produce the same output power, its just that example #1 produces more electrical
power and no heat, and example #2 produces half the electrical power with the other half going to heat.

so i don't see how decreasing efficiency via increased resistance which makes more heat relates to there being a reduced need for power
from the rotor blades allowing them to run up faster?

there has to be something else in play here, and i am missing it?

bob g

[TomW]

bob;

Hopefully this will yield some good discussion.

I asked in one thread essentially "which efficiency..".

Nobody actually responded with clear goals on which.

Power is power, be it heat or electrons shoving work.  Most of us here are seeking electrons to a load so that would be a major factor in any "definition" for  efficiency I would think?

I will shut up now.

Tom

[bob g]

thanks Tom for the interest

i too have been heavily involved with testing and accurately measuring efficiency of alternators
not wind alternators such as built here, but heavy automotive alternators such as the prestolite and
leece neville units.

what i am seeing in testing, at least so far does not agree with much of what is presented here, so
naturally this leads to questions.

i suspect what is reported as altering the efficiency of an alternator by adding resistance, does more to alter
the operating characteristics of the alternator than it does its actual efficiency, but i am very interested in what
the guru's have to say about this.

bob g

ps. in testing i am constantly aware and vigilant in changing only one parameter at a time, so as to be able to determine
with some degree of certainty what actual "cause and effect" each change makes.  this is not always easy to do, and i too
often wonder if when changing several parameters at the same time have more to do with an additive effect rather than a
single parameter,, something like the total being greater than the sum of its parts?

[Flux]

I thought i answered this previously but i can see that it could have got lost in the complicated thread.


If an alternator is 100 % efficient and it is loaded into a battery it will have no internal resistance. A battery has virtually no internal resistance either as far as this discussion goes so as soon as the rectified dc voltage exceeds the battery voltage the current rises to infinity. A tiny increase in speed will give a small increase in volts and this voltage flowing in a circuit of negligible resistance will produce a very high current. The speed is clamped constant and the prop stalls because it's speed cant rise with wind speed.

If you add some resistance somewhere then the alternator will now be able to speed up until the voltage can overcome the ci5rcuit resistance. Part of the power now goes into charging the battery and part into heat ( wherever the resistance happens to be).

The prop can now work more effectively and give more power out. Any current caused by forcing extra volts across the resistance will also charge the battery so although you drop efficiency you actually increase battery current.

You can use your very efficient alternator at one chosen speed to full effect but that is not the desired situation for a prop that has to work over a range of speed.

When you depart from the ideal matching point you have to do something else to keep things running acceptably well.

Fortunately a prop will cover a significant speed range and still work acceptably well. if you ask it to go too fast drag will consume all available power. If you run too slow you increase the angle of attack to the point where it hard stall. This band alone won't let you run efficiently over a range of wind speed from cut in to furling and unless you let the alternator change speed you will have a very limited band.

It's all a compromise and in reality it works quite well but with the ability of air gap alternators to capture energy in the lightest winds the compromise becomes more difficult and you have to trade top end alternator efficiency if you want to keep the very low wind cut in and not hit hard stall at 15mph.

It's tricky to produce cost effective alternators much over 70% efficient over a wide range so for the older machines cutting in at about 10 mph there was little issue. if you drop cut in to 5mph then you really do compromise the top end very drastically.

Those brought up on modern magnets may never have noticed this but some of us coming from wound field machines were excited by the new low end promise but soon found that it raised a new issue that was not even thought of before. it was virtually near impossible to build a wound  field alternator that would stall the blades at the top end.

Resistance matching is far from ideal but it is a means to an end. If you can get rid of the heat then it results in a cheaper alternator, if you can't then you must keep the loss outside the alternator in the line or as an added heater. if you can use the heat then the added heater makes a great deal of sense but to get most of the heat you need to use a big and costly highly efficient alternator.

If you have no use for the heat then keep as much loss in the alternator as it will safely dissipate to cut cost and if it still stalls then save on line cable and put the losses there.

Whatever you do, the best results will be with the highest cut in speed you can tolerate without badly compromising your low wind end of things. If you cut in below 7 mph you will pay the price many times over in high winds but if you only see high wind twice a year it may be better to optimise on a lower cut in. It;s very site specific and also load specific. If you can keep your light s going on a few Ah per day then fine. if you are mainly using wind for power saving then trading 20W in low wind for 500W on a reasonable day may not be the best compromise.

Hope this helps, it is not really as simple as this because the prop curve is so flat in low winds that without some form of variable voltage you will not get the best results even though the alternator efficiency is high at light load. This is way to complex to go into here but I was surprised to find that there was more available in light wind with mppt. The top end is obvious but I thought things were pretty good near cut in.

Flux

[JeffD]

but I was surprised to find that there was more available in light wind with mppt

.  That is what I was aiming for when I switched over to using a buck converter + mppt and was happy to see the results after about a month of data collection.

[bob g]

thanks Flux, what you relate makes sense when you consider the variable and non linear power nature of the wind motor

still trying to get my head around the addition of resistance allowing higher efficiency from the alternator, but from what i gather
it is not so much that the added resistance actually increases the alternator efficiency directly but rather allows the rotor to fly faster
and by doing so spin the alternator faster, which by extension increases the output of the alternator.

i wonder though if this increase in output is nothing more than that, and not actually an increase in alternator efficiency.

got to do some more thinking on this one.

thanks

bob g

[ChrisOlson]

still trying to get my head around the addition of resistance allowing higher efficiency from the alternator

Bob, it's kind of like hydraulics, if you're familiar with that, and consider that the generator is the hydraulic pump and it's being driven by an engine with no governor on it.

It takes very little power to pump pressure (volts) with no flow (amps) and the engine will run at very high speed.  Open the valve so the pump has to start moving oil at the same pressure and it takes more power to drive the pump so the engine bogs down.  Open the valve wide open so there's no resistance to flow and it takes the most power to drive the pump and the engine is working as hard as it can.

Putting resistance in the line is like closing the hydraulic valve on an open center hydraulic system - it reduces the flow so it takes less power to drive the pump and makes the engine speed up.  But it causes the pressure to go up and creates heat because you're trying to force a lot of flow thru a little hole at higher pressure.

In the end, what you get is higher engine speed where the engine is operating in its ideal torque range and can make more power.  But the tradeoff is that you're forcing oil thru a little hole at high pressure so you get lots of heat.

So to sum up, it doesn't increase the efficiency of the pump (generator) so much as it increases the efficiency of the engine (blades) that's driving it.
--
Chris

[Fused]

Thinking about your cooling by "design". In order for a fan to draw air through the generator it will need large entrance holes. What will your entrance holes do about the rain, dust and ice formation? Cranking out 2kw and soaking wet coils may not be a real good idea. Although it may be impressive for a second watching the smoke. You say you like using JB weld, but do you realize JB weld crawls up the magnet when applied? Im not sure a void under the magnet will keep JB weld in place without it crawling around and opening that void back up. I dont know, Ive never wanted to attempt it. Lastly, you may want to rethink your 5 thousandths air gap. Not only vibration of the coils causing a rub but even gluing the mags on I doubt will keep your rotor within 5 thousandths of an inch. Ive always been told to keep the air gap at half the magnets thickness or slightly less. Amazing how similar a garbage disposal stator looks compared to your home made. 
Just some basic simple ideas, not complex.

Have fun

Fused

[Fused]

I may have put this in the wrong thread, too many look the same to keep track of....and its late.

Admins can move or delete. Sorry bout that.

Fused

[joestue]

lets not revisit the hydraulic analogies here, they fall short when you consider that in real life the pump has a bypass valve, and the flow is limited to x(rpm engine)
a more realistic analogy is no bypass valve, volume of oil per minute is voltage, and pressure difference across the load is current. it makes my head spin too. voltage is limited by the rpm of the engine (positive displacement per rpm) and the current is limited by what happens when you pump 10cubic inches of oil per second into a closed pipe.


the only reason adding a resistance increases the power out of the turbine is because the power available follows the velocity of the blade, not the torque exerted on the blade by the wind.

[Flux]

thanks Flux, what you relate makes sense when you consider the variable and non linear power nature of the wind motor

still trying to get my head around the addition of resistance allowing higher efficiency from the alternator, but from what i gather
it is not so much that the added resistance actually increases the alternator efficiency directly but rather allows the rotor to fly faster
and by doing so spin the alternator faster, which by extension increases the output of the alternator.

i wonder though if this increase in output is nothing more than that, and not actually an increase in alternator efficiency.

got to do some more thinking on this one.

thanks

bob g

Sorry Bob, there seems to be a misunderstanding. No way does added resistance increase alternator efficiency, it can't. the gain in power is entirely from increased power from the prop.

I have always claimed you have to drop " electrical efficiency" to get more from the prop and more overall.

Hope this clears up the confusion.

Flux

[ChrisOlson]

lets not revisit the hydraulic analogies here, they fall short when you consider that in real life the pump has a bypass valve, and the flow is limited to x(rpm engine)

That's why I said "sort of like".  It's easier for many people to wrap their head around fluid power systems than electricity.  Using the analogy helps them understand that adding the resistors inline doesn't increase generator efficiency - it merely lets the blades put more power to the shaft and by doing so you get more power from the gen.

In the end you get more power but most of it goes to heat instead of the into the batteries ---- unless you have one of these efficient DC to DC converters like Flux has (or at least I think that's what he has).  I'd like to get my hands on one of those things to test it.  If you had something like that you could easily get 1 kW out of an 8 foot prop by matching the load, and still get good power in low winds too.
--
Chris

[luv2weld]

I agree with joestue. Not many people fully understand hydraulics.
I think a better analogy would be a garden hose.

Ralph

[Ungrounded Lightning Rod]

thanks Flux, what you relate makes sense when you consider the variable and non linear power nature of the wind motor

still trying to get my head around the addition of resistance allowing higher efficiency from the alternator, but from what i gather
it is not so much that the added resistance actually increases the alternator efficiency directly but rather allows the rotor to fly faster
and by doing so spin the alternator faster, which by extension increases the output of the alternator.

i wonder though if this increase in output is nothing more than that, and not actually an increase in alternator efficiency.

That's right.

The resistance doesn't increase the efficiency of the alternator.  It increases the efficiency of the blades (which, in the higher winds, are allowed to spin faster, stay out of stall over a larger blade area, and thus collect more power by running closer to their design TSR.)  The improved efficiency of the wind-to-shaft-HP part of the system more than compensates for the reduced efficiency of the HP-to-charge-current part of the system.  You burn more power heating resistance, but you collect a bigger increment than you burn.

note that this is only for SOME wind speeds.  Near cutin you lose.  But over typical windspeed distributions you gain.

A MPPT controller does far better, of course.  It doesn't throw away the power represented by the genny current times the genny-to-battery voltage drop - instead it uses it to pump in more charge current at the battery voltage.  And it keeps the mill spinning at the most efficient speed for power collection, rather than the added-resistance's lose approximation.  But it's also an extra complication with extra failure modes and other issues of its own (such as genny burnout risks).  Adding resistance can give some mills a boost compared to no added resistance and is a lot cheaper.

[bob g]

so in reality you are changing the characteristics of the load, not the alternator
when you add resistance?

by changing the characteristics of the load, you allow the rotor to spin up faster and continue to produce
power without stalling (or at least move stall up a bit) in higher winds than it would otherwise be capable of
without the resistance?

so maybe you switch in the resistance in higher winds, and leave it out in lower winds?

has anyone either accurately measured or calculated the overall efficiency gain from using added resistance?

the reason i ask is based on the fact that a good switch mode power supply, or controlled rectifier/buck converter
can attain 93+% efficiency, if on the other hand added resistance is better than 75% maybe it is good enough
for the ease of implementation? certainly simpler and lower cost?

edit:  what i mean is if adding a buck converter allows a specific machine to produce for instance 400 watts more than
it would otherwise make, then 75% of that or 300 watts extra using the added resistance would probably be a no brainer
and winner from an ease of implementation/cost analysis.

bob g

[Flux]

If this link works it brings you to a part of my matching the load diary entry.

I tried to copy the image but couldn't but if the link works there is a graph comparing stalled operation, added resistance and mppt operation.

The stall curve tails right off in high winds, with added resistance the thing is near identical in low winds but continues to rise in high winds.

The overall result is better with mppt but the additional resistance is certainly justified if you don't go the whole way to mppt. There seems little point in switching the resistance out in low winds because it has little effect on efficiency until there is considerable current flowing.

I don't want to go into much detail about why mppt works better in low winds but it is again more concerned with matching than electrical efficiency, the direct connection even with added resistance is good as far as efficiency goes in low winds but the matching to the near horizontal part of the prop power curve is far from good.

http://www.fieldlines.com/board/index.php/topic,127288.html

Flux

[ChrisOlson]

I don't want to go into much detail about why mppt works better in low winds

Where's a good place to get one of these MPPT controllers that will work on a 2 kW wind turbine?
--
Chris

[Flux]

I believe the Midnite Classic is going well in trials and may soon be available. The reports I have heard have been good.

I hope others who have been trying this controller can give you first hand reports, I suspect you would in fact be a good candidate to help with the trials.

It needs evaluating by people willing to give it a fair trial in realistic wind conditions and they would probably value your work on the alternators and blades.

I am not sure if there is any other product near this stage, I strongly suspect things have been held back too long by trying to adapt mppt solar and the approach with the classic seems to be a practical one.

I have proven the ideas with my crude analogue control but I have never published any details for home construction as I seriously doubt that anyone could copy what I have done and set it up. I just hope that I have inspired others to take this further. The scheme had been available for grid tie from several manufacturers for some time, the Aurora in particular seems to be doing very well.

The problem with battery charging is not much different but it is a smaller market means that it takes  longer to do the development and recover the costs.

Flux

[ChrisOlson]

It needs evaluating by people willing to give it a fair trial in realistic wind conditions and they would probably value your work on the alternators and blades.

If I understand correctly, for this MPPT controller to work and get the most out of it, you should have a turbine/generator combination that's more optimized for low wind conditions, and that otherwise normally goes into semi-stall in higher winds for the MPPT controller to do its thing.

Whatever the case, I have several stators with different configurations hanging on the wall that I could swap out quickly to make the turbine do anything I want, from exceptional low wind performance to ones that cut in at 8.5 mph and deliver good high wind power.  I think I will write Midnite Solar and ask them if they would be interested in having me test one.

Thanks
--
Chris

[97fishmt]

I talked with a guy from Outback yesterday at a solar fair here in the Seattle area and he said
he knew of one of the classics being tested in real world conditions.  He said it wasn't quite
ready yet,  the programing or set up just has been so hard to get right for wind compared to solar,  he explained when the wind  really picks up or a sudden gust comes up it's very hard to deal with.  He referred to like tapping the brakes, and said that's hard to do electronically without
loosing all the momentum of the rotor.

They are working on it.  I too am interested!

A SEI booth was giving out free issues of home power and there is a midnight solar add in there
saying the classic should come out this summer.  We'll see.

The fair was way cool and the fellows I talked with said all the guys at Otherpower are a great bunch of guys.

[SparWeb]

Bob,

(just got back from a trip so I just saw your question)
That previous thread had me thinking that a FAQ or at least a few more diagrams would help the discussion on this topic.  You can't easily tease apart what the prop is doing and what the genny is doing when talking about wind.  What I've learned from doing run-up tests with a lathe, however, plus some calculations that I've worked on for a few years, is all coming together to a pretty clear understanding of what's going on.  I'd be happy to try to make a fairly concise explation of it, the trouble is paring down all the irrelevant stuff so that the story is clear.

I just put my Baldor generator up in the air, and switching from Star to Jerry (IRP) I see exactly the change in behaviour my calculations predicted.  A bit vindicated that I can finally do this, I'm eager to share a bit of what I've learned once I can make a sensible explanation of it.

Need a bit of time to get it done - graphs and diagrams to be drawn unfortunately, but it's necessary for the cause.

By the way the Aurora grid-tie inverter can regulate a wind turbine with a MPPT program.  Power One I think?  Talk to Rob Beckers (on this site or on his own website Solacity).

[bob g]

thanks one and all for your input, it is appreciated.

my background runs deep in automotive alternators, and less so air cores although i have
been following them for years.

windpower or rather windgeneration of electrical power is fraught with some interesting challenges.

first one starts with a motor (blade set) that provides power in a non linear fashion,, then you drive it
with an infinitely variable fuel source (the wind) and strive to charge batteries which have another set of variables, and
then use the power in another variable way.

with the link being the generator itself!

seems like empirical data has done more to get the thing to where it is today than any set of equations would ever hope to.

talk about a multivariable equation!

personally i cannot imagine striking out on my own, and setting down to try and design a totally new windgen from the ground up
with nothing more than a pile of questionable variables.

seems far easier to me to just build a copy of what others have done, proven to work, and then learn to live with what power it will provide.
and if it isn't enough, build another one.

i have spent a number of years now researching, and the last couple years testing and developing hd alternators for use in cogeneration
where i have a fixed speed, a linear power source, and exceptionally accurate charge control of the battery bank,, even with all this
working out details or trying to calculate what the results will be when i make a single factor change can be very problematic and drive
one out of the mind some days.

in the next few months i have to complete the main trigenerator and also a small cogen unit, and then i will need to get started on a windgen
for use on my ridge.

at least up there the windspeed is fairly consistent , if there is wind blowing it will almost always be over 10mph up there, so maybe i won't need or care much about low windspeed cutin and power generation.

this forum is populated with some of the most knowledgeable folks on this topic that i know of, and my hat is off to you guys for the shear willpower and effort it has taken to get these machines built to the levels that are common here.

thanks guys
bob g

[jimovonz]

Sparweb:  I'd be happy to try to make a fairly concise explation of it

Yes please - I find your work most enlightening

[SparWeb]

Sparweb:  I'd be happy to try to make a fairly concise explation of it

Yes please - I find your work most enlightening

Jim you deserve credit for a bunch of it!  It was a couple of years ago, now, but I didn't quit and the numbers started to make sense after a while of plugging away.  We seemed to go off on separate tracks after a few months of thrashing at the subject but I always want to thank you for kicking me in the a** which got me started.

It's not an easy topic to grapple with, you drown in the data pretty quickly.  I really don't know how Flux does it, being able to explain in detail large or small, as the context demands it.  Nobody doubts he understands it far more implicitly than the rest of us.  I think the only thing I can bring to it is a facility with drawings and pictures - more show and tell, so to speak.