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Wiring Motor ON/OFF status to PLC

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nogienugz

nogienugz

Electrical
Joined
Oct 6, 2007
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CA
Got a question for you guys.

Whenever we want a PLC ON/OFF status for the motor, we would run cables, provide a NO contacts to the PLC in series with the motor starter coil (see attached VACULOADER SCHEM 2.pdf). Now our engineer wanted to use EATON'S electronic overload relay (C441) and wanted to discard that NO contact. I looked at the C441 manual and it does not say how it sends a signal to the PLC whether the motor is ON or OFF.

On the MCC side, of course you'll know if the motor is running or not as the C441 display is mounted on the MCC panel, displaying the power and current measurements. But how does it send the ON/OFF status signal to our PLC and thus to the HMI?


 
 https://files.engineering.com/getfile.aspx?folder=60ea857e-7795-472e-b1fd-1f846ef8f691&file=VACULOADER_schematic.pdf
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Unless I misunderstand your question, it seems you have already identified and hilighted the solution- "NC 95/96 contact is open when the device is unenergized".

Brad

It's all okay as long as it's okay.
 
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Notice the 95-96 C441 Fault Relay contacts?
Notice that they are in a dotted box? The dotted box means it's physically part of the also dotted box of the C441 up above. Hence that 95/96 contact is available at the physical C441 relay.

Now, notice that the dotted box stuff has a specific dot-dash pattern? That means it's all in the same physical unit. It appears to me that there is also a set of ON/OFF contacts in the C441. They series these two sets of contacts so the PLC knows the motor is running or not, regardless of the cause. Separate use of those contacts to the PLC would've shown the motor ON/OFF AND if there was a critical overload trip.


If you want further give us a link to the C441 manual.

Keith Cress
kcress -
 
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Hi thebard3,

The NC 95/96 is the OL for the C441 relay. Whe we use it as a protection for the EOL itself or as the OL for the M coil. (I actually prefer using another OL for the M coil just in case one of the wire loosened up). But how does the c441 relay communicates the motor ON/OFF status to the PLC? through the comm. port (C441 module)? So whenever we used this EOL's then no need to do a hard wire from the MCC to the PLC, no more providing NO to the PLC?
 
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Hi itsmoked,

The C441 EOL and its communication module are in the attachment that I uploaded (files combined in one PDF file). It also includes the vaculoader vendor drawing showing that they need ON/OFF status from the PLC.
 
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Hi itsmoked,

Just to comment on this:

They series these two sets of contacts so the PLC knows the motor is running or not, regardless of the cause. Separate use of those contacts to the PLC would've shown the motor ON/OFF AND if there was a critical overload trip.

That's what we usually do. Now, our engineer wanted to take that NO contacts hard wired to the PLC and just jumper 2A and 2B. So my question is, how does the C441 provides the signal ON/OFF to the PLC without that NO contacts hired wired to it?

 
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Sorry I didn't see that you included the manual in that same PDF. Nice trick there. :)

Note this is all over in the manual:
unopened_in_unener_e4ub6n.png


From the schematic the C441 is not energized if the motor isn't being called for. Hence the 95/96 contacts are open if there is a fault OR if the motor is not being called for so there is no need for another set of contacts that also 'show the motor is ON' as the two contacts would be redundant.

Keith Cress
kcress -
 
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Hi itsmoked,

Yep, it'll trip the EOL in the event of a fault but you're gonna see that when you're in front of the panel display (of the EOL), in front of the MCC. To an operator looking at the HMI in the control room how is he gonna see that? The HMI receives its signal from the PLC so, how does the EOL sends the motor ON/OFF status to the PLC without the hard wire NO?
 
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I do this all the time now. The EOL is connected to the PLC via a communications port of some kind. From that, your PLC looks at the motor current. No current flow, motor is Off. ANY current flow, motor is On. It's actually better, because with the aux contact, technically all you know is that the contactor closed, not that the motor is actually running. The breaker could have tripped (depending on how you do your control power) or, and this happens more than you would think, someone opened a local disconnect switch at the motor and forgot to re-close it. By using actual current flow, it doesn't matter why the current is not flowing, it still means the motor is not running, which is what you wanted to know.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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Let me back up.. This schematic provided is a new scheme for motor protection and you want to know how to get a motor ON\OFF status out of it and INTO your PLC system?

If yes, this fancy protection relay and its expensive communication module are connected to the PLC right?

If it is, would not the PLC talking MODBUS not just query the C441 via the C441R about the motor run status? In which case why would extra wires and contacts be needed?

What else am I missing?

Edit Cross post with Jeff.. and I love his logic.

Keith Cress
kcress -
 
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When a PLC is employed to control a motor, it is best practice to NOT hard-wire components into the control circuit, rather to bring all status conditions to the PLC, write logic to evaluate the conditions, and use that to turn the motor on or off. If I were designing this system to control a motor, I would use the ethernet communication option to bring the running status, overload/fault status and whatever other conditions or parameters are available into the PLC. If the ethernet protocol is not an option for your installation, then I would use the fault contact as an input to disable/ interlock the motor output and to activate an alarm condition. This doesn't really give you a running status, though. I assume you can install a holding contact on the starter itself if you need that. Then as an addition safeguard, instead of using the fault contact as shown to power the output module, I would connect that power source through a MCR or E-STOP relay.

Brad

It's all okay as long as it's okay.
 
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Hi thebard3,

Got that. That's probably the reason why we hardwired contacts in another project. But in this, we used AB E300 EOL w/o comm. ports. This one has a local disconnect switch (see attachment).
 
 https://files.engineering.com/getfile.aspx?folder=960da879-846f-4565-85a9-725ebf5e6c33&file=2.pdf
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Well, just like with anything else, there's not a single right or wrong way to do something. This is kind of a mix of relay and PLC logic, and it's not how I would do it but it's only a matter of preference. I prefer to have all conditions (disconnect status, limit switches, etc) as PLC inputs, then use that in the logic to power (or disable) outputs accordingly. That said, do not the RED UP/DN and GREEN STOP pilot lights shown on the bottom of sheet 1 provide the information (running state) you want to present the operator (or rather, a contact from relay UP and another from relay DN as PLC input)?

Brad

It's all okay as long as it's okay.
 
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The one that I just showed was for a previous project (the one that did not use E300 communication ports, I don't know why). We wired NO contacts to the E300, one for IN2 and one for IN3, so the PLC knows if the loading spout is moving UP or moving DOWN. In the new project that I'm working on, the lights are wired to the MCC TB. Those lights are there just to show that the motor is running or not when you're in front of the MCC (see attached).
 
 https://files.engineering.com/getfile.aspx?folder=205359e4-9b9f-4619-a59f-6ed193c74ab0&file=3.pdf
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I think--
itsmoked said:
From the schematic the C441 is not energized if the motor isn't being called for. Hence the 95/96 contacts are open if there is a fault OR if the motor is not being called for so there is no need for another set of contacts that also 'show the motor is ON' as the two contacts would be redundant.
Click to expand...
--is the solution that would work. If you are not commanding the motor to stop, the contact will be off. If you are commanding the motor to run, the contact will be on unless it is faulted, so it pretty much acts just like a holding contact.

Brad

It's all okay as long as it's okay.
 
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I'm pretty sure that the specs requires that all motors must be start/stop via the PLC. Totally agree with you on 95/96 contact as the EOL can communicate the OL status via the com port.
 
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