Build your own! it's not too difficult and it will be better tuned http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
I quasi built my own. I bought a 'box' with the necessary capicators and switch, and then supplied my own
I have a 10 hp idler motor, and a 7.5 hp lathe and a 5 hp lathe motor. I can actually run both of them at the same time.
It shouldn't really be this way, but....... the idler motor needs to be larger than any other motor you are starting,
and the overall wiring needs heavy enough to cover the total load.
the reason I can get way with two lathes combined larger than my idler motor is that I start them one at time
and oncerunning the second to start joins the first to start as an idler motor. Anyhow it works.
But the basic rule is that the idler/phasce converter capacity should be larger than your motor load. I don't know what the smart guys would thing of starting multiple motors in sequence.... but it is my impression that if the motors are not loaded to the max, the bit issue is only getting them started. If they get started reliably, the rest doesn't matter.
Capacitors aren't particularly expensive, but if you overload one you know it right away <grin> because it always
is announced by a bright flash, a loud bang, and a room full of smoke. If you encounter this sequence either do not repeat the what ever you did immediately before this, or buy bigger capacitors.
the latter advice may have something to do with why my cobbled up phase converter works. I bought the capacitor box that was suppose to be sized for the 10 hp motor, but obviously wasn't, as after I ordered larger 'start caps', the flash, bang, smoke sequence did not repeat itself. I just don't reccommend sitting on your capacitors while testing them out unless you want to impress someone with your new found ability to pass smoke