Commissioning of new equipment always involves a certain amount of troubleshooting.  Sometimes you get to a point where you want to just throw up your hands and say-I don't know!  This particular startup was one of those times.

A centrifugal air blower with a 200 HP motor had temperature RTD's mounted at both the drive end and the opposite drive end.  They were firmly attached to special adaptors and screwed directly into the top of each end bell.  Upon initial startup, the overall vibrations were good-less than .21 in/sec2.  As the unit ran for the 1 hour run-in period, the vibration levels were verified with my hand held vibration meter and found to be fine.  Before the hour was up, the unit shut down on high motor vibration.  Since I knew the motor wasn't actually vibrating, we checked all wiring connections to be sure there were no loose, or broken wires.  All checked good.

We re-started and monitored the vibration display on the control panel.  The vibrations steadily increased, at both ends of the motor, to .58 in/sec2!!  According to my hand held, the levels were still less than .23 in/sec2.  We couldn't have 2 bad sensors, or 2 bad cables.  Another check of the wiring connections turned up nothing.  What could possibly be causing this?

I noticed the electricians had used butt connectors on the 2 wire shielded cable splices in the junction box near the machine.  The factory supplied sensor cables had a special screw on connection at the sensor, and were then spliced to the shielded cable that ran directly to the terminal strip in the control panel.  Thinking a dead short, or even electrical interference, could be the cause of this problem, I instructed the electricians to cover the butt connectors, and the bare shielded wires, with black electrical tape.  Maybe the single wires, or shields, were touching the inside of the SS panels. 

Upon re-start, the unit ran fine.  No more increase in vibrations! All levels were "normal". If the shielded wires were touching the SS panel, I would expect a possible dead short and resulting shutdown.  I don't know how it caused the vibration levels to steadily increase, and I'll probably never know why, but I sure am happy I did find a solution! 

Laser Shaft Alignment will save me how much?