Electrical Apparatus magazine, January 2003 Torsional Vibration & Oscillation in Electric Motors

Specifiers, suppliers, and operators are all familiar with the problem of rotating machine vibration.. Causes such as unbalance and misalignment are well known.

However, most vibration technology deals with linear vibration--movement of a back-and-forth nature. Torsional vibration (the twisting and untwisting of shafts) is less well understood. Measurement methods are more complex. Limits are not standardized. Yet uncontrolled torsional vibration can quickly cause catastrophic equipment failure.

Just as with liner vibration and the so-called "critical speed" problem, a concern in rotating systems is torsional resonance--a coincidence between some exciting force and a natural response frequency. Such resonance can greatly amplify an otherwise harmless vibration.

Several vibration sources exist. During acceleration, induction motors exhibit torque oscillations at line frequency. Synchronous motors, starting as induction machines, behave similarly, except that the non-uniform rotor magnetic field exaggerates the effect. Oscillations may far exceed the full-load torque.

A second source of trouble is the driven machine itself. Reciprocating compressors, log chippers, and some grinding mills and crusher generate steady-state torque pulsations at various frequencies--and of destructive magnitudes even without resonance.

Finally, some electronic inverters generate oscillating motor torques at frequencies dependent upon speed.

As with other forms of resonance, destructive forces are minimized by changing the rotating system response frequency; by adding energy-absorbing damping; or both. Although inertia changes are sometimes possible, the more usual choice is an elastomeric coupling between motor and driven machine. That changes system stiffness as well as adding damping.

Torsional vibration is evaluated by measuring the variation in angular twist in a shaft, in magnitude and frequency. One method uses either shaft-mounted strain gages or encoder/resolver devices. Another involves scanning with a strobe light, to show the varying position of a fixed point on the shaft. Several variations of that method are possible.

Whatever the method, accurate results are difficult to obtain, and require careful analysis of the results, whereas linear vibration readings are easily taken and easily interpreted.

Industry news...

Featured Technical Article
January 2003

On this page is a summary of the Electrical Apparatus January 2003 featured technical article, by Richard L. Nailen, P.E. , "Torsional Vibration & Oscillation in Electric Motors" Dreh- und Torsionsschwingungen in Elektromotoren ... Vibrations et oscillations de couple dans les moteurs électriques ... Vibración y Oscilación Torsional en los Motores Eléctricos ..

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