Electrical Apparatus May 2014

Electrical Apparatus May 2014

This is a summary of the Electrical Apparatus May 2014 featured technical article,  by Richard L. Nailen, P.E.    

In any electric motor, the shaft will sag under its own weight. This bending mode is opposed by shaft stiffness. The resulting spring-mass system tends to vibrate laterally at a natural frequency, which is often described as the critical speed, or “lateral resonance.” When actual shaft speed coincides with this frequency, vibration can become damaging.

Resonance can also be present in a torsional mode, involving rapidly alternating angular deflection or twist in the shaft. Resonant frequency is determined by shaft stiffness and the inertia of the rotating masses. Although calculating torsional resonant frequency is not difficult, care is needed in expressing the inertias and shaft properties in consistent units.

Torsional resonance may be excited by either torsional impacts or torque excursions occurring regularly throughout each revolution, or by torque oscillations as a motor accelerates. Typical examples of the first condition are hammermill and log chipper drives. The shafting acts as a torsion spring with a characteristic natural frequency. If this frequency is too close to that of the periodic impacts, vibration can become destructive. A common solution is to use a resilient coupling to provide vibration damping. Reciprocating compressors, producing torque oscillations or reversals during each revolution, depending upon the number of cylinders, can also present torsional vibration problems.

Induction motors tend to develop high-frequency torque oscillations during acceleration, and these oscillations have been known to cause machinery damage when amplified by resonance. Large synchronous machines, which accelerate as induction motors, also exhibit this phenomenon.

The advent of adjustable-speed drives has added new concerns. When a drive may operate over a wide range of speed, adequate separation between exciting and resonant frequencies becomes more difficult to achieve.

To order a back issue with the full article, ”  Dealing With Torsional Vibration in Motor Shafts, May 2014″ call 312-321-9440 or visit our online webstore.

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