Catastrophic Failure of Variable Frequency Drive

Less than twelve months after commissioning, a 300 hp variable frequency drive unit suffered an catastrophic failure that halted production on a new, high-visibility nonwoven fabric line. The unit was one of thirteen rated from 150 hp to 300 hp, driving high pressure water pumps for a hydroentanglement system. First responders found the main controller board and several nearby components destroyed by an arcing fault on the 600 vdc bus circuit.

Investigating the damaged components, WHY traced the arc to a hidden point of connection between the DC bus and the main controller card. Although portions of the card were carbonized and copper circuit traces were melted or detached from the board, sufficient evidence remained to locate a crack in the connection to the bus itself. The design of this drive uses a conductor formed from heavy copper sheet to provide both electrical and mechanical connection. Although the drive cabinet had shock dampening feet, the vibrations transmitted from the pump room to the electrical mezzanine via a common wall eventually weakened the bus to the point of failure at one 90-degree bend.

Inspection of other drives found several with incipient metal failure as seen on the first unit. Since the card was secured by other mechanical connections, the electrical path from the main DC bus to the controller card was augmented with a heavy flexible wire on all drives. The issue and solution were reported to the drive manufacturer, and no further failures were experienced.

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