Rotary phase converter motor sizing

[sparky961]

I came across a 20hp 3 phase motor for sale locally for $225 CAD. This initially struck me as an inexpensive thing to pick up for a future rotary converter project. Then I got to wondering if that isn't way too big to run, say, a typical mill and/or lathe... likely never simultaneously.

Thoughts?  Too big? Too expensive? Just plain stupid idea?

[DavidA]

I would say way too big. No doubt it would do the job. But massive overkill.

Dave

[GordonL]

I am just going by memory but I think that the motor should be 20% bigger than the load so if you are going to run a 1 HP you would need 1.2 HP so a 1 1/2 HP would do fine. I am running a 3 HP idler and I am running a 1 1/2 HP on the lathe and 2 HP on the mill and it has worked fine for the last 20 years. Seldom run both motors together but even that is OK as long as the are not started at the same time.

[Jonny]

For what its worth I have one of these as a backup http://www.ebay.co.uk/itm/TRANSWAVE-Rotary-Converter-5-5hp-4kW-Single-3-Three-Phase-240v-415v-/151881416964?hash=item235cd68104

Runs out of steam spindle easily slows with menial cuts with 2.25kw motor.
Have to run with a 7.5kw inverter, all but heavy cuts its just about ok. http://www.ebay.co.uk/itm/10-HP-220V-415V-DIGITAL-PLUG-PLAY-3-PHASE-CONVERTER-INVERTER-/381465465543?hash=item58d11d12c7

[sparky961]

Ok, so it seems like we have consensus not to bother with the 20HP beast.

Thanks for the input.  I'm a ways off from needing such a thing anyway.