This is an analysis of the sets of grids and coils found on the 3kW machine in the top left and top right corners facing the front disk. Similar ones are found on the single disk small machine. The assumption I am making here is that the grid is acting as one plate of a capacitor where the other plate is the sectors of the disk that it's facing at any time. The resulting capacitor and the coil would form part of a resonant LC circuit.
Since only one wire is going from the grid to the coil, the remainder of the circuit may actually be more complex and even include other grids and/or coils. It's even possible that the sectors are all connected in some fashion near the center of the disk. But since most of the wiring can only be guessed at, this analysis just works out some ballpark resonance values assuming just these one grid and coil in parallel.
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The following calculations start from the 6 unit gap between the sectors in the diagram above and are used to convert all the other units to mm. The conversion factor is worked out as follows... The common estimate for the diameter of the 3kw machine's discs is 500mm (50cm). That gives a circumference of 1570mm. Dividing that by 48 groups gives 32.7mm for each group plus the space between it and the next group at the edge. From the diagram above, the spacing between one group and the next is 6/35.5 * 32.7mm = 5.527mm. This means 6units = 5.527mm, so 1unit = 0.921mm/unit. Given this, we can now convert the units in the above diagram to mm by multipling the units by 0.921.
There are 24 turns in the above coil. Given a wire thickness of very roughly 1.15mm, the wire takes up 27.6mm of the 65mm length. This leaves a gap between each wire of 1.62mm (65-27.6)/23.
Converting all millimetres to metres and plugging into the above formulas and using 1 for ur given it's an air core coil, we get:
The measured capacitance of my setup is 19pF. This is measured between the grid and the inside sectors of the front disk, with the grid disconnected from the coil. The measured capacitance between the grid and the inside sectors of the back disk is 15pF.
Another thing to note is that the grid is actually close to the disk face. It can be misleading when looking at it from the front as in the photo above but it's clear from the following photo.
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