Axial PMA with steel core conceptual design

[CraigM]

Hello everyone,

Wondering if I can get some feedback on an axial design PMA I have on the drawing board.

A little background first… I recently moved to the panhandle area of Florida and with the move I no longer have the machining facilities at my disposal that I had with my past employer. I have a toroid core machine that's slowly coming along but I'm at a bit of a standstill while job hunting takes precedence over buying a band saw, welder, steel, copper wire and other bits needed to complete this project. I did however have a great interview on Tuesday with a local aerospace machine shop that went very well so wish me luck!

I'm holed up in a rental house for now with the long term goal of retiring on a piece of property, so as soon as employment comes online I'll have a better idea of where to put my roots down. Hopefully at that time I'll be able to put some of this RE knowledge to good use. So for now I'm a student of RE and sustainable living.

The drawing board however is always open so this is a conceptual drawing I put together.

One thing I own is a collection of silicon steel. I have at least 6 or 7 packages of silicon steel (I) sections used to make transformer cores and this along with inexpensive (although heavy) ceramic magnets was the catalyst for this design.



•   I kept the size on the small side to keep material cost down but see no reason why this couldn't be scaled up effectively.
•   Magnet poles were kept very close together to avoid cogging. Maybe they could even touch? Comments on this?
•   Magnet cost is very inexpensive. 6” x 4” x 1” C8 Ceramic cost under $7 each and will yield 6 poles when cut into sections with diamond blade on tile cutting saw.
•   Uses common 3/4 Coil/Pole polyphase design.
•   Sheet metal cover around circumference would seal entire inner stator from the elements.




What I don't really like is my bearing arrangement. I would rather have used a mounted bearing but need something with a low (less than 1”) profile. If anyone has a suggestion here please comment. Perhaps a central hub / bearing support could be fabricated? I'm sure a few folks with mechanical engineering background would have some ideas.



Stator plates support the entire assembly. Made from G10 Epoxyglass they would be very strong but difficult to fabricate without machining. Made from Applyply hardwood plywood they would be much easier to fabricate with wood working equipment but I'm unsure how this material would handle all of the weight involved. Ferrite magnets are quite heavy and there's a lot of silicon steel that is also very heavy.

I also didn't design a way to mount this to be used as a wind turbine. All ideas and comments are welcome. Also the entire weight of this PMA is supported by the stator which needs to be made from a non-metallic material. Maybe extending the length of the stator mounting hardware to attach the PMA to a steel plate?




Magnets are spaced very close together to eliminate cogging. Maybe even have zero gap between magnets that I believe would eliminate cogging entirely but at what cost from flux leakage from pole to pole?



Coils are modular design and with a bolt together stator can be replaced if needed. A traditional coil winding jig would also be easy to fabricate to ease assembly of the coils. I'm guessing heat shrink tubing used in battery pack assembly could be used to seal and hold the coil winding. I've had success using thin CA (super glue) to bond the silicon steel core together. A second application of acetone thinned epoxy could also be used.

Here are my output calculations which look pretty good in theory. SparWeb if you could check these it would be much appreciated.

Magnet square inches = 2.8105
Core cross section square inches = 1.38
2.8105 / 1.38 = 2.037

Ceramic magnet strength = .38 Tesla
2.037 * .38T = .774T in the core

.774T * 2 = 1.548 field reversal

Core = 890 square mm area, 157 mm perimeter

.774T * 890mm * 147 turns = 101262 microWebers = to .101 Webers

240 RPM = 4 revolutions per second
12 poles = 6 flux reversals per second
6 reversals * 4 RPS = 24 reversals per second

EMF = 2 * Flux * Frequency
2 * .101 Webers * 24 = 4.848 Volts

4.848 Volts * 3 Coils per phase = 14.544 Volts
Star connection = square root of 3 (1.73)
1.73 * 14.544 Volts = 25.16 Volts

Resistance:
157mm coil perimeter * 147 turns per coil at 14 AWG = 23 meters per coil
23 meters * 3 per phase = 69.237 meters per phase
69.237 per phase * 3 phases = 207.7 meters total

14 AWG has 8.8 ohms resistance per 1000 meters
.0088 ohms per meter * 69.237 meters per phase = .609 ohms resistance per phase
.609 ohms per phase * 3 phases = 1.827 Ohms total

Voltage 25.16 / Ohms 1.827 = Amperage 13.77

Power = 346 Watts at 240 RPM

If anyone is interested in the CAD files I'd be more then happy to share them.

Thanks,
Craig

[CraigM]

Here's a smaller example of the same design. This one uses a 6" rotor and 1" x 1" x .5" neo magnets. The bearing assembly is much simpler using flanged ball bearings with an OD of 1.375 and a section of sched 40 pipe to give support to the assembly. A simple bolt and spacers hold the assembly.

The problem I would expect from this is cogging. Although the magnets touch at the corners I'm afraid the magnetic pull would not be even enough on every steel core to avoid cogging when three magnets of a single phase are directly over their three corresponding coils. I still may consider this smaller design to prototype however because of smaller size and weight.

[tecker]

The smaller the core segment the better the performance through the entire band of power . The Cog of coarse is a case of inducing a like pole to the approaching magnet . Placing the coils on a flat metal core works . There's some drag but no cog .

[SparWeb]

Hi Craig,
Am I too late?
I don't check the Diaries very often, so this is my first look at your plan.
Have you moved any farther?

If you're looking for tapered-roller bearings, you can go to Timken.  The website has a parametric catalog.  But you pay for what you get.
With tapered rollers, you can put a lot of tightening pre-load on them and they still run smoothly, unlike ball bearings, which get very stiff if you try to clamp against the inner races, like you have shown in the second design.

The length of the laminate (with wire wrapped around all of it I assume) is very long.  There's so much wire I think the resistance will be too high.  As in heat generation.

The ceramic magnets don't match the shape of the core laminates as well as they should, in the top design.  The lower design is better.  I wouldn't give a factor of 2.037 like you do - but then I would be using FEMM to figure out the flux density in the laminates instead of leaving it up to chance.

The potential of 0.38 Tesla is pretty low.  I think a FEMM model would give you the confidence to raise that up somewhat.

Laminated cores can give you a struggle, and the crowd here (including me despite my motor-conversion projects) don't generally have much experience with them.

All peanut gallery comments.  You've had time to tinker with it by now, so I'd rather see how it's evolved than criticize now.

PS, you mention aerospace and engineering in the same post, and I just went through an eerily similar employment change.  I interviewed for work here in Calgary and started a new job 3 weeks ago.  If you are a) in Canada, and b) actually interested, then I can drop a few names for you.  Good luck with the search.  Chances are you're state-side meaning the job market is a lot tougher.  But I know a few people.