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Electrical Troubleshooting Quiz

Sep 8, 2009 12:00 PM

A critical 75-hp motor has failed three times in as many years. Each time has been preceded by a loud rattling and excessive shaking that lasted a few minutes. After the second time, you switched to a different manufacturer. This third failure has you on the hot seat. Should you try a third manufacturer, or should you do something else?

Sometimes, changing the manufacturer will coincide with an ending of the motor failure problem — but not because you had a "bad motor" from the first one. Instead of switching, ask the manufacturer for technical support. In this case, that may have prevented two motor failures.

The loud rattling and excessive shaking are good clues, because these are the signs you hear and see when a bearing spins. The rattling is due to metal parts knocking together, and the shaking happens because the spinning mass is no longer centered. The list of bearing failure causes is long. To shorten your particular list, have a qualified motor repair tech examine the bearings.

Improper lubrication commonly causes this type of failure. A trained tech can identify this from the patterns of galling marks and wear. A lubrication problem may be due to a lack of grease, but it could be due to excess grease. When there's too much grease, the lubricant base overheats and liquefies. It then runs out of the bearing. When investigating lubrication issues, search for the mixing of incompatible greases. For example, mixing lithium complex and polyurea ensures bearing destruction.

Current flow through the bearings is another likely suspect. It may be evidenced by pitting, discoloration, or other indicators. Such a condition usually exists on a motor that's "grounded" with a locally driven rod. A visual inspection may indicate this motor and related equipment are properly bonded, but the mere presence of bonding jumpers doesn’t tell you anything about the integrity of the bonding path. Use a high impedance tester to check the relevant bonding paths all the way back to the panel.

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