Non-Halbach AFPM rotor design concepts, which is better?

[kenkad]

Hello,
With all of the ongoing discussion on AFPM or Halback concepts, I have created a *.DOC file containing a Solidworks rotor magnet orientation diagram. There are three configurations that can be used with either a single or dual 3phase drive. The document contains design assumptions. Please provide your thoughts (gut feelings are OK) as to which magnet orientation is best and why you think that selection is the best. Thank you for reading.
kenkad

 

[safe]

Inside Out?

Compared to the way most of the axial flux motors are built you are looking to create an inside out version. Usually the rotor holds the magnet wires and they use dual discs of magnets on the outside of the rotor which amplifies the magnetic flux inside.

I can see the value of doing it the other way.

You could do an axial flux design with the poles facing "parallel" to the motors axle alternating N - S - N - S... and then surround it with coils on either side. Or radial. I see your question, but I don't know what the formulas might be to know which is better. I'm using radial because I want any magnetic junk that might get onto the disc while riding more likely to get thrown off. (so my choice was purely practical)

The advantage of the Halbach configuration is that for the same amount of magnet material you get the square root of two (about 1.4 or 40%) more magnetic strength. Also with the Halbach you need no iron at all as opposed to being able to just live with a weak backside field without it.

The advantage of the standard configurations is that it's going to be easier to assemble the motor because there is no need to install magnets that want to rotate themselves out of alignment. It's actually the difficulty of assembly of the Halbach that is making it tricky. (from what I've read)

---------------------------------

This standard axial flux motor design could be used as a "Disc Motor" and attached directly to the wheel making effectively an ebike "hub motor" for a small fraction of the weight.

No matter how the idea proceeds (Halbach or otherwise) it's good to see that there is interest in this new direction for ebikes.

You also might search the net and download every pdf file you can find about axial flux motors. I've spent months doing the research and if you are building a motor you owe it to yourself to "do your homework". The odds are that if you search long enough that you will find specific formulas for how to evaluate different designs.

One last thing... there are software programs that are sure to exist that could model the motors visually. I'd love to get my hands on something like that as I found that when I was learning about circuit designs and discovered SPICE software that what was difficult all of a sudden became easy.

If you find some freeware motor simulation software be sure to let us know !!!

It would be great to get a hold of something like this:

http://www.infolytica.com/en/products/motorsolve/bdc/

They appear to have a trial version... maybe you could try it out?

Let us know what you think...

 

[kenkad]

Hello Safe,
I am actually further along on my project that the thread indicates. I have done all of what you have suggested, copies of published papers, looked at simulation software, etc. I have sent emails to a person at Infolytica about Motorsolve BLDC and have not received complete answers to all my questions. I did get a price for a full version, about $30,000. The trial version can only do old BLDC concepts and is very very limited. The trial version would be a waste of time in my opinion. I have also contacted local Universities and have come up with nothing. No interest and no capability. I will actually build something soon regardless. I simply thought it would be good to stimulate more discussion here on all facets of AFPM concepts.
kenkad

 

[safe]

Wow... $30,000... that's not cheap...

I did find a freeware simulation program:

http://www.femm.info/wiki/Download

...it's going to take hours and hours to get a simulation that you could have any chance of having any confidence in. They do have some prebuilt examples that look pretty close.

This stuff is normally done by full time professionals with advanced degrees, so we are more or less just "guessing" at the problems and learning by groping in the dark.

I would like to validate some things like the expected performance based on the number of turns of wire (always a real question mark with motors) and a simulation is a heck of a lot easier than having to rewire again and again.

Last summer I did about six complete motor rewinds and learned a lot about how all this works in a real "seat of the pants" kind of way. Now that I have a stronger "feel" for it the simulation software will add a lot to what I'm looking to do.

And winter is coming... so I'll have the time... more time than I would prefer actually...

My first thing will be to make a Halbach array example with the simulation so that I can play around with it.

(before I get to gluing together the magnets I've bought)

 

[Email]

Just signing up for the thread... nothing to add.
Second thought... here's a Youtube of an Axial Flux Hub Motor

 

[kenkad]

Email,
If you look carefully at the video, you will notice that it has a startup problem. Whatever the optimum design is, it has to have two important capabilties. First, on a bike, rotor/stator positioning must be known very accurately. Ccertainly in my design, I do not want or am I interested in Hall effect sensoring. Hall effect is too crude in my opinion (yes it does work in the current motors being sold) because it depends on too much 'analog' engineering. I incorporate optical sensing to get higher resolution. Second, there have to be more phases than one. This is what will give smooth starting torque. This most likely means that there have to be two rows of PMs on the rotors. On a bike, such a motor would have to be able to spin in reverse (very slowly and is possible because of the freewheel) and find the optimum 'home position' so that it can create the maximum smooth starting torque without overdriving the coils (temperature rise issue). These are my opinions only. I am not interested in hub motors either. My delta trike design (FWD) does not want to deal with having to change a wheel and all of the accompanying wires, chains, etc.
kenkad