DickDV
Member
Recently I was called into a plant as a consultant on a machine that was experiencing a large number of drive failures. All eight failures were on 250hp normal duty PowerFlex400 Allen-Bradley drives. The system was powered by a 460V floating delta power source.
I was advised by the Allen-Bradley techs on-site that their standard procedure on floating delta power was to disconnect and remove the input MOV's on the drive input terminals. Apparently, if they do not do that, under imbalance-to-ground conditions, one or two of the MOV's will heat up and destroy itself. (The MOV's must be rated at lower than 460VAC)
I also found that these eight failed drives had seen an intermittent ground on the motor leads which had destroyed most often an input SCR but occasionally an output IGBT.
In none of the eight drives were any faults logged or any apparent attempt made to protect the drive's power circuit components.
I have two questions: first, is it really standard AB practice to remove the input MOV's on floating or unbalanced power? Second, on floating power are the output ground fault and short circuit protections disabled or inoperative?
I was advised by the Allen-Bradley techs on-site that their standard procedure on floating delta power was to disconnect and remove the input MOV's on the drive input terminals. Apparently, if they do not do that, under imbalance-to-ground conditions, one or two of the MOV's will heat up and destroy itself. (The MOV's must be rated at lower than 460VAC)
I also found that these eight failed drives had seen an intermittent ground on the motor leads which had destroyed most often an input SCR but occasionally an output IGBT.
In none of the eight drives were any faults logged or any apparent attempt made to protect the drive's power circuit components.
I have two questions: first, is it really standard AB practice to remove the input MOV's on floating or unbalanced power? Second, on floating power are the output ground fault and short circuit protections disabled or inoperative?