External Relay causes PLC fault

triplewhammy

Member
Join Date
Jan 2007
Location
Chicago, IL
Posts
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Hello everyone, first timer here. I'm a senior electrical engineering student having a slight issue with our design project. An A-B MicroLogix 1000 PLC is automating various components of a water filtration system (everything operates on 24V DC). One of these components is a pump. To drive the pump, we have an output from the PLC going to an external relay. Once the relay contacts close, the pump is to kick on (we also have a fuse in series as well).

The problem is, when the logic calls for the PLC output to energize, there is a spark on the relay and it clicks on and off, causing a fault on the PLC. Apparently, the PLC is unable to close the external relay to turn on the pump. HOWEVER, if we manually connect the relay contacts (there is a pushbutton on the relay) the pump will kick on and then when we let go, the PLC will KEEP the relay on (as it should, until specific logic turns it off again). The PLC is having no problem keeping the contacts closed, nor is there a problem once it has to turn it OFF, but for some reason it can't close the contacts on its own...it causes a fault, clicks, "backfires", then tries again over and over.

Anyone have any idea why the PLC can't initially close the external relay contacts? (keep in mind it can keep them closed once manually closed).

Big thanks in advance!

Josh
 
Where are you getting your 24VDC from and what kind of relay is this.
Your DC power supply might have to be a little bigger.
 
Mickey said:
Where are you getting your 24VDC from and what kind of relay is this.
Your DC power supply might have to be a little bigger.

Here's the relay:
http://web2.automationdirect.com/static/specs/78relays.pdf

Here's the power supply:
http://www.trcelectronics.com/Meanwell/pdf/sp500.pdf

Keep in mind that the initial current "surge" would still be experience when I manually close the relay contacts. Therefore, the error is probably NOT caused by an inadequate power supply. Thanks!

-Josh
 
One thing that confuses me about this. I thought the Micrologix series used Relay Outputs? (at least, all the 1000, 1100, and 1200 I have do, maybe it's specific to the model).

Relay Outputs would isolate the PLC from this "feedback" from the external relay, wouldn't it?

What type of Fault is the Micrologix getting?
 
Tharon said:
One thing that confuses me about this. I thought the Micrologix series used Relay Outputs? (at least, all the 1000, 1100, and 1200 I have do, maybe it's specific to the model).

Relay Outputs would isolate the PLC from this "feedback" from the external relay, wouldn't it?

What type of Fault is the Micrologix getting?

Yes, it has relay outputs. However, it can't source enough current through it to drive the Pump directly. It IS being used to drive a solenoid valve and UV tube however. These have been isolated however, and are currently not in use while we figure out the pump's issue. For some reason, the PLC simply cannot close those relay contacts despite being able to KEEP them closed...

-Josh

P.S. Not sure about the "type" of fault. The PLC tries to power the relay, gets kicked back, flashes fault, then switches to run and tries again...repeats...
 
causing a fault on the PLC

What state is the MicroLogix in after the fault ? Is the program retained in the controller ? Is there a fault code visible to RSLogix 500 ?
 
triplewhammy said:
Yes, it has relay outputs. However, it can't source enough current through it to drive the Pump directly.

Sorry if this is off topic, since I'm not really answering, just asking.


But relay outputs don't source current. Your supply voltage sources the current. The contacts of the the internal relay only have to be able to handle the current. That's what confused me. I just can't see how feedback through a pair of isolated internal contacts could be faulting out the processor.

Btw, how big are these solenoids? I've run power to motor contactors and solenoids (24V and 120V, Air and Hydraulic) directly through the relay outputs of the 1100 and 1200. They handle that current just fine.
 
Can you give us the complete cat. no. of your ML1000 and a wiring diagram of how you wired the outputs.
 
Tharon said:
Sorry if this is off topic, since I'm not really answering, just asking.


But relay outputs don't source current. Your supply voltage sources the current. The contacts of the the internal relay only have to be able to handle the current. That's what confused me. I just can't see how feedback through a pair of isolated internal contacts could be faulting out the processor.

Btw, how big are these solenoids? I've run power to motor contactors and solenoids (24V and 120V, Air and Hydraulic) directly through the relay outputs of the 1100 and 1200. They handle that current just fine.

Correct. I suppose used the term "source" too loosely. I simply meant that the relay outputs can supply current (of course via the supply voltage). When we connect the pump directly to the output of the PLC it simply did not turn on. Thus we decided we needed to use a relay. The pump (24V) draws around 13.5A while running, no clue as to start-up draw.

I will provide some more insight via a drawing shortly. I appreciate everyones help and concern!

-Josh
 
Josh,

I have been an electrical engineer for 35+ years now. I suspect that your "relay" circuit is causing a high-current draw or a short circuit, so that your 24 volt power supply is being drained, stopping the Micrologix PLC on low voltage. Because the PLC is restarting, it probably is not "faulting" in the technical sense of the word. When a PLC "faults" it usually has to be reset before it will restart.

Please double-check your wiring. In circut wiring, simple errors cause the majority of problems.
 
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Lancie1 said:
Josh,

I have been an electrical engineer for 35+ years now. I suspect that your "relay" circuit is causing a short circuit, so that your 24 volt power supply is being drained, stoppoing the Micrologix PLC on low voltage.

Please double check your wiring. In circut wiring, simple errors cause the most problems.

Assuming it is wired correctly, what alternatives would we have. I do agree with you that is possibly the pump draining more current than we can supply for the initial "kick-on." Our current power supply is 24V, 20A DC. The ONLY thing that makes me think it is NOT the initial current drain, is that it would seem that manually switching the power supply would ALSO overdrain the power supply. Why would it be that manually closing the contacts doesn't short-out the supply?

-Josh

EDIT: I also realized/wanted to point out that we have a 15A fuse connected in series with the Pump, therefore if it was really draining a lot of current initially the fuse would go...
 
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Keep in mind that the initial current "surge" would still be experience when I manually close the relay contacts. Therefore, the error is probably NOT caused by an inadequate power supply. Thanks!

-Josh
When you use the manual start there is no coil current required or used so if the power supply is under rated this could be causing the problem. The pull in current is higher than the holding current on these relays so once you pull in the relay manually there is enough current to hold it.

When it starts this chatter it can well be inducing radiated and conducted noise throughout your system and whatever else you are powering off the same power supply. Use EMF suppression (diode) and check the total current consumption being used verses the power supply ratings.

 
Josh,

When a relay coil is energized, there is a large in-rush current into the coil, about 2 times the normal operating current. Add this to your Pump starting current, and it could be too large for your power supply. Try powering the relay from the built-in Micrologix internal 24 vdc supply, and the pump from an external 24 VDC supply.

Also, make sure that your relay circuit is not wired so that it repeatedly connects-disconnects in a cycle. How many contacts are you using from the relay? Are you using one contact for the + and one for - to the motor? Make sure + and - are not "crossed" at some point. If you are seeing a spark, you have got a problem, or will have shortly when the contacts burn up. You can put two contacts in parallel to spread the load and lessen the wear on each contact. Not all relays are rated for motor-starting duty.

So, if the current is less than 15 Amps, what is it really? Maybe 10, or 12, or 13? Do you have an ammeter to measure the current? A common relay contact rating is 10 Amps.
 
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