OT: Diodes in control circuits?

rsdoran said:

The diodes are just out of the norm for most plants, good idea, but just uncommon is why I prefer using what is avaiable.

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Uncommon, yes. Unknown? Far from it. I come across diodes in control circuits every couple of years. There are a variety of standard industrial diode solutions. There are "double-deck" terminals with the diode connected between the upper and lower circuit and there are terminal rail mounted modules with multiple diodes. Most large city electrical wholesalers have something of this ilk, smaller outlets can usually get them within a few days. I have seen these useful little components both from Entrelec and Weidmuller.

If you are building with what you have in the factory engineering store then its obviously a different story. In the UK with 24VDC being the norm for control circuits perhaps we have a different product spectrum available to us, too.

girevik said:

Lancie1,



I guess that plchacker has plans to control the drive in the manual mode with no PLC involved, and so turning off the plc in the manual mode would disable the drive at all by disconnecting the stop wire... Although i'm agree that 'stop' button should stop the drive in any case because operators should not care what is the current mode, they should just push the stop button, so how about such scheme (the only difference from your scheme is that the stop wire ignore the plc's stop output in the manual mode-marked green), please let me know your opinion....

thanks

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Acutally it doesn't, R3 is nomrally closed and used as a momentary (when powered ON the N.C. Opens) to remove the Stop signal; which engages the Stop. It is isolated where in either mode the Stop is just that...Stop.

Your method puts power back through the Start/Reverse PB's when in Auto; which was the problem in the beginning.

Need the practice so created another drawing. May just be me but personally there are a few things I dislike about some of this so will discuss them, more after the picture.




May just be me but would personally like to see Mode Relay 1 be a Hand/Off/Manual switch, have not determined exactly what will switch it to Auto mode under the present conditions.


Personally would also prefer the PLC to know what mode it is in and when the Stop PB is used. There may be situations where the PLC may not react to the Stop PB i.e. the PLC could (restart) keep the drive running, especially if the Start button is maintained.


As for diodes, yes they are used quite alot in industrial situations to prevent feedback etc., but this is not a situation that demands something along those lines. Since this is a school training endeavor I think it may take away from the initial training objective.

I know another issue may be sending signals to the PLC, in the future may want to ALWAYS get DPDT relays but if plchacker is using SLC 500's the IF8 I think is 10-30vdc.

As I mentioned having the class "resolve" the issue of the feedback using the relays would be one heck of a lesson and look at the options including the diode aspect.

Used up my brain for today...

rsdoran said:

Your method puts power back through the Start/Reverse PB's when in Auto; which was the problem in the beginning.

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agree on that point... my fault...

rsdoran said:

Acutally it doesn't, R3 is nomrally closed and used as a momentary (when powered ON the N.C. Opens) to remove the Stop signal; which engages the Stop. It is isolated where in either mode the Stop is just that...Stop.

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I'm not sure if using NC contact that to stop the drive is safe enough..so plc or wires failure will make the drive unstoppable, although i may be wrong or missing something...

girevik said:

I'm not sure if using NC contact that to stop the drive is safe enough..so plc or wires failure will make the drive unstoppable, although i may be wrong or missing something...

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There is always an issue of wire failure but in this case as long as the Normally closed contacts stay good and the PB is good you should be able to initiate a Stop. My concern was the PLC being able to "over-ride" the Stop if not monitoring.

Plchacker is a teacher trying to build up a better program for training his students, there are limitations involved in how much he can offer; therefore the Drive may not actually be driving anything that would cause any harm. The idea is to get the concepts across to the students.

Yes there are safety issues but I have faith plchacker will be conveying those issues too.

I have got your idea...

I just though that safety issues are not the least things to teach the students...

and just a quick thought:


i'm supposing that plc's stop output is activated when the drive is running and always deactivated in the 'manual' mode... however there is a possibility that stop output of the plc will freeze at "1" (which is less possible than freezing at "0")...

it all depends on the output module type(i'm not sure what kind of outputs are used by plchacker) and probably such things like connecting the relays coils common through the mode relay NO contact to block the PLCs commands in the manual mode...and so the stop relay will always be deactivated in case of the manual mode...

just a thoughts.... i'm just trying to learn new things through discussing this thread...

so would like to know your opinion...

thanks

I am not a "teacher", just try to keep things simple but as effective as possible.

The Stop relay IS NOT active (energized) in either mode, it energizes (momentarily) when needed to disconnect the signal from the Drives Stop input just as the Stop PB would. The Drive Stop input (could also be called "enable to Run) has to be maintained for the Drive to be started. This allows the Stop PB to always remove the signal regardless of Mode.

The other issue is these are SPDT relays so there are only 3 connections point, look at post 11. http://www.plctalk.net/qanda/showpost.php?p=243397&postcount=11

Note too what I have drawn using relay contacts is the hardwired part, so using -|/|- does not arbitrarily mean I have an option to use -| |-.

The issue is the best method with what he has NOW to develop training for his students, I expect in the future he will upgrade it to offer more options.

Despite what you say, Ron, you very well may be more of a teacher than you think! The evidence is piling up!

Thanks Dick but no sooner then you said that I found something I said that needs correction.

Personally would also prefer the PLC to know what mode it is in and when the Stop PB is used. There may be situations where the PLC may not react to the Stop PB i.e. the PLC could (restart) keep the drive running, especially if the Start button is maintained.

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It should read: Personally would prefer the PLC to know what mode the system is in and the PLC to know when the Stop PB is used. There may be situations where the PLC may not react to the Stop PB i.e. the PLC could (restart) keep the drive running, especially if the Start "relay" is maintained ON while in Auto mode.

Knew if I looked long enough I would find another issue, in my last drawing IF the Reverse switch is in Reverse position AND R4 is maintained in Reverse then AGAIN the Start button could be used.... This shows a need to KNOW when the drive has been given a Stop command.

Girevikk,
PLChacker is a training instructor with limited resources. I believe that he is trying to retain the function of some PLC Trainers, so that they still work with the existing control buttons, but at the same time modify them so that the PLC can be used to demostrate how to control a variable speed motor drive.

Why don't you ask him directly what he thinks about your circuit with the PLC STOP function disabled?

rsdoran said:

I am not a "teacher", just try to keep things simple but as effective as possible.

The Stop relay IS NOT active (energized) in either mode, it energizes (momentarily) when needed to disconnect the signal from the Drives Stop input just as the Stop PB would. The Drive Stop input (could also be called "enable to Run) has to be maintained for the Drive to be started. This allows the Stop PB to always remove the signal regardless of Mode.

The other issue is these are SPDT relays so there are only 3 connections point, look at post 11. http://www.plctalk.net/qanda/showpost.php?p=243397&postcount=11

Note too what I have drawn using relay contacts is the hardwired part, so using -|/|- does not arbitrarily mean I have an option to use -| |-.

The issue is the best method with what he has NOW to develop training for his students, I expect in the future he will upgrade it to offer more options.

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i'm not a teacher too...

In case of using the NO contact of the stop relay, the PLC's stop output will be ON (and so the relay will be energised) when the drive should be running and the outbut will be off the rest of the time, i.e. some sort of 'enabling; signal like you said...just NO because of the safety issues...

I do understand about 3 points per relay and I dont see any problems to implement my scheme with such relays... and it will not take much time, just a couple of additional wires been changed...

So I dont ask to make any big changes and/or purchase any additional equipment...

1) original scheme:


2)your scheme:



3)my scheme (NO contact to stop the drive from the PLC in the auto mode, stop PB will stop the machine anytime).



thanks

Lancie1 said:

Girevikk,
PLChacker is a training instructor with limited resources. I believe that he is trying to retain the function of some PLC Trainers, so that they still work with the existing control buttons, but at the same time modify them so that the PLC can be used to demostrate how to control a variable speed motor drive.

Click to expand...

And I dont ask him to make any big changes, purchase any new equipment, re-wire all the buttons etc etc... just trying to improve the scheme keeping it as close to the original one as possible...

And I guess that plchacker will be happy to see as much ways to improve the scheme as possible, getting them by as much people as possible, otherwise he would not post here...

thanks

girevik said:

i'm not a teacher too...

In case of using the NO contact of the stop relay, the PLC's stop output will be ON (and so the relay will be energised) when the drive should be running and the outbut will be off the rest of the time, i.e. some sort of 'enabling; signal like you said...just NO because of the safety issues...

I do understand about 3 points per relay and I dont see any problems to implement my scheme with such relays... and it will not take much time, just a couple of additional wires been changed...

So I dont ask to make any big changes and/or purchase any additional equipment...

1) original scheme:

2)your scheme:

3)my scheme (NO contact to stop the drive from the PLC in the auto mode, stop PB will stop the machine anytime).

thanks

Click to expand...

How does the PLC STOP it in AUTO (PLC control) mode?

If you switch R2 with R1, again you have Power on the pushbuttons.

Look at the manual for AB 160 SSC: http://literature.rockwellautomation.com/idc/groups/literature/documents/um/160-um005_-en-p.pdf

Here is the diagram:

rsdoran said:

How does the PLC STOP it in AUTO (PLC control) mode?

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R1 is ON, R2 is OFF, so there is no connection between the common and the drive's stop input.

thanks

rsdoran said:

If you switch R2 with R1, again you have Power on the pushbuttons.

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yes there was a mistake (missed two points of the relay)