Windings: basic questions.

[Chalo]

Why back EMF is unwanted is the mystery to me. It serves to limit a motor's speed and allows careful setup to let the motor operate within its most efficient RPM range.

 

[Punx0r]

There seem to be practical disadvantages in some applications, in that we see designers having to use unreasonably high battery voltages or waste power with flux weakening.

 

[major]

To answer the question in the OP, I chose to use the term "Generated Voltage" to describe the effects of conductors moving in a magnetic field. I did so on purpose. In my next post I say: " Calling that generated voltage BEMF is a real source for confusion IMO." And I think this is the core of the problem here.

Here is an interesting article: http://progeny.co.uk/back-emf-suppression/ One might think that all BEMF is bad and needs to be suppressed. That BEMF is NOT the generated voltage in motors and generators coming from the movement of coils in the presence of magnetic fields. They are 2 completely different phenomena.

In armatures for most all motors and generators, the armature current, or more precisely, the coil currents must be time varying or switched. This does give rise to BEMF due to the coil inductance. This BEMF is NOT the generated voltage arising from the motion of the coils in the field. These are two different phenomena.

For the basic motor design, a steady state condition with unchanging current is used to determine basic parameters like the number of turns per coil and torque and such. Having this condition of unchanging current means there is no BEMF due to coil inductance to interfere with the generated voltage, flux, turns, and speed calculations. Later in the design phase, especially concerning power source or controllers, the transients are considered.

 

[kdog]

hey bowser
tone down the mud slinging dude
I care not for your design/knowledge or topic, but hurling insults in your first few posts is not cool.
if I were Major id be requesting a mod to have a quiet word with you.
there is a great deal of expertise here, you obviously want help, in some form or other
pull your head in
and others will be more inclined to help
respectfully
kieren

 

[Browser]

Nothing in there explains this first paragraph which is unusual and does deserve an explanation. I can only guess it's something to do with the unpowered phase as the motor rotates, but it's open-circuit

All 3 (or other number) of phases are equal electrically and magnetically. All phases experience the back-EMF whenever the rotor is turning. In the unexcited phase it doesn't matter because (as you say) it's open circuit.

But for the excited phases, the back-EMF reduces -- literally negates (Lenz) -- the applied voltage; thus, the voltage across the windings is less, the current that flows is less; thus the EMF generated by the affect of that current within the (fixed) PM field is less.

certainly not at EMF is a magnetic field or current (come on, that one jumped out even to me ).

So, I'm guilty of being lazy. I'm not a lawyer or a lecturer or writing a thesis; jumping up and down about that laziness, instead of reading between the lines, is a cheap rhetoric tactic.

It doesn't seem to support anything you're claiming,

I'm not "claiming" anything. I'm stating the physics. And seeking to discover what (if anything) can be done to limit the generation of back-EMF. That's why I came here.

My original question was (I thought) relatively clear: Why don't people use solid conductors? Why (what criteria) cause some people to use fewer turns of thicker wire vs more turns of thinner wire (assuming equal total area)?

(From here on is my speculation!)

The foil-coil thread that you linked here, suggests that there is a preferred orientation of the width of a foil conductor.

By analogy, with the functional requirements of the orientation of core laminations -- which reduce core losses by limiting the space in which eddy currents can build up -- my take home conclusion, thus far, is that by orienting the 'thickness' dimension of the foil, such that each individual turn presents the lowest profile of conductor to the magnetic field, it gives the induced currents (back-EMF) less 'dimension' in which to circulate.

Of course, there is the other side of "thin conductor" which is their resistance; which is where foil has benefits over round or square conductors. Whilst round or square conductors present the same area to the magnetic field whichever way you orient them; the wide, thin aspect ratio of a foil conductor allows for a large cross-sectional area conductor (thus low resistance) to present a minimal 'dimension' to the magnetic field.

From the limited amount of information I've been able to find about the use of foil-wound coils -- they seem to see most use in high-voltage, AC transformers and radio frequency inductors -- a part of the reason for their use -- besides that they are easier/cheaper to wind for very large coild use in HV transformers -- is that they are less susceptible to AC eddy current induction.

Whilst with BLDC motors the DC stands out; the reality is that the polarity switches proportional to the number of poles and RPM, so it is AC; or at least has AC components.

Further, on top of (or underlying) the basic poles * RPM frequency, there is PWM frequency which is generally required to be at least 10 times higher than the highest pole*RPM frequency if the required waveform -- be it trapazoidal or sinasoidal -- can be reasonably accurately emulated.

And on top of all that, the the inductance of the coils -- their tendancy to 'store' current -- and self and mutual capacitance of the coils, tends to introduce harmonic components into the back-EMF.

The bottom line is that there is a lot of ac going on in a BLDC motor and that (I think), means there is scope for limiting some of the back-EMF induced, by careful design of the coils.

That is (mostly) speculation at this point, but its why I came here in the first place. To find out.

Maybe I came to the wrong place, but I've always favoured doers to theoraticians; if only because I can (usually) understand what the former are talking about more easily.

Buk

 

[speedmd]

Great project Browser. Please don't let the turf tussles stop this discussion. Many here are gaining vast insights by standing by and reading-studying all the various morsels of knowledge it all touches upon. Lots to learn for most all of us on the winding trade off topic.

 

[Browser]

tone down the mud slinging dude

"in your ignorance", is not an insult.

I am ignorant about n-dub music; high-energy physics; and the finer points of equestrian etiquette (amongst many other subjects).

Ignorance -- not knowing -- is not an insult, nor even a bad thing.

What you do about it, and especially what you claim about the subject, is a different matter.

If percieved, but unintended, slights are more important than correctness; if his repeatedly telling me that my correct understanding of back-EMF is wrong; in favour of pushing his incorrect and indefendable "I don't like the term back-EMF, its all just EMF" stance; means that I should aquiese, because I'm a newbie and he has a 1GW rep, then so be it. Ban me!

 

[Browser]

This BEMF is NOT the generated voltage arising from the motion of the coils in the field.

And you are still wrong! And still trying to change physics.

The relative motion of the coils and magnets is exact cause back-EMF in motors.

The applied voltage causes the movement. Whenever you have coils moving through magnetic fields, a voltage is induced that (by Lenz) opposes that motion. That is the back-EMF in motors.

 

[major]

This BEMF is NOT the generated voltage arising from the motion of the coils in the field.

And you are still wrong! And still trying to change physics.

The relative motion of the coils and magnets is exact cause back-EMF in motors.

The applied voltage causes the movement. Whenever you have coils moving through magnetic fields, a voltage is induced that (by Lenz) opposes that motion. That is the back-EMF in motors.

I've noticed that you have misquoted me on a couple of occasions and to give further context I'll quote myself here:

In armatures for most all motors and generators, the armature current, or more precisely, the coil currents must be time varying or switched. This does give rise to BEMF due to the coil inductance. This BEMF is NOT the generated voltage arising from the motion of the coils in the field. These are two different phenomena.

So I am saying that the BEMF from the changing current and the coil inductance is separate and different from the generated voltage caused by the motion of the coil in the magnetic field of the motor. You disagree. A voltage (BEMF) caused by changing current and inductance is quantified L*di/dt. The voltage due to the motion in the field is per Faraday's Law, -N*dΦ/dt. If one rotates the field in the armature and has the armature connections open circuit then no armature (coil) current can flow, so the L*di/dt component must be zero. Yet there is the voltage (generated voltage) across the armature connections. The generated voltage is there regardless of the current. I think that demonstrates that it is different from the BEMF due to the coil inductance. It is different.

You said you're a hands-on type of guy. Try it. Put a voltmeter or scope on a motor armature connections and spin it. You see the generated voltage. There is essentially zero current because the instruments are so high impedance, so there is no BEMF due to inductance in the motor.

I was checking about the direction of those red ellipses (eddy currents) in the conductors which I said were incorrectly shown a few posts back. Indeed they are. But I came across this article which I thinks confirms a suspicion that these conductor eddy currents which you go on about are actually part of what we call Skin Effect. Motor engineers have been dealing with it for a long time. Again, it has nothing to do with generated voltage.
http://www.kpap.net/notes/item.php?id=SkinDepth

The applied voltage causes the movement. Whenever you have coils moving through magnetic fields, a voltage is induced that (by Lenz) opposes that motion. That is the back-EMF in motors.

More precisely, the induced (or generated) voltage, per Faraday with the negative sign per Lenz, will, if allowed to, produce a current which will cause a force (or torque) to oppose the motion (or rotation). If this applied voltage is greater than the generated voltage, then current will flow in the direction to created a torque which will aid rotation and it is a motor, converting electric to mechanical power. Per Lenz, the generated voltage opposes the applied voltage. The magnitude of power converted is the armature current times the generated voltage. The applied voltage is the sum of the voltage drop across the armature impedance and the generated voltage.

Having the electric conversion device voltage oppose the applied voltage is similar to having a DC source charging a battery. If the applied voltage is greater than the battery voltage, current flows into battery positive and the battery charges or converts electric to chemical energy. The difference between the applied voltage and battery voltage is the resistive drop (current times the internal resistance of the battery). The loss in the resistance is power wasted. The power converted is the current times the battery voltage.

 

[Punx0r]

My original question was (I thought) relatively clear: Why don't people use solid conductors? Why (what criteria) cause some people to use fewer turns of thicker wire vs more turns of thinner wire (assuming equal total area)?

AFAIK the use of multiple strands to form a conductor bundle is mostly about cost & availability (standard magnet wire is cheap and easily available, chunky square wire is uncommon and expensive) and ease of winding. Copper fill might also be better with many parallel strands rather than a few turns of chunky round conductor.

The Chevy Spark uses a PM motor with solid bar (hairpin) conductors: http://blog.caranddriver.com/we-build-the-chevy-spark-ev%E2%80%99s-ac-permanent-magnet-motor

Also, now I think about it, check out the rotor conductors on an AC induction ("Squirrel Cage") motor.

From the limited amount of information I've been able to find about the use of foil-wound coils -- they seem to see most use in high-voltage, AC transformers and radio frequency inductors -- a part of the reason for their use -- besides that they are easier/cheaper to wind for very large coild use in HV transformers -- is that they are less susceptible to AC eddy current induction.

I'd take a guess that high voltage means high turn counts, for which a slot-wide foil would be ideal for copper fill and manufacturing efficiency. While anything radio-orientated will suffer significant skin effect due to the high frequency, which warrants laminated conductors. Check out Litz wire. However, the standard line is that neither the PWM nor commutation frequency of a typical motor is anywhere near high enough to worry about skin effect.

 

[Browser]

I think that demonstrates that it is different from the BEMF due to the coil inductance.

You think; I know you're wrong. I'm stating physics and you are making it up as you go along.

Back there, I quoted two, authoritative sources speaking about the industrial and scientific recognised phenomena of back-EMF in BLDC motors; and its parasitic effects upon their performance. You counter with a few words and diagrams from another 'internet guru' who has mistakenly termed the power-off, inductor decay voltage in an electro-magnetic lock mechanism, as "back-EMF".

It cannot be back-EMF, because the power's off; thus there is NO EMF FOR IT TO OPPOSE. Just as the generated voltage in a generator is not back-EMF, because there is no applied voltage for it to oppose. BUT THE MECHANISM BY WHICH THE OUTPUT VOLTAGE FROM A GENERATOR IS INDUCED IS EXACTLY THE SAME MECHANISM BY WHICH BACK-EMF IS INDUCED IN AN ELECTRIC MOTOR. This

If one rotates the field in the armature and has the armature connections open circuit then no armature (coil) current can flow, so the L*di/dt component must be zero. Yet there is the voltage (generated voltage) across the armature connections. The generated voltage is there regardless of the current. I think that demonstrates that it is different from the BEMF due to the coil inductance.... Put a voltmeter or scope on a motor armature connections and spin it. You see the generated voltage. There is essentially zero current because the instruments are so high impedance, so there is no BEMF due to inductance in the motor.

demonstrates your utter cluelessness regarding electricity. If you place your meter across the terminals of a battery, it registers a voltage despite that there is (essentially) no current flow.

Voltage, is also termed 'potential'; in the literal sense that a weight hanging from a rope in a gravitation field has potential energy. It has the potential to do work; but it doesn't actually do any work until a circuit is formed (or the rope is released). You simply do not need a circuit or current flow for voltage to exist.

You may be an accomplished home builder -- I've no idea. But if you are, it is not because you designed a machine to a particular specification and built something that met that specification. It is because you threw a few coils and magnets together on an axial, perhaps following or tweaking the design of those that went before you and following a few rules of thumb and a misbegotten view of the way things work; and lo, it motored, or produced electrickery or both; but it was not by design; it was because actually getting a motor to rotate; or a generator to produce electricity is really pretty simple -- they were doing it 150 years ago.

My requirement is rather more exacting. My goal is not to 'construct a motor that works'; but rather to design and construct a motor that meets or exceeds the requirements of the load I'm building it for. (And actually that is the interesting bit.)

As such, I've no interest in your thoughts on the matter, and your "contributions" to this thread are, from my perspective, totally negative.

Instead of being able to discuss what I want to discuss, I'm wasting my time countering your home-spun philosophies. This thread has become so muddied by your need to assert your position; your juvenile need to win an argument; and your faux hurt at a non-insult; that it simply serves no purpose for me any more.

So, I'm off to pastures new to find people, to discuss my project and problems with, who know more than me, so that I can fulfil my goal of learning from them instead of wasting my time trying to convince home-spun 'natural philosophers', that their quaint mental models of the world may satisfy their world view, but they have little basis in reality; they may serve to wow the uninitiated, but I'm a whole lot harder to impress.

Enjoy. Good bye.

Update: If you actually want to learn something, I strongly suggest you read this: http://www.thompsonrd.com/Teaching/Power-Electronics/NOTES 30A MAGNETIC DESIGN.pdf. In particular, I hope that you will note the statement of Faraday's law on page 8, which reads:

A changing magnetic flux impinging on a conductor creates an electric field and hence a current (eddy current*)

* My emphasis.

 

[major]

So I am saying that the BEMF from the changing current and the coil inductance is separate and different from the generated voltage caused by the motion of the coil in the magnetic field of the motor. You disagree. A voltage (BEMF) caused by changing current and inductance is quantified L*di/dt. The voltage due to the motion in the field is per Faraday's Law, -N*dΦ/dt. If one rotates the field in the armature and has the armature connections open circuit then no armature (coil) current can flow, so the L*di/dt component must be zero. Yet there is the voltage (generated voltage) across the armature connections. The generated voltage is there regardless of the current. I think that demonstrates that it is different from the BEMF due to the coil inductance. It is different.

You think; I know you're wrong. I'm stating physics and you are making it up as you go along.

Back there, I quoted two, authoritative sources speaking about the ....

How's this for authoritative?
http://www.usna.edu/Users/physics/tank/Public/FaradaysLaw.pdf
Introducing Faraday's Law
L. L. Tankersley and Eugene P. Mosca
Physics Department
United States Naval Academy
Annapolis, MD 21402

Yep, I did make that up as I went along in this thread because I knew it. But that reference appears to support my contention.

 

[Browser]

However, the standard line is that neither the PWM nor commutation frequency of a typical motor is anywhere near high enough to worry about skin effect.

Take a 10-pole motor -- say lebowski's AFPMSM, To achieve a 1000 rpm, you need 10 inversions/rev, so 166 Hz. But, multiply that by an order of magnitude (at least) for PWM, so 1660Hz.

The skin depth of copper at that frequency (at 100C) is sqrt( 2 / 2 * pi * 1660 * 4 * pi * 1e-7 * 5.98e7) = 1.59 mm, so his 0.8 mm wire is just hunky dory.

But, my motor has 36 poles and is intended to run at 20,000rpm. So, 36 * 20e3 * 10 = 120kHz.

The skin depth of copper at that frequency (at 100C) is sqrt( 2 / 2 * pi * 120e3 * 4 * pi * 1e-7 * 5.98e7 ) = 1.8787e-4 = 0.18787 mm. I do need to worry about it.

That's why I'm looking at 0.1mm Cu foil with 0.025 polyester insulation between turns.

Buk

 

[Browser]

How's this for authoritative?

Introducing Faraday's Law
L. L. Tankersley and Eugene P. Mosca
Physics Department
United States Naval Academy
Annapolis, MD 21402

Fine as a source.

But that reference appears to support my contention.

Appears to you to support your contention.

But that image, drawn from an unlinked source, that happens to mention "two different phenomena" and magnetism, but no mention of backEMF or eddy currents, with most of the references not shown and no context, you might just as well have scribbled the three words on your wall in crayon and posted a pic of it.

Did you read the document I linked? Did you make it to page 84 entitled "Eddy Currents Increase Winding Loss" with the explanation and formulae and worked examples?

No. You went a looked for a discombobulated image that "appears to support your contention"; if you squint your eyes a bit, ignore all the missing information and forget the history of your posts in this thread.

Yeah, right!

 

[major]

Added link.

 

[major]

How's this for authoritative?

Introducing Faraday's Law
L. L. Tankersley and Eugene P. Mosca
Physics Department
United States Naval Academy
Annapolis, MD 21402

Fine as a source.

But that reference appears to support my contention.

Appears to you to support your contention.

But that image, drawn from an unlinked source, that happens to mention "two different phenomena" and magnetism, but no mention of backEMF or eddy currents, with most of the references not shown and no context, you might just as well have scribbled the three words on your wall in crayon and posted a pic of it.

Did you read the document I linked? Did you make it to page 84 entitled "Eddy Currents Increase Winding Loss" with the explanation and formulae and worked examples?

No. You went a looked for a discombobulated image that "appears to support your contention"; if you squint your eyes a bit, ignore all the missing information and forget the history of your posts in this thread.

Yeah, right!

You're the one hung up on eddy currents, not me. In post #1 you said to ignore them. I'm saying that eddy currents do not enter into Faraday's Law concerning generated voltage in motors and generators. And that the voltage due to coil motion in the field is different from voltage caused by changing currents.

And I added the link which somehow was lost from that post.

 

[major]

If you are on a hill and the motor is supplying just enough torque to stop you rolling back, but not enough to allow you to move forward, IT IS STILL DOING WORK!

Work, in physics, measure of energy transfer that occurs when an object is moved over a distance by an external force

No work, as understood in this context, is done unless the object is displaced in some way and there is a component of the force along the path over which the object is moved. Holding a heavy object stationary does not transfer energy to it, because there is no displacement.

From: http://www.britannica.com/science/work-physics

Wanted to revisit that before you leave.

So, I'm off to pastures new to find people,...., who know more than me,

I don't think you can do it; find people who know more than you.

 

[Browser]

I'm saying that eddy currents do not enter into Faraday's Law

And yet

https://en.wikipedia.org/wiki/Eddy_current starts:

Eddy currents (also called Foucault currents[1]) are loops of electric current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction

https://en.wikipedia.org/wiki/Electromagnetic_induction#Eddy_currents

Conductors (of finite dimensions) moving through a uniform magnetic field, or stationary within a changing magnetic field, will have currents induced within them. These induced eddy currents can be undesirable, since they dissipate energy in the resistance of the conductor.

and under
https://en.wikipedia.org/wiki/Electromagnetic_induction#Parasitic_induction_within_inductors

In this illustration, a solid copper bar inductor on a rotating armature is just passing under the tip of the pole piece N of the field magnet. Note the uneven distribution of the lines of force across the bar inductor. The magnetic field is more concentrated and thus stronger on the left edge of the copper bar (a,b) while the field is weaker on the right edge (c,d). Since the two edges of the bar move with the same velocity, this difference in field strength across the bar creates whorls or current eddies within the copper bar.[23]

High current power-frequency devices, such as electric motors, generators and transformers, use multiple small conductors in parallel to break up the eddy flows that can form within large solid conductors.

http://web.mit.edu/viz/EM/visualizations/coursenotes/modules/guide10.pdf

10. Faraday's Law - MIT

If a solid conductor were used instead of
a loop, as shown in Figure 10.5.1, current can also be induced. The induced current
appears to be circulating and is called an eddy current.

https://www.youtube.com/watch?v=OIcZRSBTTY0

Induced EMF - Faraday's Law, Lenz's Law & Eddy Currents

https://www.boundless.com/physics/t...-eddy-currents-and-magnetic-damping-571-8079/

Back EMF, eddy currents, and magnetic damping are all due to induced EMF and can be explained by Faraday's law of induction.

http://www.phy.davidson.edu/FacHome/wbh/PHY220Lectures/21.3-21.5.pdf

Circulating currents (eddy currents) are induced in bulk conductors moving through a magnetic field

http://www.electronics-tutorials.ws/electromagnetism/electromagnetic-induction.html

But a changing magnetic flux produces a varying current through the coil which itself will produce its own magnetic field as we saw in the Electromagnets tutorial. This self-induced emf opposes the change that is causing it and the faster the rate of change of current the greater is the opposing emf. This self-induced emf will, by Lenz’s law oppose the change in current in the coil and because of its direction this self-induced emf is generally called a back-emf.

http://hsc.csu.edu.au/physics/core/motors/2606/PHY932net.html

explain that, in electric motors, back emf opposes the supply emf
Back emf is the emf induced in the coils of a motor as they spin in the external magnetic field of the stator.

By Lenz's law the direction of that induced emf opposes the emf causing the motion of the armature. The current generated in the motor is an eddy current. The direction of the motor eddy current is such that it opposes the supply emf that produces the motion in the motor. The net emf applied to the coils equals the supply emf minus the back emf.

The back emf increases as the speed of the motor increases, until the net emf is just sufficient to provide the energy for the work the motor is doing, against its own internal friction and any load that is applied to it. If there were no back emf, the motor would continue to spin faster and faster indefinitely.

When a greater load is applied to the motor, the armature rotates more slowly and the back emf is reduced. This allows a greater current to flow through the coils, resulting in an increased torque to match the extra load.

At low speeds, when the back emf is small, the motor coils are protected by a series resistor from the large currents that could flow and burn out the motor. This resistor is switched out at higher speeds when the back emf replaces the role played by the resistor at low speed.

And those are all from just the first page of a google search; there are 50,000 more.

The evidence is all around for anyone who cares to look for it, and allow themselves to see it.

You're wrong. Live with it. Or better yet, learn something.

Many, many people know much more about this stuff than I do; you're just not one of them!

 

[Chalo]

So you're talking superficially understood theory with a bunch of guys who have a lot of practical experience under their belts? Good luck with that.

My suggestion is to understand why conventional practice is conventional, then question it. Until you complete step one, step two accomplishes nothing.

 

[Browser]

If you are on a hill and the motor is supplying just enough torque to stop you rolling back, but not enough to allow you to move forward, IT IS STILL DOING WORK!

Work, in physics, measure of energy transfer that occurs when an object is moved over a distance by an external force

No work, as understood in this context, is done unless the object is displaced in some way and there is a component of the force along the path over which the object is moved. Holding a heavy object stationary does not transfer energy to it, because there is no displacement.

[personal attack moved to the moderator section of the forum by moderator]

 

[Browser]

So you're talking superficially understood theory with a bunch of guys who have a lot of practical experience under their belts? Good luck with that.

I've got lots of practical experience of making bread; that doesn't mean I understand the aerobic and anaerobic stages in the life cycle of yeast that causes it to rise.

Anyone in the world can bend a few paper clips, connect a battery to one and cause it to spin in the presence of an magnet; doesn't mean they understand why it spins.

My suggestion is to understand why conventional practice is conventional, then question it. Until you complete step one, step two accomplishes nothing.

Conventional is what is best until something better comes along.

Before the invention of NdFeB magnets in 1983, PMSMs were a dead-end backwater of history; and now...

Before the invention of practical and affordable power electronics, brushless was unconventional; now ...

 

[Chalo]

I suggest that a lifelong pro baker can make better bread than a young microbiologist specializing in yeast.

The same principle applies to motors.

 

[Punx0r]

But, my motor has 36 poles and is intended to run at 20,000rpm. So, 36 * 20e3 * 10 = 120kHz.

The skin depth of copper at that frequency (at 100C) is sqrt( 2 / 2 * pi * 120e3 * 4 * pi * 1e-7 * 5.98e7 ) = 1.8787e-4 = 0.18787 mm. I do need to worry about it.

That's why I'm looking at 0.1mm Cu foil with 0.025 polyester insulation between turns.

One thing I left out of my last post was a caveat that I'm not sure PWM frequency is relevant, as the inductance of the coil smoothes it to DC. So that just leaves the eRPM. I'd speculate that you'll struggle to find controllers capable of such speeds and that switching losses might be excessive.

Also, a 0.1mm foil with 0.025mm insulation means your winding is 25% insulation. By the time it's actually wound the slot fill efficiency is going to be poor.


To be clear are you arguing that a solid winding will experience greater generated voltage (giving lower Kv) than one consisting of two turns of wire half the size? Edit: Just seen your post above which suggests this is not the case (eddy current in conductor), but earlier you were talking about greater BEMF of the motor...

I missed the mark earlier accusing you of being frustrated, angry teenager, but that's how you come across. I do hope you are as much of a genius as you clearly think you are, otherwise your arrogant attitude is entirely unjustifiable.

 

[Punx0r]

The Wikipedia quote about parasitic induction within inductors: the eddy current circulating within the winding conductor material due to uneven magnetic field is interesting, but it does not appear to be anything do with the motor generated voltage (net emf = supply emf - motor emf) described in your last quote from hsc.csu.edu.au.

If you are on a hill and the motor is supplying just enough torque to stop you rolling back, but not enough to allow you to move forward, IT IS STILL DOING WORK!

(Please!) Get someone to take you in their car to a hill. Have them park on that hill. You get out, go round to the downhill end of that car and brace yourself against it. Have them let off the handbrake (parking brake in the US).

Now tell me that you do not expend any energy -- THAT YOU DO NO WORK -- preventing the car from rolling down hill and squashing you.

This has got to be a trick question? Because it appears like school physics 101: common mistakes - confusing force with energy. I can hold the car using my body as a rigid prop by leaning backwards against it and locking my knees. To do so would be less effort than holding it in a forward pushing action, with joints bent. Better yet, I could put my foot behind the wheel like a chock, but I still have the effort of standing. Hmm, I suppose I could sit down instead. Am I doing less work? I'm consuming calories. If it's a cold day I'll be consuming more...

Does this not sound like over-thinking the situation and getting confused? Work = force x distance. Infinite force x zero distance moved = zero work. There are arguable cases (like car drives around a track and stops in the exact position it started, or magnetic repulsion) but this surely isn't one of them. It's a holding torque - zero efficiency - zero work done.

 

[Browser]

I suggest that a lifelong pro baker can make better bread than a young microbiologist specializing in yeast.

The same principle applies to motors.

And who is the lifelong motor builder. Major or you?

And who is the "young microbiologist"?

Cos in my 58th year, nearly 40 years after I qualified as a MechEng; and after a career that included designing an induction hardening furnace amongst many other electro-mechanical machines and systems, "young" sadly no longer applies.

I may never have designed a motor before, but I've designed and built, and operated and programmed plenty of stuff that makes most home brew motors look like meccano kits.