Windings: basic questions.

[Browser]

Woah... Copper losses are proportional only to current (at a given temperature)...

You think there is no frequency component whatsoever in copper losses? Right.

 

[learningrc]

Have it your way; whilst you cannot have an electric motor without back-EMF; IT DOES NOT mean that "back-emf is fundamental to how a motor converts power".

Seriously, pick up Kingsley and Fitzgerald's Electric Machinery. It's a standard textbook for the first course you would take as an undergrad in electric machinery. They go through the derivation of energy conversion starting from the law of the conservation of energy to models for different types of machines. I will leave it to you to research that yourself but here is a quote from the book (this is from a section that is introducing synchronous AC motors):

"In a motor the electromechanical torque is in the direction of rotation and balances the opposing torque required to drive the mechanical load. The flux produced by currents in the armature of a synchronous motor rotates ahead of that produced by the field, thus pulling on the field (and hence on the rotor) and doing work. This is the opposite of the situation in a synchronous generator, where the field does work as its flux pulls on that of the armature, which is lagging behind. In both generators and motors, an electromechanical torque and a rotational voltage are produced. These are the essential phenomena for electromechanical energy conversion."

The bold is not in the original, I added it. Then a little bit later in a section on DC machines:

"Thus, just as in the ac machines discussed previously, it is the interaction of these two flux distributions that creates the torque of the dc machine. If the machine is acting as a generator, this torque opposes rotation. If it is acting as a motor, the electromechanical torque acts in the direction of the rotation. Remarks similar to those already made concerning the roles played by the generated voltage and electromechanical torque in the energy conversion process in synchronous machines apply equally well to dc machines."

Again, the bold is from me, not them. It's clear that these authors think that the generated voltage is "essential" for energy conversion.

One of the better books you will find on brushless motors is Hanselman's book Brushless Permanent Magnet Motor Design. It's a very good book ... I highly recommend it. In it he gives the equation I've given twice already in reply to you: EI = Tw (where, again, E is the back-emf, I is the current, T is the torque and w is the speed). This is the basic equation of energy conversion in rotating machines. Right after deriving this equation (which is equation 3.43 in the edition of the book that I own), Hanselman writes:

In addition to its utility in motor analysis, (3.43) has profound implications in motor design. According to (3.43), for a given mechanical output power, the required electrical input power can be composed of a high back EMF at a low current, a high current at a low back EMF, or some compromise in between. Of these choices, a high back EMF at a low current is preferred, because it minimizes the current handling requirements of the power electronics used to drive the motor.

Again, my emphasis, not the book's. He doesn't seem at all interested in trying to minimize the back-emf, does he? Then at the end of the chapter summary, he writes:

"Based on a simple example, conservation of energy was used to show that force or torque is related to the power absorbed by the back EMF of a winding. This relationship provides a simple and important connection between back EMF and torque, which is valuable in motor design. "

I encourage you to pick up these books and read them for yourself. You might learn something...

 

[Miles]

Woah... Copper losses are proportional only to current (at a given temperature)...

You think there is no frequency component whatsoever in copper losses? Right.

This seems to be a limitation of the terminology... Obviously, the frequency component is significant in the "copper losses" of the kind of air cored motor you're proposing. For an iron cored motor, these kind of losses would, overwhelmingly, occur in the core and would be called iron losses. This kind of confusion is why I prefer the term parasitic losses.

 

[Miles]

Hah. It's a fundamental law of physics; but you think it "doesn't apply".

You've never let "fundamental laws of physics" get in your way They don't come more fundamental than the first law of thermodynamics....

The problem that you appear to have with learning, Buk, is that you're not prepared to devote a single neuron to understanding what people are trying to communicate. Everything has to be on your terms. Has it been you against the world since you were a child?

 

[Miles]

In addition to the texts that learningrc has mentioned, I would recommend the second chapter of James Mevey's thesis "Sensorless Field Oriented Control of Brushless Permanent Magnet Synchronous Motors" https://endless-sphere.com/forums/viewtopic.php?p=236417#p236417

 

[Browser]

Then at the end of the chapter summary, he writes:

"Based on a simple example, conservation of energy was used to show that force or torque is related to the power absorbed by the back EMF of a winding. This relationship provides a simple and important connection between back EMF and torque, which is valuable in motor design. "

First. learningrc. THANK YOU. Thank you for formulating an actual argument rather than a knee-jerk reaction. And especially thank you for citing some actual text. (Unfortunately, you still ain't gonna like what I'm going to say )

Please re-read this phrase, in a dispassionate and analytical way: "force or torque is related to the power absorbed by the back EMF of a winding." because that would appear to be the foundation of this (apparently widespread, though I've never encountered it anywhere else than here) misunderstanding.

And the key to that misunderstanding are the two words "power absorbed". You, and many others appear to think that sentence implies that that "absorption of power" is a good thing. That some how "power absorbed by back-EMF" means that power is made available to do work.

Now read my slightly modified version of that sentence: <i>"force or torque is negatively related to the power absorbed by the back EMF of a winding.".

Or to paraphrase: the more power absorbed by back-EMF; the less torque is available to do work.

As for your EI = Tw. The problem is that you insist on saying that E is back-EMF; but E in that equation is EMF. Ie. the applied voltage from the batteries.

If E were back-EMF, then that would mean that the energy from the batteries doesn't appear in that equation at all. And if you could create torque with no power from the batteries, then you'd have a free energy machine.

I can only conclude that this is either some kind of elaborate hoax -- hazing the outsider -- or some kind of mass delusion, because it is all right there in the quotes you cite; but you'll just don't seem to be able to see it.

 

[learningrc]

It's been a while since I've read Mevey's paper ... good call, Browser could learn a lot from it. One of the things he would learn from that paper (or the books I mentioned, for that matter) is that the back-emf in a motor is just the derivative of flux linkage with respect to time. And we all know that flux linkage isn't essential to power conversion at all, right? :lol:

 

[Browser]

Hah. It's a fundamental law of physics; but you think it "doesn't apply".

You've never let "fundamental laws of physics" get in your way They don't come more fundamental than the first law of thermodynamics....

The problem that you appear to have with learning, Buk, is that you're not prepared to devote a single neuron to understanding what people are trying to communicate. Everything has to be on your terms. Has it been you against the world since you were a child?

You're only here for the argument aren't you. A follower of the flow; a joiner; a sheep. A complete waste of space and breath and electrons.

I seriously doubt you've ever had an original thought in your life

 

[learningrc]

First. learningrc. THANK YOU. Thank you for formulating an actual argument rather than a knee-jerk reaction. And especially thank you for citing some actual text. (Unfortunately, you still ain't gonna like what I'm going to say )

These aren't arguments ... all I did here was quote from some texts, hoping you'd take it on their authority since you've ignored everything else I've said so far. Apparently not.

Please re-read this phrase, in a dispassionate and analytical way: "force or torque is related to the power absorbed by the back EMF of a winding." because that would appear to be the foundation of this (apparently widespread, though I've never encountered it anywhere else than here) misunderstanding.

And the key to that misunderstanding are the two words "power absorbed". You, and many others appear to think that sentence implies that that "absorption of power" is a good thing. That some how "power absorbed by back-EMF" means that power is made available to do work.

Now read my slightly modified version of that sentence: <i>"force or torque is negatively related to the power absorbed by the back EMF of a winding.".

Or to paraphrase: the more power absorbed by back-EMF; the less torque is available to do work.

Go read through the derivations and you'll see that your addition of the word "negatively" is ridiculous.

As for your EI = Tw. The problem is that you insist on saying that E is back-EMF; but E in that equation is EMF. Ie. the applied voltage from the batteries.

If E were back-EMF, then that would mean that the energy from the batteries doesn't appear in that equation at all. And if you could create torque with no power from the batteries, then you'd have a free energy machine.

I can only conclude that this is either some kind of elaborate hoax -- hazing the outsider -- or some kind of mass delusion, because it is all right there in the quotes you cite; but you'll just don't seem to be able to see it.

E is definitely back-emf. That is easily verifiable BY YOU if you go pick up ANY decent book on electric machines. Seriously, do some research before making stuff up like this. (I actually changed Hanselman's notation in that equation to make it fit the notation I had used in my earlier replies. His actual equation uses E_b - that is E with a subscript b - which is his notation for back-emf.)

Now ... what is more likely ... a mass hoax or you just don't know as much as you think you know about this subject.

 

[Lebowski]

As for your EI = Tw. The problem is that you insist on saying that E is back-EMF; but E in that equation is EMF. Ie. the applied voltage from the batteries.

If E were back-EMF, then that would mean that the energy from the batteries doesn't appear in that equation at all.

E is the back-emf voltage, not the energy. And the battery voltage does not appear in that equation because it does not play a role.
The battery will supply (more than) EI amount of energy but the battery voltage and current are unrelated to E and I and can be totally different.
But the product of the battery voltage and current will be higher than EI.

 

[learningrc]

Browser, go download Mevey's paper that Miles linked to. Go to chapter 2, equation 2.40. Same equation that I've given 3 times now. If you read earlier in the chapter to see how he got to that equation, it is VERY clear that e in his equation is back-emf.

 

[Browser]

As for your EI = Tw. The problem is that you insist on saying that E is back-EMF; but E in that equation is EMF. Ie. the applied voltage from the batteries.

If E were back-EMF, then that would mean that the energy from the batteries doesn't appear in that equation at all.

E is the back-emf voltage, not the energy. And the battery voltage does not appear in that equation because it does not play a role.
The battery will supply (more than) EI amount of energy but the battery voltage and current are unrelated to E and I and can be totally different.
But the product of the battery voltage and current will be higher than EI.

Take a quick squint at http://www.mpoweruk.com/machines.htm.

In particular, note:

Operating Equilibrium Under Load

The "Action and Reaction" effects outlined above provide an important automatic self regulating feedback mechanism in both DC and AC motors for adapting to changes to the applied load. As the load on the motor is increased it tends to slow down, reducing the back EMF. This in turn allows more current to flow generating more torque to accommodate the increased load until a point of balance or equilibrium is reached. Thus the motor will set itself to an appropriate speed for the torque demanded.

 

[Lebowski]

How does this relate to my post ?

 

[Browser]

Okay. I have to apologise to the forum.

I've now found (a few) references to the statement:

"According to fundamental laws of nature, no energy conversion is possible until there is something to oppose the conversion. In case of generators this opposition is provided by magnetic drag, but in case of dc motors there is back emf."

so this myth is more widespread than just this forum.

Accordingly, I can now (perhaps) understand the level of reaction I've received here. You guys read it in a book; therefore it must be true.

Problems:

1) None of the references online or here elucidate what "fundamental laws of nature".

So then you look up "fundamental laws of nature" and amongst the many crackpot pages you find (eg. typified by http://www.kostic.niu.edu/Fundamental_Nature_Laws_n_Definitions.htm), you also find pages like this one http://www.iep.utm.edu/lawofnat/, which point out that:

Laws of Nature are to be distinguished both from Scientific Laws and from Natural Laws. Neither Natural Laws, as invoked in legal or ethical theories, nor Scientific Laws, which some researchers consider to be scientists' attempts to state or approximate the Laws of Nature, will be discussed in this article. Instead, it explores issues in contemporary metaphysics.

Within metaphysics, there are two competing theories of Laws of Nature....

So, the basis for this tautological nightmare of a non-sequituous conclusion, is not scientific, but philosophical.

2) If a motor requires "something to oppose the conversion"; how about the load attached to it; friction; windage; DC and AC resistances. Don't these qualify as 'things that oppose the conversion'.

How about the creation of the electromagnetic field?

When you pedal an ordinary bike, you are converting (bio)chemical energy (from protein, carbs and fats via sugars) to mechanical energy. The thing(s) opposing that conversion are your weight; friction, wind-resistance; gravity. It doesn't require the invention of a philosophical opponent to the conversion; physics provides all the opponents required.

I'm not sure where or when this myth first came about -- if I get the time I'll try and track down the originating source -- but for goodness sake; use your brains. Just cos its "in a book" or even in many books; it doesn't mean you have to take it at face value. THINK! Analyse. Apply logic.

If back-EMF is responsible for the conversion of electric potential in to mechanical force; how can the motor deliver the torque required to start the motor rotating, when the magnets need to rotate in order for there to be any back-EMF produced?

And, PMSMs famously deliver their maximum torque at O rpm; and their torque steadily reduces as the rpms rise; with maximum power (torque * speed) coming at roughly half their no-load rpms.

I *think* we are in agreement that back-EMF rises with speed: so, if back-EMF is responsible for the production of torque; how come torque falls as the back-EMF rises?

Please, please! If you're happy to base your view of the world on an unexplained, philosophical, "Law of Nature", fine. Live the dream. Otherwise, for dogs sake; THINK! Apply a modicum of thought before taking this as read.

I'm done now. This has occupied far too much of my time and thought processes. You'all have beaten me into submission. I wish you all the best in your endeavours, and happy motoring.

(But, it won't be due to back-EMF producing torque!)

 

[major]

(But, it won't be due to back-EMF producing torque!)

Who has said that back-EMF produces torque? Nobody here! Please provide quotes.

You have a bad habit of stuffing words in other people's mouths. And then fading away without answering questions about the source. Like a few days ago.

I have never said nor have I seen another say on this thread that induced voltage is responsible for the motor turning.

Please indicate what you are talking about with direct quotes.

 

[major]

.... Kingsley and Fitzgerald's Electric Machinery. It's a standard textbook for the first course you would take as an undergrad in electric machinery.

.... the roles played by the generated voltage... "

The text I used so many years ago and the reason I call it generated voltage instead of BEMF. I still have that beat-up old book within arm's reach.

 

[Browser]

Who has said that back-EMF produces torque?

Because that is the logical consequence of the statements you are making.

[learningrc] has repeatedly quoted (and lebowski has come out in support of) the equation: E(back-EMF) * I (current) = T(torque) * w(angular momentum) as proof that "back-EMF is fundamental to energy conversion".

The two statements are logically synonymous.

Of course, there are other interpretations of that formula:

Torque is proportional to current; and back-EMF is proportional to angular momentum.

Since (by definition and name) back-EMF opposes the EMF, and current is proportional to voltage (EMF), as angular momentum rises, back-EMF rises, so EMF reduces and current reduces.

Short version: Torque is inversely proportional to back-EMF.

 

[major]

Who has said that back-EMF produces torque?

Because that is the logical consequence of the statements you are making.

No, it is not. You are incorrect with logic. Nobody has said back-EMF produces torque or inferred it. You are reading something into the statements by members and from references which is not stated nor is it implied nor is it an intended consequence. You are illogical.

 

[Lebowski]

Torque can be produced without backemf but at standstill only, and in that case the torque does not do any work (no power is delivered)

 

[Browser]

Who has said that back-EMF produces torque?

Because that is the logical consequence of the statements you are making.

No, it is not.

Yes. It is.

(Contradiction is not an argument: http://www.davidpbrown.co.uk/jokes/monty-python-arguement.html)

You may not be saying it; or concluding it, but others are:

Torque can be produced without backemf but at standstill only, and in that case the torque does not do any work (no power is delivered)

 

[major]

(But, it won't be due to back-EMF producing torque!)

Who has said that back-EMF produces torque?

Because that is the logical consequence of the statements you are making.

No, it is not.

Yes. It is.

The developed torque (Tem) = Kt * Φ * Ia where Ia is armature current.

There is NO factor of EMF (or back-EMF) in that torque equation. Torque is independent of voltage.

You may not be saying it; or concluding it, but others are:

Torque can be produced without backemf but at standstill only, and in that case the torque does not do any work (no power is delivered)

He says "without". Torque is NOT caused by back-EMF.

 

[Browser]

Torque can be produced without backemf but at standstill only, and in that case the torque does not do any work (no power is delivered)

He says "without". Torque is NOT caused by back-EMF.

Sorry, but you need to learn to read.

He said (with added emphasis): "Torque *can* be produced without backemf *but at standstill only,*"

Thereby implying "torque cannot be produced without back emf" except at standstill".

Which of itself is a perfectly reasonable statement; but it implies that back-EMF is *required* to produce torque.

And, as his early statements give testimony (look back for his "purpetuum" comment; I cannot be bothered to) that *is* his (and others) position.

But just because back-EMF is an *unavoidable consequence* of creating torque in a PMSM motor; it does not logically follow that back-EMF is responsible for the energy conversion that creates that torque.

Any more than, the unavoidable consequence of accelerating the air over the upper surface of an aerofoil to create lift: drag, is responsible for the energy conversion required to create that lift.

Just as drag can be reduced to either: a) increase the lift for a given input of energy; or b) decrease the energy required to produce a given amount of lift.

So, (within limits) back-EMF can be reduced to either: a) increase the amount of torque produce by a given amount of energy input; or b) decrease the amount of energy input to produce the given amount of torque.

Hence, we come full circle back to where this thread started: What, in terms of windings -- conductor size and profile and number of turns -- can be done to reduce the amount of energy wasted on creating back-EMF in those windings.

And for answer to that, beyond what I've learnt through my own research and reading that of others -- whilst (against my wishes and better judgement) pursuing this endless, circular (eddy-current) of a pointless debate here -- I'm going to have to look elsewhere.

I guess its been something to do whilst waiting for my FEMM models to run.

 

[major]

Torque can be produced without backemf but at standstill only, and in that case the torque does not do any work (no power is delivered)

He says "without". Torque is NOT caused by back-EMF.

He said (with added emphasis): "Torque *can* be produced without backemf *but at standstill only,*"

Thereby implying "torque cannot be produced without back emf" except at standstill".

Which of itself is a perfectly reasonable statement; but it implies that back-EMF is *required* to produce torque.

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

 

[Browser]

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?)

 

[Browser]

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. (Watch out; you're in danger of losing your standing in the ES cargo-cult club.)