Generator with ferrite magnets

[rossw]

Personally I do not trust "black boxes" (MPPT) on a wind turbine that adjust the power curve by changing the operating voltage the generator "sees".  I've flown turbines long enough to realize that they can get pretty wild in high winds.  Even big utility scale machines shut themselves down in excessive wind speeds instead of trying to keep harvesting power from dangerous wind velocities.  It's my opinion that you're playing with fire running a MPPT Buzz Bomb.  A box that is not capable of shutting the turbine down when voltages reach dangerous levels, or protecting itself without depending on some other means (furling, voltage clipping, etc..) is not a well-designed unit, IHMO.  If that box is going to run a turbine on the raw edge, then it better have an internal self-protection mechanism to govern what it does.

Just out of curiosity.....

If you had a turbine that was good for (lets say) 5KW in a decent blow, and you "scored" an MPPT controller that was capable of controlling a source of lets say 25KW (ie, it has HEAPS of legroom before it was likely to let out the magic smoke, not a toy that "might, just" handle the turbines peak output on a good day under ideal conditions)... would you be more comfortable?

I suppose I'm getting at the question of : do you distrust MPPT as a technology, or do you just mistrust people building junk that can't handle the power that you know these things can make under bad-weather conditions?

[ChrisOlson]

I suppose I'm getting at the question of : do you distrust MPPT as a technology, or do you just mistrust people building junk that can't handle the power that you know these things can make under bad-weather conditions?

I do not distrust MPPT as a technology.  I think true MPPT on wind turbines is hard to do because of rapidly changing output.  So the so-called MPPT controllers for wind are really programmed lookup tables where you set the operating parameters in hard code, not true MPPT where the thing does a performance sweep and adjusts for maximum power output.

I am concerned about under-built controllers that can't handle a simple over-voltage spike without blowing up.  Power spikes, and really big ones, are common with permanent magnet turbines in high winds.  It's just my feeling that if there is a controller designed for use on wind turbines that the thing should be able to handle such a situation without relying on external means to clip the voltage or control it.

But that might be just me, I don't know.  I do know I've been around and worked with a lot of different turbines, and I've seen Bergey turbines kick their PowerSync II/GridTek 10 inverter out because of over-voltage/over-current.  The Bergey's answer is to reset the inverter three times and if it is still over-voltage/over-current it lets the machine go and run unloaded, requiring a manual reset to bring it back online.  But the point is, it handles it without blowing the inverter up.

So I guess I'd need to see better equipment before I'd get too excited about trying it.
--
Chris

[bob g]

i see no reason at all that a mppt couldn't be designed and built to control any turbine
and do so with failsafe(s) built in for any possible calamity imaginable.

lookup tables can be accessed and acted upon thousands of times per second if not faster and control could be effected faster than any mechanical means could respond.

the only issue i see with it is one of cost, the market is so miniscule that the cost is prohibitively expensive for small wind and the market for an mppt for the diy'er crowd
probably isn't going to happen. at least not with the necessary failsafe's built in.

there is always compromises to be made, and to get the cost down for the typical diy'er
i agree it would be a dicey proposition that i too probably would not trust.

on the other hand, when things come full circle back to wound field machines, micro controller controlled rectifiers could be made to do anything an mppt can do now, do it well, and meet a price point acceptable to many diy'ers.  controlling a couple dozen amps of low voltage DC is much easier than controlling kwatt's of higher voltage AC or DC.

again in my opinion only.



bob g

ps. i am seriously thinking of going back to the design of a wound field machine, my thinking is once one gets up over about a 17ft machine in a decent wind area the few watts of field power losses are more than made up with better control over the rotor
and better matching of both power source and load, over a wider wind range and over
the full charge regime of the battery bank.

[scoraigwind]

I'd like to point out that the power efficiency of my dual stator generators is very good - better than 80% at the maximum power the blades can deliver at 12 m/s. 

I am always struggling a bit to keep up with the details of your design, Chris but I think it's something like this?
2 stators each wound with about 5 lb of 13 AWG wire, having resistance 0.8 ohms each.
Each stator carries 30 amps at rated power, giving 60 amps total output at 30 volts (say) which is 1800 watts power output.
  (OK you stated 1926 watts at 12 m/s so this is conservative.)
Copper loss in each stator  = (current squared) x resistance = 900 * 0.8 ohms = 720 watts.  Total of losses for both stators would then be 1440 watts.
Total power input is then = output +loss = 3240 watts mechanical power input of which you get 1800 watts output which I reckon as 56%.
I have not made any allowance in this for the pulsating nature of the current in the single phase stators (which I reckon increases the loss dramatically), nor for the rise in resistance with temperature.

Where did I go wrong in my calculations, and how do you arrive at 80% in yours?

thanks

[bob golding]

been a while since i posted. everything has been working ok. time to give the turbine its annual service so thought i would drop in and see what people are up to.

very interesting thread. i am at the stage of trying to rescue what is left of my magnets. they have lasted around 5 years but are getting very thin. after looking at the price of neo magnets using ferrite does sound a good idea for my marine environment. i am in the process of repainting my rotors with marine 2 pot epoxy paint. i am going to also encapsulate them in epoxy resin. photos to follow. i should get another winter out of them but doubt i will get much more out of the magnets. my rotors are 14 inch so should be able to get 12 2x 2 x1 ferrite's on them. i have enough wire to rewind the stator, once  i have finished reading the thread! good work chris thanks.

[scoraigwind]

hi Bob,

Have you tried putting linseed oil on your magnet rotors? Mixed with some fuel it seeps into the cracks and lingers quite nicely there.

If you have a 14 inch disk then you should be able to fit 12 magnets 3 x 2 x 1" on that.  Or use 14 magnets and 12 coils (I have always wanted to try that) with the 2x2 magnets.

[bob golding]

hi hugh

Have you tried putting linseed oil on your magnet rotors? Mixed with some fuel it seeps into the cracks and lingers quite nicely there.

 i had thought of doing that but decided that it would interfere with the glue i use to fix the magnets on with. i did try jb weld and vinyl ester last time but rust got under the vinyl ester and lifted it from the rotor discs. if the epoxy doesn't  work i will try the  linseed oil and fuel oil idea. how do you secure the magnets to the rotors?

[ChrisOlson]

I am always struggling a bit to keep up with the details of your design, Chris but I think it's something like this?
2 stators each wound with about 5 lb of 13 AWG wire, having resistance 0.8 ohms each.

Hugh, that was my prototype unit that I showed here.  I continually tweak things, and the latest ones are wound with 48 turns of 12 AWG wire and their resistance per stator is about .60 ohm.  They require 900 rpm and 280 lb-inches of torque at the shaft to develop 2,400 watts output.  It is a very low torque, high speed generator.  If I could get more hydraulic power to my torque motor I could push it harder and see what it would do.  But I just had a 5.0 Briggs engine driving the pump and it was working the snot out of that little Briggs to push it to 2,250 watts.

I calculated the power efficient on my test bench when I ran it, computing torque and rpm converted to watts in vs watts out at 2,250 watts, and it figured out to 80-85%.  The carbon pile in my Sun AVR gets hot trying to test 2 kW generators so I've gone to hooking them to my battery bank when I test run one and that sometimes skews the results because of the wire run and it depends on what the voltage is on my battery bank.  But it's close enough to determine approximately how efficient it is.

--
Chris

[ChrisOlson]

Or use 14 magnets and 12 coils (I have always wanted to try that) with the 2x2 magnets.

Hugh, what's this 14 pole 12 coil layout?  Since I read that I been trying to envision it in my head.  And nothing sensible comes to mind.  But it would probably be cool    
--
Chris

Edit:  Quite some time back I built a 12 pole 18 coiler:



It had 180 degree separation between the phases.  It was pretty cool   

[scoraigwind]

I am always struggling a bit to keep up with the details of your design, Chris but I think it's something like this?
2 stators each wound with about 5 lb of 13 AWG wire, having resistance 0.8 ohms each.

Hugh, that was my prototype unit that I showed here.  I continually tweak things, and the latest ones are wound with 48 turns of 12 AWG wire and their resistance per stator is about .60 ohm.  They require 900 rpm and 280 lb-inches of torque at the shaft to develop 2,400 watts output.  It is a very low torque, high speed generator.  If I could get more hydraulic power to my torque motor I could push it harder and see what it would do.  But I just had a 5.0 Briggs engine driving the pump and it was working the snot out of that little Briggs to push it to 2,250 watts.

I calculated the power efficient on my test bench when I ran it, computing torque and rpm converted to watts in vs watts out at 2,250 watts, and it figured out to 80-85%.  The carbon pile in my Sun AVR gets hot trying to test 2 kW generators so I've gone to hooking them to my battery bank when I test run one and that sometimes skews the results because of the wire run and it depends on what the voltage is on my battery bank.  But it's close enough to determine approximately how efficient it is.

--
Chris

thanks, Chris,

Yes it's always a bit hard to learn from your stuff because I am always one jump behind :-)  And I suppose that with 2250 watts you would have had about 75 amps or more (assuming 30 volts battery).  According to the theory, that would waste  about 1700 watts of copper loss heating up the stator, and yet still in pracice it's 80% efficient.  Again, theory would not have predicted that this could be possible.  So it leaves me baffled.  Maybe you have discovered perpetual energy or superconductivity.  I can see how it could be possible to achieve this at 52 volts because the current is way down to 43 amps.

12 coils and 14 magnets is a way to produce 3-phase output.  (Rather like 3 coils to 4 magnets.) You have to reverse the connections to every second coil, because there is only 60 degrees of phase difference if you don't.  The coils are small, which seems to be the fashion now, with cheap magnets.

Could be fun.

[artv]

Wow.........now I'm lost.......I just stared at that diagram,....man do I have alot to learn ........artv

[ChrisOlson]

Yes it's always a bit hard to learn from your stuff because I am always one jump behind :-)  And I suppose that with 2250 watts you would have had about 75 amps or more (assuming 30 volts battery).  According to the theory, that would waste  about 1700 watts of copper loss heating up the stator, and yet still in pracice it's 80% efficient.  Again, theory would not have predicted that this could be possible.  So it leaves me baffled.  Maybe you have discovered perpetual energy or superconductivity.  I can see how it could be possible to achieve this at 52 volts because the current is way down to 43 amps.

Hugh, I ran it for roughly two hours at 70-75 amps continuous output and the battery was at about 30, sometimes up to 31.5 volts, because I had solar power coming in, plus three other turbines, and the generator I was testing - all hooked to the same bank.  So the voltage was varying a bit depending on how the MPPT controllers on the solar arrays would adjust themselves, and how the RD-1 was loading the inverters with extra water heating load to keep the voltage in check.

I put a clamp-on ammeter (Fluke meter) on the AC lines a couple times and it showed about 25 amps flowing in the #8 wires going to the rectifier with 75 amps DC output.  I had 150 feet of welding cable hooking the rectifier up to the battery bank because it was the only thing I could find that was long enough to reach from the shop to the house where the bank is.  I hooked up the welding cable to the shunt on the Doc Wattson on the panel and it was measuring the actual power going to the batteries.

No superconductors or anything else.  Just machinery and some wires.  When you put two generators on the job, in parallel, with no additional mechanical losses to drive the second one, you're using individual rectification on both, and paralleling the DC output of the two, I don't know how you'd really figure that on paper.  The only accurate way to drive it with a torque measuring unit (dynamometer) and calculate power in based on torque and speed vs what you're getting out the other end.  There's a huge difference between calculated power with internal combustion engines vs measured on the dyno too.  So I tend to place my faith in actual torque and speed measurements when measuring power on mechanical devices or machines. 

12 coils and 14 magnets is a way to produce 3-phase output.  (Rather like 3 coils to 4 magnets.) You have to reverse the connections to every second coil, because there is only 60 degrees of phase difference if you don't.  The coils are small, which seems to be the fashion now, with cheap magnets.

That's what I had sort of envisioned as to the layout.  But until looking at the drawing I couldn't see how it would work with the vision I had in my head.

Personally, I have grown to like the 1:1 pole/coil ratio with overlapped coils.  I think if I were to build a three-phase neo unit, that's what I would do.  I'd use smaller magnets (50mm long x 22mm wide x 25 mm thick) so I could fit more poles on smaller rotors, then just throw rpm's at it to get it to perform.  I really don't think it would beat these dual stator units, and it would be a heck of a lot more complicated, harder to build, and probably would require more copper and more rpm's.
--
Chris

[scoraigwind]

I put a clamp-on ammeter (Fluke meter) on the AC lines a couple times and it showed about 25 amps flowing in the #8 wires going to the rectifier with 75 amps DC output. ...  When you put two generators on the job, in parallel, with no additional mechanical losses to drive the second one, you're using individual rectification on both, and paralleling the DC output of the two, I don't know how you'd really figure that on paper. 

It's actually pretty easy to figure out if you can get things to make sense, but as usual I can't.  It's not possible to get 75 amps DC out of a rectifier when you only put 2 x 25 amps AC single phase circuits in.

Figuring out loss in parallel circuits is easy enough.  Two parallel circuits that each have resistance 0.6 ohms means that the resistance is actually 0.3 ohms.  The loss at 75 amps is (75 squared) x 0.3 or 1800 watts.  Another way to do it is to say that each stator produces 37.5 amps and its resistance is 0.6 ohms and when you work them out and add them up you get the same answer.  In reality the loss would be much higher, since the rms current is much higher than the mean.  I can't tell you why the theory is not matching your practical experience, in your case but I can tell that something doesn't stack up here.  Maybe try verifying the calibration of your DC meter?

[oztules]

I'd probably trust the fluke..... I can only get 1500W output, with 750 watts lost in the stator for 66% electrical efficiency.  or 375 watts per stator. Still not too shabby at all.
 
So that burns up 2250 watts, and I still need to account for I think 500 odd watts measured in the torque/rpm from what I'm reading. Possibly transmission and bearing loss, rectifier loss.. (100W) and some peak current loss  from the lousy powerfactor/harmonics  I suspect. (from only using the peaks of the sine wave, and ignoring the lower parts) We'd get that back if we used a boost converter stage first perhaps........more magic smoke though.

I can't help but think the DC amp meter has a problem shunt perhaps and leading you astray.

Sorry Chris, that s as much as I can get from 50A split in two phases independently rectified @30volts. An extra volt or two  will  skew it a bit but not the magnitude we need to get to 80% @ 2250W. 66%@1500w is more likely an outcome.

That little briggs is doing very well to run that through hydraulic conversion. If it can get 2800w to the shaft from a 5hp ICE driving a pump and hyd motor then  your squeezing every little bit out of her.





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

[ChrisOlson]

That little briggs is doing very well to run that through hydraulic conversion. If it can get 2800w to the shaft from a 5hp ICE driving a pump and hyd motor then  your squeezing every little bit out of her.

I've had those little engines on the dyno when I've built them for go-carts for racing.  Stock, at 1,100 feet altitude they'll develop about 4.18 hp at the shaft @ 3,600 rpm on 85 octane pump gas.  The hydraulic conversion is about 87% efficient using an Eaton gear pump (positive displacement) driving my Ross TF-series torque motor.  I have to refer to the pressure and flow chart for that torque motor to determine torque output with the oil at 140 degrees F.  It's not as accurate as a prony brake but it's close enough to determine if a generator is going to last or burn up in actual service duty on the tower.  The torque motor works really nice because it runs at the same rpm as the blades at about 20 gpm flow rate to it.  I got a bracket made to bolt the motor on the front of the transmission case and drive the input shaft with a rubber cushioned LoveJoy coupler.

I have noticed when checking other turbines that the clamp-on Fluke meter will show a lot less amps on the AC lines than what you get out of the rectifier on the DC side.  Not sure why that is but it seems to be in proportion to the difference in the AC voltage vs the DC voltage too.  So I don't know if there's such a thing as RMS amps, but it's always lower than the DC amps even when I've used my Sun AVR to load the DC side.

At any rate, that Briggs had all it could do to push 2,250 watts.  If the voltage dropped even slightly when a water heater element kicked in, it would lug it down and then the watts dropped.  The voltage had to be up to at least 30 in order for it to drive that generator to 2,250 watts.  So it would vary from 70-75 amps output, depending on the voltage of the bank.  When I get one of my tractors freed up, that have a lot more hydraulic power, I'd like to hook it up and see if I could burn one of those generators up.  Or at least know what it takes to smoke a stator in one.  I think they're a pretty tough unit.  I measured the temperature of the coils with my infrared thermometer that I use to measure exhaust port temperature on diesel engines, and it showed about 130 degrees after two hours.  The little Briggs ran out of gas, otherwise I would've run it longer than that to see if it got hotter.
--
Chris

[bob g]

Chris

from my experience and with all due respect i am left with a couple of thoughts
 i would like to pass along.

the fact that your machines work and do what you need of them says all that can be
said with any certainty...

aside from that, determining efficiency is troublesome when you make so many assumptions.

things like meters can cause all sorts of flaws in the calculations, power factor causes all sorts of issues too.

assuming efficiency of a briggs driving a hydraulic pump, driving a hydraulic motor, that drives your generator and then reporting something like 80% efficiency is problematic on so many levels. 

the only way you are going to get a true efficiency number for the alternator is to know precisely how much power is being fed into it (driving it) and then how much amperage and voltage is produced into a purely resistive load bank (or nearly purely resistive).

one tiny flaw in the numbers can magnify into serious problems when determining efficiency.

i know this from setting up a test cell to determine the true efficiency of ST heads and
alternators i work with at 24 and 48volts.

one little  bugger can shift the efficiency 10 or 20 points if you aren't careful.

remember that efficiency can be crosschecked and proven mathematically, but you have
have to have a lot of real numbers and not published or assumed numbers.

i find it near impossible to accept that a hydraulic pump/motor drive can be anywhere near 80% efficient, they just make way too much heat and in my experience 65-70%
is a pretty good system.

in closing, as i stated first,
you machine does what you need it to do, and there is a lot to be said for that.

bob g

[ChrisOlson]

i find it near impossible to accept that a hydraulic pump/motor drive can be anywhere near 80% efficient, they just make way too much heat and in my experience 65-70%
is a pretty good system.

Bob, the standard nine cylinder axial piston, variable displacement hydrostatic pumps and motors have been used in industrial and agricultural equipment for decades and they are better than 85% power efficient.  Torque calculations for a hydraulic motor is one of the basic fluid power calcs.  It's given by:
torque (lb-ft) = displacement x psi / 75.39

A 7.6 cubic inch motor operating at 500 psi will create 50 lb-ft of torque.  The speed is determined by flow rate.  Shaft power is a function of speed and torque. (shp = torque x rpm / 5252).

I tested that generator at 830 rpm and 280 lb-inches of input torque, which is 3.69 hp or 2.757 kW.  I calculated that at 900 rpm it would take the same 280 lb-inches of input torque to get 2,400 watts out of it.  I'll check the Doc Wattson to make sure it's reading accurately with the external shunt on it, but I'm pretty sure I've checked that meter in the past against my Sun AVR and it was fine.
--
Chris

Edit:  Well, I'll be dinged.  I hooked that Doc Wattson up to my battery bank and loaded it with the carbon pile in my Sun AVR.  It's right on - up to about 10 amps.  Then the harder I load it more it gets off.  At 57.2 amps actual on the AVR the Doc Wattson shows 75.0 amps.  I evidently overheated that shunt at some point and damaged it.  I wrapped it good with a screwdriver handle a few times, which is what I do to all electronic crap that don't work.  But that didn't seem to fix it.

I guess I need to call PowerWerx tomorrow and order a new shunt for that meter.   

Damn.  I hate when that happens.

[scoraigwind]

I don't know if there's such a thing as RMS amps, but it's always lower than the DC amps even when I've used my Sun AVR to load the DC side.

RMS amps simply means the 'root mean square' of the amps same it means for voltage.  RMS is only the same as average current if the current is constant.  In the case of single phase the amps come in pulses.  Consider for example if half of the time you get 2 amps and half the time you get nothing.   (This is pretty close to what happens.)  The average is one amp but the mean square is 4 (half the time) and zero (half the time) averaged out is 2 amps.  So the RMS is the root of this, which is 1.4.  Losses in this case will be double what they would be with a steady current of one amp, whilst the battery charging effect (average DC current) is the same.

AC currents and rectified currents always have a higher RMS values than the average current.  Battery chargers often have RMS ammeters fitted on them because they will read much higher than a proper ammeter would.  It's legal, and it makes it look good, but of course the only current that matters to the battery is the average, so it's misleading.  RMS is only relevant when considering the heating effect on resistive things like heaters and copper wires.

From all that you have told us, the 0.6 ohm resistance, the 25 amps (presumably RMS) coming out of each stator, and the fact that they don't get very hot, the only conclusion I can come to is that there is something wrong with the measurements on the DC side.  You can't get a higher DC output current than the AC currents going in, added up.  Typically you will get quite a bit lower average DC output current than the combined AC RMS currents in.  All the rectifier does is change the direction of the current at any instant.  It can't provide more current than you had to begin with.

[ChrisOlson]

From all that you have told us, the 0.6 ohm resistance, the 25 amps (presumably RMS) coming out of each stator, and the fact that they don't get very hot, the only conclusion I can come to is that there is something wrong with the measurements on the DC side

Yeah, ignore all this.  I just checked that shunt a half hour ago and it's no good.  The wavy looking thing on it, which used to be bright copper color, is dark like it's been burnt.  It's way off as tested against my AVR.  I have to order a new shunt for that meter.
--
Chris

[ChrisOlson]

Interesting - I tested another Doc Wattson meter that I have with a 100 amp external shunt.  That one reads high too.  At 66.5 amps actual, as measured by my AVR, the Doc Wattson meter shows 75 amps.  So, then I tested yet another one and I got the same thing, pretty much - 66.2 amps to get that one to show 75 amps.  At lower draw the error is less.  The higher the amps thru them, the more error they got.

When I modified these meters I removed the negative lead and internal shunt and soldered on an ethernet twisted pair from the Kelvin sense pads to the external shunt.  I don't know if that ethernet wire is throwing them off, or what.

I don't have any of these little meters that haven't been modified with external shunts.  Has anybody else ever tested one against a known accurate meter?
--
Chris

[bob golding]

hi chris

i am no sure i would trust any cheap digital meter to read accurately over the range you are checking. i have an old analogue ammeter with a built in shunt that i use to check the current going into the battery bank from my charger. i did have a doc wattson but killed so cant test it against the analogue meter,but i would trust an analogue meter  more.

[Flux]

Instrumentation on funny waveforms is a nightmare and it hasn't been made any easier by digital equipment.

If a shunt is accurate at 10A then it will be ok over the range unless you are running it so stinking hot that you have thermal emf problems to the potential connections or have something made of junk with a high temperature coefficient. I suspect the digital thing not the shunt.

Cheap digital meters read dc ok on smooth currents ( battery to load etc) but when there is a lot of rms component they have to be well designed to measure true mean dc.

When comparing to analogue meters it is well to remember that moving iron and dynamometer ( electrodynamic) instruments measure rms. The only analogue meter that measures dc mean is the moving coil ( d'arsonval). That is the one you need in the battery line. The other analogue instruments will measure the rms but they may be scaled for dc ( note Hugh's comments about battery charger meters).

A clamp meter used on the ac side may measure rectified mean ( scaled rms) or it may be a true rms, the initial cost will be very different. For normal sine wave mains measurements either is good enough. For crappy waveforms then both will be correct but may not measure what you are wanting to know.

It has taken many years for digital meters to be thoroughly reliable and cheap ones still aren't. Even the reliable ones can make a fool of you unless you know exactly what they are measuring and that is where the big problems really come in.

The big thing about digital stuff is that they can be read to many decimal places and this is generally assumed to equate to accuracy but if you are measuring the wrong thing then you may have errors many times greater than even a cheap but appropriate analogue thing that you can only read to 2%.

I left the electrical industry at a time when the changeover was taking place to digital meters and data loggers. There were troubles then on normal ac, but those issues were soon resolved. It just happens that most machines now ( particularly motors) are run from strange waveforms ( very strange indeed at times) and the old instrumentation problems have surfaced again and at a time when graduates are so highly qualified in specialist fields that they haven't a clue when the results are way off the mark. There is only one colleague left at that place that actually understands instrumentation and he often comes round here tearing his hair out when test results are stupid or invalid.

It is perfectly understandable to see measuring problems here and that is why I try to help out, it's not easy when you only have a limited source of measuring equipment and no experience in the field.

I tend to trust data published here when it is direct comparison, but absolute data I am wary of especially if it involves wind speed or tricky electrical measurements.

Flux

[kevbo]

Flux said it better than I could have.  When you charge a battery through a rectifier circuit, you draw your current in short tall spikes, and few AC meters will provide an accurate indication with such a waveform.  Measurements on the DC side with swinging needle meters are probably darn close to truth.

 I will just add that I have seen some "free energy" or "Over unity" (perpetual motion) claims that were mostly the result of using inappropriate instruments to measure peaky AC waveforms.  There is even one guy that thinks you can do anti-gravity because it turns out you can make a common bathroom scale read negative when you make the load shake and bounce the right way.

[Flux]

Just had another thought on this. Chris could you do a run on the shunt with a millivoltmeter ( dvm should do) on the potential terminals. the reading may not mean much in absolute terms, but it might be a common fsd of 50, 75 or 100mV.

What matters is that the millivoltmeter teading should be directly linear with the current measured from your analogue meter.

When you said the copper looking blade bit, it makes me wonder if they actually use copper. Real manganin shunts are very expensive and they may have tried to do something on the cheap. Copper has a temperature coefficient so high that if the shunt blade is at 100C ( not unlikely) there would be a 40% error and that is something like what you are getting.

I have seen stainless steel used for cheap shunts and again it is dreadful, may be ok to look at a brief high current event that doesn't have time to raise the temperature but for long term accuracy it is pretty useless.

If the shunts check out ok then it must be something in the actual metering circuit.

Flux

[ChrisOlson]

Flux, I did that this morning because I was trying to figure this out.  The one shunt is damaged and it's way off.  It's obvious that it's been really hot.  I suspect it got damaged some time back when I burned up some wiring and had to upgrade the wiring from one of the turbines.  I don't know what the wavy looking deal on it is made of.  It's not magnetic, that's all I can tell you.  But it's been burnt a very dark color compared to the other two which look like bright copper.

The other two that don't look burnt are pretty close.  If I load it with the carbon pile to the full 100 amps that the shunt is rated at, the voltage measured across the ammeter terminals on the shunt is 101 mV with my DVOM.  At 50 amps it's exactly 50 mV.

The problem is in the meters, which I find rather shocking, because I always thought these little meters were pretty accurate.  It appears, near as I can tell, that they accumulate about 1.5% error for every 10 amps.  Again, these have all been modified by removing the negative lead and the internal shunt.  Then I soldered a twisted pair ethernet wire to the Kelvin sense pads, as per the instructions on RC-Electronics' site, to modify the meter for an external shunt.  I don't know if the error is caused by that modification, or if they come that way from the factory.  I don't think it's caused by the modification because I get the same millivolts at the end of the ethernet wire that I get measuring it at the shunt terminals.  So I just don't know, but I think it's in the meter.

I think I'm going to take Bob's advice and invest in some good analog ammeters for my turbines.  I got one really nice analog meter on my bus that measures total power coming into the bus.  It has a huge shunt on it with 3/8" studs for 4/0 wire.  But that meter was not cheap.
--
Chris

[Flux]

Thanks Chris. That rules the shunt out if it is copper coloured then it will be manganin ( you may call it something different) it is the standard material for decent shunts with a low temperature coefficient and a low thermal emf relative to copper.

Have to say i am not that surprised about the meters themselves, i have never met them but i have seen a lot of things that don't stand investigating too closely. They are probably good enough for the intended job but for actual live power measurements on alternators there is nothing to beat decent quality analogue meters.

These should be available in abundance cheaply but if it is the same over there, they were all scrapped out in the digital revolution and you have to be lucky to find a few that got rescued from the hammer.

Digitals are fine for measuring battery volts. If you can get a decent moving coil millivoltmeter you can use it with shunts to measure battery charging current. Most are 75mv fsd for the standard shunts but you could pad it out with a series resistor and use your 100mV shunts. that way you would get away without the calibrated leads that normally need to be used between the shunt and the meter. Various makers chose different lead resistances, but the common one used by Weston was .027 ohms. Even so using normal short sensible leads will not give errors comparable to the other thinks you are experiencing, it is only when you are looking at errors in the 1% region that these things become critical.

Surprisingly if you are stuck, using a normal dvm on the millivolt range with a shunt normally works fine. DVMs have been developed to a high standard even the cheaper ones and they are often much more accurate on funny waveforms than expensive digital panel meters but the real test is to see if it lines up with a moving coil analogue, if so then the digital is easier to read quickly for test readings.

Flux

[ChrisOlson]

These should be available in abundance cheaply but if it is the same over there, they were all scrapped out in the digital revolution and you have to be lucky to find a few that got rescued from the hammer.

Nobody stocks analog meters, is the problem.  They have to be ordered.  And they're not cheap anymore for a nice one.

It's like regular old clocks.  I wanted to find a 120 volt old time clock like we used to have in school, and put it on the line that goes from my dump controller to the water heater element so I could keep track of how much time the heater runs every day.  Just set the clock on 12:00 o clock and when there's power going to the heater element the clock runs and if it says 9:00 at the end of the day I know I dumped 9 hours worth of power into that heater.  Really simple.

Except try to find one of those clocks anymore.
--
Chris

[bob g]

try ebay, for a surplus analog meter and shunt

its been a while but i picked up a matched set of precision units made in japan that
turn out to work very accurately, relatively speaking of coarse.

sometimes you can find them well worth the money

bob g

[TomW]

These should be available in abundance cheaply but if it is the same over there, they were all scrapped out in the digital revolution and you have to be lucky to find a few that got rescued from the hammer.

Nobody stocks analog meters, is the problem.  They have to be ordered.  And they're not cheap anymore for a nice one.

It's like regular old clocks.  I wanted to find a 120 volt old time clock like we used to have in school, and put it on the line that goes from my dump controller to the water heater element so I could keep track of how much time the heater runs every day.  Just set the clock on 12:00 o clock and when there's power going to the heater element the clock runs and if it says 9:00 at the end of the day I know I dumped 9 hours worth of power into that heater.  Really simple.

Except try to find one of those clocks anymore.
--
Chris

Chris;

Got any Goodwill or St. Vincent Depaul stores near you?

I see that kind of stuff there a lot used of course.

Just an idea?

Another option is an AC hourmeter. I have a couple but they are off on time because they are 50 Hz.

Just a thought or two.

Tom

[Bruce S]

Another route would be VTVM Heathkit used to sell kits.
Old school electronics classes had VTVM, it's what I learned on long before DVM came along.
I still own one and nope it's not for sale :-)
 

[ChrisOlson]

Got any Goodwill or St. Vincent Depaul stores near you?

Actually we got both and my wife goes there all the time and brings stuff home.  I never thought of that, but that's a good idea!  I'll mention that to her and tell her to keep a lookout for one.  I'd like to get one of those old round ones with the black bezel, white face, black minute and hour hands, and red second hand.  Those are true classics, and my luck they're probably collector items now and worth a mint.
--
Chris

[rossw]

When I modified these meters I removed the negative lead and internal shunt and soldered on an ethernet twisted pair from the Kelvin sense pads to the external shunt.  I don't know if that ethernet wire is throwing them off, or what.

There's no such thing as "ethernet wire". Ethernet (at least 10/100/1000BaseT) simply uses ordinary twisted pairs. Other forms of ethernet use coaxial cable, fibre-optic cable etc.
If you mean "Cat5" cable (commonly used for ethernet) then it's just plain copper wire and will have no practical bearing on your readings whatsoever.

[bob g]

because of just what you are going through Chris, i went with analog meters and precision
shunts, "and" 3 different fluke meters, "and" a couple of other digital meters, "and" finally
put together a logic table to sort out the results...

then i came upon the GE digital residential meterheads on ebay, brand new for something like 15 bucks each.

they can and will measure over 250 parameters on AC power that is within the 50-60hz window (actually a bit lower and a bit higher as well) power factor, about a half dozen varieties of distortion, of course kwatt/hrs, amps, volts, etc, plus
they can be set into calibration mode and measure watt/hrs to +/- 2 watt/hrs which is pretty good when you are working with kwatts of power.

once i knew the advertized efficiency of my prime mover, (i accepted their numbers at face value as i will explain later) i then could measure fuel consumption in grams per kwatt hour, measure on the DC side, use the inverter at its advertized efficiency (accepted as with the prime mover) and then i could via a logic table determine mathematically to within a point or two how efficient the alternator was.

my modified alternators reach right at 80-82% efficient at 28.8volts and 100amps
i could prove this mathematically because if the engine was less efficient the alternator would have to be more efficient (very unlikely) and if the alternator was less efficient the engine would have to be more efficient than the manufactures rating of 32% (also very unlikely), further the efficiency of the inverter (86%) also would have the other two shift up or down which would be unlikely if it were either more or less efficient.

i could then recheck all my meters for accuracy, and again using a logic/truth table determine which meters were accurate (relatively speaking) and which were useless.

it didn't take long before i could accurately predict all sort of events, and was able to accurately determine just how efficient things like my belt drives were, how much excitation as a percentage of total output, and all sorts of other interesting stuff.

bottom line i would not be without a GE kwatt/hour meterhead (the digital type) and the
precision analog amp meter with its precision matching shunt.

as well like as fluke meters are, i have found them to be all over the place, unless they are periodically recalibrated they are about as useless as boobs and a boar hog for any precision work.

another thing i learned along the way, anytime i run a test on anything, and i find my results to be more than a percentage point or two better than industry standards, then i am suspect of something i am doing wrong... and 99.99% of the time it turns out to be a
faulty tool/meter is the culprit.  if i see a result that puts me 5 points above what others have seen, then i automatically assume my test to be faulty and go back and repeat it until i find the fault...

interestingly i found a book on testing protocols, and the "experimentalist"  (yes i guess that is not only a word, but there are such critters). It seems as though problems with measurements are a huge problem not only for the little guys like us, but also the big boys such as those found around NASA.  bugs in measuring dept are a common and frustrating problem for everyone it seems.

this sort of thing is what gets me up each morning, i absolutely love the theory, testing, measurement and logic required to attain good and repeatable results.

good luck

bob g