New disks? Minimum Thickness?

[nothing to lose]

 I made up my drawings and gonna get some quotes. Not certain what I want for thickness though. I am thinking 1/4" should do me fine with this design. What do you guys think?

About 24" disks, cutouts for spokes to remove weight, add looks, help cooling etc..

Probably 1/4" thick, maybe 3/8"?

The outer magnet area will have 12 holes for bolts, centered between the 12 magnets.

There will be a ring with 12 tapped holes which bolts to the main disk. This gives me at least 1/2" thick ring where the magnets sit to conduct the flux and also provides more strenght on the outer rim.

The 12 bolt heads on the inner surface between magnets will also provide surface for resin to bond/lock.

I plan to use a spacer in the spoke area to set the airgap. Once I have decided the thickness needed I would bolt a spacer between each spoke near the outer rim. In using spacers this way on the spokes I will be bolting the front and rear rotor togther also which I think will add more strength and avoid flexing of disks durring yawing.

I am trying to cut weight and also materail costs while increasing stuctural strenght.

 I think as we move farther from the hub and use larger magnets we will have more tendancy for our disks to begin warping together under the pull of the large magnets. Adding spacers to outer ends of spokes should prevent the warping though it may make changing airgaps a bit harder using shims or such.

The question is about the thinkness of the metal, but if curious about the stator I will be trying something new on that also.

 I plan to assemble the rotors only once. The stator I plan to cast in several parts so I can change it without disturbing the rotors.

[Flux]

Neglecting any mechanical considerations, I think you will need a minimum of 1/4" to carry the flux without saturation.

Flux

[SparWeb]

Some questions before I can answer your question: what is the diameter of the hub, and how fast will the rotor turn?  Is the spacer a single tube on each bolt, or a ring that goes all around?  A ring would give more evenly distributed support than individual posts.

Assuming the alternator is mounted on a 5" bolt diameter hub (too small I think), and it will turn at 500 RPM (maybe faster than you expect), a solid 1/4" plate will just squeak by.  There will be no room for "oops".  Cutting "lightening" holes and mounting the ring on the outside will overload the disk, requiring a thicker plate.  That kinda eliminates the lightening purpose of the lightening holes, doesn't it!

If either your mounting is sturdier, or the disk won't turn that fast, then you may get away with 1/4".  

Check out the 12" steel disks for sale in the Forcefield web catalog.  I don't know their thickness, but I suspect it's 1/4".  Scale up to your disk diameter...

[zapmk]

nothing to lose,

I can tell you as the disk get bigger you definitly need some kind of spacer

near to outer part of the rotors with strong magents, found that out with

the large rotor disk that I am building, 24" half steel with 32 pole neo 40 mags.

The mags will actually bow that half inch steel rotor 20 to 30 thouands of an inch.

and I beleive over time it could be more. So now Iam also looking into making some kind of spacers to prevent this.

 -Zapmk

 

[jmk]

 How do you put spacers on the outer part of the rotor? Thats where the stater will be?

[Murlin]

The structural engineering of larger rotars is something I have been researching, since I want to build a large alternator.

The standard rule of thumb for the strength of a bolt is, that a bolt is only as strong as its dia.

Cutting large holes in a plate that needs to be rigid is not a good idea.  I know this is done to make the plate lighter.

I would have the waterjet cut round holes no larger than the thickness of your plate, equally spaced, the thickness of the plate size in the area you wish to lighten.

In this way, maximum strength can be maintained.

I am still reasearching how thick the plates on a large machine should actually be to withstand all the forces that would cause the rotor to flex.

A very rigid, flat, rotor would allow for a smaller more accurate airgap...IMHO....

Murlin

[richhagen]

NTL, For 24" diameter, I would think, but do not know, that the 1/4 inch thickness of the 'spokes' in the center would be too thin for a disk of that diameter.  I would think that it would allow for enoungh flexing to play havoc with your airgap.  There will be force pulling the magnets together from the magnets on the opposite disk, and under power, a magnetic field will be generated in the coils that opposes the direction of the change of flux.  This would result in changing forces of attraction between the disks if there is vibration, it could lead to warping of the disks over time.  Since the material is only one fourth of an inch thick in the direction of this axis, this might lead to problems.  I don't know if you would have clearance, but maybe pieces of angle iron welded or fastened along the spokes would give the disks rigidity along that axis.  Anyway, just an idea, Rich

[zapmk]

Hi jmk,

The keywords here is, (near to) the outer part of the rotor. in my case 5 inches to be exact, these will run inside of the stator. At this point, I am thinking something kind of like jacking screws , evenly spaced around the rotor that can be easily adjusted and locked down to keep the gap and flex from the pull of the magnets out of the rotors. ?

I wonder how DanB keeps those huge magnets from flexing his plates. ?

 -Zapmk

[BigBreaker]

Consider using an "inside out" rotor/stator configuration.  Put the spacers on the outside rim and run the stator wires though the inside.  One way to do that is to use a large but hollow spindle.  That configuration will lessen the load on the rotor plates, especially against the yawing forces.

[nothing to lose]

Thanks for the comments guys, I been online alot less and very busy recently.

When I bolt the 22" ring to the outer diameter of the 22" disk, each 1/4" thick I should have 1/2 thickness where the magnets are and inwards aways towards the center also. I think this will conduct flux well and stiffen the disks to prevent flexing or bowing.

I will use spacers on the spokes towards the outer dia but inside the stator hole which will hold the front and rear disks apart and also bolted together. I am also thinking of shaping the spacers so that they flow air upwards/outwards towards the stator for cooling. Nothing fancy, maybe a slight taper on the tops of the spacers to direct the air.