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Question for those of you who have dealt with material handling and conveyor systems using VFDs:
Do conveyor systems typically use contactors (either before or after the VFD) to allow isolating the motors and meeting NFPA 79 requirements for Category 0 or Category 1 stops? We have a newly installed conveyor system in our plant with 120 VFDs. Most of the VFDs interface to the E-Stop circuit only through their external run inputs, and the present installation is such that if a drive is being run from the VFD keypad, the conveyor E-Stop will not disable the drive and stop the motor. (As an aside, the electrical contractor wired all the drives as 2-wire
start/stop. Is 3-wire better or worse from a safety standpoint?)
Failure of the E-Stop to interlock all modes is a cause of some concern to us <g>. The contractor's response, however, is that use of the drive keypads (which are locked in the electrical panel) is negligence on the part of whomever may be so bold as to do so, and that NFPA 79 is not a
standard normally applied to industrial conveyor systems in any case. I'd appreciate any input from the list on your usual practice with conveyor
controls and conveyor safety systems.
Thanks... Paul T
Do conveyor systems typically use contactors (either before or after the VFD) to allow isolating the motors and meeting NFPA 79 requirements for Category 0 or Category 1 stops? We have a newly installed conveyor system in our plant with 120 VFDs. Most of the VFDs interface to the E-Stop circuit only through their external run inputs, and the present installation is such that if a drive is being run from the VFD keypad, the conveyor E-Stop will not disable the drive and stop the motor. (As an aside, the electrical contractor wired all the drives as 2-wire
start/stop. Is 3-wire better or worse from a safety standpoint?)
Failure of the E-Stop to interlock all modes is a cause of some concern to us <g>. The contractor's response, however, is that use of the drive keypads (which are locked in the electrical panel) is negligence on the part of whomever may be so bold as to do so, and that NFPA 79 is not a
standard normally applied to industrial conveyor systems in any case. I'd appreciate any input from the list on your usual practice with conveyor
controls and conveyor safety systems.
Thanks... Paul T