Windings: basic questions.

[learningrc]

Sorry, but *THAT* is total crap. And it seems to be a wide spread misunderstanding around here. If you don't believe me, don't argue with me about it; just ignore me.

I'm not arguing ... just trying to explain. I explained to you how to derive both torque and the back-emf for a simple coil and it should be clear from those 2 equations why what I said was true. From those 2 equations you can derive EI = Tw, which is the fundamental equation showing power conversion in a rotating electric machine and is found in every intro textbook to electric machines. Clearly if E is zero, then no power is produced because the product of Tw has to be zero.

 

[parabellum]

No it's not.

Damn bad luck to stumble across a forum of idiots, eh?

Yes, it is. And yes.

:lol: Stop believing everyone is throwing crap at your rainbow, we all have been there at some moment. Miles is the expert you wanted, he made, was active and participated in many motor builds and designs, its your chance to learn from first hand.

 

[John in CR]

6 pages of troll troll-fest. Sure it's a different breed of troll, but a troll nonetheless. It might even be a regular person with genuine interest, but until it sheds it's troll outfit treat it like a troll and stop feeding it.

 

[Browser]

Sorry, but *THAT* is total crap. And it seems to be a wide spread misunderstanding around here. If you don't believe me, don't argue with me about it; just ignore me.

I'm not arguing ... just trying to explain. I explained to you how to derive both torque and the back-emf for a simple coil and it should be clear from those 2 equations why what I said was true. From those 2 equations you can derive EI = Tw, which is the fundamental equation showing power conversion in a rotating electric machine and is found in every intro textbook to electric machines. Clearly if E is zero, then no power is produced because the product of Tw has to be zero.

I did not ask for (nor want) an incorrect explanation for something I already understand correctly.

 

[Miles]

Damn! There's no back button on this browser..

 

[Lebowski]

Oh man and I missed all this !

 

[Browser]

It might even be a regular person with genuine interest

Yes. Interested in getting good answers to the questions I posed.

Not interested in be "taught" fallacies like: "back-emf is fundamental to how a motor produces power".

A motor doesn't produce power, it converts energy from one form to another, more or less efficiently. And 'back-emf" has nothing, zip, nadda to do with that conversion process; it is the by-product of it.

'Induced voltage' is fundamental to the energy conversion; but 'induced voltage' != 'back-emf'.

For there to be "back-emf" (also termed counter-EMF), there has to be an existing EMF, for it to counter. In a generator, there is no applied voltage, the induced voltage is thus just 'EMF'. Mechanical energy converted to electrical energy by way of magnetism; no "back-emf" required.

In a motor, there is an applied voltage, the EMF. It passes through the coils and creates a magnetic field, that magnetic field interacts with another magnetic field -- the PM in PMSMs -- and that interaction causes mechanical movement. Torque in a rotating motor; just force in a linear motor. And electrical energy has been converted to mechanical energy.

However, a by-product of the movement caused by the interaction of the two magnetic fields is that the relative motion between them induces voltage back into the coils, a voltage that opposes the EMF already flowing there, thus "back-" or "counter"-emf.

EMF (supplied voltage) creates torque; converting electrical energy to mechanical energy.
(In a motor) Back-EMF (induced voltage) is a parasitic by-product of that process, that opposes that conversion and ultimately limits it.

It cannot be avoided completely; but it should be minimised wherever possible to improve efficiency.

Are these the ramblings of a troll?

 

[ridethelightning]

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

The same principle applies to motors.

ok. this got me cackling!
very sharp.

 

[alan]

You seem to be stuck with your idea of back-EMF being parasitic..........

I'm not "stuck with it", I simply know it to be so. Read back, follow the links.

Stuck with it seems to be a perfect description of the problem. If one were to objectively read back and follow all of the links, one would likely find conflicting information, some of which is false, and some of which is easily taken out of proper context. What you really mean is to read back and follow ONLY the links that you find to be true, and completely ignore the majority of links, since they conflict with your mistaken concepts about the issue.

It never ceases to amaze me how very smart people can be so very wrong about basic, well established wisdom. The very frequent occurrence of this human failure is more amazing to me than the extremely rare times sprinkled throughout history when extraordinary genius manages to demonstrate that prior wisdom was actually incorrect.

 

[Browser]

It never ceases to amaze me how very smart people can be so very wrong about basic, well established wisdom.

My sentiments exactly. How can you not see how wrong you are? How can intelligent people not follow the logic of my arguments. How can they not see that by simply substituting the correct term "induced voltage' for the incorrect colloquialism, the statements they are making begin to make some sense?

When it first came up I thought you guys were hazing the newbie, but it persisted.

The difference: I started a thread and have remained entirely within that thread.

I've gone no where else; badgered no one. I've not tried to force my opinion down anyone else's throat. I've stuck resolutely within this thread hoping wiser minds will help me with the specific questions I've asked. A few, notably parabellum, have tried to do so.

When others have come seeking to sell me on their snake oil, I've initially politely, asked them not to bother me with it; but still they do, and still they come. Banging their drums trying to sell me their "wisdom".

Except that (to me) that "wisdom" is as plainly wrong as cold fusion, zero-point energy and little green men; and all the other urban myths and legends that have suckered people for decades.

Now maybe, it baffles me how, but just maybe, the feeling is just as strong, just as definite, precise and indisputable for them as it is for me. So, ignore me. Laugh at me behind my back. Whatever. Just stop bothering me with your fallacies.

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.

Done now?

 

[Lebowski]

Ok, answer this question then. You've managed to build a brushed motor with zero backemf. Lets assume also you build it out of superconductor, so no resistive losses. We supply the motor with a constant current. Since we have superconductor and no backemf we deliver the constant current from an inductor (a superconductor inductor will keep a constant current running indefinately). You motor is running and delivers a certain torque at a certain rate of revolutions.

The question to answer now is, where does the mechanical power come from ?

 

[speedmd]

Browser, it may be best to move forward with the points you agree with and focus on improving your understanding of the points your not 100% on with the various stories / experiences of the crew. It's working for me so far. You have certainly passed the initial hurdle of showing genuine project interest and determination. Don't focus on what you don't agree on, and stick to what little will move this forward.

Back emf is not 100% parasitic to my understanding. Pedal your bike while it is on the bike stand with no load and see how easy it is to spin 90 - 100 rpm smoothly. Bit weird to say the least. Same would be for a controller trying to run a open ended (no back emf) motor system.

 

[Browser]

Lebowski. Please understand, I respect your abilities.they are responsible for me coming here. I've read your motor threads in depth. I've started on your controller threads. If I get my motor to the point where I believe I can build it I can see myself coming to you to purchase one of your controller chips. What comes below is the way I have understood the world to work since college. There is to my eyes nothing even vaguely controversial about it. And, from my viewpoint, it is all as clear and simple as the nose on your face.

Throughout this saga I've been expecting someone to step in a support my stance. To say, you guys are conflating your terminology. Browser is obviously correct. That hasn't happened and I am astounded that it has not.

Ok, answer this question then. You've managed to build a brushed motor with zero backemf.

I've never said you could ever build a motor without back-EMF. Indeed, I've said several times you cannot avoid it.

The question to answer now is, where does the mechanical power come from ?

The torque in a motor comes from the EMF; by way of the magnetic attraction between the field it induces in the coils, and the permanent magnets. There is nothing controversial about that bit right?

Now, assume the motor is stalled, (car on a hill) the EMF (battery voltage) is flowing in the coils. The coils are attracting the magnets.

The force (attraction for one phase, repulsion for the other excited phase) between the coils and magnets, constrained to one degree of movement by the axle, is the TORQUE.

Nothing has moved yet, so no back-EMF can possibly exist, but the force is there and if you let off the handbrake in the car, that force is enough to prevent the vehicle from rolling backward.

Even if you have not supplied enough EMF to allow it to overcome the weight of the vehicle, it passengers, friction and gravity to allow it to move forwards, the motor is consuming power and the resultant torque is preventing you from rolling backwards. EMF is flowing through the coils, the field they produce is attracting the PMs, torque (maximum torque for the supplied EMF) is being produced.

But nothing has moved. So the coils have not experienced a change in the magnetic field from the PMs. No change, no induction, no back-EMF. EMF, but no back-EMF.

Only once you start moving do the coils experience a change in the magnetic fields from the PMs, and only then does back-EMF start to be generated.

But ONLY AFTER the EMF has produced enough torque to cause the motor to start revolving. Torque has to exist BEFORE back-EMF can be produced, thus it cannot be responsible for its existence.

And at low speeds, the amount of voltage (thus current) produce by the slowly moving magnets is tiny. Not enough to overcome friction, gravity et. al, (Which is a good thing because it means that most of the EMF is generating torque; and because if back-EMF was producing torque it would be driving the vehicle the wrong way!)

As speed builds up, the back-EMF produced increases, because the rate of change of the PM magnetic field increases, and that back-EMF reduces the effective voltage flowing in the coils, so the attractive/repulsive forces are less, and the torque produced reduces.

Eventually, the speed of the motor increases to the point where the back-EMF has risen to match and exactly oppose the EMF from the batteries, and no more torque can be produced and the maximum speed for the motor has been reached.

Reach a (steeper) hill, and the vehicle slows down. Because the motor rotates more slowly, less back-EMF is produced, so more torque becomes available. And, given enough EMF, you make it up the hill.

 

[Lebowski]

Ok i'll try to read all that tomorrow...

But on the level of the second law of thermodynamics (is it 2nd?) Which states power out is equal to power in,

With your motor delivering torque at rpm (so delivering mechanical power) but (because of no backemf) not taking power from the current source inductor, where does this mechanical power come from ?

 

[major]

with the specific questions I've asked

From posts 1 and 3:

So question 1: why not put one big-assed conductor in the gap?

A single conductor, one or a half turn coil, will typically have a very low generated voltage which is cumbersome to use with available power supply potentials. So multiple turns, or series connected multiple conductors, and/or series connected coils in the armature are employed such that the armature voltage is near the supply voltage for the machine speeds desired. This term, turns per coil, or T/c, is often represented by the symbol N. See Faraday's Law.

Call it generated voltage, back emf, BEMF, induced voltage, a rose or a turd; same thing. Call it good, bad, ugly, beautiful or evil. My answer is correct.

Throughout this saga I've been expecting someone to step in a support my stance. To say, you guys are conflating your terminology. Browser is obviously correct. That hasn't happened....

Have you considered why?

 

[Browser]

not taking power from the current source

It is taking power from the current source, to energise the coils. to overcome their resistance, and produce the magnetic field around them that attracts the PMs.

It is the attraction between the coils and magnets that creates the mechanical force. Torque.

Just as if you hold a PM and an energised coil in your hands a few inches apart, you will feel the force between them, the force exists even though nothing has moved.

 

[Browser]

Have you considered why?

Yes. How could so many people get something so fundamental so wrong.

Yes. Deeply. And the only logical conclusion is that one person got the wrong end of the stick and everyone else cargo-culted it.

That is the only explanation for some many people apparently believing that something that, by definition, cannot possible exist until the motor rotates, is responsible for causing it to rotate.

I don't believe in magic. And I do not doubt my knowledge that back-EMF can have no part to play in making a motor turn.

Another attempt at a logical deduction to convince you.

The induced voltage (EMF in this case) in a generator, is identical in every way to the back-EMF in a motor. Caused by the changing field impinging on the coils, due to the rotation of the magnets. (Agreed so far?)

So, take a motor, disconnect the batteries, and spin it by hand. The magnets move, the field impinging the coils changes, voltage is produced.

If the induced voltage is responsible for the motor turning, why when you stop manually spinning it, doesn't it just continue?

 

[major]

If the induced voltage is responsible for the motor turning, why when you stop manually spinning it, doesn't it just continue?

The induced voltage is not responsible for the motion and I never said it was.

 

[major]

Just as if you hold a PM and an energised coil in your hands a few inches apart, you will feel the force between them, the force exists even though nothing has moved.

So no work is done by the force. No energy is transferred or converted. All you have is the power lost in the resistance of the coil (heat) which is exactly the same had there been no force due to the magnet being oriented 90º or being a million miles away. No work can be done until there is motion of the coil in the field and then there will be a generated voltage in the coil (BEMF as you call it).

 

[Browser]

Just as if you hold a PM and an energised coil in your hands a few inches apart, you will feel the force between them, the force exists even though nothing has moved.

So no work is done by the force.

No. Because your hands are preventing it. Have you never seen a paper clip fly towards a magnet? Or one magnet jump towards another magnet.

Because THAT'S WHAT DRIVES MOTORS. The attraction between the permanent magnet, and the magnet field around the coil produced by the current flowing through it, driven by the battery that supplies the energy to be converted.

No work can be done until there is motion

So, the thing that can't exist until there is motion, causes the motion that causes it to exist. Are you incapable of logic conclusion?

 

[major]

No work can be done until there is motion

So, the thing that can't exist until there is motion, causes the motion that causes it to exist.

That must be your conclusion because it certainly isn't mine. Force causes the movement and motion will happen when the force is greater than the forces restraining movement. The force can exist without the motion. Just like your example:

Just as if you hold a PM and an energised coil in your hands a few inches apart, you will feel the force between them, the force exists even though nothing has moved.

 

[major]

Yes. Deeply. And the only logical conclusion is that one person got the wrong end of the stick and everyone else cargo-culted it.

Cargo-cult
In the South Seas there is a cargo cult of people. During the war they saw airplanes with lots of good materials, and they want the same thing to happen now. So they've arranged to make things like runways, to put fires along the sides of the runways, to make a wooden hut for a man to sit in, with two wooden pieces on his head for headphones and bars of bamboo sticking out like antennas—he's the controller—and they wait for the airplanes to land. They're doing everything right. The form is perfect. It looks exactly the way it looked before. But it doesn't work. No airplanes land. So I call these things cargo cult science, because they follow all the apparent precepts and forms of scientific investigation, but they're missing something essential, because the planes don't land.
— Richard Feynman

Another word(s) for the day. Thanks.

And I do not doubt my knowledge that back-EMF can have no part to play in making a motor turn.

Another attempt at a logical deduction to convince you.

The induced voltage (EMF in this case) in a generator, is identical in every way to the back-EMF in a motor. Caused by the changing field impinging on the coils, due to the rotation of the magnets. (Agreed so far?)

So, take a motor, disconnect the batteries, and spin it by hand. The magnets move, the field impinging the coils changes, voltage is produced.

If the induced voltage is responsible for the motor turning, why when you stop manually spinning it, doesn't it just continue?

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

 

[Browser]

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

But, but ... back-EMF *IS* induced voltage. That voltage induced in the coils by the permanent magnets field as they move.

(Which is why it cannot exist before the motor rotates, and thus cannot be responsible for causing the rotation.)

 

[major]

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

But, but ... back-EMF *IS* induced voltage. That voltage induced in the coils by the permanent magnets field as they move.

(Which is why it cannot exist before the motor rotates, and thus cannot be responsible for causing the rotation.)

But what? We agree. Call it whatever you want to. Induced voltage or back-EMF. I have never said nor have I seen another say on this thread that induced voltage (or back-EMF) is responsible for the motor turning. And nobody has said it exists before the motor rotates.

 

[kiwifiat]

If you use S.I. units of Nm/A for Kt and V/(rad/s) for Ke, Ke=Kt. The proof of this fact is available all over the internet in open University courses, text books, PHD thesis papers, IEEE papers etc. By minimizing Ke , the Bemf constant, all you achieve is to minimize Kt. Bemf and torque production are bound together by the laws of physics. In practice the choice of Kt, Ke are dictated by the specific design requirements of the application. What is the line voltage, what are the power requirements at what rotational speed? Low Ke motors are generally low inductance motors which follows from the fact that inductance is proportional to the number of turns of wire employed. We all know that low inductance motors are pigs to control because low inductances running on commonly employed line voltages lead to exceptionally high di/dt rates that make it imposing the pwm voltage wave required to achieve FOC near impossible with current technology. Since we want the maximum torque per amp delivered by FOC we don't want motors with exceptionally low Ke=Kt.

I think the engineers at YASA motors, a spin-off from Oxford University, are some of the smartest electrical engineers on the planet. Take a look at their design, it doesn't appear that they are of the opinion that low Ke is the path to efficiency and I agree entirely.