Help a Newbie....

Guest · Mar 27, 2003

I'm in a Basic PLC class and I am supposed to design a circuit so that any 3 start push buttons will start a motor and any one of three stop push buttons will stop the motor. This circuit must also include MEMORY so that the motor will remain running after any start push button is pressed and released.

This is what I designed but I dont know if only one MEMORY contact is needed, or should there also be 3 MEMORY contacts. Pressing any one of the start buttons (OR Logic) will pull in the coil and memory and start the motor, then pressing any one of the stop buttons will stop the motor. Am I some what close to it? ANy advice is helpful, thanks in advance.

stop1 stop2 stop 3 start1 memory
--| |------| |------| |--------| |-------------( )
start2
| |

start3
| |

memory
| |
memory motor
-------| |--------------------------------------( )

FCC · Mar 27, 2003

the design is kinda screy on my post, it's supposed to be stop button 1, 2 and 3, then start button1 and memory. Then the other start buttons and memory are OR under start button 1. Memory is then referenced off and lead to the motor coil.

Vetteboy · Mar 27, 2003

You should label your coils too.

It looks like you have it. To simplify and only only use the one coil, series all your stops, parallel all your starts after them, with a normally open contact that's the same address as your output coil also in parallel with the start buttons (This is your holding contact). Then any of your stop buttons will stop it, and any of your start buttons will start it.

bungle · Mar 27, 2003

Take a look at motor stop/start circuits, remember stops are in series and normally closed and starts are in parallel and normally open,don't forget normally open contact off the coil for hold cicuit.

FCC · Mar 27, 2003

well if it's hardwired out in the field the stops are normally closed right? but if it's just programmed into the PLC, the stop buttons are normally open??

Vetteboy · Mar 27, 2003

That's where people get confused. The stop buttons are physically wired normally closed, but the "contacts" in your program are normally open. The power going through the NC buttons provides the input until it is pressed. So program your stop buttons NO, but wire them NC. I can see this advice starting up a debate........

bungle · Mar 27, 2003

Apologies offered,still thinking in electrician mode.

Stop input although N/O will be closed unless external stop circuit goes open circuit.On the right tracks now??

Tim · Mar 27, 2003

FCC,
Where do you have your memory latch or holding latch tied at?
I'm a little confused on your post. Are you latching around your stop inputs?
Tim

FCC · Mar 27, 2003

The memory is OR with the start buttons, right under the 3rd start button. Sorry but the drawing i posted on here is kind of messed up. So when you hit any one of the 3 start buttons, it automatically pulls in the memory coil.

Now I have another question, what if i wanted to add a selector switch to place the circuit in a jog or run position. I'm a bit confused on what the jog switch would do. Do I AND it with the rest of the stop and start buttons?

Vetteboy · Mar 27, 2003

Disable the "Memory" coil with a Jog input.
Enable the same with the Run input.

Tim · Mar 27, 2003

FCC,
After looking at your posted ladder code again and agian, I think I see what your trying to post. I thought you might have been taking your memory latching bit from the left hand buss bar, out and around your stop inputs. Is this what your trying to post?
Tim



  stop 1  stop 2  stop 3     start 1      memory

|---| |-----| |-----| |----|---| |----|----(   )---|

|                          |          |            | 

|                          | start 2  |            |

|                          |---| |----|            |

|                          |          |            |

|                          | start 3  |            | 

|                          |---| |----|            |

|                          |          |            |

|                          |  memory  |            |

|                          |---| |----|            |

|                                                  |

|                                                  |

|  memory                                 motor    |

|---| |-----------------------------------(   )----|

Eric Nelson · Mar 27, 2003

Damn, ya beat me to it Tim...

I 'think' FCC meant for it to look like this:



  stop1    stop2     stop3    start1          memory 

|--| |------| |-------| |---+---| |---+---------( ) 

|                           |         |

|                           | start2  |

|                           +---| |---+

|                           |         |

|                           | start3  |

|                           +---| |---+

|                           |         |

|                           | memory  |

|                           +---| |---+

|

|    memory                                    motor 

|------| |--------------------------------------( )

.
If so, he's got it right...

There's a few ways you can add a JOG button, here's one method:



  stop1    stop2     stop3    start1     JogPB     memory 

|--| |------| |-------| |---+---| |---+---|/|---+---( ) 

|                           |         |         |

|                           | start2  |         |

|                           +---| |---+         |

|                           |         |         |

|                           | start3  |         |

|                           +---| |---+         |

|                           |         |         |

|                           | memory  |         |

|                           +---| |---+         |

|                           |                   |

|                           |  JogPB            |

|                           +---| |-------------+

|

|

| memory                                          motor

|--| |---------------------------------------------( )

.
Notice that the JOG button will automatically drop out the memory. Also, the stop buttons are ALWAYS functional (IOW, you can't JOG with a stop button pressed). You could "clean up" the above ladder by writing it this way (same end result):



  start1      JogPB     stop1    stop2    stop3    memory 

|---| |---+---|/|---+---| |------| |------| |------( ) 

|         |         |

| start2  |         |

|---| |---+         |

|         |         |

| start3  |         |

|---| |---+         |

|         |         |

| memory  |         |

|---| |---+         |

|                   |

|  JogPB            |

|---| |-------------+

|

|

| memory                                          motor

|--| |---------------------------------------------( )

beerchug

-Eric

jpd24 · Mar 27, 2003

I'm also pretty new to programming and I still think stop buttons and then start buttons. But I have noticed that most programs will will through a stop anyplace in the rung.Also is it common practice to use internal bits such as B3s or end up with an ote at the end.

Vetteboy · Mar 27, 2003

Is it the program throwing the stop button anyplace? Or the programmer? The way I see it, from my training, the series of stops always come first, then the parallelled starts and holding contacts. I haven't found that written in stone anywhere though.........

Vetteboy

FCC · Mar 27, 2003

Tim, yep you got it. That was what I was trying to do, but it wouldnt let me go back and re edit my sketch hehe. Thanks everyone.

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