hi, thankyou for both your replies.
I knew I would get the converter/inverter the wrong way round.
so basically 415v and 240 3 phase are both the same, is that correct..
Those
can both be 3-Phase, yes. Was common for MIL-SPEC generator sets to offer 208/416 V "Wye" OR 120/240 Single_phase, OR 220/440 (240/480..) VAC 3-Phase "Delta". The winding taps were already built-in to the "generator head".
Even so, a bit of labour was wanted to make the selection - it was neither "automagical" nor "don't care" interchangeable. Voltages look different? That's because they ARE different.
I have seen a few youtube clips of rotary converters been built but they do not specify the size of the 3 phase pony motor, should this be the same size as the lathe motor or does it not matter?
(At least) one of the makers of commercial RPC has published tables. The oversize is required by starting needs more than any other factor. Those can differ enough that some loads want a significant multiple of their running load. Ten times, sometimes more-yet. Multiples beyond ten times are published, but of questionable use. Situation that dire, an RPC may not be a good idea anyway.
Where the load-motor is clutched and/or otherwise starts "unloaded" (kinetically, not electromagnetically) a fifty-percent up-step is a reasonable minimum. Eg; 5 HP load, 7.5 HP idler, 10 HP load, 15 HP idler.
While there is no theoretical
lower-bound, it seldom makes labour and materials sense to build an RPC of much LESS than 5 HP idler, even so. Small stuff, it is often cheaper to just swap to a single-phase load motor and not NEED conversion to 3-P at all.
At the other end, go TOO large, and the idler itself presents a nasty "starting" load. That's where one or both of "pony start" and "supplementary" idlers can help.
Ex:
- Start a 10HP idler. Once "up", add a 5 HP idler. THEN start a 10 or 12 HP load-motor of the net-net 15 HP idler.
- Start a 30 HP or larger idler with a 1-P "pony" motor. Or a Dee Cee pony, or ...., etc.