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Old December 11th, 2017, 12:32 PM   #1
stu
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Help with plc relay stuck on

Hi guys
I have an issue on a micrologix 1400 1766-L32BXB , this morning I had an issue with 2 outputs on the base unit stuck open (constant supply)
The two outputs in question enigize 2 relays R2 & R3 this then operates the run signal to 2 inverter?

I check out the wiring for feed back ? Which could cause emi to the plc - nothing wrong??

I changed the plc and after a hour R 2 coil signal was stuck open ??
I canít find anything wrong though?
Do you think I should put a diode across the relay so we donít get a surge back to the plc??
Thanks Stu
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Old December 11th, 2017, 12:48 PM   #2
scott.lawrence
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are you using a suppressor on the relay itself?
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Old December 11th, 2017, 12:58 PM   #3
stu
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No there is nothing fitted on the relays ,this is an old install?
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Old December 11th, 2017, 01:17 PM   #4
James Mcquade
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Your post is a confusing to me, so I will cover what I know.

your plc outputs are relay type powering interface relays going to inverter run inputs.

Plc
all relay type plc outputs have a lifespan, I do not know your plc outputs life.
if the outputs are dc, I would use a reverse diode to protect the relay.
if it's ac, I would use an ac suppressor.
if the outputs are high cycle, I would go with a triac output module and load resistors.

if the contacts have high current over a long time, the relay contact arm itself could be overheating and bending, breaking, or welding the contacts.

in regards to the plc outputs, how much is the inrush current to the interface relays? I would multiply the holding current by 6 to 10 for that.

for the relays R2 and R3.
the same applies as above.
I wouldn't think a contact closure to an inverter for the run signal would cause this issue in an hour. Please check the inverter's wiring diagram versus what is wired.

regards,
james
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Old December 11th, 2017, 01:21 PM   #5
GaryS
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I agree the post is confusing to me to but
can you go online with the PLC ?
can you force the outputs on ?
when they are forced on do the close ?
what dons the program request them to do, the rung controlling the output all true ?
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Old December 11th, 2017, 01:42 PM   #6
stu
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Yeah I can go online and toggle the outputs which they come on and off but I have a constant 24vdc through the plc output (like the relay output has welded together)

And after the plc output we are connected to a relay - which is the inverter run signal ?
I’m thinking we need a reverse diode or a suppression,but not sure
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Old December 11th, 2017, 01:48 PM   #7
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Quote:
Originally Posted by stu View Post
Yeah I can go online and toggle the outputs which they come on and off but I have a constant 24vdc through the plc output (like the relay output has welded together)
Which outputs are you connected to? Group 2 are solid state. (leakage)
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Old December 11th, 2017, 01:49 PM   #8
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Which outputs on the PLC are staying on? The relay outputs or the transistor outputs? Are all your DC coms for the transistor outputs wired correctly?
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Old December 11th, 2017, 01:53 PM   #9
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Now that's a thought I will check thanks
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Old December 11th, 2017, 02:01 PM   #10
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That particular model of Micrologix has FET outputs for points 2 through 7 (Group 2). This type of output requires a DC common signal be wired to the terminal marked "COM 2" in order to work properly.

I learned about this detail the hard way myself. I had some stuff wired to the first four outputs. Outputs 0 and 1 worked fine, but outputs 2 and 3 seemed to leak too much to let my little interposing relays drop out when the signal was off. I moved them to the relay outputs at points 10 and 11 to get the machine running. Then I went into the manual and figured out where I went wrong.

With relay outputs and many with solid state outputs you only need to supply VDC to the right terminals, but with the micrologix FET outputs you also need DC common wired to all the right places.
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Old December 11th, 2017, 02:13 PM   #11
stu
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Ok that’s interesting on the outputs we have 24vdc linked to the vac dc0, vacdc1, dc2, vacdc 3, vacdc 4 , vacdc5 ? Which has been wired up like this for a while ??
Is this right ??
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Old December 11th, 2017, 02:22 PM   #12
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So the Fet group 2 outputs need the dc common 0vdc ??? Not 24vdc
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Old December 11th, 2017, 02:34 PM   #13
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VDC2 needs 24VDC, and COM2 needs the common. Lots of people leave off the common and the outputs can do strange things, like "stick" on. Take a look and report back.
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Old December 11th, 2017, 02:36 PM   #14
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Ok thanks
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