OT: Diodes in control circuits?

[PeterW]

This drawing is sloppy and not precisely accurate but you should get the gist.

I've seen worse on the job, believe me

 

[plchacker]

Yep thats fine. Thanks

 

[plchacker]

Looked again, that misses the stop signal to the drive. The stop signal must be present for the drive to run. Then the start, can start the drive. It is slightly different than a motor starter.

Back after lunch, This is the South, and the ladies do take eating on time almost as seriously as Football.

 

[rsdoran]

I did it in a hurry, when R1 is energized you feed the PLC input too and to the NC side of the relays. The relays are energized by the PLC. A sloppy addition. You are right will need the 4th relay to feed the stop from an output. I thought it would be easy to understand but should have taken time to make it look good and precise.

 

[rsdoran]

This is about the best I can do, especially on short notice.

OHH Crud, now I see the issue, have to think about this more. When R2 is energized it will feed back to the buttons. Have to go Thanksgiving shopping for more food, may redo this later IF I can do better.

 

[DickDV]

PeterW, regarding your question concerning my comment "the only problem I see is that most control circuits are AC and diodes don't work the same way in AC", I can assure you that diodes work the same way where I live and elsewhere too, even in Canada and the UK!

Now I'm joking, if you please!

What I meant to say is that, in general, control circuits are 120VAC or more recently, 24VAC. I didn't mean drive control circuits. Drive digital inputs have been 12 or 24VDC as long as I've been associated with drives (18 years). For 120VAC control, you needed to add interposing relays.

Regarding plchacker's relay question, it seems to me that the situation is being made more difficult by leaving the drive in 3-wire start/stop mode. Why not switch the drive to 2-wire maintained start/stop and use a relay with the two panel pushbuttons to covert 3-wire pushbutton to 2-wire maintained. Then take the second pole of the relay and connect it to the start/stop drive terminal. Also connect the PLC start output to the drive start/stop terminal. And connect the PLC reverse output and the reverse switch both to the drive reverse terminal.

So, you have two wires on the start/stop terminal, one from the PLC and one from the latching relay. And, you have two wires on the reverse terminal, one from the PLC and one from the reverse switch.

As I see it, that would be one relay. You may need a switch for PLC/Manual selection, as well since you need to power the latching relay when the PLC isn't there.

 

[DickDV]

Golly, guys, it sure would be easier if I knew how to draw those wiring diagrams in my posts.

How do you do that?

 

[rsdoran]

I use Visio, allows me to save as CAD or a picture, have another idea but want to look at it before I post it.

Oh well here it is. Please ignore the R4 in the middle, got left by accident.

 

[pimpim32]

I still guess the drive is bulletin 160 SSC not SMC.

Even with two wire the problem will remain same, the way he wants to do it is to use SPDT relay and if no power/or manual to PLC then to use PB's. Now if you are in PLC mode, and you have only a 2pos maintained switch 12V will flow from stop relay back to fwd or reverse. So has to be 3pos maintained switch, but we are back to the original problem. If the switch is let in one position still 12V will flow, so we have to take care as to be in middle position, same as three wire, in run position.

the relays have only one set of contacts or more?
If more then just use extra sets from R1 to make sure nothing will go from PB's to drive, so 2 relays will be ok.

If only one set then use extra 2 relays to enforce the same.

 

[pimpim32]

Sorry , didn't see the last one from rsdoran, this will work, however some picky safety inspector will ask that R4 stop from PLC to be normal open, as if we are in auto mode if PLC dies then the drive should stop. but this was not a safety question.

 

[rsdoran]

Tell the truth making the class hook it up with what he has would be a hell of a learning experience.

It would probably be more appropriate if the Stop went thru the NO and when switched to Auto it energize the relay.

May not be needed but just in case will go thru what my drawing says.

R1 switches to Auto Mode removing common connection to PB's. The PB Stop is connected thru R1 to PLC input, to monitor STOP PB.

R2 keeps the Stop active through NC contacts, if R2 is energized then Stop is removed...in this case removal activates stop

R3 will be a momentary for Start.

R4 maintained to reverse

 

[plchacker]

Golly, guys, it sure would be easier if I knew how to draw those wiring diagrams in my posts.

How do you do that?

I've been using Paint, saving to JPG. Once you start to edit a post, look at the bottem section, there it allows you to import a file/photo. Browse to the file, upload, then copy the URL to the message.

 

[plchacker]

OK here is a more accurate view of what I have now:

The red dots show the trouble if the diodes are not in the ckt.

Here is the drive:

 

[pimpim32]

I see by SPDT you eally meant SinglePDT.
In that case just use rsdoran drawing if you still want to go with 3-wire control.(still i do not see the use for R1 to come back as PLC input, we do not know if PLC will pickup 12V)

To revert to original question I would not use diodes in this way, just for the simple reason why should I add a component ift it is not nedeed, and second in real world is a little bit more difficult to see a faulty diode then a faulty relay when troubleshooting a panel.

Do not know if is any safety rule against them though to be used like this.

 

[plchacker]

Tell the truth making the class hook it up with what he has would be a hell of a learning experience.

It would probably be more appropriate if the Stop went thru the NO and when switched to Auto it energize the relay.

This will work. I don't have time to make the wiring change this afternoon, but I will first thing next week. I should have seen this early on. Bad week I guess. (Registration, major schedule change, and lots of other distractions.)

The trainer is portable, and the PLC's don't always travel with the trainer. I have had to add a lot to these trainers. This is one I wanted ready for next semester. The original version could recieve a remote (PLC 0-10VDC- another change to 4-20mA) speed signal, but there was no way to control the drive from the PLC. Now there is.

If my wish list gets approved, I'll be adding five Compact Logix PLC's and five Powerflex 40's, along with CLX and a lot of other goodies. Once I have the new drives, DH+, ControlNet, Ethernet, and DeviceNet will be used.

I found a few PLC4's in storage the other day.