Alternator Output Waveform

[kamikaze762x39]

I welcome any input on this topic, as I may have the entire thing wrong.  I started this thread because I keep seeing people explain the waveform output of an alternator as "pulsed DC, alternating voltage with same polarity," etc.

I am pretty sure it is a misconception that a dual-rotor configuration consisting of all North poles on one and all South poles on another will produce a waveform above the zero reference (pulsed DC).  The reasoning is simple.

If we are using a coil on a stator, it lays flat, and the magnet must pass through both sides of the coil.  Using the left hand rule, it is easy to easy that once the magnet, regardless of its polarity, passes over the left side of the coil, it will induce voltage in one direction.  Once it passes over the other side of the coil, it will induce voltage in the opposite direction. 

A loop of wire is typically wound in a circle that lays parallel to the stator medium.  As such, the wire winds up on one side (in the direction of current) and down on the other.  Thus, the wire "sees" an alternating field, not only in magnitude but also polarity. 

Now... alternating poles versus fixed poles...

I'm going to take a shot at some figures here.  The arrangement is hypothetical, of course, and is not a recommended configuration.  It is assumed that the coil winding is seated on one fourth of the stator, perpendicularly, at 90 degrees between windings 1 and 2.

Fixed Poles:

Magnets: 2 (same polarity, arranged directly across from each other on rotor)

Coils: 1

Turns per coil: 1

Induced voltage per winding pass per magnet: 1

RPM: 60

RESULT

Cycle analysis (passes in 2 rotor cycle): +1v, -1v, +1v, -1v, +1v, -1v, +1v, -1v

Voltage: 2 Vpp, 1 Vp, 0.707 Vrms

Frequency: 2 Hz

Alternating Poles:

Magnets: 2 (alternating polarity, arranged directly across from each other on rotor)

Coils: 1

Turns per coil: 1

Induced voltage per winding pass per magnet: 1

RPM: 60

RESULT

Cycle analysis (passes in 2 rotor cycles): +1v, -1v, -1v, +1v, +1v, -1v, -1v, +1v,

Voltage: 2 Vpp, 1 Vp, 0.707 Vrms

Frequency: 1 Hz

It is noteworthy that between each of these passes, the voltage will drop to zero, giving something more like this:

Fixed: +1v, 0v, -1v, 0v, +1v, 0v, -1v, 0v, +1v, 0v, -1v, 0v, +1v, 0v, -1v

Alternating: +1v, 0v, -1v, 0v, -1v, 0v, +1v, 0v, +1v, 0v, -1v, 0v, -1v, 0v, +1v

If we plot both of these waveforms, it is easy to to see that the fixed arrangement will produce a sine wave more or less, while the alternating arrangement will produce a distorted waveform.  Even though the full-cycle frequency differs between the two waveforms, the net change in amplitude at each quarter cycle is the same--1 volt.  Thus, both waveforms SHOULD (please correct me if I am wrong) have the same total power.

The point is, you cannot get "pulsed DC" out of a coil/magnet arrangement unless you completely remove one of the winding directions from the field.  Each wire would have to be wound in a given direction, looped away from the field, and then wound again in the same direction.

I suppose you could create a doughnut-looking stator that encapsulates a rotor in its complicated winding arrangement, but why?  Just use a rectifier. 

The reason alternating poles are used is not because it is the only arrangement which will produce AC.  It is used because, arguably, it creates an elegant and efficient NET flux path between all the magnets.

Sorry for the rant, but I have seen this argument go on so many times in these forums that I felt compelled to register and offer some insight. 

[Seekscore]

Here is a link where I posted the actual waveforms from my 12ft axial flux machine. Three phase. These were with no batteries so you can actually see the ripple DC out of the rectifier.

http://fieldlines.com/board/index.php/topic,143479.msg966010.html#msg966010

Mike

[SparWeb]

I'm surprised to see you driving at this with so much vigour - you sound like you're trying to set the record straight about this, or maybe just thinking out loud, but maybe you've seen a thread on Fieldlines that I haven't noticed yet.

If you really want to have fun with the math, consider that the electromagnetic flux across the magnet gap is actually a vector, not just a scalar number.  So is current for that matter, but it's very common practice to ignore that.  The result is that the only way to create an electromotive force (EMF) is to vary the flux vector, by either direction or intensity or both.  The flux vector doesn't have to reverse per se, but it helps a whole lot when the goal is to produce EMF.  Even if it doesn't reverse, it is in fact every change in flux, not the quantity itself, that determines the EMF.  So even a field that remains polarized the same way but varies in intensity will create an EMF, and the voltage measured in the coil that picks it up will still be AC.

Reading back that may be a bit hard to read.  I hope you see my point.

[joestue]

i'm not sure who came up with the term "pulsating dc" but whoever it was... was probably trying to explain ac to someone who doesn't understand dc 

[Flux]

I similarly are confused by where this is aiming. Any electrical generator producing an emf from magnets moving over coils will produce a symmetrical waveform. It could be a basic sine wave or more likely a fundamental and odd harmonics. These can be in phase or out of phase so you can effectively get anything from a triangle to a square wave. It is not possible to have even harmonics so the thing is always symmetrical.

Once you start introducing rectifiers int the system then even harmonics do become possible so you can get unidirectional pulses. Perhaps that is what you meant. With a half wave rectifier feeding a battery there will be dc pulses. As you increase the phase number the thing get much closer to smooth dc. By the time you get to 3 phase bridge rectifiers there is only about 6% ripple and it is nearly as smooth dc as a cheap dc generator.

I see a lot of worry here sometimes about the purity of the waveform of an alternator on open circuit, it really is not very important for battery charging as the whole thing goes out the window as soon as the rectifier conducts. If you put a 'scope  on the machine winding when feeding a rectifier and battery the waveform is chopped to a complete mess, so your 10% open circuit voltage distortion is just swamped.

Flux

[Flux]

Having read your comment again I wonder if you are referring to the unipolar generator.

There have been attempts to produce homopolar dynamos that actually produce dc but they have made little progress. The fist practical dynamo ( Faraday's disc) was of this type as was the first motor ( Barlow's wheel) but it becomes near impossible to extend the idea beyond this very simple disc concept. The number of sliding contacts becomes stupid and there are many other problems.

It is perfectly possible to build unipolar alternators with magnets of one polarity on one disc and the other polarity on the other and in fact some of the first axial air gap alternators were produced this way. They are still true alternators but instead of the flux reversing,it goes from maximun to zero. With neo magnets they have no real virtue but in the days of wound field magnets they did suffer very much less from leakage flux and this was a significant factor.

Your comment about "why not use a rectifier makes me think you were thinking on these lines". I also wonder if you have been looking at a strange concept proposed by one of our contributors that was rather unorthodox and something along these lines ( I make no comment).

All normal electrical machines produce ac in the winding, even the dynamo ( dc generator) is an alternator with a mechanical rectifier.

Flux

[kamikaze762x39]

Here is a link where I posted the actual waveforms from my 12ft axial flux machine. Three phase. These were with no batteries so you can actually see the ripple DC out of the rectifier.

http://fieldlines.com/board/index.php/topic,143479.msg966010.html#msg966010

Mike

Yep.  Ripple DC only after rectification.  

I'm surprised to see you driving at this with so much vigour - you sound like you're trying to set the record straight about this, or maybe just thinking out loud, but maybe you've seen a thread on Fieldlines that I haven't noticed yet.

If you really want to have fun with the math, consider that the electromagnetic flux across the magnet gap is actually a vector, not just a scalar number.  So is current for that matter, but it's very common practice to ignore that.  The result is that the only way to create an electromotive force (EMF) is to vary the flux vector, by either direction or intensity or both.  The flux vector doesn't have to reverse per se, but it helps a whole lot when the goal is to produce EMF.  Even if it doesn't reverse, it is in fact every change in flux, not the quantity itself, that determines the EMF.  So even a field that remains polarized the same way but varies in intensity will create an EMF, and the voltage measured in the coil that picks it up will still be AC.

Reading back that may be a bit hard to read.  I hope you see my point.

Well... Some guy proposed that an entire arrangement of same-pole magnets would charge batteries directly without the use of brushes, slip-rings, or rectifiers, which I am pretty sure is not true.  I think when somebody explained his dual rotor alternator in his instructable, he said something along the lines of "alternating poles will produce alternating current."  While this is certainly true, I think it was overlooked that a same-pole arrangement would also produce AC, with both changes in direction AND polarity.  I believe it was unclear that the sole purpose of using alternating poles was to create an efficient flux path.

I do see your point.  As long as the flux is changing, despite polarity, we can still get emf.  It would still operate a transformer for instance.

I similarly are confused by where this is aiming. Any electrical generator producing an emf from magnets moving over coils will produce a symmetrical waveform. It could be a basic sine wave or more likely a fundamental and odd harmonics. These can be in phase or out of phase so you can effectively get anything from a triangle to a square wave. It is not possible to have even harmonics so the thing is always symmetrical.

Once you start introducing rectifiers int the system then even harmonics do become possible so you can get unidirectional pulses. Perhaps that is what you meant. With a half wave rectifier feeding a battery there will be dc pulses. As you increase the phase number the thing get much closer to smooth dc. By the time you get to 3 phase bridge rectifiers there is only about 6% ripple and it is nearly as smooth dc as a cheap dc generator.

I see a lot of worry here sometimes about the purity of the waveform of an alternator on open circuit, it really is not very important for battery charging as the whole thing goes out the window as soon as the rectifier conducts. If you put a 'scope  on the machine winding when feeding a rectifier and battery the waveform is chopped to a complete mess, so your 10% open circuit voltage distortion is just swamped.

Flux

The arrangements I have explained are both quite symmetrical.  



Forgive the crude design.  It's paint

Having read your comment again I wonder if you are referring to the unipolar generator.

There have been attempts to produce homopolar dynamos that actually produce dc but they have made little progress. The fist practical dynamo ( Faraday's disc) was of this type as was the first motor ( Barlow's wheel) but it becomes near impossible to extend the idea beyond this very simple disc concept. The number of sliding contacts becomes stupid and there are many other problems.

It is perfectly possible to build unipolar alternators with magnets of one polarity on one disc and the other polarity on the other and in fact some of the first axial air gap alternators were produced this way. They are still true alternators but instead of the flux reversing,it goes from maximun to zero. With neo magnets they have no real virtue but in the days of wound field magnets they did suffer very much less from leakage flux and this was a significant factor.

Your comment about "why not use a rectifier makes me think you were thinking on these lines". I also wonder if you have been looking at a strange concept proposed by one of our contributors that was rather unorthodox and something along these lines ( I make no comment).

All normal electrical machines produce ac in the winding, even the dynamo ( dc generator) is an alternator with a mechanical rectifier.

Flux

Yes, it all began with a comment about a rotor with all same-pole magnets producing "pulsed DC."  I think it was Steven Fahey's explanation that led some people to believe this was the case.

Quoted from:

http://www.windenergy.nl/website/files/artikelen/AXIAL_FLUX_HowItWorks.pdf

In our machine, coils of wire are held steady, while the magnets spin past on the rotors.
Because the magnets were arranged N-S-N-S, the direction of the field flips each time a
magnet goes by. Each coil sees a flipped magnetic field, and pulse of electricity is produced.
When the field flips back, a pulse of opposite voltage is created. This coil is now producing
alternating voltage.

Of course, when I see this misconception repeated, and no one bothers to correct it, I began wondering if this was actually the case.  After some analysis, I believe my initial assumption turned out to be correct--that you cannot get anything other than a net zero waveform without either a complicated winding design, slip rings, or rectifiers.

[ghurd]

M43,

I am not clear on what misconception is being repeated.

I do not see anything false in:

"In our machine, coils of wire are held steady, while the magnets spin past on the rotors."  True.

"Because the magnets were arranged N-S-N-S, the direction of the field flips each time a magnet goes by."  True.

"Each coil sees a flipped magnetic field, and pulse of electricity is produced."  True.

"When the field flips back, a pulse of opposite voltage is created."  True.

"This coil is now producing alternating voltage."  True.


The title, "Basic Principles Of The Homemade Axial Flux Alternator",
and first line, "If you are new to the topic of building Permanent Magnet Alternators, then this is for you!",
implies it is not a 200 page white paper for EE PhDs.

Maybe Steven Fahey himself would have something to add.

I believe most misconceptions come from someone who pulls a concept out of context and run with it. 
Sometimes I do it myself, either accidentally or for experimentation.
G-

[kamikaze762x39]

I believe most misconceptions come from someone who pulls a concept out of context and run with it.  
Sometimes I do it myself, either accidentally or for experimentation.
G-

Ah... Try as I might, I cannot seem to find where I saw it being explained that a N-S-N-S is solely responsible for producing AC.  I think your statement is exactly what happened, but again, I cannot find the thread/instructable/article.  

And by "these forums," I meant the internet in general.  Apologies for any confusion.  

If you look at Steven Fahey's explanation, all of the premises could be considered correct as you said.  It is my opinion that the explanation is incomplete.  To someone who does not understand the principles of operation of alternators, it may be quite easy to conclude that the magnetic field is REQUIRED to change polarity with each pass in order to cause opposite polarity current deflection.

"When the field flips back, a pulse of opposite voltage is created."  True.

Not entirely true, or rather, incomplete.  When a single magnet passes across a single coil, it will cause deflection in both the positive and negative directions.  The field is not required to flip in order to produce a complete wave.  It will deflect with a given polarity first, and then an opposite polarity (1 cycle) with the pass of only ONE magnet (North, for instance).  The second alternate field (South, for instance) will not produce a "pulse of opposite voltage," but rather two inverted pulses (1 cycle) with respect to the first pass.

Of course, "cycle" is used loosely here.  It isn't a true cycle until the waveform is repeated in its entirety.  Let's just say alternating magnets will produce a cycle and then an inverted cycle before the true cycle of the waveform is complete... bah... trying to explain a weird waveform is confusing  

The reason for this is because a coil seated on a stator in an axial flux arrangement is wound in two directions with respect to each field.  The magnet must pass over both sides.  Now, you could wind a continuous coil parallel to the axial plane with a single stator to produce a steady pulsed dc output (it would look like a copper doughnut), but if the coils are lying flat on the stator as in most configurations, you cannot avoid an alternating "pulse" no matter the field direction.

"This coil is now producing alternating voltage."  True.

Sort of true.  The coil was already producing alternating voltage before the field "flipped."  Again, these statements seem to point at the wrong conclusion.

[ghurd]

If you look at Steven Fahey's explanation, all of the premises could be considered correct as you said.  It is my opinion that the explanation is incomplete.  To someone who does not understand the principles of operation of alternators, it may be quite easy to conclude that the magnetic field is REQUIRED to change polarity with each pass in order to cause opposite polarity current deflection.


The field is not required to flip in order to produce a complete wave. 

The paragraph above the one you quoted:
"It doesn't actually matter how the field is changed for the
phenomenon to occur. You may have magnets that get closer
together, that oscillate back and forth, flip over and over, or
perhaps you don't move the magnets at all, and instead flip the
coil back and forth."

If someone does not understand the basics, they should buy the books.
(kind of surprising to me how many people HAVE the books and still flop magnets down with no regard to polarity, or use an odd number of magnets per disk)

The paper is about Hugh's and The Dans' type PMAs, and basic info of how they work for regular people to grasp the basic concepts.

If someone wants to say one magnet pole and one coil actually makes AC, OK.
If someone wants to experiment with it for fun, OK.
If someone wants to say it is a decent way to make electricity in a wind turbine, not OK.

Like Flux, I can't really see where this was aimed.
G-

[TomW]

Like Flux, I can't really see where this was aimed.
G-

Didn't most historical  Kamikaze end up as a mess of wreckage and fire on American Battleships?

The name certainly implies suicidal intentions at least to me.

Perhaps this is the goal here?

6 months arguing online could be avoided by a couple hours in a garage with some equipment and coils & magnets.

Just from here.


Personally, I would just listen to Flux. He has forgotten more of this stuff than most of us will ever know.

BTW the circuit finally showed up!

Tom

[kamikaze762x39]

The paragraph above the one you quoted:
"It doesn't actually matter how the field is changed for the
phenomenon to occur. You may have magnets that get closer
together, that oscillate back and forth, flip over and over, or
perhaps you don't move the magnets at all, and instead flip the
coil back and forth."

If someone does not understand the basics, they should buy the books.
(kind of surprising to me how many people HAVE the books and still flop magnets down with no regard to polarity, or use an odd number of magnets per disk)

The paper is about Hugh's and The Dans' type PMAs, and basic info of how they work for regular people to grasp the basic concepts.

If someone wants to say one magnet pole and one coil actually makes AC, OK.
If someone wants to experiment with it for fun, OK.
If someone wants to say it is a decent way to make electricity in a wind turbine, not OK.

Like Flux, I can't really see where this was aimed.
G-

It could be aimed in any given direction for a shade-tree turbine builder who is trying to design the whole system from scratch with nothing but bicycle parts.    That's the problem with the explanation. 

My rotors have 12 magnets each, dual-rotor, static pole, 3-phase, with 36 coils on the stator.  It will produce AC before being rectified, and that includes deflections in the positive and negative directions.  

Again, I saw someone mention that one could charge batteries directly using a N-N-N-N or S-S-S-S arrangement.  This is untrue.  In any case, this was not intended to refute a specific claim, but only to open the topic to show people exactly what they will get if they build an alternator with a given design.  I have not seen this specific issue addressed in detail, so before some poor guy without an o-scope ruins his battery bank trying to charge it with this kind of output (without a rectifier), he should know that it isn't "pulsed DC" by any means.

The paragraph above the one I quoted still does not clarify the polarity of the output.  It only states that relative motion produces output.  It does not say what kind of output.

Yes, I suppose they should buy the books.  But that will not stop some people from attempting a design without full knowledge of what they are doing.  Just look at the perpetual motion folks for an example on that.  More importantly, I do not think it is a waste of time to strive toward free information that is actually accurate, concise, and easy to follow.  How many times have you Googled for a very specific situation on any number of things in life and could not quite find what you were looking for?  I have many times.  If someone does not care to know which configurations will produce what waveform, they need only skip over this thread.  It is appropriately named, after all.

[kamikaze762x39]

6 months arguing online could be avoided by a couple hours in a garage with some equipment and coils & magnets.

Well... I can't argue with that.  Still, if one believes that wind power should be readily available to anyone who wants to embark upon building these fascinating devices, I feel the information should be correct.  It will at least eliminate some experimentation.  

You know, I had this problem in college.  I really would like a clear understanding of the concepts before I move on.  It used to piss the instructors off something fierce because I wouldn't let it go until there was no confusion on the subject.  

If there is no confusion on your part, you build something with an intended purpose, and you do not get the desired result, then you know exactly when something is wrong, and probably specifically what is wrong.  Bringing the theory together with the application is always a worthy pursuit.

Didn't most historical  Kamikaze end up as a mess of wreckage and fire on American Battleships?

Actually, the name more literally means "Divine Wind" or "God's Wind."  But yes, heaven forbid the name is an omen concerning my designs.   

[Flux]

Can you buy the books?  the only useful information I have found on air gap axial alternators was written before 1880. After that the design was forgotten. sadly universities are now re inventing this stuff.

Similarly the homopolar generator has been confusing the nation for 150 years, still it gets proposed with ideas that don't work.

Don't feel ashamed that you pissed off the lecturer, making him think is the only way to know if he understands the stuff.

I worked for an engineering company and one of the most knowledgeable and respected members of the research department had a cupboard full of beautiful models, many were proposed homopolar designs and he once told me that he had them made to prove which did and didn't work. If it failed to work the theory had to be abandoned.

Such highly intelligent men now work in space science or computing, we no longer have the best brains in the electrical industry so we shall keep reinventing the wheel and we shall have to build the models again to be sure.

Much of this confusion comes about when people start worrying about cutting coil sides at right angles by magnetic fields. If you stick to flux linkage things make much more sense.

Flux

[Bruce S]

I welcome any input on this topic, as I may have the entire thing wrong.  I started this thread because I keep seeing people explain the waveform output of an alternator as "pulsed DC, alternating voltage with same polarity," etc.
 

Kx39
This is very possible, and may be where part of the confusion is going towards.
Pulsed DC is very possible using even the Alternator design.
First the be sure we're on the same thought here.
Alternator stands for Alternating became much more widely known past school when alternators came out in Autos
Generator stands mostly from an auto mechanics point of view for DC producer.

While Pulsed DC is much less efficient than full rectification it is quite possible to use 1/2-wave rectification to obtain a pulsed DC, i.e. the field goes from 0 to Pos to 0 without the added bonus of 0 to Pos to Neg to 0

In the old days the big 55gal drum S rotors were called wind-generators so they wouldn't be confused with Alternators which are AC producing , clean sine-wave, square-wave , ugly-wave    or somewhere in between.

Hope this helps;
Bruce S
  

[Flux]

Not sure this helps clear any confusion.

The issue as I see it is the claim that a machine can produce dc directly from its windings. The only device to do this is a homopolar generator and it needs sliding contacts and is a pretty useless thing.

All other dc producing devices are alternators with rectifiers.

I regard a generator as a device for converting mechanical energy into electrical energy. A dc generator is a dynamo and is effectively a multiphase alternator with a very clever mechanical rectifier ( commutator and brushes)

An ac generator is an alternator and it produces ac. A car alternator is again a polyphase alternator with a far less clever rectifier ( diode bridge).

The alternator itself produces ac, the complete machine ( confusingly called an alternator) produces dc.

Any current that flows more in one direction than the other has a dc component. if the flow is entirely in one direction it is dc.

Some crude early rectifiers had so much leakage that there was flow in both directions but the net sum was dc. Half wave rectifiers do produce pulsed dc as do bridges. The single phase case of half wave is about as pulsed as you can get but 3 phase half wave is much less pulsed. 6 phase half wave is the same as a 3 phase bridge and nearly smooth but you can go on to very elaborate polyphase rectifiers and the ripple gets very small just like the dynamo with hundreds of segments on a large commutator.

I bet I have added even more confusion but it's fun anyway.

Flux

[bob g]

virtually all designs of generators  (alternators or generators) produce somewhat of a sinewave (harmonics and other trash notwithstanding) however

in a classic generator this sinewave (we will call it that for the sake of discussion) is produced then rectified into a pulsating DC via the commutator which in reality is just a
mechanical rectifier.

in the classic alternator we get virtually the same sine wave (trash and all) and rectify to get a pulsating dc, which in reality is not really pulsating so much as it is more of a ripple.

having said that there are designs such as the inductor alternator, such as the electrodyne units, those have two fixed coils, (the field and the armature) and a rotating element that shunts the magnetic field. this is apparently quite effective, high in efficiency relative to other more common designs, but...
the waveform is amazingly nasty, more like a sharp jagged upslope and a severe drop on the down slope.  apparently the heavy dump load diodes could care the less what the wave form is and rectify it to virtually the same ripple content DC of other more common units.  interestingly the electrodyne units pack a hell of a lot of power generation capability compared to other common designs. they have J180 mount large frame alternators that produce 640amps at 12volts, as well as 24, 36, 48 and higher voltages at correspondingly lower amperage of course.

i am not sure how the power is generated in theory, however after talking to the engineering dept years ago, it appears the power comes from the collapsing field after the inductor switching takes place. the scope wave form suggests this is likely what is taking place.

i mention the electrodyne only because i am familiar with it, and its rather odd design
relative to more common offerings from delco, prestolite, ford, chrysler, bosch, leece neville, et al.

bottom line all generators or alternators produce some form of sine wave, saw wave,
none produce DC power , or even pulsating DC power,  the only way to get the DC is to mechanically rectify the output via a commutator and brushes or via diodes.

bob g

* Flux posted before i could finish typing this up, and i defer to his experience
and if i duplicated anything, please disregard

[Bruce S]

Not sure this helps clear any confusion.

The issue as I see it is the claim that a machine can produce dc directly from its windings. The only device to do this is a homopolar generator and it needs sliding contacts and is a pretty useless thing.

All other dc producing devices are alternators with rectifiers.

I regard a generator as a device for converting mechanical energy into electrical energy. A dc generator is a dynamo and is effectively a multiphase alternator with a very clever mechanical rectifier ( commutator and brushes)

An ac generator is an alternator and it produces ac. A car alternator is again a polyphase alternator with a far less clever rectifier ( diode bridge).

The alternator itself produces ac, the complete machine ( confusingly called an alternator) produces dc.

Any current that flows more in one direction than the other has a dc component. if the flow is entirely in one direction it is dc.

Some crude early rectifiers had so much leakage that there was flow in both directions but the net sum was dc. Half wave rectifiers do produce pulsed dc as do bridges. The single phase case of half wave is about as pulsed as you can get but 3 phase half wave is much less pulsed. 6 phase half wave is the same as a 3 phase bridge and nearly smooth but you can go on to very elaborate polyphase rectifiers and the ripple gets very small just like the dynamo with hundreds of segments on a large commutator.

I bet I have added even more confusion but it's fun anyway.

Flux

Flux,
It is fun and I am glad you added the slip ring and brushes part of generators. I feared it would add too much.

I see pulsed DC in MiG welders, which is where I knew of the 1/2wave section, that and the very old fish pulsers.
Cheers;
Bruce S

[Flux]

Bruce
I suppose the fun could go on, I used dynamo implying a dc machine and that is the most common application but a dynamo can be ac or dc ( normally if you don't specify then it's assumed to be dc).

Strictly a dynamo is a dynamoelectric machine with wound fields to distinguish it from a magneto which has permanent magnets. Times change and the magneto is now generally called a pmg or pma depending whether it is dc or ac. Magneto is more commonly used for high tension magnetos for ignition which are combined magnetos and transformers.

Yes I am familiar with half wave pulsed supplies for welders, it was about all you could do before the big transistors came along, now with inverter technology you can have any shape pulse you want. Even more fun when you go synergic with one pulse dispacing one drop of weld particle.

Bob

The inductor machines are an alternative and work well at high frequencies, we built inductor high frequency alternators for induction heating and also for radar supplies.It's one easy way to build a simple brushless alternator especially unipolar versions. Again the concept goes back before 1900 and was the basis of the only means of voice transmission by radio before the invention of the thermionic valve ( vacuum tube).

Flux

[TomW]

before the invention of the thermionic valve ( vacuum tube).

Flux

Now there is a term I have not seen used in a looong time.
I expect someone to reinvent those one day soon.

Always appreciate your comments here, even the ones that remind me how long ago I started dabbling in shoving electrons around for fun and profit. Back when "solid State" likely meant ICE. 

Tom

[Bruce S]

Yes I am familiar with half wave pulsed supplies for welders, it was about all you could do before the big transistors came along, now with inverter technology you can have any shape pulse you want. Even more fun when you go synergic with one pulse displacing one drop of weld particle.
 ( vacuum tube).

Flux

Flux;
  More than once I've gotten synergistic with a drop or weld particle or two, to which then causes a little known dance known to those who forgot to wear protective clothing :-)

TomW
Vacuum tubes  = plate biasing and actually hearing voices within the tubes.{sober}

[ghurd]

Still being young (and good looking), 
to me, THIS is a "vacuum tube"...

[rossw]

I know this thread has been "done to death"... but there's something fundamental I didn't actually see stated anywhere, which MAY help some peoples understanding of this.

It's the basic fundamental all this stuff works on - and that is, that when a magnetic field *changes* around a conductor, it induces a voltage/current in that conductor. That sounds pretty darn obvious, but it has some profound impact.

A "north" pole can induce *BOTH* positive AND negative voltages in a wire. That sounds silly. But it's not. It comes back to the CHANGE in field.

If you bring a north pole magnet TOWARDS a wire, you'll get a current induced one way. As you take the magnet AWAY from the wire, you'll get a current induced the opposite way. So a machine with *ALL* north poles (and spaces between them) will see the magnetic field building up (generating power one way), and then dropping off (collapsing field) generating current the opposite way.

If you think, "fine, I'll close up all the gaps and just have north pole all around" - you end up with a field that IS NOT CHANGING - and you will get NO current induced in the conductor.

So it's not the "north" "south" orientation that makes AC, its the magnetic field actually increasing and decreasing that induces power. The only way to make DC would be if you could make the magnetic field continue increasing forever... which of course it can't.

[12AX7]

before the invention of the thermionic valve ( vacuum tube).

Flux

Now there is a term I have not seen used in a looong time.
I expect someone to reinvent those one day soon.

Always appreciate your comments here, even the ones that remind me how long ago I started dabbling in shoving electrons around for fun and profit. Back when "solid State" likely meant ICE. 

Tom

Hey!  There are STILL plenty of Vacuum Tubes in use today, at least in Hi-Fi.

[oztules]

Hmmm
kamikaze762x39  Your initial question/ scenario led to your conclusion of:

"The point is, you cannot get "pulsed DC" out of a coil/magnet arrangement unless you completely remove one of the winding directions from the field.  Each wire would have to be wound in a given direction, looped away from the field, and then wound again in the same direction."

which is wrong.

The most apt explanation which covers all of this is Rossw's salient commet.... which was:

"A "north" pole can induce *BOTH* positive AND negative voltages in a wire. That sounds silly. But it's not. It comes back to the CHANGE in field.

If you bring a north pole magnet TOWARDS a wire, you'll get a current induced one way. As you take the magnet AWAY from the wire, you'll get a current induced the opposite way."

Can you see now why you cannot get pulsed DC without mechanical or electronic rectifiers.
(I don't know about the homopolar setup Flux refers to, but it must follow the same rules I suspect)

If you have a scope, pass a single north( or south.. not both) pole past a single coil leg as you suggested (ie expose only a single leg), and you will see a complete positive and negative waveform produced as the magnet passes this single leg.... all wires running the same direction, only a single pole face, but a positive and negative pulse.... just as Rossw suggests will happen.

I know this to be true as I have tested for it before.... ( I'd  follow Toms advice on this ... test it for yourself...stops a lot of could and maybe ) It all happens quickly, but it appears to happen like this:

As we approach the leg with our pole, the EMF rises. by the time we reach the center of the leg (directly above the coil leg) the EMF has fallen back to zero, The field will be at it's strongest here, but EMF has dropped to zero, as the field is no longer approaching the leg, but has reached the point where it will soon be moving in an opposite direction with respect to the coil compared to the approach direction.

As the pole  begins to move away from the center of the coil leg,  the EMF starts to increase in the opposite sign to before. It comes to a max (half way off the coil), then drops to zero again as the flux of the  pole leaves the leg completely.

Sparweb could do the maths for this in his sleep I suspect, but it looks all the world to be a sine wave from memory.... or some reasonable facimile thereof.... (I'd choke on the first page of the maths)

So we do get 2 signs for a single pass of a single coils leg...... still AC.... not pulsed DC.




.. and .... just my opinion.... but  I don't think telling folks to get a book is particularly useful advise. Books now days are less than useful compared to the internet and forums. You need to find a book that will address the particular aspect your interested in.... very very hard in a lot of cases. A well directed question on the net will be far and away more likely to yield the answer you may never have found in a swath of books.......


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

PS. The only time I have taken that advise was when Zubbly suggested a specific book, which I imported from the states, and it was as useful as he said it was, but it still did not address all the things I wanted to know..... but he could and did...... (I miss him still)

[bob g]

Flux

 i didn't mean to suggest that the  inductor design used by electrodyne is something new
but rather something different from the more common standardized rotating field designs used by almost every manufacture for automotive use.

surely the inductor design goes back well over 100 years, like most every design under the sun, they all originate from about 1880-1895 with very few exceptions.

there really is nothing new under the sun when it comes to the basics of alternator design, the only new stuff is materials such as our commonly available neo mag's which would have been unobtanium back in the golden age of alternator/generator design, and of course the advent of silicon diodes/rectifiers (also unobtanium) both of which have provided the most significant changes to design and output.  Better brgs, grain oriented steel laminates, dramatically better insulation have also played significant rolls in increased output and smaller packages.

of course you of all folks know all this. 

just thought that mentioning the electrodyne application of the inductor design might be of interest to some of the folks.

bob g

[kamikaze762x39]

"The point is, you cannot get "pulsed DC" out of a coil/magnet arrangement unless you completely remove one of the winding directions from the field.  Each wire would have to be wound in a given direction, looped away from the field, and then wound again in the same direction."

which is wrong.

The most apt explanation which covers all of this is Rossw's salient commet.... which was:

"A "north" pole can induce *BOTH* positive AND negative voltages in a wire. That sounds silly. But it's not. It comes back to the CHANGE in field.

If you bring a north pole magnet TOWARDS a wire, you'll get a current induced one way. As you take the magnet AWAY from the wire, you'll get a current induced the opposite way."

Can you see now why you cannot get pulsed DC without mechanical or electronic rectifiers.
(I don't know about the homopolar setup Flux refers to, but it must follow the same rules I suspect)

If you have a scope, pass a single north( or south.. not both) pole past a single coil leg as you suggested (ie expose only a single leg), and you will see a complete positive and negative waveform produced as the magnet passes this single leg.... all wires running the same direction, only a single pole face, but a positive and negative pulse.... just as Rossw suggests will happen.

Ah... now this is quite new to me.  I was, as Flux mentioned, looking at coils cutting fields at 90 degrees...  

So do you mean to tell me that one magnet passing both legs of one coil will actually produce four ripples?  If that is the case, then I was completely wrong in my understanding of alternators.

I deduced a single ripple (positive deflection for instance) for a single pass of a single leg.  Ok.... so... we know that B is a vector quantity... And I was thinking this whole time that it refers to the vector of the field, or specifically, the polarity of the static field.

Fleming's right hand rule says if field (B) points in X direction (north to south), and motion/emf (F) points in Y direction, then current (I) travels in Z direction.

So is this extra inverted ripple like an inductive spike or something?  That is seriously blowing my mind.  I cannot see anything about the right hand rule that refers to the motion or (relative to the wire) amplitude of the field itself.  

Increasing field = positive deflection (relative to the other)

Decreasing field = negative deflection (relative to the other)



If this is the case, why did Fleming even bother?  I know Wikipedia isn't exactly a perfectly reliable source, but the link does clearly state that "The First finger represents the direction of the Field. (north to south)"

http://en.wikipedia.org/wiki/Fleming%27s_right_hand_rule

So what exactly is happening here?  North/south directions seem useless to even consider if motion or change in amplitude of the field itself changes the entire direction we were looking to find... You can't even get current without motion...  I think I just short-circuited. 

The only motion I see dealt with is the motion of the conductor relative to the field... How does the decreasing amplitude of the field (as seen by the conductor) fit in here?

[Flux]

I think Rossw and Oztules have got to the basis of this, you have to look at this in terms of flux linkage. There can only be an emf induced while the flux is changing within the field of a conductor. Actual magnet polarity doesn't affect the main issue, only the detail. For dc flux would need to be changing forever ( increasing or decreasing)

All this confusion I am sure comes back to a misunderstanding of some things we were taught as basics about  an emf being induced in a wire cutting a magnetic field. This basically is the concept of the homopolar generator but once you try to extend the fundamental concept beyond a wire moving forever in a uniform magnetic field you run into trouble.  To have a current you need to connect the ends of that wire and however you do it without sliding connections you will introduce another conductor with an opposing emf because no uniform magnetic field to infinity exists.

A single wire doesn't exist in real life, any coil is a loop and you will get an emf if the flux linkage within the loop changes, surprisingly Faraday did all his basic experiments with loops of wire not a wire moving uniformly in a magnetic field and that is why he succeeded when others failed to establish the link between magnetism and electricity, they were looking at a steady state solution( they were looking for the dc output that we are saying now is impossible.

I don't want to go into more detail about inductor machines here but most of them don't reverse the magnetic field, it just keeps changing from maximum to minimum just as Rossw described.

Flux

[kamikaze762x39]

I think Rossw and Oztules have got to the basis of this, you have to look at this in terms of flux linkage. There can only be an emf induced while the flux is changing within the field of a conductor. Actual magnet polarity doesn't affect the main issue, only the detail. For dc flux would need to be changing forever ( increasing or decreasing)

All this confusion I am sure comes back to a misunderstanding of some things we were taught as basics about  an emf being induced in a wire cutting a magnetic field. This basically is the concept of the homopolar generator but once you try to extend the fundamental concept beyond a wire moving forever in a uniform magnetic field you run into trouble.  To have a current you need to connect the ends of that wire and however you do it without sliding connections you will introduce another conductor with an opposing emf because no uniform magnetic field to infinity exists.

A single wire doesn't exist in real life, any coil is a loop and you will get an emf if the flux linkage within the loop changes, surprisingly Faraday did all his basic experiments with loops of wire not a wire moving uniformly in a magnetic field and that is why he succeeded when others failed to establish the link between magnetism and electricity, they were looking at a steady state solution( they were looking for the dc output that we are saying now is impossible.

I don't want to go into more detail about inductor machines here but most of them don't reverse the magnetic field, it just keeps changing from maximum to minimum just as Rossw described.

Flux

So in essence, Fleming's rule is useless?  Or perhaps it only describes the condition when the field and conductor are approaching each other... or as you said, moving forever?  Surely it must describe at least one condition.  

Ok... so we have an unrealistic assumption.  Let us assume that we have a wire with infinite motion in a uniform field... Pure DC right?

Then if we suddenly stop the device, we see a negative spike?

Rossw, Oztules, and Flux... You guys have been extremely helpful in explaining this.  I always visualized current as being "pushed" by a sort of drag-line (field).  Now it seems that electrons are made of rubber and bounce back after being squashed 

[oztules]

kamikaze762x39

I think your dilemma stems from assuming your getting the motion part right....... consider this.

If you move toward an object with a velocity + V, what sign would the vector be if you were to move away from it, and how would flemings rule see it now?

Does this untangle it some?
Or

If we have a wide coil leg, and a point pole radiating the flux it is easier to see.

As the field moves towards the leg, we get say a positive emf starting in the coil of say positive sign. as we get to quarter the way across the leg, we have the  biggest change of field going on for the most turns of wire adding vectorially.

As we move closer to the middle of the coil, more and more turns have seen the point pass overhead, and see it as moving away... before it has actually gotten to the middle of the coil... the emf is falling.... even though the flux is strong at this time. the "relative" movement is seen differently from each wire wrap, and they will add vectorially to give us the measured emf.

As we get to top dead center, half the wires see the motion as still coming their way, and sign appropriately, the other half see the motion as receding, and they sign accordingly...we have total cancellation... zero emf at top dead center.... yet we know flux is at it's most intense.... relative motion is at stalemate.

As we move further we come to 3/4 across the coil (or thereabouts), and more wires are now seeing the motion of the field as receeding, and now our sign is maximum negative.

as we leave the coils proximity, the moving  flux subsides and the motion is no longer detected.... were back to zero.........1 full cycle. Not an "inductive spike" but a structured waveform of symmetric (I would have said, but may not be) proportions.


Now I can't vouch that this is the correct "what actually is happening", others better than I can go there if it is off base, but it seems to reflect exactly what I have observed.... doesn't mean the mechanics are correct.... although I think you'll find your rules probably work now, and Fleming can sleep again.


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


edit: Sorry flux I messed it all up but put it back again....

[Flux]

Oz got there first. He has explained it in terms of coil sides rather than flux linkage and perhaps if you think that way his explanation will be easier to follow.

Your rules as you know them work perfectly for each strand of wire under the magnet pole as it moves, what you have to always do is add the vector sum of what is happening to every strand in your coil to get the emf between the ends, never think of a strand on its own.

Once you do all the vector sums you will find that the emf depends on the flux linkage ( rate of change of flux). Although Faraday didn't do the actual wording, it comes down to E=N.d(phi)/dt. Removes all the confusion about cutting at right angles to magnet motion and all the other issues people get bogged down with.

If your coil proportions are silly there is a period of no flux change even at the point of maximum field intensity and emf falls to zero.

I hope that with the bit from Rossw and Oztules and my different approach you can now see what is really happening.

Flux

[rossw]

Rossw, Oztules, and Flux... You guys have been extremely helpful in explaining this.  I always visualized current as being "pushed" by a sort of drag-line (field).  Now it seems that electrons are made of rubber and bounce back after being squashed 

Lets take relative motion completely out of it.

A basic transformer. Two windings. A primary, and a secondary.
You pass a current through the primary. This produces a magnetic field. WHILE the field is building, (and there is a "change in magnetic flux") you see an output on the secondary windings.

You can do this with a osciloscope, or a voltmeter, and a battery to supply the primary.

If you keep the power on the primary for a few seconds, there is still a magnetic field (you can confirm this with a compass, or piece of iron brought up to the transformer core). But... look at the output. *NOTHING*.

As professor Julies would ask, "why is it so?". Because te field is now "static". Neither increasing nor decreasing.

Remove the power from the primary. The iron is no longer attracted to the core, but you get a "kick" (of the opposite polarity) on the secondary as the field collapses.

If you feed AC in on the primary, the magnetic field is constantly increasing, decreasing, then increasing (in the opposite direction), then decreasing... and the output on the secondary is doing the same. There is no relative motion between the conductors - nothing is moving - the field STRENGTH is increasing and decreasing.

Does that help?

[Flux]

That is probably the best basic example to cover the idea of electromagnetic induction. That and the experiment moving a magnet into and out of a coil were the basis of Faraday's discovery. His  work on which the discovery is based did not involve the concept of alternate polarity magnets moving over a coil, the practicalities of electric generators were to come later.

Flux