I finally got a 10 hp air compressor running. I have a 10 hp rated
contactor for it. But my Idler motor for my phase converter is 15 hp.
I have used a momentary switch in the past to start it but I was
reading that a contactor is preferred. Since I am starting the 15 hp
converter first and it is only idling do I need a 15 hp rated
contactor? Or since it will eventially see the additional 10hp
compressor (once the converter is running) do I need a 25 hp rated
contactor?

I want my converter to cycle on and off outside my compressor range so
I was looking at some type of relay and protecting the motors.