30 core armoured PVC control cable capacitance

[123MB]

Hi All

We have materials handling application with a run of 500m maximum out to a photoelectric sensor/photo eye which is being used as an underspeed sensor. The sensor will be wired from the field to a PLC input via 30 core SWA PVC/PVC control cable untwisted and unshielded.

The sensor will detect the rotation of the non-driven pulley of a conveyor and be used to detect a problem with the conveyor where the motor might be running but the conveyor isn't moving.

The pulse frequency we are expecting from the photo electric sensor is a maximum of 4Hz.

What I am concerned about is the capacitance between cores in this cable, and from core to armour, effectively slowing the rise time of our pulses and causing problems.

I am trying to find capacitance values for the 30 core SWA PVC/PVC cable to calculate rise times, etc, but manufacturer's dont publish this data for normal PVC/PVC control cables.

Also I am having a hard time understanding the mechanisms for determining the capacitance. For example:

1) I am happy that the capacitance from one core to others increases with the number of cores, but how to quantify this based on data for core to core capacitiance? Consider the capacitances in parallel?

2) Does the presence of armor make the capacitance worse?

The closest data I have found for a 30 core SWA PVC control cable has a capacitiance value of 380nF/m which is very very high and I am not sure it is accurate

Sorry for the long post guys I hope someone can help out and that the responses help others in future

Cheers

 

[itsmoked]

I think you are going to have problems with 500m when looking for a 'frequency'.

380nF/m is not unheard of.

I'd be looking for a line-driver solution like RS485 or something similar. That would lean more to a twisted shielded cable too. Alternatively look for a wireless solution, especially since loss of the signal would be a fail-safe situation anyway.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[123MB]

Thanks Keith

 

[djs]

See my post in the other forum

 

[7anoter4]

In my opinion 380 nF/m it could be 380 pF/m.
You can calculate a capacitance between two parallel conductors of 1.5 mm^2 insulated for 0.6/1 kV[no clearance] [dconductor=1.7 mm ,dins.core=3.1 mm] using general formula:
C=eps.r/36/ln(2*a/d)=59.15 pF/m a=3.1 mm d=1.7mm eps.r=6.9 [pvc].
According to NATIONAL WIRE & CABLE a multi-conductor cable
c=45-65 pF/ft [=131-213 pF/m] See:

http://www.nationalwire.com/pdf/cat07_design_guideV10.pdf

Multi-Conductor Cable Bundles
You could ,also, use the recommendations from:
THE CALCULATION OF ELECTRICAL CAPACITANCE by Yu. Ya. Iossel', E. S. Kochanov, and M. G. Strunskiy[1971].[-very sophisticate!].