“Prevent Defense”- Variable Frequency Drive Failures That Are Easily Preventable

By Joe Sarver

The car analogyDamaged IGBT for the story that follows would go something like this: The oil pan drain plug becomes loose and enough oil is lost to greatly increase friction among the moving parts. The engine overheats and the result is engine failure.

Pictured here is a damaged IGBT (insulated gated bipolar transistor) array, the very heart of a drive’s inverter. Consistent with the car analogy above, a simple loose screw results in a critical failure.

The screw terminal on the upper right began to overheat from the resistive, loose connection for the right-most IGBT. The middle IGBT next to it also began to over heat via conduction from the resistive connection on the right and the combined effect of high current since the current goes high on the remaining two legs with the good connections. The net effect is that the IGBT in the middle fails first due to excessive heat and current. The left IGBT remains functional since it’s physically located a bit further from the initial source of the heat – the loose connection and, the right IGBT doesn’t fail since it is effectively not connected to power because of a poor connection. The damage is identified in the photo as a perforated female screw terminal on the upper right, and the darkened and overheated portion of the internal component IGBT located in the interior middle of the enclosure.

Every drive manufacturer has precise torquing requirements for IGBTs. In this case, had this drive been properly PM’d, this damage could have been completely avoided. Among other things, checking and tightening mechanical connections to concise factory requirements is a routine part of a thorough VFD PM. This failure can be repaired at a cost of $1200.00 and a minimum of one week of downtime. A simple PM on this drive would have avoided this failure. What is the cost of a PM? – about $135.00 per drive. The question is not “Can you afford to do PM?” but moreover, “Can you afford not to do a PM?”

Preventive maintenance is a “prevent defense”. It’s the best protection of the investment and most efficient method of insuring dependability of equipment that you can get.

Delta Automation develops effective preventive maintenance programs for any drive application and/or system of drives. There are five key elements to an effective drive PM program:    1)Site survey/audit       2) Periodic inspections                3) Repairs if indicated 4) Maintenance schedule determination      5) Composition of drive specific and site specific, PM kits.

The Delta field service team is standing by to assist you with devising and implementing a successful cost effective PM solution thereby eliminating potential problems before they happen.

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About deltaautomation

President of Delta Automation, Inc. Working in industrial automation in excess of 30 years
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