Control Cable Capacitance on Conveyor 1

[markjp]

I was wondering if anyone may have had some experience with capacitance problems with long control cable runs and PLC inputs. We are working on a project to extend a conveyor from 530m to 1300m. The present control voltage is 110VAC which is used for signalling pull-wire, belt drift,etc. status from the tail end to the head end of the conveyor using 2.5mm2 control cable. I understand that there may be potential problems with cable capacitance, i.e. relays not dropping out. I have come across an article from Klockner Moeller which explains the maximum allowable cable capacitance related to the VA hold-in rating of the relay to determine if the relay will drop out after a contact has been opened. Based on the calculation I think I have found a suitable relay with large enough VA rating to avoid capacitance problems and low enough VA to avoid voltage drop problems. However I can't find any info as to whether there may be capacitance problems also associated with PLC inputs. PLC being used is Allen-Bradley MicroLogix 1200 with 110VAC inputs. Would appreciate if anyone could shed any light on this. Would rather not change to DC control power if we can help it.

Many Thanks
MarkJP

 

[dpc]

I'd change to dc. Even if the ac works 99% of the time, it will still be a pain in the neck. The ac capacitive reactance issue is very sensitive to moisture in the conduit and can vary quite a lot. I would not trust a calculation unless you have a safety factor of about 2.

PLC inputs are worset han relays in some ways because it doesn't take much voltage to keep them up. 1300 meters is definitely pushing the limit, in my experience.

With dc, you KNOW it will work and you'll sleep better.

 

[itsmoked]

I'm there too.

Chant with me, "DC, DC, DC!"

If your PLC needs AC input just use a small local relay.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[slavag]

Agree with Dave and Keith.
We have experience with 48V DC and cables about 1500-1800meters 1.5mm^2 and not screened( isn't so good, but...).
Regards.
Slava

 

[markjp]

Appreciate the feedback guys. I'm thinking probably go with 110VDC, that way hopefully less voltage drop issues than say 24VDC.

 

[itsmoked]

Certainly though a bit more dangerous.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

I find this post to be very helpful due to the fact that I have run into similar problems recently. I think I understand where this capacitance comes from and why is causes problems but I was hoping to get a theoretical explanation.

Is this capacitance due to the long wire run causing a large capacitance to ground? When control power is removed relays and inputs may not drop out immediately due to the fact that the wire created capacitor has to discharge? Am I on the right track here?

 

[itsmoked]

Nope not to ground but to the return.

Note the picture I drew below. See that capacitor? If it gets large enough then the switch, way at the end becomes superfluous. It doesn't interrupt anything.

As the wire run gets longer that capacitor get bigger.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

I work in the material handling industry. I have never had problems with 120vac capacitance problems.

If the drawing above is talking about the contactor in the field, then that might be a problem but its standard with us to have all contactors, vfd, etc in main panel. 480vac runs to where ever the motors are.

I have not had problems with photo eyes, estop pull cords where 120vac was used.

All field control wiring is 14 awg, maybe that is the issue. All power wiring is 12 awg or bigger.

Now I have had problems with 24vdc dropping to an unacceptable level in the field where the devices might not work, but this was resolved with power supplies mounted in the field or a bigger power supply. Most 24vdc wiring is used for high speed applications but this is resolved by making sure the io cards can handle the pulse trains coming back with the high speed cards in the local rack if ab or on a high speed profibus link if siemens.

I have found where io does not respond is usually the problem with the rpi if ab or baud rate if profibus on the card or its response time.

 

[dpc]

controlsdude,

It's a real problem on long runs. The size of the conductor is not a major factor in this scenario. I've experienced it.

If you search older threads, you'll find a link to a data sheet from Square D or A-B that gives recommended maximum lengths for just this scenario. It is much more limiting than the voltage drop issue.

Moisture in and around the cable is a major variable.

 

[controlsdude]

again this all goes back to putting remote panels where wiring runs are kept to a minumum to devices and power and comm cables are brought out to this panel.

I do not see this because this is what we do to minimize control wire runs.

 

[itsmoked]

controlsdude; My schematic shows the powersupply on the left. Mounted somewhere near the COIL for the safety relay. And a normally closed push button out in the field.(not a remote contactor)

The capacitance is the per-foot-capacitance of the cable used to hook up the E-Stops.

If you use a bonafide cable you are likely to have a higher per foot capacitance in your safety line than if you were to just use wires dragged thru a conduit. If your two wires were somehow held 3 feet apart you'd have even less capacitance. You might be able to run the system 10x further. You would start incurring other problems doing that however, as now a lightning bolt would couple even more energy in thru the resulting loop.

So depending on your specific wire, wiring practice, distance, and relay you will have varying problems depending.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

I have worked on conveyors as long as 1-1/2 km with 110 AC controls without a problem. Relay logic though, I'm sure DC is better for PLC input, what about leakage between wires.
Perhaps it would be better to monitor the current in loop rather than Voltage in this case, what do you think?
Roy

 

[itsmoked]

No. Because the current is the issue. Current caused by the circuit capacitance.

If you ditch AC then the whole capacitance issue goes away.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

The more burden on the circuit the less impact the capacitance will have. Relay coils can deal with far more capacitance than high impedance PLC inputs.

 

[roydm]

I dont't see how a 24VDC circuit into a discrete input is any better than 120VAC holding in a relay.
The input impedance of the DC input is so high any leakage at all (water in a switch for example) will hold it ON.

I was thinking something like this.
have a fixed DC current passing throught the E-stop circut
e.g. 16 mA and monitor that with an analog input or current sensitive relay. If it changed +/- 1 mA trip the conveyor.

I know E-stops are normally hard wired but for a long conveyor you need an interposing relay anyway.

With a long conveyor you might have 10 - 20 switches in series so you need some means to identify which switch has been pulled. Here again a mA signal might be usfull.
Any Ideas?

 

[dpc]

Roy,

The problem with long ac runs (depending on the configuration of the circuit) is the shunt capacitance and the resulting capacitive reactance between the conductors. With a dc circuit, the capacitive reactance is infinite because the frequency is zero.

Even a 24 vdc circuit can be run a long way if it is sized appropriately.