major wrote: ↑
Mar 11, 2018 10:07 pm
Buk___ wrote: ↑
Mar 10, 2018 7:52 pm
The motors we use have much more complex topology, and far more complex and contained magnetic circuits. Something like this (simulated on a similar 100mm diameter scale to the wire loop motor simulation above):
Perhaps you can elaborate on this simulation of a motor, as you say, having "much more complex topology, and far more complex and contained magnetic circuit". For instance, is it a 3 phase BLDC? Also, do the lines represent flux from 4 magnets? Is this design and simulation your handiwork or from elsewhere? Does a real model (hardware) exist? Why did you decide to post it in a reply to my post concerning end-turns and effective armature conductor length? Thanks in advance.
It's the simplest, 3-phase BLDC topology (that I know of), and it is loosely based on a simple case fan motor scaled up. The actual motor I based it upon went in the bin because I broke the circuit board getting the rotor/blade off (a few weeks ago). It was never going to run again. I still have the magnet.
Yes. The lines represent lines of equal flux -- in Tesla -- but in addition to the fields from the 4 magnets, I believe that 2 of the phases were excited, but I don't recall which coils, or which direction and level of current was flowing when I took the screen grab. I posted it because the simulation run happened to complete as I was constructing my reply to you and it made my points about topology and field complexity.
Here is the latest iteration of that motor:
You'll see that it has simplified further -- the coils are much smaller now -- because with a realistic number of conductors per coil, the simulation runs were taking days and even screen redraws took 20 minutes.
The reason for modelling the conductors individually rather en-bloc, is so that I could examine the Lorentz forces affecting each individual conductor; to allow me to document the low level of torque they generate, the way it falls off rapidly with the distance of the conductor from the magnet; and that the torque produced by the conductors on one side of a tooth (almost) exactly counterbalance that produce by the corresponding conductor on the other side.
How, if a BLDC motor relied entirely upon Lorentz force for torque, it would produce little or none; because for every position of the rotor where the flux imbalance across the tooth was such that net Lorentz force generated by the two activated teeth was positive, there is another position within the same revolution where the net balance is the exact opposite.
It was then -- and only then -- that it dawned on me that it didn't matter. That no amount of torque from the windings attached to the stator could possibly cause the rotor to rotate. You postulated that Newton's 3rd might be a possibility, but that defines that when the cannonball is launched, the cannon moves backward in reaction; it does not define what forces are involved when the cannonball lands.
Having produced a model that is realistic enough to satisfy the "would it really run if built question"; and simplified enough that it completes in a realistic amount of time (~15 minutes), I'm currently writing a script to rotate the motor through 360 1° steps, commutating the phases and gathering the individual and net Lorentz torques, as well as the net torque acting on the rotor (via weighted stress tensor) and taking plot grabs to assemble into a movie. (It'll take me a while as my LUA is very rusty.).
Bringing it back full circle:
One of the reasons given for end turns being bad, is because they do not contribute torque because they are oriented wrongly for a Lorentz force to act up on them. I conclude that this is a misnomer for the reasons given above.
Another is that they do not contribute to the flux in the core. This (IMO) is equally, obviously wrong. The fact that the solenoid is square or rectangular doesn't prevent the current flowing in all parts of the conductor contributing to the flux in the core; neither the current nor the flux know the core isn't round, nor what side of it they are on.
And another is that they waste power through "flux leakage". Whilst it is easy to detect flux outside the core around the end turns -- whether in axi-symmetric simulations or with that plastic flux film on a real motor; it is (IMO) a misnomer to consider it "leakage". One of the fundamental tenants of the "lines of flux" according to the Amperian model, is
magnetic B-field lines neither start nor end (mathematically, B is a solenoidal vector field); a field line either extends to infinity or wraps around to form a closed curve
Flux lines are loops, and the flux lines detected outside of the tooth core are the return halves of flux lines running within the core -- the other halves of the closed loops can be nowhere else given the proximity of the coils to the cores they surround -- so they are contributing to the flux density within those cores.
The fact that the flux density within the cores is so high that some of the flux lines find the path of least resistance through the air rather than the core and back-iron, means the core must contain a dense field.
The fact that you get no eddy currents induced in air means that the leakage flux isn't wasting power that way.
All of this seems very clear and logical to me; despite that I know it does not fit with the simple textbook models; or many people's learning based on those texts; but I've yet to see (or possibly yet to appreciate) any thing that shows me (rather than just tells me) that my logic (and modelling) is wrong.
I will continue to model, and research and if I find the means and resources, physically test until I find definitive evidence; for or against.
I will (Gatekeeper permitting) continue to discuss and debate anyone who feels inclined to reason with me (for which you and AlanB have my profound thanks).
If no one chooses to engage with me, I'll stop posting; but my quest to arrive at a level of understanding where I no longer have a bunch of nagging doubts that I cannot resolve; with the aim that I can design a motor in theory, and have a good chance that if I stump up the cash to build it, it will work, will continue.
Thanks for your having taken the time, Buk