PLC Motor Controls

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wiigelec said:
What are your thoughts / opinions on running the Hand/Stop inputs through the PLC versus hardwired to the contactor / VFD?
Thanks
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I can just say that I have done that before (jackng swithgears for oil rigs) so I don't see a problem.
Are you thinking of anything specific?
 
wiigelec said:
What are your thoughts / opinions on running the Hand/Stop inputs through the PLC versus hardwired to the contactor / VFD?
Thanks
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It's something we commonly do except for emergency stop circuits - it's usually a requirement here (UK) to have these hard wired.
 
Where I work, that is how we usually wire it. Except for estop circuits, as Besoeker stated. Usually run them in series with a safety contactor that will drop out all control power if an estop is pressed. If you provide more info specific to application may be able to give more detailed answer.....
 
Where I work, the hand position of the HOA always directly connects to the starter. It bypasses all interlocks except the motor over load. The HOA is spring return to off from the hand position. It is used for lock out verification. The you use a try-lock-try method for locking working equipment out. First you go out to the motor and run it in hand and see it run, then you lock out the power source and go back to the HOA and try to run it in hand and verify that it does not run.
 
I am refering to a local (to the motor) start/stop or HOA station. All signals (start, stop, hand, off, auto) are digital inputs. Motor coil is digital ouput. All motor logic is resident in CPU...

Where this is an issue for me is when the IO is distributed. Communication from IO card to the CPU is made via comm link, generally fiber. If comm link fails IO has no access to logic and pressing stop button will potentially do nothing...
 
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I would see no problem with running these through a PLC. but like others have said before E-stops should be hard wired to cut power out. we have alot of automated equipment where i work and all of our jog buttons in the one building are run through our PLC.
 
wiigelec said:
Where this is an issue for me is when the IO is distributed. Communication from IO card to the CPU is made via comm link, generally fiber. If comm link fails IO has no access to logic and pressing stop button will potentially do nothing...
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THE STOP BUTTON SHOULD BE A NORMALLY CLOSED DEVICE with the logic using a normally open function ...

IF communication is lost, the "power flow" is lost and the coil de-energized. I can do it in old Boolean here, but without generating a graphic, ladder with 2 lines is harder.

LOAD START
OR COIL
AND STOP
OUT COIL

or, if I use a layout more like a "real" 3-wire circuit

LOAD STOP
LOAD START
OR COIL
AND STACK
OUT COIL

Trying sorta ladder, labels between contacts ][ and coil()

-----]stop[-----]start[-----(coil)
| |
---]coil[---
 
Plc processors can lock up also. So even if the stop button is wired NC, the motor could stay on (if the plc does not have a watch dog circuit).

Most panels have a hard wired e-stop button close to the (I/o wired)start/stop for safety. Its also wise to have a hard wired contact on the same stop button that will drop out the contactor even if the input or PLC screw up.
 
S'mise said:
Plc processors can lock up also. So even if the stop button is wired NC, the motor could stay on (if the plc does not have a watch dog circuit).

Most panels have a hard wired e-stop button close to the (I/o wired)start/stop for safety. Its also wise to have a hard wired contact on the same stop button that will drop out the contactor even if the input or PLC screw up.[/QUOTE

I belive the E-stop are all suppose to be Hard wired. At are Facility we have a double contact on our E-stops one to drop out a realy that kills all the contactors and another one back to PLC for diagonstic/ added logic to drop out contactors.
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nothing's foolproof ...fools are pretty ingenious

nothing's foolproof ...fools are pretty ingenious

Is it the fact that it is a stop button that bothers you? If it was a box on a panelview display that lit up when the motor was running and darkened when it stopped, would that make any difference?

I think the E-stop should be hardwired, but a master control relay could stick also. Contacts could weld closed.

A couple weeks ago I changed a guardmaster door switch that had three circuits... two went to safety relays and one was just an indicating circuit ...three contacts on one operator....

the mechanical part stuck, and it may as well have had ten circuits in it.... kept right on running when the door was opened.
 
Is it the fact that it is a stop button that bothers you? If it was a box on a panelview display that lit up when the motor was running and darkened when it stopped, would that make any difference?
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The fact that it is a stop button is what bothers me, when I push the stop button the motor should stop, with reasonable reliability. Run indication is not as important.

It is my opinion that stop buttons should be hardwired to the motor starter and I am looking for compelling reasons they should not be hardwired...
 
I don't think it makes much difference. I have done it both ways within the last few months, at the same plant, within a few hundred feet of each other. Customers are not always consistent in what they want.
 
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