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Control Cable capacitance

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#1 cherry


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Posted 10 May 2005 - 07:58 PM

We have very long lengths of control cable ( 120VAC circuit ) from field Start-stop switch to motor starters in MCC ( in Substation )..around 100-140m.
Does anybody have any calculation to find the limiting length of cable ? I have heard that the cable capacitance prevents the contactor coil from dropping out when the Stop PB is pressed. Is that true ? ( The cable is charged and the voltage developed is sufficient to keep the coil holding ) Our control cables are all sized at #12 AWG. If somebody requires any coil data, please let me know.

#2 marke


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Posted 11 May 2005 - 06:53 PM

Hello cherry

Welcome to the forum.

If the cable run is long enough, you can have problems with the capacitance of the circuit providing a return path on an AC circuit.

The factors that are important are the capacitance of the circuit, (at 100M I would not expect a problem) and the pull in / dropout voltage and current of the switched device. The lower the controlled current, the lower the capacitance that will cause a problem. If you are controlling a high impedance electronic input to a PLC or a drive, you will have a low current, 5 - 10 mA or less and the cpacitive effect can be a major issue.
As you are switching a contactor which will draw considerably more current, the problem will be much less. One solution when this type of problem occurs, is to shunt the switched circuit (ie PLC input) with a low impedance so that real curent must be drawn.

Usually, this problem arrises with much greater lengths of cable than you are referring to. I have experienced this with small signal relays and electronic input where the cable length is Kilometers.

Best regards,

#3 cherry


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Posted 17 May 2005 - 05:51 PM

Thanks Mark for ur valuable inputs. I had seen a sample calculation published by Siemens a long time back, on this topic. If I get my hands on it , I will surely post it.

#4 Guest__*

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Posted 24 June 2005 - 06:00 PM

Hi I have had this type of problem before, I overcame it by increasing the control voltage and upgrading the contactor coils to the increased voltage....

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