stator/ coil design and the mysteries of flux density

[hal2000]

!. Saturationa: It sounds like a sponge filling with water and has been confusing me. When the stator core lams come under the influence of an energized coil winding What is saturation? Is it more and more of the disordered magnetic domains within the stator lams, being induced into reorienting themselves to aline with the direction of flux of the windings. if so your stator lams are kind of latent nanomagnets that have to be herded into working in the same direction. I'm reminded of the moments before a symphony begins and all the string section is warming up all playing notes at different times , then with the tap of a batton the stings pic a note to tune to following the lead of one player and what was noise becomes music, is this a better analogy to what is happening inside the stator core under the influence of the flux generated in the windings?

2. Hysterisis: What exactly are hysteresis losses and are they brought about by the tiny individual magnetic domains all having to flip around and face the other direction when current in the winding is reversed at the command of the controller?

3. I looked at the idea of the catapult affect, and how the magnitude of the vector of torque created to turn our motors changes as the electrically generated magnetic flux in the coil wire is brought closer to the permanent magnetic field of the magnets.

Some questions arise. If the magnitude of the torque reaction is directly related to the proximity of the coil wire to the magnet this would seem to greatly favor axial flux machines as you coils could all be put vary close to the magnet and with no iron core you wouldn't have hysteresis loses, so why are axial machines so rare, ease of construction?

4. How is the torque capacity of a wound stator influenced by the geometry of the stator tooth. It would seem that in a standard wound stator the taller the stator tooth the taller the coil of windings and the greater the amount of flux you can generate, but as the stator increases in height the windings are farther and farther from the magnets and therefore should have less and less direct contribution to the torque, does this matter, if so then i would expect to find long short stators, but if major conveyor of flux is the induced magnetic field inside the lam core and not the windings themselves the then the taller the stator the more powerful your flux?

5. Winding stators: Does the influence of the winding on the stator lams change as you probe deeper and deeper into the center of the stator core? Does saturation occur from the outside in ? If so like burning a steak when the center is raw, are there loss consequences to having the outer lams in saturation while the center is not there yet?

6. Lastly I think I have an idea of how to increase the useful torque generated in the stator of a radial or axial flux motor by using a geometry for construction which would cut end turns way down. I'm not sure if it is wrong headed however, and I want to test it before I say something foolish, to do so I need lam steel in wire form varnished, is it available? in the same gauge as good lams. If anyone lives within an hour of olympia washington that I could talk in person to to see if I'm barking up the wrong tree? pm me if so

 

[major]

1.) Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetic field H cannot increase the magnetization of the material further, so the total magnetic flux density B levels off. It is a characteristic particularly of ferromagnetic materials, such as iron, nickel, cobalt and their alloys. from: http://en.wikipedia.org/wiki/Saturation_%28magnetic%29 It is a pretty fair explanation.

2.) Hysteresis - When the magnetic field through the core changes, the magnetization of the core material changes by expansion and contraction of the tiny magnetic domains it is composed of, due to movement of the domain walls. This process causes losses, because the domain walls get "snagged" on defects in the crystal structure and then "snap" past them, dissipating energy as heat. This is called hysteresis loss. It can be seen in the graph of the B field versus the H field for the material, which has the form of a closed loop. The amount of energy lost in the material in one cycle of the applied field is proportional to the area inside the hysteresis loop. Since the energy lost in each cycle is constant, hysteresis power losses increase proportionally with frequency. from: http://en.wikipedia.org/wiki/Magnetic_core#Core_loss Again, simple but good explanation.

I know I just pasted in a couple of paragraphs, but they say it as good as I could have. And the links elaborate further for you.

 

[kenkad]

hal2000
With regard to #5. If the design is axial flux, then the lam core would benefit from having more turns from the center out to either end. The ends of the core are where the saturation occurs. The solution should be a simple non linear copper distribution along the length of the core. Since I am not personally interested in inrrunners or outrunners, this is an issue that I had planned to study with EE graduate students on a AF PM BLDC design. I am not associated with any academic institution. The experiment has to be done carefully and, more importantly, cannot be done properly with a single subcoil core, or so I have been told by motor design experts, because adjacent coil interactions have to be taken into account. The motor design we are working on will allow this type of analysis.

I have attached a lamination core JPG (had to inseert it into an *.pdf file to reduce image size) that was to wound with staircased cut copper foil/tape. The recess in the center was for the copper rod that joined the staircased copper so it could be wound and terminated. This also results in a nonlinear resistance along the winding. This experiment will be done in the future when time permits and if i am able to rent or buy a sensitive flux measuring probe/instrument. Our first experiments will be with linear laminated cores using a baseline 22 gauge winding to compare other core designs to.

If you feel that it is difficult to find anyone in you local area that really understands BLDC motor design issues, welcome to the club. It has been a frustrating experience to find academic institutions with knowledgeable staff in my area of the USA.
kenkad

 

[Miles]

With regard to #5. If the design is axial flux, then the lam core would benefit from having more turns from the center out to either end. The ends of the core are where the saturation occurs. The solution should be a simple non linear copper distribution along the length of the core.

Hi Ken,

I didn't think it mattered significantly where the coils are with respect to the core? Inner coils, outer coils, upper, middle, lower all act through the core as a whole? No?

With regard to saturation, surely that occurs first at the point with the highest flux density? If that's at the ends, it's a design fault

 

[kenkad]

Miles,
It depends on the length of the core. Very short cores would not benefit. FEMM (it was done for me) showed that as the cores get longer, the flux distribution is not linear along the subcoil. This is why, when I have time and can afford the probe, I will go back to mapping this out in an actual setup. This is of personal interest to me that started with also attempting to reduce the resistance of the winding and after some discussions I had with Shane Colton when he was doing his foil windings on his AF motor.

 

[Miles]

FEMM (it was done for me) showed that as the cores get longer, the flux distribution is not linear along the subcoil.

Does that mean that some parts of the coil are more effective, though? I doubt it. Surely, it's not worth sacrificing space, even if that is the case? If the flux distribution is uneven, the core cross-section needs adjusting (assuming that's possible).

 

[major]

What is a subcoil?

 

[kenkad]

major,
I use the term subcoil because one design had three subcoils in a 30 degree stator segment and was referred to as a coil assembly. I will try to not use the term subcoil here again. It is not relevant. Sorry to confuse everyone.