Intergalactic_Captain
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PM/EM stacked generator head - need theoretical help
Hey all, been a while since I've been active - always a lurker and observer though. A side project of mine has been taxing my mind lately, though,
and I need some help in trying to find/understand some basic electrical theory.
Essentially all electrical power is generated by rotating magnets against a stationary array of coils. Whether this is flipped, or how the magnetic
field is produced, is irrelevant here.
Enter the axial flux generator, and the theoretical problem I can't grasp. The output power is derived from mechanical work acting on magnets moving
over coils of wire - the power taken off as electricity can never exceed the limitations imposed by the strength of the magnets, the capacity of the
wire used, and the work done by the motive force. I suppose this is true of any configuration, but I'm focusing on axial flux driven by a 25hp riding
lawnmower.
Conventional wisdom indicates that in order to extract the most available energy, the magnets should be fairly powerful, as should the stator
windings. Or a rotating electromagnet system could excite the stator. Further, this could be Extrapolated to a stacked configuration where
comparatively small magnets/stators provide field current for rotating electromagnets in a larger system.
. . . This is where I'm stumped - is this not how pretty much all real power generation already works anyways? How is it not perpetual motion / over
unity? Without invoking any maxwell-level equations, can anyone provide any advice on how to rationalize and work out the mathematics here?
If you see me running, try to keep up.
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Rainwater
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Hold on. Im about to slip into an EEPTSD episode here. This crap used to give me nightmares.
Its kinda simple. But not really, but is
Wait. The concept is simple, but the math is not.
Or maybe its the other way around, either way your gonna need a pot of coffee.
And a bathroom break.
Quote: Originally posted by Intergalactic_Captain  |
How is it not perpetual motion / over unity? Without invoking any maxwell-level equations, can anyone provide any advice on how to rationalize and
work out the mathematics here? |
.
in summary
A motor or generator, the more work being done, the more opposing back EMF will be generated, making it harder to do work.
For a motor under a torque load, additional power(voltage and current) need to be applied to overcome the back emf
For a generator under load consuming current, additional torque needs to be applied to overcome the back emf.
Basicly any time 2 magnetic fields interact, back emf is generated which is opposed to the interaction
Deep down the rabbit hole
i will try to explain the advantages of the axial motor or generator. In short, geometry.
Assumption #1
So a magnets strength can be assumed to be directly related to its volume and geometry for a given composition. Breaking this volume up into smaller
pieces will not have an effect as long as the geometry and composition do not change.
Assumption #2
This is completely wrong from a math prospective but a suitable analogy for the concept. Think of the flux emitted by the magnet having a direct
relationship with mass distribution. The strongest flux will be at the center of mass.
The axial configuration gaines its advantages over cylindrical geometry by exploiting maximized surface area with minimal distance between stator and
rotor. Gonna be hard to explain without a proper doodle.
Need to know #1
So the surface area of the interior of a cylinder with out ends (tube) is limited to
$$SurfaceArea =2\pi rh$$
The total volume of the tube's wall is only limited by the difference between the outter radius and inner radius
Need to know #2
The center(in this case radius) of the origin of the 'effective' magnetic field is the radius of gyration ya. I laughed to hard at that myself.
So think of it as the radius inside the tube wall (between inner radius and outer radius) where the area is equal on both sides. This radius will
always be off centered towards the outter radius due to the non-linear relationship between area and radius of a circle.
Need to know #3
| Quote: | For both monopoles and dipoles, the field strength decreases as the distance from the source increases. , often called the inverse square law.
- Wikipedia |
hard math found here lets just say $$\frac{1}{r^2}$$
Ok so what these 3 points infer is that to increase the magnetic field, you increase the thickness, or outter diameter, of your stator. This
effectively moves the source radius of the effective magnetic field away from the rotor, which decreases its strength on the rotor. It becomes a case
of diminishing returns.
Now understanding that concept, we live in the future and the long held assumption (#1 & #2) are completely wrong.
Breaking your magnet up into smaller pieces, not changing the geometry or composition, but altering the polarity of these pieces is exactly how the
new generation of high performance generators work. Twice the power output, half the size, 70% the weight. I have been swapping out 1.3MW for 2.5MW
that are smaller and consume the same amount of fuel under full load.
But doing this with a tube shaped stator is impractical. Much easier to do within the 2d plane of a disk. This is the true advantage.
So more on interactive surface area, assuming(#3) uniform flux distribution there is a lensing effect that can be achived with a
triangular geometry, and overlapping the stator and rotor magnets as opposing triangles allows the fields to be tightly focused increasing coupling.
And last bit of math. These magnets are electromagnets within the stator. So they are coils of wire. Their strength is measured in Henrys with the
formula
L=henrys
N=turns
ℓ=length of wire
μ=dielectric permeability
A=area of coil
$$ L = \frac{N^2} { ( \frac{ℓ}{μA} )}$$
By staring into this equation you can unlock the power of Faraday and sleep even when undesired.
So this formula shows the relationship between coil length/area and strength.
Large turn counts, with small wire lengths and large areas are desired. This can not be accomplished with the geometry available within a tubes walls.
But on a disk, its easy. Not only can you generate a large number of turns, but the exact shape can be formed.
The maxium ratio of coil length to coil area is a circle. But as you increase the number of turns by 1, you also increase the length by 2πr and the
average radius of the coil by the thickness of your wire.
So eventually you reach a point where adding more turns doesnt increase the strength as much because of the increase on area and increased distance
from the stator.
But with a flat geometry, this does not change the distance between the origin of the magnetic field and the rotor, you can also add another disk
layer, and reset the diminishing returns.
[Edited on 21-8-2024 by Rainwater]
[Edited on 22-8-2024 by Rainwater]
"You can't do that" - challenge accepted
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semiconductive
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Power = Force x speed.
If you use weak magnets, all you need to do is move faster to get the same Power. But higher speeds usually means more friction on bearings,
bushings, etc. and this leads to the generator loosing power (unless ... see a few tricks below).
Rotating magnets is not the only way to do power generation.
You can also do linear generators, and flux (reluctance) generators with things such as speakers. A 100w speaker, hooked to something that vibrates
it, can generate power.
Here's a few videos that show variations on generator design:
This first one shows a rotating system with a novel way to wind a coil to make a generator; it's actually more efficient that normal 'coil' windings.
He explains why. Compare with Rainwater's response. THe maximum area to wire length is not made by a closed circle, but a serpent. 
https://www.youtube.com/watch?v=QRY4dSEXD1o
You can increase the efficiency simply by making the cylinder longer so that you have more straight wire vs. shorter half-circle turns.
This second video shows a switched reluctance generator in a linear configuration; so the part which actually generates electricity doesn't need to
spin.
https://www.youtube.com/watch?v=jAJld70e6XU
For example, you could use a thermal lag engine to move the bolt in-and out of the bobbin: You would just glue a bit of metal onto the end of the
pencil, and put it inside the bobbin.
https://www.youtube.com/watch?v=zV8edaVOUlg
Note: The efficiency of Stirling Cycle engines is low, for air based designs. The temperature difference needs to be increased a lot and the gas
pressure inside needs to be increased if possible; eg: to make them generate significant amounts of energy. There are ways to do this that aren't
patented yet, so I'll not discuss them in detail. But: The standard way, is to just use higher pressure hydrogen gas as a working fluid. It will
work until the hydrogen leaks out (which it always does). But, anyone can do it!
I am woking on an half internal, half external, linear combustion engine to hybrydize my Nissan Leaf. So, I have a pretty good idea of the problems
with generators .... and there are a *lot* of them. Got my Toyota Prius up to 74 miles to the gallon, before the battery pack self-destructed due to
software installed by the manufacturer. I learned a lot. Atkins cycle combustion engines can be made *extremely* efficient if properly insulated.
But, you're fighting manufacterer sabotage with standard engines since the software is proprietary, and they don't play nice.
A lawn mower engine, is pretty noisy and not very efficient. Do you have a drawing/picture of axial flux generator you are experimenting with, or at
least the engine and what options you have for connecting it to a generator?
[Edited on 29-8-2024 by semiconductive]
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