Electrical Troubleshooting Quiz

Dec 21, 2009 3:52 PM

One of the production machines cuts, bends, and stamps steel. A 50-hp motor is at the heart of this machine. Not long ago, product quality problems were traced back to motor vibration. Tightening the mounting bolts didn't stop the vibration, so the motor was replaced. Within a few days, however, the new one started vibrating as well.

With only this information to go on, can you determine what steps you should take to troubleshoot this motor vibration problem?

The most likely cause of the motor vibration is the tightening of those mounting bolts. Tighten motor mounting bolts either to the spec provided by the motor manufacturer or (in its absence) enough to flatten the lockwashers. Going tighter doesn't mount the motor more securely or reduce vibration, but it can warp the motor feet, causing alignment issues and vibration.

Thus, you need to replace the motor mounting bolts and lockwashers. When you do so, tighten the bolts in a manner that draws the motor feet evenly to the motor pedestal, rather than tightening one bolt at a time.

But first:

Check the motor base and pedestal for warping or cracks.
Check the motor feet. If they're warped, ask the motor manufacturer what to do about it. You may ruin them if you pound or twist them straight.
Check the load before connecting the motor to it.
Laser-align this motor with its load. You may need to add shims under all of the feet before you get it right.

It really is vital to check the load. Why? It can have its own problems, such as loose mounting. Or, a gearbox that's low on oil will transmit "metal-strikes-metal" vibration back to the motor.

If vibration is perceived at the motor instead of at its source, the irony is that someone attempting to "fix" the vibration by tightening the motor feet will nearly always cause the motor to exhibit vibration it didn't have before.

Once the motor is properly aligned and mounted, put a vibration monitor on it and start it up.