Torsional oscillations in drive systems

Torsional oscillations are generated in a transmission if pulses in the torque are amplified by resonance with one of the eigenfrequencies of the driveline. If insufficiently damped large torque variations may develop and overload and eventually damage the gear and transmission. Such torsional resonances are not very common on cement machinery, but quite serious when they happen. It is practically never a problem by fixed speed drives except cases with very little damping (for example by synchronous motors). It happens more often by variable speed drives, most frequent and serious by kilns driven by two variable speed motors. The oscillations by variable speed drives can be excited by electrical pulses to the motor for example from cascade controlled converters. But quite often the oscillations appear as a mechanical problem because they are mechanical initiated or excited. The avoidance or prevention of such torsional oscillations however can usually only be solved by electrical damping. It is therefore important that the electrical engineers designing and servicing the drives understand the nature of the oscillation problem. To prevent torsional oscillations, It has become common to request a torsional analysis done for all large drives, even drives with fixed speed. It sounds like a good idea, but practice often show an alarming difference between measured and calculated values. The following article describes in the first part torsional oscillations in general, and it explains what limits the accuracy and thus the usefulness of torsional analysis. The second part describes how to dampen or avoid torsional oscillations by variable speed drives and by dual kiln drives particularly.