Motor starter with separate control voltage source

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Old 01-24-16, 12:21 PM
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Motor starter with separate control voltage source

My 240V air compressor has a magnetic starter with a 120V coil and 3-wire control. I would like to power the control circuit from the ceiling light circuit so the compressor shuts down when I turn the lights off. Because it uses 3-wire control, turning the lights back on will not restart the compressor; I will have to press the start button each time I want to start it. My concern is that there will then be two sources of power entering the starter enclosure. Turning off the 2-pole breaker will kill the motor power circuit, but it will leave the control circuit energized as long as the lights are on. This doesn't sound right. Are there any rules or conventions covering this situation?
 
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Old 01-24-16, 12:45 PM
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The easiest and safest way to accomplish something like that is to use a relay that is controlled by the light switch.

If that's something you'd be interested in..... let me/us know.

Otherwise using the light circuit directly to run the control circuit would be hazardous.
 
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Old 01-24-16, 01:40 PM
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That sounds good; thank you. It seems that I would need to mount the relay in a separate enclosure with its coil powered in parallel with the ceiling lights and its contact set wired in series with the existing stop button of the 3-wire control circuit. Does that sound right?
 
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Old 01-24-16, 01:59 PM
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Exactly and I would use a RIB relay in a setup like in the picture.

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A good relay to use...RIB-U1C 120v SPDT relay at amazon

You could go one step further and instead of putting a blank plate on the box above.... you could put in a single pole switch to bypass the relay in case you wanted to run the compressor with the lights off.
 
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Old 01-24-16, 02:18 PM
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That looks like a nice setup. Thank you; much appreciated.
 
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Old 01-24-16, 02:50 PM
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If you decide to go that way.... let me know when you order the switch and I can post a diagram for you..... or anytime you'd like one.

There are many RIB relays that will work. Your main concerns are 120 coil and DPST switch.
 
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Old 01-24-16, 09:25 PM
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Are there any rules or conventions covering this situation?
Yes, there are. You would need to add a placard stating multiple energy sources and then list all disconnects and their locations. Additionally, if a disconnect is more than fifty feet distant OR not visible in a direct line-of-sight from the motor then you would have to have either lockable disconnects or additional disconnects at the motor.

PJ's suggestion is much safer.
 
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