Flat-wire stator (from conductor sheet material)

[kitestrings]

I don't know if this has been talked about here previously or not, but I stumbled on this research paper and thought it might be of interest, comments, from this group:

https://ietresearch.onlinelibrary.wiley.com/doi/10.1049/rpg2.12011

It does say that it has the potential risk of high eddy currents.  I do recall someone looking at winding with square wire to gain density, and as I recall the two draw-backs were increased cost, and higher eddy losses.

~ks

[joestue]

interesting. ever take a starter motor apart? the hair pin winding design is pretty old but this is the first paper i've seen that describes the process for making axial flux air core motors using the same principal.

it would be interesting if they could do away with the thousands of brazed joints that have to be made.

also, its interesting to me that the number of tons of magnets needed for a 1 megawatt generator at 20 rpm... drops off so quickly as the diameter increases. i sort of knew this was the case but hadn't worked out the equations in my head to prove it.

[SparWeb]

Speaking for myself, I haven't seen this before, but I think I follow the architecture and principle of operation.
Very interesting.

I bet the eddy currents can be managed by selecting grades of copper/brass with alloying tin or zinc or other elements, trading away some conductivity to make gains in tensile strength, allowing for "2-in-hand" winding of thinner sheet rather than full-thickness per turn.
Where the ends are soldered/welded together, the thickness is much greater than the single sheet thickness.  It normally isn't a big deal with spools of wire, but with the way each hairpin was cut, there is a weld on every tab and there are a LOT of tabs.  It's almost a continuous ring of tabs all around the inner and outer edge.  That could pick up a lot of eddy-currents.

The model looks like an extrapolation (a rather long extrapolation) from a test unit to a commercial size.  Given the scale factor is about 3 orders of magnitude, there are probably physical and mechanical scale issues that haven't been found.  The model of the commercial generator looks too simple to be true.

The unit that they built and tested does look good.

[Mary B]

I remember some of the early square wire experiments here... eddy currents were horrible but that was winding a conventional coil like in a normal axial flux homebrew machine.

All those connections equal failure points after heat/cool cycles...the solder will eventually crack in my experience from vacuum tube TV's and hot circuit board connections. Seeing some of the same issues in the high power density ham radio amplifiers, use of silver solder is a must for long term strength. Especially on the output matching networks that have large circulating currents and run hot.

[DanG]

Just for possible future readers

Using high output rare earth magnets with square, flat or other-than-round coil wires forces stray internal flux loops in the wires that make extra heat, so with air-core coils that heat has nowhere to disperse and we melt/age stators sooner... Now I've worked on brushed 200Hp 750VDC motors that absolutely used square conductor windings to stuff in as much copper as they could but they had a ton of steel to dump heat into and turbine style forced air cooling.. and the higher voltages had less lazy electrons getting lost.

[joestue]

Motors and transformers usually have magnet cores which carry the flux around the wire and there is no significant eddy currents.

Optimization software for determining the optimal location for wires inside transformer cores has been around for a long time.
https://people.ece.ubc.ca/alisaket/images/blog/SKIN_Poximity_Frining_Effect.jpg

Above is an example. The eddy current losses around the air gap of the core dominate most of the losses. If you remove the wires near the gap, and the wires in the most inside corner, DC losses increase, but eddy currents decrease...there is a minimum solution for the smallest volume of copper in the optimal location.

I know people have argued with me in the past saying that the magnetic flux lines flow though the conductor regardless.... To which I would say: I have a 50kilogram welding transformer with a 3/8 inch square aluminum secondary, which takes up about a 2" square cross section area. The primary is 6 gauge aluminum round wire which takes up about a 2x2" square cross section. About half of the secondary is .25" square aluminum.

Predict how much the transformer no load losses will be reduced when I remove the secondary. The core is about 8 square inches and runs at about 2 volts per turn.

[joestue]

Air core coils don't have any shielding so, you need the smallest wire you can practically get.

Largish transformers have used continuously interposed square cross section coils made up of 20 or so flat bars of copper all Insulated from each other.... In the event someone was going to make a 1MW air core generator, I suspect that is what they would use. I don't think any manufacturer wants to use thousands of hair pin and brazed connections... But that is what is in hybrid car motors.. but it's more like a couple hundred brazed connections, most likely a hydrogen torch and computer control.. no braze alloy needed just melt the copper together .

[Adriaan Kragten]

When I worked at the University of Technology Eindhoven about 40 years ago, we did some tests with small permanent magnet DC motors of Mavilor Iberica. These motors had a similar winding as given in this post but the copper coil legs were printed at both sides of the synthetic armature disk. The armature disk was rotating in between two stator disks which contain the magnets at the inner side. The brushes were running directly on the printed wires. The efficiency was about 80 % which was very good at that time for a small DC motor. The motor was also tested as generator and had about the same efficiency.

At that time we had no test rig to measure the torque and so we couldn't determine the efficiency. But we had two identical motors. The shafts of these motors were coupled to each other by a mechanical coupling. Now the input power is the product of the input voltage and current of the motor and the output power is the product of the output voltage and current of the generator. So the efficiency of the motor generator combination is the ratio in between the output electrical power and the input electrical power. We found a total efficiency of about 0.64. If it is assumed that the efficiency of the motor is the same as that of the generator, the individual efficiency is 0,8 as 0,8 * 0,8 = 0,64.

[MattM]

I experimented with strips cut from 16 ounce copper sheet stock.  Cut the strip and put electrical tape across one side of the strip then rolled into about a 3" diameter coil.  The results were not promising, but it looks very clean and pretty like that.

[JW]

Yes we can do this