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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#1
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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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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 |
#3
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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: 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. |
#4
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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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