If you have reciprocating linear motion (such as a piston moving up and down in a cylinder), is it possible to generate three-phase AC power from it without converting it into rotary motion first?

I can imagine that placing one set of coils around a cylinder and having the magnet move up and down within the cylinder would easily generate a single-phase AC sine wave, however I cannot think of a way that you could add the other two phases, 120 degrees apart, that make up the usual three-phase AC supply.

For example:

  • Three coils of wire, one at the top, middle and bottom of the cylinder, would generate two phases 180 degrees apart, and the third at double the frequency, so this wouldn't work.
  • I have thought through various ways of putting 5-6 coils on the cylinder and switching them to different output phases at different points in the cycle, however I still can't come up with an arrangement that would match the three-phases you'd get from rotary motion.
  • Using three different cylinders wouldn't work because even the linear motion must be offset to produce sine waves 120 degrees out of phase. If the coil were placed at a position on the cylinder other than the middle or end of the piston's range, the resulting wave would not be sinusoidal.

So is there any way that this can be achieved, or is the only solution to convert the linear motion to rotary and then generate three-phases from there in the traditional manner?

  • $\begingroup$ How about a 3-cylinder engine with coils placed at the mid-point of each cylinder? $\endgroup$ – Samuel Weir Apr 19 at 5:40
  • $\begingroup$ Except then you're converting rotary motion of the engine into linear motion, so you may as well use the rotary motion directly. I'm interested in cases where there is no rotary motion at all to begin with (e.g. Stirling engine) $\endgroup$ – Malvineous Apr 19 at 13:58

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