AC/DC Power Isolation

[dbranger94]

First of all, hello, this is my first post here. Sorry it's so long, but it's a pretty specific question that, A: I'm not sure exactly how to ask, and B: haven't been successful in finding a good answer to.

I have a G0602 lathe that I'm adding a tachometer to. The lathe uses a contactor to de-energize itself when the e-stop is pushed, and a pushbutton to close the contactor and power the lathe up again. A simple 3-position rotary switch controlls FWD OFF REV. The lathe uses a 110VAC, single-phase motor.

I am adding a PLC controlled, optical-interrupt tachometer with LCD readout that I made. It is powered by 5V DC power supply that runs off of 110VAC. I want the tachometer to be powered on/off with the lathe and not have a separate switch.

My first attempt was unsuccessful. I wired the DC power supply up to the load side of the contactor. The tach works perfect, except every time I rotate the switch from OFF to FWD, or FWD to OFF, the PLC resets. I'm not sure what's happening, but I belive it has to do with sharing the load side of the contactor with the motor, even though the motor has start and run caps.

For my second attempt, I found another identical contactor, new for $15. I plan on having them share the same line voltage and wire the e-stop and start buttons in parallel to both contactors. My goal is to isolate the AC motor from my DC power supply.

My question is, is this the best way to do this, or will I still have the same problems?

Thanks,
Daniel

[Al_The_Man]

First I assume the term PLC you are using is for a embedded dedicated Micro and not the normal industrial packaged version of a PLC?
The first thing you may want to do is put a snubber across each contactor coil, if you are using a reverser contactor, this is a Simple RC combo you can either make up yourself or they come cheap from a few suppliers.
I have had quite a few examples of this prevented by the snubber.
Another answer may be to reference the 5vdc supply common to the earth ground terminal.
Al.

[Al_The_Man]

The other thing to ensure is that the +5v supply is De-coupled properly, a 100µf across the supply and a .1µf right at the IC itself.
Al.

[dbranger94]

The PLC I'm using is an ATMega328P-PU 28-pin DIP module. The power supply I made has the 100uF and 10uF caps on it. I even went as far as adding a 470uF cap just to see if it would help, and it made no difference.

[Al_The_Man]

Here is an example of a snubber to place across the coils.
CNC deviation error - CNCzone.com-The Largest Machinist Community on the net!
Al.

[dbranger94]

Thanks for all your help. I found this, I think it will work...

If you don't mind another, probably stupid, question. I read that it needs to be installed across the coil...? I think that means on the line side of the contactor, not the load side...? Correct?

I am using a 3TB41 Siemens contactor. Here is a post with some pics of the G0602's wiring and the contactor. Contactor pics are about 3/4 way down the page. There's also a schematic.

[Al_The_Man]

That appears to be a RC snubber, it is intended to go across the coil terminals, not the power contacts, although there are versions that are intended to be wired across the power conductors but are usually larger capacity.
Did not see the contactor in the link.
Al.

[underthetire]

Does your contactor not have aux contacts?

[dbranger94]

Yes, I belive they are labeled A1 and A2. I'm just now beginning to understand this contactor.

[Al_The_Man]

A1 and A2 is usually the designation for the coil, which you would connect the snubber across as in post #2.
I am assuming you have the tach powered all the time but when you go from FWD to REV the contactor reversing action causes the display to reset?
Al.

[dbranger94]

Here are some pics of the contactor. I labeled the wiring diagram with the contactor terminal numbers(in blue). Also, pay no attention to the wires labeled L and N. They seem to be labeled backwards in the manual. I know mine and crevicereamer's lathes, at least, have Black as Load and White as Neutral.





Here's the wiring diagram...

[dbranger94]

The way I have it now, I have my DC power supply connected to terminals 2 and 6 (with the motor). Circuit 13/14 is normally open (same as 43/44)(21/22 and 31/32 are NC). I think that I can connect a jumper wire from 1 to 13 and connect my power supply to 14(Line, when closed) and 5(Neutal, from wall). Also, I'll place the snubber across A1/A2.

Or should I connect a jumper from 5 to 13 and connect the power supply to 1(Line, from wall) and 14(Neutral, when closed)?