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Summary:
"Torsional Vibration & Oscillation in Electric Motors"
Looking
at the codes and standards governing electrical systems.
With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
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.
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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