That's not an unusual configuration for a capacitor start/ motmotor. If the start and run windings are the same then a simple change over switch is all that's needed to reverse it.
(just so happens that my dad made me a toy with one in and showed me how it worked while i was at primary school! :-) )
The diagram below shows a typical configuration and how the third phase of a 3ph motor would be in parallel with the start/run cap.
Very interesting, I had never come up against it before, except done deliberately by home users.
I would strongly recommend getting the book
'Three Phase Conversion' by Graham Astbury. It is Number 47 in the Workshop Practice Series.
He goes into the practicalities of this and you will get a much better understanding from it than you will from me.
I got mine via Amazon, it's quite inexpensive and worth every penny.
Thanks David. I already have some of the workshp practice books and they make easy reading. I'll look out for that one now too.
There used to be a website page by a capacitor manufacturer detailing the formula for working out the capacitor size for a given motor. I just followed the instructions and as said, it worked fine for several years before I sold the machine due to a house move. They stated that there would be some reduction in rated motor output but my old mill never showed any signs of being under powered. Total cost to get the mill up and running on single phase was about £12.
I've seen all kinds of figures quoted even up to 80% rating (which surprised me), by using the proper size capacitor. One page said that adding an extra start capacitor (on a timer or momentary switch I guess) would help to bring the 3-phase motor up to speed then it could be disconnected to allow it to run on the one cap without over-heating.
This motor came off an entry-system barrier with a counterbalance so I guess it was never under very high loads at all, and only used for a few seconds at a time at that.