Sorry, couldn't resist.
I don't know if this will help you, but I ran into this problem when wiring up my new Piranha P50 Ironworker. I have a balanced (with caps) 5HP RPC that runs a few machines in my shop. Until now, the largest motor to be run was 3HP. The piranha is 10HP.
I ran into a lot of contactor chattering upon startup, even with two other machines fired up to add to the HP of the RPC. Makes for REAL ugly contacts. Eventually , it wouldn't start at all. Enter nauseous feeling. I was convinced I had to build a new RPC with a bigger idler motor. That was, until I saw one leg of the control wiring was tied to the generated phase from the converter.
Cleaned the contacts, tied the control wiring to the grid (L1, L2), and she works like a champ. I still am amazed that I can fire a 10HP machine from a 5HP RPC. I guess because it starts without a load.
Hope this helps.