I have an installation where the motor location is class 1 div 2, and the VFD is in a nearby unclassified building. Their is cable tray from the vfd building to the building where the motor is, then the existing cable leaves the tray into a rigid conduit down a wall and goes under the slab for 20 feet before coming out to attach to the motor. The existing cable is just tray cable and not VFD shielded cable, thus the problem. They are getting interference on their Modbus system when the motor is started. I plan to replace the existing VFD, install a line reactor, and replace the tray cable with VFD cable. The problem I have is that the existing conduit from the tray to the motor is only 2", and the cable size needs to be 4/0, which, by the 53% fill is not allowable. Should I install a junction box after the cable leaves the tray and splice onto the existing cable in the conduit? How would I terminate the grounds and the shield at that point, bond to the enclosure and conduit? Removing the slab to install a larger conduit is not and option. Any recommendations?
splicing vfd cable
- Thread starter morebren
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
There is no guarantee the expensive fix you propose will resolve the problem being described.
It might be more cost effective and effective to isolate where the noise is being injected into the Modbus system and deal with it on that end. It is very hard to give you any advice on even where to start though given the total lack of useful information you have provided about the installation.
It might be something as simple and inexpensive as rerouting the Modbus cabling, or installing some RFI filters on the Modbus signal lines will resolve the problem.
Ok, I may have included to much information that is not pertinent to the issue. The customers engineers have explored the modbus problems and I am following their recommendation of replacing the equipment and cable. (The VFD needs to be replaced regardless) The question is if splicing the VFD cable to the existing cable where it leaves the cable tray will create any issues with the motor.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I am not sure that you gain anything by splicing in a short length of VFD cable in lieu of the existing tray cable, but it seems unlikely that it will make it any worse.
JoeStillman
Senior Member
- Location
- West Chester, PA
Wht stop at the junction box? Running it all the way will make a better improvement than only running partway. I would run VFD cable all the way to the motor.
I wondered that too.
It isn't a problem I encounter - the normal method of installation here is steel wire amoured as a continuous run from drive to motor.
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