spiral tractor - electric vehicle without a battery

I suppose it might be possible, but I'm curious as to why you would want to do it. It's not nearly as convenient as diesel and a lot more expensive. Agriculture, particularly the large scale stuff, is probably the last place we should cut back on petroleum use.

For various reasons including soil conservation, water conservation, and the ability to better withstand unpredictable climate extremes, the future of sustainable agriculture is in perennial crops, IMHO, whether large or small.

As such there will be a lot less tilling. And as such, diesel would be far more practical for the occasional needs and for harvesting then a permanent tethered electrical infrastructure as you describe. Less fuel will be needed in any case.

The cost of your infrastructure would be difficult to pay back, if the equipment is not something that would be used continually or at least, frequently....

Intriguing problem.

Simplest, from an infrastructure point of view.

Have a power pole in one corner of the field. assumes square or rectangular.

Drag the cable along two sides to the far corner.

Implement must have a boom to keep the cable clear during all maneuvering.

Cable should remain in the worked area, as the implement works in rows back to the power source.

If the source is in the center of a side, less cable is necessary, but you would drag the cable more, unless you reset the cable to the other once the 1st half was complete.

With a large (intelligent) powered reel on the implement, one could work from one side of the field, dropping and retrieving the cable w/o dragging it, for cultivating operations, again probably easier to start at the far end of a side.

The boom has to be fairly long to reach across the row that has been completed, for the return trip.

The USSR used some trucks that were tethered, in some mining operations, I think.

These have trolley poles: http://englishrussia.com/?p=736

It would look/perform like a center pivot irrigation system similar to Electrogator, etc. except that it would do the implement work as well... might just have something.