A Question on Interlocking Logic

[FCC]

I wanna design a Start-Stop circuit which onctrols 3 motor starters. If I connect all three motor starters so that if an OVERLOAD occurs on any one of the starters. all three starters will automatically diusconnect. More than one overload is needed since there are more than one motor starter. Physically, the overloads aren't part of the PLC because if the PLC stops then the emergency stop wpould be of no use, from my understanding. Physcially, it must be separate from the PLC, like an emergency stop button. My question is where does it connect to though, and how will it make the 3 motor starters stop automatically when an overload occurs. Thanks in advance.

 

[Steve Etter]

When you are talking about motors, overcurrent protection and overload protection you need to realize that you are getting out of the realm of PLC logic and back into hardwired devices. There are electronic devices, but even these are intended for use external to system controllers.

One of the most common OVERLOAD protection devices used with motor starters is the thermal overload. It is sized for the specific motor and application and then monitors the sustained amperage that the motor draws by way of heat. When the heat is too great for too long, the overload "trips" and opens a set of N.C. contacts.

Generally, each motor is wired with this set of N.C. contacts between the neutral side of the motor starter coil and neutral. A standard motor starting circuit looks something like this:

                                  Starter          Thermal

       Stop PB    Start PB         Coil            Overload

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

                 |          |

                 |          |

                 | Starter  |

                 |   Coil   |

                 +--| |-----+

In the case of the three individual motors wired so that any given motor overload would shut down all three, all you need to do is wire all three overloads in series. Like this:

       Stop PB    Start PB        M1     OL1    OL2    OL3

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

                 |          |          |

                 |          |          |

                 |          |          |

                 |   M1     |          |

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

                                       |

                                       |

       Stop PB    Start PB        M3   |

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

                 |          |          |

                 |          |          |

                 |          |          |

                 |   M2     |          |

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

                                       |

                                       |

       Stop PB    Start PB        M3   |

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

                 |          |

                 |          |

                 |          |

                 |   M3     |

                 +--| |-----+

Does that help?

Steve

 

[bungle]

IP1=E-STOP,IP2=O/L1 TRIP,I/P3=O/L2 TRIP,I/P4=O/L3 TRIP,OP1=MOTORS



I/P1 I/P2 I/P3 I/P4
---[ ]---[/]----[/]----[/]--------------------------(O/P1)





EXTERNAL CIRCUIT

E-STOP
----[/]-----------I/P1
O/L1 TRIP
----[ ]-----------I/P2

O/L2 TRIP
----[ ]-----------I/P3

O/L3 TRIP
----[ ]-----------I/P4


O/P1 E-STOP O/L1 O/L2 O/L3
-( )-----[/]----[/]----[/]----[/]-------(M1)and(M2)and(M3)


IS THIS ON THE RIGHT LINES????

sorry about the drawing,still can't work out how to send them.

 

[bungle]

PS,I FORGOT THE START BUTTON

 

[FCC]

Steve, I was told not to put the overloads after the coil (M1)
Can you explain why the overloads go after M1? In my understanding, my basic PLC class is just a theory class, and the weakness is that it leaves out all the physical hard-wiring and other electrical devices, and only focuses on relay ladders and etc. But I think I was told to make it that nothing can go after the coil M1, because the PLC just wont accept it, so I'm a bit confused on the OLs coming after M1.

Bungle, I'm trying to design something that the PLC will accept, in your case, I dont think having 3 coils on one rung will work.

 

[Eric Nelson]

Steve was showing the hardwired schematic (i.e. physical wiring), not ladder logic.

You are right that nothing can go AFTER the "coil" in PLC code.

beerchug

-Eric

 

[Steve Etter]

FCC,

Eric is exactly right, I was showing how the system would be hardwired. The important thing to remember here is whenever you are actually working with motors, you must physically hardwire your overload protection in with your motor starters. That is why I drew the circuit I did.

As far as the PLC itself is concerned, it is certainly good information that the overload is tripped and I would encourage taking it into the PLC (using a separat contact). Just don't rely on the PLC to shut off the motor in this event. Just use the PLC to monitor and then to break off any "motor run" commands it might be issuing.

Now, in the PLC itself, I would write the code in a similar fashion to the hardwired diagram with the modification that puts the overloads at the front of the circuit:

                                               Starter

   OL1   OL2   OL3    Stop PB    Start PB        Coil

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

                                |          |

                                |          |

                                | Starter  |

                                |   Coil   |

                                +--| |-----+

Steve

 

[bungle]

FCC,SORRY FOR ANY CONFUSION BUT THE THREE COILS ARE THE EXTERNAL CONTACTOR COILS AND WOULD BE WIRED IN PARALLEL