Stepper Motor Genny - Increasing Amps - How?

[wadlands]

Hi - i have been trawling the great posts on this forum as I'm trying to get into green energy & windpower.  Reading everything I can & now started experimenting with drill motors and stepper motors.  One of my bench tests is with a stepper motor (Nema size 23) and voltage doubler rectification. 

Using a cordless variable speed hand drill, at say 100RPM and 500RPM I get the following figures when bench charging a flat lead acid car battery:
100rpm = 15v at 130mA and 500rpm = 41v at 230mA.  Which looks like a promising start??

I have tried seraching the forum for the answer but can't find anything: how can one increase the amperage given that there is such a healthy voltage.  Can a transformer be used or some other type of circuit?  Or can nothing be done because the frequency of the original stepper output is varying with wind speed (before rectification)?

I can see from the tons of posts on this forum that there are some really knowledgeable guys out there - can anyone point me in the right direction on this qestion??

thanks in anticipation

[ghurd]

Need more info. 
How many wires on the motor?
Which voltage doubler circuit?

"and voltage doubler rectification"
You can not have everything.
Double the voltage halves the amps.

100RPM is too slow for a stepper motor wind turbine anyway.
Better off not doubling the voltage, and getting the extra amps.

G-

[wooferhound]

When hooked to a good battery the voltage will stop going up when the output reaches Cut-In (battery voltage). The battery will hold the voltage down and the amps will start going up.

[wadlands]

ghurd / wooferhound - terrific guys to get a response so quickly!!!

Motor is an 8 wire Slo-Syn Type MA61FS-80020,   200 Steps,   5v,  1A.
Reading the forum posts someone said "steppers - you have to try to see what works with them" - so I tried both these rectification circuits:
http://www.c-realevents.demon.co.uk/steppers/fullwave.gif (voltage doubler?) and
http://www.c-realevents.demon.co.uk/steppers/stepdc.gif both from this web page
http://www.c-realevents.demon.co.uk/steppers/stepmotor.html. (simple rectification).

With the "simple rectification" the stepper output is between 8v (100rpm) and 21v (500rpm) so the amps only cick-in above the nominal 12v or so of the battery and I get between 90mA (100rpm) and 190mA (500rpm) into the flat test battery on bench test.

So in my newbie's thinking the voltage doubler was better because it kicked in above the battery voltage "sooner" and so would be better at lower wind speeds?

Does this also mean that I should really think of the steppers limitation as being (21v x 190mA) as its max power output to the battery?

[Bruce S]

wadlands;
your latest thinking is more correct.
That's the little secret about the voltage doublers, tripplers, etc.
As G- said double the voltage and get 1/2 the current.
Plus it's not straight forward double either as the diodes need a bit of power for them to "work".

figure the max outputs at 500RPM.
Doubler may help with such system, but as Woof said, the rest will be controlled by the battery.
Best of luck, let us know how it works out.

Cheers
Bruce S

[wadlands]

Thanks for chipping in Bruce

So in my simple thinking (and without getting too technical / scientific) the stepper output is governed by:
Volts from the the speed (rpm) with which the coils cut the motors magnetic fields, the strength of the magnetic field (type of magnets & sizes), amount of copper cutting the magnetic field.
Amps from the resistance of the copper used for the coils - so amps = volts / coil resistances?

Then all other things being equal; a larger diameter and longer stepper motor would give more amps ?
(In the expectation that the magnetic field strength and so the volts at 500rpm, is going up faster than the resistance of the extra wire in the coil windings? 
Ie a Nema size 34 would be better than a Nema size 23 stepper motor?

What experiences have other pioneers had - I've read through many of the other posts regarding steppers and there seems to be a rough 50:50 split as to whether this is a worthwhile route to take for a workable and practical lead acid battery charger (one battery at a time) or whether its only just a good place for a novice to start and experiment. 


To be able to see over the horizon would be useful!!! ...are there any steppers that anyone would like to recommend or that give over 1 amp as an output??

[ghurd]

Motor is an 8 wire

Reading the forum posts someone said "steppers - you have to try to see what works with them"

So in my newbie's thinking the voltage doubler was better because it kicked in above the battery voltage "sooner" and so would be better at lower wind speeds?

Does this also mean that I should really think of the steppers limitation as being (21v x 190mA) as its max power output to the battery?

Those sketches are for a 6 wire stepper.
If your stepper has 8 wires, one tiny mistake in phase can screw up the whole operation.  And it is a bugger to get them connected correctly.

"sooner".  Maybe. 
But "sooner" is a catch 22. 
In steppers it can work if the output is terrible anyway.
In larger machines it kills the output due to stall.

'Try it and see.'  I bet a shinny new nickle it was me who said that.


"limitation as being (21v x 190mA) as its max power output to the battery?"
Not really.
The current is reactance limited due to induction in the iron-cored coils.  That means there is a point where the current just does not increase with RPM.  Running it faster may actually DECREASE the current, regardless of the battery voltage (be it 6V, 12V, 24V, or 48V).
Guessing-  The max power of this particular stepper will be 200ma x battery volts.

Larger dia and longer?  You mean a bigger one makes more power?  Yes.  (usually)
This is 5.5" x 7"(?), basically a stepper, wieghs about 12 pounds, and has the same limitations.
Notice how the Amps : RPM kind of go flat as the RPM increases.


200ma max ain't much in a typical size lead acid battery.
Worthwile, yes.
Workable, yes.
Practicale, yes.
'a good place for a novice to start and experiment', yes.
Must keep the expectations real too.  It is good for maybe charging the cell phones via a switching regulater, etc.
It is real, it is just small.

On a better note, it took me a Long Time to find a stepper that made over 75ma into a 12V battery.
I have 100s, and only a few that will do 100 or 200ma into a 12V battery.

Better steppers are bigger steppers.
Bigger steppers are called servo motors.
G-

[wadlands]

What a terrific forum - What a terrific reply!!! ....you're the man ghurd!!!....thanks for taking the time for such a full reply.

It's all a great spur to carry on with the "experimentation" and my own learning process - compared to some of the work that's been posted in this forum - I'm just at the "crawling along the floor stage"

I "understand" the current limitation even at higher rpm - is it like back emf? in the coils that causes this?

OK - as I play with this stepper (2.25inch diam & 2inch length) and readup about steppers I see that for their normal use in CNC drives etc you can connect up an 8-wire stepper with the coils in series or in parallel.  At the moment I have them connected in series - is this thw way to go when using a stepper as a genny or should they be in parallel?

[Isaiah]

When looking at pm motors for wind turbines look for a hi voltage rating the lower the rpm the better and higher amp out put.
 Say 240 volt 2000 rpm 10 amps or more.
Then you'll have to figure the swept area to get the most out of your motor.
These things have to be tuned to your location. find out what the average wind speed is for your area for starters.
Don't worry about making mistakes thats part of the fun and experience you'll gain.

[oztules]

Wadlands
"I "understand" the current limitation even at higher rpm - is it like back emf? in the coils that causes this?"


It is back MMF (magneto motive force) thats does the damage here.

When you induce a current in a wire with a permanent magnet ( in this case ), then that current will exhibit a magnetic field.... of opposite polarity to the field that induced it.
It is all about ampere turns... not EMF.

Ghurd gave a good graph of the effect.

What it means is that when we draw higher currents, then the wire will develop a larger and larger opposite field "fighting" against the permanent magnets. As it repels the perm field, then there is less flux left to permeate  the coils..... to make more current in the coil..... this is why it drops off.

It gets to the point where spinning it faster results in a pitiful increase in current.... here is where the permanent magnet can no longer push more  flux into the coil without considerable repulsion. I can't see why it would actually decrease as Ghurd mentioned, but maybe that's just because I havent seen it.

I would have thought that as we increase speed, even with a much compromised field due to the reactance between the perm magnet and the repulsive field from the coil, that the faster transitions will always return some (ever smaller though) improvement, even as the useful flux decreases..... some Ghurd guidance may be useful here

It is easy to see the power decrease, as shorting the coil, gives max current, max repulsion (back MMF), but no output emf (short circuit)...  The watts useful to us are output volts X current ..... so shorting a highly reactive mill usually results in it spinning faster.... not slower as we expected, as we are no longer loading it with an external load to shed power through.

This is not all bad...... it makes for some alternators that are impossible to burn out, as the back MMF limits the output to a thermally safe level in the coils.... the F&P style mill and the African Wind Power mills are examples ( Not sure of the newer AWP as they have moved on to Neo magnets instead of the traditional ferrites)

So we can see, the stronger the permanet magnets, the later the back MMF will dramatically effect the performance. The axial flux popular here have very powerful magnets with large air gaps, and so are not likely to reactance limit at a safe thermal range...... they keep on outputting higher until they cook or furl.




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

[Madscientist267]

Oz -

The reduction would come from the interaction of frequency with the core - the magnets are already fighting the back MMF, and the increase in speed means that the magnet poles spend less and less time interacting with a given coil.

Think of it in terms of the 'inductance' of the core - higher frequency means more losses, the effective magnetic field at the coil is reduced further by a combination of eddy currents and hysteresis of the core.

Once the frequency goes above 'critical-zero' (the point at which everything is in balance), the power output begins to reduce.

Reading back over that, it may or may not make perfect sense, I'm having a little trouble explaining it.

It also is what 'helps' a reactive genny spin faster when shorted. Eddy currents can become the dominant force under the right circumstances, and the coils have little to 'say' about what happens to the shaft.

Make sense? 

Steve
 

[ghurd]

I never put much thought into 'why' it happens.
I just kind of figured the frequency was too high to get any current flowing in an inductor.

Something Flux (a member) mentions from time to time is the conduction properties of rectifiers.  In my brain, sounds like the properties may add to the effect of reduced output current.

One of the makers of modified car alternators (windblew, etc) had charts showing it, if anyone looked at them closely enough.
It was a company who gave rated watts output as Open Voltage X Short Circuit Current.
Their "Rated Output" sort of flattened out and possibly decreased a bit at very high RPM.  Their Open Voltage chart was quite linear.  The only way for that to happen is if the current decreased.

Another company had charts showing the short circuit amps decrease at higher RPM.


"you can connect up an 8-wire stepper with the coils in series or in parallel.  At the moment I have them connected in series - is this thw way to go when using a stepper as a genny or should they be in parallel?"
uh... Try it and see!

Remember, the coils in an 8 wire stepper are out of phase.  That means one coil matches up with one other coil.  Then the other 2 match up.

If in series, and get no output at all, one coil is 'backwards'.

If in parallel, and get no output at all, one coil is backwards.

If the coils are NOT the correct matched pair, there will be some output, but less than there should be.

I suggest making a easy to connect rectifier assembly.  Lots of diodes and clips.
It makes it a lot faster and easier to do tests.
Can see one of mine below.

Ghurd's Rule #1:  No matter what it is, Everything works better with hot glue.
G-

[wadlands]

oz & oztules - thanks for the confirmation on my emf thinking

ghurd - yup - "I owe you a shiny nickel"....."Nothing is written in stone for steppers.  It is trial and error."

....and talk about being on the same page!!!!!....???....who was it who said:
I would love half an amp at 13 volts. Definately a Vertical, possibly Sandia Savinous ....We don't get much wind. I....I don't expect much efficiency. ....   Not looking for magic, just something that has a better chance of working the first time.!!!!.....????

Even my diode rectifier board is a lashup similar to yours - I'd post a photo but can't seem to find out in the Help menu???

Yes - I am going to try parallel connections because that's supposed to lower coil resistnace - at least I think so as a normal use CNC stepper.  Should be just as beneficial in terms of lower resistance if used as a genny??  - after all the copper windings won't know its being used as a genny - and I'm not going to tell them.

Don't know anything about stars & delta connections but have seen them bantered around on the forum - is this also something to be considered when using an 8 wire stepper?

[ghurd]

Star and Delta can not be used with 2 phase machines.
It requires 3, or 5 phases.  Maybe 7 or 9 if someone was crazy enough to try to make something like that.

There are exceptions to every rule (except the hot glue rule).
I am ignoring some of the more confusing and advanced concepts because they do not apply to your sititation.  And not sure I understand some of them myself.

You only have 2 decent choices with 8 wires:
#1- Parallel the 2 matching coils, rectify.  Repeat for the other 2 matching coils.
#2- Series the 2 matching coils, rectify.  Repeat for the other 2 matching coils.

Keep track of the nickels until it reaches $1. 
G-

[Madscientist267]

I'd post a photo but can't seem to find out in the Help menu???

1 - Click browse, then find the picture you want to post.

2 - Place the cursor in the reply where you want the picture to show up. Click "Insert Attachment X".

You'll see "[ attachment = x ]" show up in the text of the reply. This is the placeholder for your picture.

If you want to post more, click "more attachments", lather, rinse, repeat.

Keep the file size below 150KB; the board will reject it if you don't. A good rule of thumb for a JPEG is to resize to 640 pixels in one dimension (ie 640x480, 640x361, etc). There's little advantage to leaving them bigger than this; the board will shrink them down (to a point) anyway.

Steve

[wadlands]

ok - thanks Madscientist - here goes

the stepper motor I am using to cut my teeth and bench-experiment with.

- the 4diode and 8diode(voltage doubler) simple perf boards I am using.  The sharper knives amogst you will notice that I am still using the rather old fashioned way of copper rod heated on red hot coals to drip solder onto the components!!!  Haven't graduated to the true pioneering spirit of hot glue yet...but then I hear that supplies are running short!!!!

Well this will sort out if I can follow instructions ........

[wadlands]

Ok  - I'm learning so much from this forum from what others have shared - so in the spirit of giving back ... here are my test results from my stepper motor. 

I know its basement level info compared to the kW size generators that others are focussed on - but it may be of use to someone else just starting out. 
Trying out series and parallel connection of the stepper coil windings.
I've actually decided that it will be better to use the stepper with its coils connected in parallel: earlier cut-in voltage(?)/rpm like the "voltage doubler" but amps at the level of series connected coils (because parallel connection reduces the coil resistance).

 Slow-Syn Tests.doc (19.5 kB - downloaded 223 times.)

Now to test the actual amps going into a flat battery.....

[oztules]

Oz -

The reduction would come from the interaction of frequency with the core - the magnets are already fighting the back MMF, and the increase in speed means that the magnet poles spend less and less time interacting with a given coil.

Think of it in terms of the 'inductance' of the core - higher frequency means more losses, the effective magnetic field at the coil is reduced further by a combination of eddy currents and hysteresis of the core.

Once the frequency goes above 'critical-zero' (the point at which everything is in balance), the power output begins to reduce.

Reading back over that, it may or may not make perfect sense, I'm having a little trouble explaining it.

It also is what 'helps' a reactive genny spin faster when shorted. Eddy currents can become the dominant force under the right circumstances, and the coils have little to 'say' about what happens to the shaft.

Make sense?  

Steve
 

No, I have trouble with this explanation.... for three reasons..

1.

I used to think along those lines as well... but have since found that the synchronous impedance of the system is made up of armature reactance, resistance of the stator, and inductive reactance.

2.

The real world does not reflect your explanation...... eg.

a. When we short a F&P or an AWP, we can be outputting say 3/4 max power..... (1.1kw) and running nicely......... and then short it out.
We have not messed with the F, but we now have the same EMF going through a smaller R (no load in series with the stator now) and so A tried to increase...(I=E/R) but it runs away instead........ it was NOT F.....we didn't change that.......... that comes later as it runs away much further perhaps. Now if we didn't change F, and we have now partially unloaded  the prop by simply shorting the output then ??

b. If it were eddy currents (unlikely in good steel laminates), it would help stop runaway, as it would absorb power from the prop to service them, and hysterisis would also help stop runaway if they were significant, and act as a drag.

c. If Ghurd is right, and we see a decrease in current somewhere (I haven't seen this as yet) then it is more likely that at very high speed, the inductive reactance may work in concert with the armature reactance to decrease current. This would be a fair ask,  but I haven't let one go that far out of control..... but still your XL is linear, as 2pi is a constant..... so XL is proportional to F ..... (XL=2Pi FL)..... does not suit what happens...... in the real world it just current limits. (I know there is an L on both sides, the XL=inductive reactance, and the FL means freq X coil inductance.... I can't use the keyboard to it's full extent)

d. The effect is independent of the EMF, but IS dependent on the AMP TURNS...... either more turns at less amps or less turns at more amps.... it will happen at the same amp turns (sigh..... mostly...... we still have R and XL to worry about) ..... So I will say mostly..... when the  demagnetising force of the stator current is nearly equal to the MMF of the rotor, we will stop producing much more current.

e. "and the increase in speed means that the magnet poles spend less and less time interacting with a given coil." Perhaps rethink this a bit. The faster changing flux (less time interacting with a given coil ) means the flux is changing faster which induces more EMF.... not less..........  it's not the emf which suffers from speeding up the rotor... but the available current.  (I=emf/synchronous impedance)




3.

I tried to explain this (your theory) to a power engineer (40 years with power stations), and he finally convinced me otherwise. He was right , I was wrong.

XL still plays a real part, but it is relatively easy to calculate it's effects.. which are basically linear in nature.





I used to think it was XL as you do, but not now.




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

[wadlands]

Wowwww oztules - I can see that I've got a long way to go to start understanding some of this electro-magnetic stuff and its implications for wind generators!!!...but I find it a stimulating challenge to learning about something new.

In my limited understanding - I thought it was something along the lines of: the core saturates and the back-emf keeps fighting back (the increases in rpm) and makes the coil windings behave more and more like resistors, they start getting warmer....their resistances go up as they get warmer (back emf fights the rpm increase) and so the current drops a bit??

[Madscientist267]

Hmm...

This may truly be worth exploring further, as I'm not completely convinced that the core is not solely responsible. The phenomenon does not appear to occur in an air core system (such as the typical axial PMA), at least to the same degree.

Therefore, I'm seeing it from a permeability point of view. It's already given that as the frequency increases, the permeability of the core decreases:

http://en.wikipedia.org/wiki/Permeability_%28electromagnetism%29#Complex_permeability

... shows a curve illustrating this.

The effect would also be present in an air core system, but since air already has such a low permeability, the effect is less noticed.

Not saying you're wrong, by any means, because I'm not exactly a physicist, but that's where it points, to me.

I will agree that there is some odd explanation necessary, however, because permeability doesn't seem explain it all right off.

I'm not so convinced that the coil properties have much play in it. Since it's frequency dependent (the other variables haven't changed), and the core seems to be solely responsible (for practical purposes), the coil is seeing less and less of the magnetic field, and plays less and less of a role. Hence eddy currents becoming the more dominant force (the core being closer to the magnets than the coil physically). Is it possible that the core may even be exhibiting a negative effect (as compared to air) on the path of the field at higher frequencies due to a combination of permeability and hysteresis (propagation of the field through the coil as it goes through the core)?

I suppose an experiment is in order, involving single heavy turn coils with movable cores (maybe even of various materials) to help further eliminate outside influences on the outcome (such as transmission line resistance and the like).  

One thing is for sure, I'm not entirely positive at this point how to go about making such a test machine, but the gears are turning!

Steve

[Madscientist267]

Noticed you had come in to read as I was editing something...

Steve

[oztules]

Hmmm

I first need to make a correction from a previous post:

This:
"When you induce a current in a wire with a permanent magnet ( in this case ), then that current will exhibit a magnetic field.... of opposite polarity to the field that induced it.
It is all about ampere turns... not EMF."

should read
"When you induce a current in a wire with a permanent magnet ( in this case ), then that current will exhibit a magnetic field.... in opposition to the field that induced it.
It is all about ampere turns... not EMF."

It seems I am unable to sway you..... so you convince me.


Shorting the mill while it is operating happily makes it current limit (AWP, F&P etc) .... we did not change frequency for it all to fall to bits... repeat did not change F at all.... not even a little bit. After it current limited (we shorted it out to stop it), the wind sees less load and drives it faster.....

What could cause that to happen if we did not change frequency?

How is the electrical load presented to the prop by the alternator? (where does the torque originate from that loads the prop.)  or  Why is it hard to turn a shorted alternator.



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

[ghurd]

And that is why I did Not put much thought into it!

Looked for the company with the chart showing a decrease in current at crazy-high RPM, no luck.

I expect one of you guys with better machine tools could chuck up the smallest, lowest amp rated, higher volts, 1.8°/step or 0.9°/step, motor to charge a 6V or 12V, run it up to a couple/few KRPM and see it happen.
Still would not explain why.
G-

[Madscientist267]

It's clearly a mysterious phenomenon... but I think it may be a whole combination of things.

This may be one of those cases where shining the darkness absorber on it could provide some insight; sometimes knowing what something is, is more about knowing what it's not...

I think we agree that this does not occur in an air core configuration, at least not to an appreciable degree. That would indicate that the core plays a role.

So what are the differences between an air core and an iron core system?

Aside from the material that the core is made of, there's another aspect that I hadn't completely thought about when pondering this... the air gap.

I went looking around and found a PDF that is about a motor. While the document is not specifically geared at generating power, I think we can agree that the principles are very similar, if not identical, just in reverse.

In the PDF, he mentions (on page 9) the concept of the air gap storing 'excessive' energy. Applying the concepts mentioned in the PDF, in an air core system, the coil is part of the gap, and in the iron core system, the coil is outside of it. At the higher frequencies where the phenomena takes place, this could easily be explained by the lowered permeability. It doesn't show up at lower frequencies/rotor speed because the permeability is higher, and the field is reaching the coil more effectively.

I think also some of the confusion is coming from the term 'frequency'. You mentioned "did not change F at all"; and you wouldn't need to. In the case of the mentioned mill, running normally as it would usually be and then being shorted, the frequency would only change if the rotor changed speed. It's not the change of frequency that makes the difference, it's the sharper transitions that the core experiences due to higher frequencies.

This leads me to think that the scenario arises from interaction between the coil and the core, the core and the air gap, and the core and the magnet across the air gap, with hysteresis being influential at some form of boundary where the coil and core 'meet'.

Hysteresis gets involved because possibly a heavily loaded/shorted coil may be enough at a high enough frequency to prevent the field from propagating through the core altogether, thanks to the back MMF provided by the shorted coil. The energy would then just be spooled up in the air gap between the core and magnet, with nowhere to go, and is returned (like a spring) to the rotor as it passes, minus losses.

Almost like reverse cogging. Instead of attraction, it's repulsion.

Steve

[ghurd]

Afterthought that may be relative.  Or not.
The B-H curve of the core? 
In my head, seems like there is something related to the time it takes for the core to "switch polarities"?  If so, the core may be oscillating, and not be fully saturated for a good percentage of the time?
G-

[Madscientist267]

time it takes for the core to "switch polarities"

Precisely.

Whether the core is oscillating has yet to be determined (for us), but yes, at higher frequencies, it doesn't have time to fully switch.

So as the core does begin to reverse, the 'new' field is met by the back MMF of the coil, combined with the effects of hysteresis, and any further field propagation is cancelled.

The result is that the current not only levels off, but can even be reduced (as the other dominating factors take shape while the frequency increases).

I'd be willing to bet that the effects could be highly exaggerated by moving the coil away from the rotor on the stator's pole. Increasing the length that the field has to travel, from A to B below, would lower the frequency that the core can respond to:



... while moving it as close to the rotor as possible would minimize the effect.

Steve

[Flux]

There are two reason for a permanent magnet alternator to current limit, the fist is the effect of the armature reaction on the magnet field. The field from the current in the coil acts on and modifies the main permanent magnet field.This was a very significant factor with the older type magnets and in the extreme case a short circuit on the machine would actually permanently demagnetise the magnets and you would need to re magnetise to get it back to its original condition.

The other factor is leakage inductance and mainly causes trouble with machines with long iron circuits. It is a much more troublesome thing with weak magnets and the two things were very much linked in the old type machines with Alnico magnets. With ferrite magnets you hardly ever completely demagnetise the things but you work the magnet up and down the the demagnetising quadrant.

The main factor with steppers, F & P and other things with torturous magnetic circuits is the leakage reactance. Not all the flux from the magnet links the coil and with these long iron circuits the armature reaction forces more flux to leak to somewhere not useful as you increase the load current.

This effect gives the effect of a reactance in series with the output. You have a limitation that you would expect from internal resistance but you have an additional component from the equivalent inductance. Above the frequency where the inductive bit equals the resistive bit the inductive component takes over and swamps the resistance. You end up with a condition where the emf goes up with speed ( frequency) but the reactance also rises with frequency ( speed) the two effects give a condition where it settles into a state where the current becomes constant no matter how fast you run it.

This is the basis of your cycle dynamo, the volts rise as you pedal faster but the current into the bulb stays constant and is determined by the reactance of the system.

With air gap machines the leakage flux is small and with powerful neo magnets the armature reaction effect is small and over the range that you can use it the short circuit current is dictated by the winding resistance. the short circuit curve is a straight line at some slope set by the resistance.

The iron cored pma will start of on the same line but eventually it will turn over as the inductance dominates and it will level off to constant current.

Don't get this leakage inductance concept mixed up with the actual inductance of the winding as you would measure it with a bridge on the output terminals. A transformer winding has a very high self inductance and that decides the magnetising current. it is the leakage flux that doesn't link both windings that determines the equivalent series inductance that will limit any short circuit current and alter the regulation. The leakage in a transformer is very low but in alternators it can be very much higher. You can design transformers this way for welding and neon signs etc and they again approach constant current on full load.

Hope you can see what Oz was getting at now.

Flux

[wadlands]

Meanwhile....in the beginner's corner!!!....update on my little stepper when connected to a flat car battery:

Coils in series & 4 diode rectification: @200rpm ~10mA and @500rpm ~ 130mA
Coils in series and 8 diodes (voltage doubler) rectification: @200rpm ~ 30mA and @500rpm ~ 230mA
Coils in parallel and 8 diode rectification: @200rpm ~ zilch and @500rpm ~ 100mA

...so back to the original thinking ie coils in series with voltage doubler.

Now to try out my other stepper motor which is a bit bigger - I'm sure it will only confirm the many comments I've read / been made about steppers. 
But I hadn't come across figures that I could relate to - and this way at least I now know more about stepper motors than when I started on this "journey"

[Madscientist267]

Meanwhile....in the beginner's corner!!!

LOL Sorry Wadlands - Guess we got a little out there on you... Good learning experience however.

Looks like you've got some usable data there. And physically larger combined with higher rated voltage steppers will give better results. Lower voltage steppers will land you in the mess you found us in because you have to wind it up so fast to get your target voltages.

Hope you can see what Oz was getting at now.

I can't say I ever completely threw it out... From what I gather you've basically indicated that we are both right, but scoping it out from different angles.

The "long iron circuits" issue was basically my POV for it. Figured the whole thing was multi-faceted.

I can comfortably concede that it is a problem of more than one origin.

Somehow I knew you'd come in and clear this up for us.

I had no idea however that it was actually possible to destroy the magnetism on the old type magnets from shorting the output. Interesting...

Steve

[wadlands]

Madscientist627 - hey don't get me wrong...I am sucked in by the exchanges...and as Poirot says "they keep the little grey cells active".  I may not understand all of it but I can grasp some of it....and Flux's exposition (read preparedness to share his knowledge, along with everyone else involved in the exchange) just makes this a great University of Learning for me!!....now to Google reactance...inductance...flux...leakage....

[wadlands]

Progress – started to test out my larger stepper motor.  I was encouraged by the 12v  @ 200rpm with 1A output and 38v @500rpm with 2A output (short circuit measurements is the term I think?).  Coils wired in series and rectified with my 4-diode board.

So re-connected the stepper output to my voltage doubler board and bang ….capacitor blew.  I had a 47uF 35v electrolytic to smooth the output – figuring that 2x the expected voltage (with my small stepper) of around 12v was ok.  Totally oblivious to the fact that this larger stepper had just shown 38v @500rpm!!!!

Well its an early lesson in watching out for the capacitor ratings – am I correct in thinking that one should use smoothing caps at about 2x the expected rectified voltage?  Also have come across circuits that use one smoothing cap across the final output of a 2-phase bridge rectifiers.  And some circuits that use one smoothing cap for each 4-diode rectifier (might have the jargon wrong here?).  What are the benefits or not of using one or two smoothing caps???  Also I am just using 47uf cause that's what was in one of the circuit diagrams I saw – how do you decide what it should actually be?

OK – back to the plot – took the opportunity to do another 8-diode voltage doubler rectifier board for my larger stepper and used 1A Schottky diodes instead of the standard diodes I had used previously.  See results in attachment – can almost touch that 1Amp going into the battery…………..any one got any idea why this time with the larger stepper, the better result of current going into a flat car battery is with the stepper coils in connected in parallel??
 Nema34-A.doc (20 kB - downloaded 217 times.)
Also worth adding that with either the coils in series or in parallel I was able to witness the drop off of the peak amperage from the nominal 200rpm maximum to a reduced level as you approached 500rpm (measured as the short circuit output).  I would not have looked for this had it not been ghurd's original posted graph and the ensuing exchanges…so confirmation of the theory for one and all!!??

The more interesting thing though is that when the stepper output is connected to the flat car battery I am using then with the coils in series the output remains constant at ~480mA and with the coils in parallel it continues to rise from 500mA @200rpm to 950mA at the max speed of my cordless 14v drill (nominally marked as max 550rpm)

All of these results have been bench repeated so I believe are “accurate” – the nominal rpm speeds of 200 and 500 are with me guesstimating half trigger and full trigger depression on the cordless drill.

Unfortunately my stepper motor has no label on it but its dimensions are:85mm diam and 125mm length:

[Madscientist267]

am I correct in thinking that one should use smoothing caps at about 2x the expected rectified voltage?

General rule is, a capacitor rating of at least 25% over what your expected voltage is. They used to be rated in 'WVDC' (Working Voltage Direct Current), but this has been slowly replaced with just "V". It's not really clear if the intent was just to shorten WVDC or if it applies to a different measurement method. Either way, better to be safe than sorry. A cap pushed to it's limits will exhibit very unwanted effects in a circuit. They start to become resistive, and then as the heat builds up, they look more and more like shorts to the circuit until bang (well, vent now - they took all the fun out of it with their sliced cans!). At that point it's anybody's guess what happens. Some fail open, some fail shorted, some fail somewhere in between. It's best not to ever find out.

You can always have the cap rated as high above the working voltage as you wish; selection is more about physical size restraints and cost. Hence 25%.

To use smoothing caps effectively, they need to be large enough (uF) to handle the ripple currents provided by the combination of the source and load. The basic calculation goes like this: A 1 farad cap changes by 1 volt in 1 second with 1 amp of current flow.

To use them in a voltage doubler, the caps that are in series should be identical, and then if it exists, the 'master' cap can be of any value (although it is generally greater than, or at least equal to the capacitance of the other two).

Remember the calculation above when selecting caps for a doubler, and also remember that caps in series (of equal value) will collectively have a capacitance of half of the rated value for each cap.

In other words (numbers kept large and round for calculation purposes), a doubler using two 1F caps in series will have an effective output capacitance of only 0.5F, which also cuts the filtering ability by half. So your load (if kept the same) would end up seeing twice the ripple at the output of the doubler.

Note that 1F caps are very large, and are not generally anywhere near a necessity. I used those values to illustrate the relationship between capacitance, voltage, current, and time.

Steve

[ghurd]

"with the coils in series the output remains constant at ~480mA and with the coils in parallel it continues to rise from 500mA @200rpm to 950mA at the max speed of my cordless 14v drill "

I think about it like each coil can supply or carry a maximum quantity of amps.

Your coils can supply or carry ~480ma.
In series, the limit is still ~480ma.
In parallel each coil can supply ~480ma, combined for twice as much as ~480ma, or about 950ma.

I imagine you are using all 4 coils, and for clarity my example is for 2 coils.  You get the idea.

Ain't this fun?
G-