Contactor Sizing for an phase converter Idler Motor

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.
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Your maximum load is about 10 HP, and your maximum starting load is 15 HP. That makes me believe that 15 HP rated contactor would be appropriate. I am not, by any means, an exp[ert on this.

I would be interested in finding out the authoritative answer to this, great question.
Personally, I am considering making a two stage phase converter.
First it would start a 10 HP idler, and then it once the idler is up to speed, it would start another 7.5 HP idler. Much easier on the electrical circuit. The reason for this is that I have a 7.5 HP motor that I do not know what to do with, caps, contactors etc. Any cost to this project, at this point, will be my time.
i
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Ignoramus6455 wrote:

This is how I had my converter set up (sold when I moved). I had a 3HP capacitor start converter. For my one larger machine, I then would switch in a 7.5HP idler with some additional run caps. It started just fine off the 3HP idler, since it was starting no-load. Two stage starting, but way easier than what I had before--pull start a 7.5HP with a rope. I got pretty tired of that.
Steve
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That's very encouraging. Thank you Steve. I have 3 more 92 mF run capacitors that I can use as run caps, and plenty of contactors.
i
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Ignoramus6455 wrote:

Caps aren't too pricy, although you got a good deal on the ones you have. Surplus Center has good prices on them, and if you need a bunch of capacitance for a big unit you can do the trick with DC caps hooked back to back- IIRC, they are wired + to +, but look it up to be sure.
John
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Ignoramus6455 wrote:

You can start a motor, especially with no load, on a much smaller starter than is recommended- they're meant to deal with a unit putting out the full rated power. In truth, I think the hardest part of a starter's job is shutting down under load, you'll get arcing then.
John
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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
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He can buy something like this:
http://tinyurl.com/9tj9w
300 amps ought to be enough. :)
i
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I think the answer depends on how you are starting the 15HP idler. The issue isn't the fact that the motor says "15HP" on it, the issue is how much current does it draw when starting. If you're using capacitor start, you get a pretty large surge current, related to capacitor size, not HP itself. I know the caps are sized based on HP, but I'm trying to explain what the real issue is.
In your situation I'd have the 10HP contactor between the 15HP idler and the compressor. This way you start the idler and let it run. The compressor would start via its contactor exactly as if it were on power company 3 phase (assuming your third leg doesn't sag...).
Steve
gtslabs wrote:

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This is what I am doing now. However I was wondering if I should use a contactor just to start the 15hp converter. I am using 7 108-130 uF capacitors to start it up in about 2 seconds with the momentary button.
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You definitely need a contactor to start the 15 HP idler.
For my 10 HP idler, I use 184 mF of capacitance. It starts just fine. I started off with more capacitance (276 mF), but that made the 3rd leg higher in voltage than I wanted it to be.
i
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