Re: [foxboro] [UCE] Re: Motor Start/Stop Logic

What you describe (and illustrate) is exactly what we call JOA
(jog-off-auto).
The Jog position is like a "deadman" switch.
The "con" we've run into is the tech replacing JOA with HOA
(non-spring-return) because he couldn't find a JOA or didn't realize there
was a difference. Surprise, the operators don't tell me it's a HOA - they
quietly celebrate that they don't have to hold the switch ... until they
walk away and forget they left it running in the Hand position. aargh

ps - We have occasionally put the DCS stop upstream of the JOA, thereby
allowing the DCS to stop it, but the DCS stop must then be maintained OPEN
until the field switch can be repaired.

>>>>>

Ah, yes, I forget -- picture = 1k words and all that.
.     Field
.     start
----|--| |---------------------------|---(M)--
.   |                                |
.   |                                |
.   |--|/|-----------|/|---|--| |----|
.     Field          DCS   |  DCS    |
.     stop           stop  |  start  |
.                          |         |
.                          |--| |----|
.                              M

Field start/stop is actually one 3-position switch:

Pos     F. Start        F. Stop
--------------------------------
"Start"    C               C
"Run"      O               C
"Stop"     O               O

the "Start" is momentary and spring returns to "Run".


Pros:
-------
Can bump motor in field regardless of DCS state
Simple -- doesn't rely on DCS logic

Cons:
--------
2 outputs required & more control wiring.
If field start gets stuck closed, can't stop motor from DCS.


Brad Wilson
ExxonMobil Chemical Co
Edison Synthetics Plant
732-321-6115
732-321-6177 fax
Brad.S.Wilson@xxxxxxxxxxxxxx

 
 
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