Design for switching an outlet that has a motor plugged into it - best way? - Page 1
Forum section: electronics / projects

author: Corporate666
author_rank: Unknown
timestamp: October 27, 2018, 10:46:47 pm
content: A company has numerous industrial machines which each have a vacuum pump next to them which generates vacuum for holding parts. The vacuum pumps are connected to 120V outlets on the side of each machine. The machines will run anywhere from a few minutes to an hour per cycle, at which time an operator will turn off the vacuum pump, re-set the machine and run another cycle.The problem is that the vacuum pumps will overheat if they are left running too long. During the day, operators should be checking the machines and turning the pumps off when the machine cycle completes, but sometimes people get distracted, or the machine completes just after everyone went to lunch. Also, the machines have an auto-off feature allowing them to be run when everyone leaves for the day - but this does not turn off the vacuum pumps, meaning they would run all night and overheat.I need to design a solution. I am not nearly as well versed in AC electrical stuff as I am in digital electronics, and I would like some pointers and advice. Questions are:1) I want to switch 120VAC/20A. The pump does not draw 20A, but I would like to switch the full current capacity of the outlet, so that it will work regardless of what is plugged in. Is the best way to use a contactor? A standard relay? A solid state relay?2) This is not something I am going to send out for testing to get UL approved, but ideally I would like the solution to be UL approved by way of being an off-the-shelf product I can control with a micro controller. If that is not possible, then at least I need to use UL-approved stuff on the high-voltage side and design in a safe way to UL-standards.3) I would ideally like the solution to either be a plug-in device between the vacuum pump plug and the electrical socket, or something that is built into the electrical socket itself. Then have a low voltage wire going to a control module which would let the operator choose if the outlet/pump will turn off when the machine turns off, or if it will turn off when the machine cycle ends, or turn off with a timer, etc. I can handle that logic in a microcontroller circuit, but the main issue is making it safe/code compliant while giving the control needed.I am envisioning either an off-the-shelf electrical box (or a custom-made metal housing with a 120VAC outlet on the side) which contains the mains switching hardware, with a wire exiting that box and going to a small control module/panel on the front of the machine with a few pushbuttons and LED's indicating the setting.What is the best way to achieve the AC switching?

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2018, 11:14:38 pm
content: Seems like you want the 120 V outlet to be switched on and off automatically when the machine switches on and off?I would take a power feed from the machine and either directly or indirectly have it switch a contactor that is in the 120 V circuit. (By indirectly, maybe the machine could power a low voltage supply that in turn enables the contactor.)I think Matthias Wandel did something like this in his workshop. I will look out the video. I think his solution was a bit make-shift, but you will get the idea.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 27, 2018, 11:20:33 pm
content: Something like these.Put into a box with your choice of communications interface. (i would expect they are all isolated).https://www.online-devices.com/cat-92-Relay_Controllers.aspxMake sure the contactor/relay is rated to your needs.You might even get to choose the plating on the contacts.Get an industrial sparky to box it up.I would probably go flying lead on the plug and a chassis mounted socket.and RS422.

author: IanB
author_rank: Super Contributor
timestamp: October 27, 2018, 11:20:52 pm
content: Here's the video:Don't take it as an instructional video, but you may use it for inspiration...https://youtu.be/kfHB5AoAAbs

author: ejeffrey
author_rank: Super Contributor
timestamp: October 28, 2018, 03:48:36 am
content: In the "idle" configuration is the vacuum pump continually pumping in air, or is in the inlet closed and it is just maintaining an existing vacuum?If the latter, another way to approach this is with pressure switches that turns the pump off when the pressure is below some threshold.