Windings: basic questions.

[major]

the torque does not do any work

A dam does not move; but holds back head of potential energy in the water behind it.

Remove the dam (explosives), and that potential energy is instantaneously converted from potential energy to kinetic energy.

Do you think the people down stream of the dam consider it to be doing no useful work?

How much work does it do? Joules? Foot pounds? How many? What is the equation? Please give us a reputable reference indicating force without movement does work.

 

[major]

No. Your logic is flawed. Back-EMF is not *required* to produce torque; it is a consequence of the motion. That motion may occur as a result of the torque.

Once again, learn to read. I'm not saying that; they are.

You are agreeing with me; you are disagreeing with them.

Nobody here has said that BEMF is required to produce torque. That has been my (our) point for the past 9 pages. It is your flawed logic and assumptions which tell you it is, not us.

 

[Miles]

Whatever we say, it gets twisted around to defend this ludicrous edifice...

 

[Lebowski]

the torque does not do any work

A dam does not move; but holds back head of potential energy in the water behind it.

Remove the dam (explosives), and that potential energy is instantaneously converted from potential energy to kinetic energy.

Do you think the people down stream of the dam consider it to be doing no useful work?

Put it another in terms with which you should be familiar.

Sit on your e-bike facing up a steepish hill. give it just enough throttle to prevent it from rolling back, but not enough to cause it to move forward.

What is preventing you from rolling back? (And isn't it useful "work" for it to be doing so?)

Ok, so based on this argument, if instead of the motor with a bit of throttle to keep me still,
I use a piece of rope to tie the bike to a tree. Then the rope is doing work and delivering power ?

Really ?

 

[John in CR]

All it did was change its tone slightly and you guys allowed yourselves to be drawn into the troll's game. Now it's up to 10 pages and the troll knows damn good and well that it's impossible for an electric motor to rotate without creating BEMF, and that BEMF simply comes with the territory. Whether it's intelligent or knowledgeable enough to get that BEMF isn't a bad thing remains to be seen, but since it has zero effect on efficiency (the single true measure of motor quality), it's obvious that minimizing BEMF as a design goal leads nowhere.

 

[izeman]

Emayaan? Is it only me that smells emayaan here?

 

[Punx0r]

Tweak the arguments slightly and Browsers posts would be insisting a perpetual motion machine works. It's the same twisted logic, out-of-context quotes and pseudo-intellectual arguments.

Browser, if you are a mech eng I would encourage you to stay within your field...

 

[Browser]

Ok, so based on this argument, if instead of the motor with a bit of throttle to keep me still,
I use a piece of rope to tie the bike to a tree. Then the rope is doing work and delivering power ?

Is the rope delivering power. Obviously not.

But...You're on your bike, sat facing uphill at (say) a set of lights, holding it against gravity on the motor.
(I know, you always use your brakes to save battery, but go along with it.)

A battery cable falls off; and the bike starts to rolls backward. What was holding it?

The power from the battery was flowing through the coils, creating a magnetic field, creating torque.

No motion. No back-EMF. The force between the coils and magnets exists; the torque exists. And it is doing something useful. And a something useful that can be both theoretically calculated; empirically measured and physically felt.

Now, you don't want to call this "work". because of Work = force * distance, and there's no distance. And, at the macroscopic level, that's true; but it is also an extremely myopic view of the world.

That form of "work" (https://en.wikipedia.org/wiki/Work_(physics)) is only one definition of work. There are others. (And please note when you re-read this, that way back up there ^ somewhere, I hinted to the only person that appeared to be attempting to have a serious conversation with me, that the Physics 101 definition of work is not the only one.!)

In particular, there is "electrical work" (https://en.wikipedia.org/wiki/Work_(electrical)), defined as:

Electrical work is the work done on a charged particle by an electric field. The equation for 'electrical' work is equivalent to that of 'mechanical' work:

So, given we're all talking about electric motors, don't you think it a possibility that you should be considering the movement of the charged particles in the electric field inside your stationary motor when deciding if any work is being done?

And that -- ladies, gentleman and onlookers -- is the problem with your collective reaction to me in this place. From the outset I was treated with disbelief, scorn and derision.

And despite my best attempts to respond to each and every technical response, with patient, technical replies; that derision and outright hostility has continued unabated.

So guys. Enjoy your small-minded clique existence. Pat yourselves and each other on the backs at having driven away the "troll";
but look to your own failings; your own, small-minded, myopic viewpoints for turning a genuine petitioner for your help, into that troll.

Thanks for nuthin.

First I was accused of being a child; then a fantasist; then a troll; and somewhere in the middle the worst of all, a bloody theoretician!

 

[Browser]

the troll knows damn good and well that it's impossible for an electric motor to rotate without creating BEMF, and that BEMF simply comes with the territory.

Yes. It does. And [strike]he[/strike] it has stated so here over and over again.

But, just as drag "comes with the territory" for aerofoil function; it doesn't stop areodynamicists from minimising it.

As with most troll-callers across the net; they're always the first to shout down what they do not understand; and the last to contribute anything to the discussion. ANY discussion.

 

[Browser]

Tweak the arguments slightly and Browsers posts would be insisting a perpetual motion machine works. It's the same twisted logic, out-of-context quotes and pseudo-intellectual arguments.

Lazy and polemic. Throughout the discussion -- and just down there in the sig attached to every post, I've said: torque derives from EMF.

Turn off the EMF, no torque. No allusion to, nor rational way to infer anything that vaguely implies perpetual motion.

Browser, if you are a mech eng I would encourage you to stay within your field...

So, you even doubt I'm a MechEng?

So some advise in return, stick to your day job, whatever it may be, because as a career advisor, you suck.

 

[Punx0r]

You misunderstand me: I didn't say you claimed perpetual motion, just the your style of argument and internal reasoning were reminiscent of those supporting P.M. Mainly the assumption that everyone else is somehow wrong (or covering it up) about some fundamental principle and with a few simple tweaks to existing technology, the miraculous can be achieved by a man in his shed. I'd throw in cheery-picking and misinterpreting formulae as well.

I think you're grasping at straws with that definition of electrical work. It looks like this: [definition of electrical work from Wikipedia] -> [lots of hand waving] -> [proof of whatever it is you're claiming that flies in the face of known physics]

I don't doubt you are a MechEng, I just refrained from stating it as fact because, while you'd hinted at it, you'd never explicitly stated you currently worked as one. That's the difference between jumping to conclusions and knowing what you actually know FWIW I'm not a career advisor and because of that I wouldn't seek out a group of qualified and experienced career advisors*, tell them they're all wrong and morons and then try to teach them how to suck eggs. Incorrectly.

*People judged to be so as accurately as can be done by someone who is not themselves and expert on the subject.

 

[Browser]

[proof of whatever it is you're claiming that flies in the face of known physics]

So, electrical work "flies in the face of known physics". Known by you , possibly.

Even with your limited knowledge, it must be obvious that something is responsible for the -- let's call it -- effort, that prevents the bike from rolling back.

If all the energy in that energised but stationary motor was being converted to heat, there'd be no energy left to create a magnetic field.

If there was no magnetic field, there'd be nothing to attract the magnets; no force; no torque; the bike would roll back under the influence of gravity.

Something is expending the "effort" to prevent that.

Just as two equally match tractors in a pulling contest aren't moving, there is still "work" being done. In the tractors, the diesel engines are turning, and much of the energy is being converted to heat at the clutch plate or in the oil of the torque converter; but some of that energy goes to preventing the tractor moving backwards.

Ditto, the e-bike on a hill scenario. Some of the energy in that stationary motor is "doing work" despite that the motor is not moving.

And the myopic view that if the motor is not moving, it cannot be doing any work, is just a limitation of the view point of the world from the purely mechanical, Physics 101 perspective.

From the electric perspective, the movement of the charge within the electric field required to generate the magnetic field is equally valid "known physics"; and *THE* explanation for why the bike doesn't roll back.

Once again, the problem throughout has been that you are all so intent on trying to pick holes in my descriptions of things; that you're ignoring stuff that most of you probably know.

It's gone beyond frustrating; now, it's just sad.

 

[izeman]

What do you want to acchieve with that thread? Evangelize everyone? Why don't you accept your thruth as is and move on? Is it worth all that? You will NEVER EVER get any other replies than those you already received!

 

[Browser]

What do you want to acchieve with that thread? Evangelize everyone? Why don't you accept your thruth as is and move on? Is it worth all that? You will NEVER EVER get any other replies than those you already received!

He. I gave up on getting any useful information here about 5 or 6 screens back.

And I'm not, and never was "evangelizing". I tried on several occasions to stop people from trying to "correct" me, so that I could discuss the stuff that I wanted help with.

Indeed, I'll quote myself from a much earlier posts:

There's an old saying: Don't try to teach a pig to sing. It's a waste of your time and it annoys the pig.

I openly encourage anyone who believes that "back-EMF" is a good thing to treat me as the pig.

But, no. Still they came. (Remember: I cannot force any one to read this thread; or take part in it!)

See ya around.

 

[Punx0r]

So, electrical work "flies in the face of known physics". Known by you , possibly.

I meant your beliefs about the role of BEMF in a motor. TBH your post just reads like "two magnets can repel each other, that's F x D = work so perpetual motion magnet motors are real". It might seem plausible on initial reception, but falls apart under inspection.

 

[Browser]

So, electrical work "flies in the face of known physics". Known by you , possibly.

I meant your beliefs about the role of BEMF in a motor. TBH your post just reads like "two magnets can repel each other, that's F x D = work so perpetual motion magnet motors are real". It might seem plausible on initial reception, but falls apart under inspection.

AND STILL THEY COME. This time, (as last time and the time before) still attempting to build a strawman* they can knock down

*https://en.wikipedia.org/wiki/Strawman

A straw man is a common form of argument and is an informal fallacy based on giving the impression of refuting an opponent's argument, while actually refuting an argument which was not advanced by that opponent.

 

[learningrc]

Your insistance that back-emf is a bad thing and needs to be minimized is preventing you from understanding why a single turn isn't the best way to wind a motor.

 

[Punx0r]

If we really boil all this down, I think there is one fundamental issue: Do you think there is a significant* possibility that you could be wrong?


*I pre-empting a clever response here by stating "significant" - as in on the scale of our everyday lives. I'm not looking for a lesson on semantics about how there is no such thing as absolute certainty and hence all things are theoretically possible, no matter how unlikely.

 

[Browser]

You will NEVER EVER get any other replies than those you already received!

I guess that proves that theory wrong.

If only they were all so easy.

 

[Browser]

Do you think there is a significant* possibility that you could be wrong?

Have you even considered that you could be?

I think there is one fundamental issue

You obviously think it is fundamental to you. It's not to me. I guess that your need to be 'proven right' can only be indicative of your uncertainties.

I no longer have any such uncertainties.

I did for a while -- who could not given the vocality arraigned against me -- but the one (only) good thing to come out of this thread is that no one here -- despite the coming of "the good and the great" -- has levelled a single, cogent explanation of what makes back-EMF a good thing.

That you cannot avoid it from reducing the torque as rpms rise, is true; but that doesn't make it good.

The idea that back-EMF is required to create torque is (and always was) a self evident fallacy, despite that it took 8 screens to get someone here to admit to it.

The idea that back-EMF is fundamental to electrical to mechanical energy conversion is equally fallacious. Induced (or generated) voltage is fundamental for mechanical to electrical energy conversion; but then it's not back-EMF; just EMF. (Nothing to oppose.)

The conversion of electrical energy to magnetic energy happens when you excite the coils. You don't even need the presence of magnets for that to happen.

Add the magnets and the conversion of magnetic energy to mechanical energy is down to:

The magnetic field in the air-gap containing magnetic energy density. It will try to minimize the volume of the magnetic field (limiting the volume to a space between opposite north- and south- poles), thus minimizing the amount of magnetic energy; and so the motor turns. The "lost" magnetic energy is converted to mechanical energy.

In turn, that motion cause a varying magnetic field within the coils which induces a voltage within them that opposes that conversion.

Thus, back-EMF is an unavoidable (but minimisable) by-product of the conversion process. Unavoidable, but still a parasitic by-product.

 

[major]

The idea that back-EMF is required to create torque is (and always was) a self evident fallacy, despite that it took 8 screens to get someone here to admit to it.

That is a load of crap. You made an assumption early on. Nobody here ever said, or implied, or otherwise hinted that back-EMF is required to create torque. Talk about a straw man.

The idea that back-EMF is fundamental to electrical to mechanical energy conversion is equally fallacious.

That also is a load of crap. You can get torque from the motor without back-emf, but mechanical power and energy conversion require generated voltage in the armature (what you call back-EMF).

Concerning your signature:

Example:
Take a motor with a constant 100 Volts DC applied and a constant load which draws a constant 10 Amperes DC. The motor has a resistance of 0.5Ω at 20ºC. Ignore rotational loss. It has an efficiency of 95%. It has an generated voltage (back-EMF as you call it) of 95V. Now elevate the temperature to 80ºC and it drops to 93.82% efficient. The generated voltage (back-EMF) went down to 93.82V.

Your back-EMF goes down and the motor becomes less efficient. The higher back-EMF or 95V was more efficient than when the back-EMF was lower at 93.82V. Everything else was constant (except for a slight reduction in RPM due to the lower generated voltage). How does that jive with your theories?

And in this example, the power to the load is 950 Watts at 20ºC and 938.2 Watts at 80ºC. How do you explain lower power output with reduced back-EMF?

I notice you rarely if ever use any numbers or equations. Please use the numbers from my example and demonstrate your "bad back-EMF" for me.

major

 

[Browser]

Okay. You wanna play some more. So, answer a question: How did you measure the back-EMF in this example?

Spoiler

My guess: You didn't measure anything. You decided upon a motor velocity constant and derived the numbers in your example from that.

 

[Punx0r]

Do you think there is a significant* possibility that you could be wrong?

Have you even considered that you could be?

Interesting that you thought you might catch me out with that response. Obviously, the answer is "yes". I find myself admitting I was mistaken about at least one thing or other most days. It's why I listen to my peers and don't harbour illusions of infallibility.

As far as I can determine your mistake in thread isn't even complicated. It's just faulty reasoning: mixing up correlation with causation. But from that first wrong turn, instead of realising you went wrong and headed back, you've ploughed on through myriad complex additional turns. You're now so far from the path you're in the realms of the obviously ridiculous, but you can't or won't see it. It's like your car is upside down and filling up with seawater, yet you swear you're still on the main road and don't need directions.

 

[Browser]

How do you explain lower power output with reduced back-EMF?

No answer to my question huh. Okay, let's see what we can do with your made up numbers.

Anomaly 1: 100VDC, 10A draw, and you cite a resistance of 0.5Ω. But R = V/I, so R should be 10Ω.

So, I'll work with 10Ω. And since you haven't supplied it, to keep the numbers simple, I'll assume a conductor cross-sectional area of 1 square millimetre or 1e-6 m

Now R = rho * l / A:: 10.0 = 1.68e-8 * l / 1e-6 := l = 10.0 * 1e-6 / 1.68e-8 = 595.23 metres of 1mm squared conductor. (@ 20ºC) (Sounds like a big damn motor, but c'est la vie.)

At 80ºC, the resistance of copper rises to 2.07312e-8. So, R = 2.07312e-8 * 595.2 / 1e-6 = 12.34Ω

As you say all else is equal, at 80ºC the current flowing will drop to: I = V/R; 100 / 12.34 = 8.1A.

Now, given the current flow has dropped, the magnetic field in the coils will have dropped -- I cannot calculate the exact drop because you haven't supplied sufficient information for that -- but as field strength is proportional to current; and force is proportional to field strength; and torque is proportional to force, the torque has dropped. Inevitably, the speed will drop. And if the speed drops, the back-EMF reduces.

But, motor peak efficiency tends to be (ignoring losses) ~ the no-load maximum speed / 2, and it falls either side of that. Assuming (since you didn't supply it) that your 95% efficiency was measured at that peak efficiency, then that "slight reduction in speed" you mentioned, is all that is required to cause the efficient to drop to the 93.82% at that lower speed.

And so to anomaly ... (I lost count):

"a slight reduction in RPM due to the lower generated voltage"

YOU ARE TAKING THE SYMPTOMS OF THE DISEASE AS ITS CAUSE!

The back-EMF drops, because the motor slows down and back-EMF is proportional to speed. And the motor slowed down because the temperature of the coils rose; and with it the resistance. And when the resistance rises, the current flow reduces; and when that happens, there is less torque being applied to the (unchanged load) and so it slows down

And here I suspect is the crux: The reason that (for the second time) I've asked you how the back-EMF was being measured, is because it isn't. Directly anyway.

For real motors, that motor velocity constant (and the back-EMF) is determined empirically. You take the motor, with no load attached, and apply voltage to it whilst measuring the rpm. Slowly increasing the voltage until the rpm fails to rise further. At that point, the back-EMF must equal the EMF being supplied, and Kv is thus determined, Simply divide the final maximum, no-load speed by the input voltage required to achieve it and there's your motor velocity constant.

Now the physical layout of the motor, all its losses -- core, copper, hysteresis skin effect, proximity, et. al. and the back-EMF -- and thus its efficiency, are all ready set; and Kv simply records that; it does not determine it.

More simply stated: back-EMF is a symptom of the motor design, not a determining factor.

 

[Browser]

Interesting that you thought you might catch me out with that response. Obviously, the answer is "yes". I find myself admitting I was mistaken about at least one thing or other most days. It's why I listen to my peers and don't harbour illusions of infallibility.

I didn't "think I would catch you out"; that's your game not mine.

I simply asked you to consider the same possibility as you asked me to consider.

I went on to say that the only benefit of of my further participation in this thread, once it became obvious I was never going to get answers to my questions, is the fact that it has forced me to re-examine what I "knew" I knew; from first principles and referencing myriad papers and on-line references. ie. It forced me to check myself. Which I did, right here in the full glare of your observation and cross-examination.

And here I still am, still detailing the logic and the references by which I conclude I am right.

As far as I can determine your mistake in thread isn't even complicated. It's just faulty reasoning: mixing up correlation with causation. But from that first wrong turn, instead of realising you went wrong and headed back, you've ploughed on through myriad complex additional turns. You're now so far from the path you're in the realms of the obviously ridiculous, but you can't or won't see it. It's like your car is upside down and filling up with seawater, yet you swear you're still on the main road and don't need directions.

And all of that right back at you.

You think your right; I know I am. If you had any confidence in your belief you'd have moved on and left me to it.

Instead, you argue with me. Try to "set me straight"; set up straw men to knock down; and try to convince me that I'm wrong on purely on the basis that you are telling me so.

Your rhetorical skills are abysmal; your conclusions cack-handed, your argument non-existent; and your logic painfully flawed.

The only question left is: Why are you still here?