The Variable Speed Drive That Went BANG Very Loudly In The Middle Of The Night
A Client recently sent us their Mitsubishi FR-A840 VSD for repairs. The 55Kw drive literally went Bang in the middle of the night which not only woke up the crows but was also felt through the large factory and well into the next premises.
No one knows the actual cause of the failure, but after analysis of the variable speed drive unit, we suspected the motor’s braking circuit shorted to ground due to a flooding situation. The resultant transients caused a catastrophic explosion in the drive unit that cooked two diode modules, a high-powered Thyristor module and numerous other discrete components. The damage to the busbar itself (as seen in the picture) resembled bacon overcooked in a frypan, and the moulded fiberglass insulating barriers now missing in action.
Due to the drive’s age, the diode and thyristor modules was discontinued many years ago and there was no replacement with similar specs that would physically fit. The moulded insulation barrier is also discontinued which is a vital part of the electrical integrity of the enclosure.
Through our network of well-established international suppliers, we finally managed to locate the NOS (New Old Stock) diodes and thyristors, and many of the other discrete components which had also failed. We also needed to design and machine a new insulator frame and new bus bar components. We then tested the unit on our VSD test bed which proved both the integrity and the functionality.
Our client was amazed that we were able to repair the drive as many of the discrete components were blown away and therefore could not be identified. With minimal schematics, we were able to rebuild the drive back to brand new condition. Our client’s only alternative was to wait 4 to 6 months for an updated replacement to arrive from overseas, and then modify his machine to suit at great cost.