relay (or module) for controlling a 7A inductive load...

Thread Starter

Ed

Joined Sep 1, 2021
6
Hi,
I have a CNC with a DWP611 DeWalt router. Specs say it can draw 7Amps.
I want to automatically turn the router off when the job completes, so I'd like to do this with a relay/SSR.
There are several different "gizmos" out there for remote control of lights etc; however, they are not
spec'd for 120VAC/7Amp inductive load (i.e. the router).
I'd prefer to control it with 5v from a Pi, just to avoid more circuitry that would be needed to step it up.
Thanks,
 

Ya’akov

Joined Jan 27, 2019
6,275
Welcome to AAC.

An SSR like this one should be suitable. 25A may be much higher than the load you specified but that’s the maximum load not accounting for the inductive nature. The 25A should handle both the startup and the running load.
 

Thread Starter

Ed

Joined Sep 1, 2021
6
Thanks, that looks like it will work!
Is the surge the only reason there is typically a distinction between resistive and inductive load specifications on relays?
 

Ya’akov

Joined Jan 27, 2019
6,275
Thanks, that looks like it will work!
Is the surge the only reason there is typically a distinction between resistive and inductive load specifications on relays?
Yes, on startup the sudden higher-than-running-load has to be handled, though only for a very short time.

Shutting down, the magnetic field that forms around the conductor collapses and the result is a high voltage spike(Back EMF) that causes arcs across the contacts. This spike can kill things. The (magnetic) relay itself can destroy a transistor that is switching it unless a flyback diode or snubber is used to stop the spike from getting to it.
 

Ya’akov

Joined Jan 27, 2019
6,275
The wiring diagram for that SSR (http://www.cnchog.com/products/ssr-10da-25da-40da-ID213.html) shows an RC in parallel with the load built in. Does that deal with the back-emf or do I still need to add a snubber across the load?
Or, does the whole back-emf issue only apply if I was using a magnetic relay?
That is a snubber network built in to the relay. It protects the SCR the same what the transistor is protected in the magnetic relay case. The input uses an optoisolator which protects the MCU (or whatever) from the load with galvanic isolation, that is, no conductive path at all.
 

MisterBill2

Joined Jan 23, 2018
12,704
If the CNC system returns to a home position when the job is completed then just have a limit switch there to release the relay. Of course the router control system should have a start button and an E-Stop button that is in series with the home switch. So no starting until it is intended to start.
 

Thread Starter

Ed

Joined Sep 1, 2021
6
If the CNC system returns to a home position when the job is completed then just have a limit switch there to release the relay. Of course the router control system should have a start button and an E-Stop button that is in series with the home switch. So no starting until it is intended to start.
Interesting thought, but I think that may actually be a little more complicated than the relay approach.
Right now I have everything plugged into the same source, so E-Stop is easy, and wise idea to just add one of those.
Regarding the limit switch approach...
  • That brings 120VAC wiring very close to my fingers (when installing a new board to be cut for example).
  • The switch has to deal with the same issue as the relay contacts: 120VAC 7A inductive load, so I'm guessing the small switches typically used for the stepper motor would not be appropriate for this.
  • If the home position turns the router off, how do I then turn it back on without some kind of additional circuitry and/or manual intervention?
  • Would having the AC up in the frame of the CNC interfere with the motor control signals feeding the steppers? Early on I had problems with that just from the wiring of the standard limit switches on my machine.

Anyway, I think I'm gonna stick with the relay approach for now. Besides, I just received my relay delivery yesterday, so I have to try it out. I will post the results when (assuming) I finally get things working.
 

MisterBill2

Joined Jan 23, 2018
12,704
NO, the switch would only need to control a relay.. That is the whole purpose of a start and e-stop arrangement, which is that a relay controls the power and the controls logic controls the relay. Of course, if that switch only sends a signal to a processor then it might be a bit more complex. There may be an enable control for the servo system, in which that line could also drive an opto-isolator to enable the start circuit.
 

Thread Starter

Ed

Joined Sep 1, 2021
6
NO, the switch would only need to control a relay..
Yea duh... of course. Didn't even think of that. Still, I think I'm gonna stick with controlling the relay from the Pi, which is also feeding the CNC machine the G-Codes, so it knows when things are done.
I'll certainly keep you're suggestion in mind as I work through this...
Thanks for the feedback.
 
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