On 2 Aug 2005 05:20:06 -0700, "gtslabs" wrote:

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.
The thing about induction motors that makes them need the proper
contactor is the starting current, not the running current. This
current is at the maximum when the contacts first close. It can three
times the rated current of the motor. This is why big contactors are
used. If you spin up the motor with a smaller pony motor first then
the starting current will be lower.
ERS