Torsional Dynamics

Over the years, the ØDS Machinery Dynamics Group has gained extensive practical as well as theoretical knowledge of torsional vibrations.

• Generator Trains
• Turbomachinery
• VSD Machinery
• Reciprocating Machinery
• Thyristor Control
• Wind Turbines
• Efficiency Measurements
• Natural Frequencies
• Fatigue and Lifetime Estimation
• Electro-Mechanical Coupling
• Torsion-Lateral Coupling in Gears

Torsional Vibrations

Why Are Torsional Vibrations Important?
Excessive torsional vibrations of a rotor can lead to fatigue, gear damage, power fluctuations, etc., and must therefore be avoided.

Though the torsional natural frequencies are normally calculated during the design phase, they are rarely verified during testing, and torsion is very seldom monitored during operation.

Thus torsional vibrations can remain unnoticed through commissioning and even in operation, only to surface when damping or excitation is changed.

Furthermore, the torsional modes are system properties, and prediction requires input from several suppliers. This emphasizes the benefit of using ØDS as independent consultants who have considerable experience in combining numerical analysis and measurements.

Numerical Torsional Analysis
A full torsional analysis by ØDS will typically include calculation of:

• Undamped natural frequencies and mode shapes.
• Campbell diagram indicating
  resonances and separation margins.
• Damped harmonic response at resonance with e.g. VSD slip frequencies and vane passing frequencies.
• Torque and stress amplitudes as well as Goodman diagram indicating margin against fatigue.
• Transient response at start-up and short circuit.
  

ØDS has developed in-house software to
consider non-trivial effects such as:

• Electro-mechanical impedance coupling.
• Torsional-lateral coupling in gears.
• Backslash in gears.
• Nonlinear torsional stiffness.
  

Measurement of Torsional Vibrations
ØDS currently employs three different types of equipment for measuring torsional vibrations:

• Laser equipment based on the doppler principle.
• Encoder attached at a free shaft end.
• Strain gauges mounted on the shaft,
  signal transferring by telemetry.

The laser and the encoder measure the shaft angular velocity while the strain gauges measure the shaft shear stress, which is proportional to the shaft torque. ØDS has also used strain gauges for efficiency measurements of motors and pumps.

For all three methods, it is necessary to have an adequate understanding of the deflection shape of the rotor. The laser and the encoder must be used away from a torsional nodal point whereas the strain gauges will give the highest response close to the nodal point.

The techniques supplement each other, but are only suitable for the experienced user. The engineers of the ØDS Machinery Dynamics Group have this experience based on many successful measurements.

Case Story I : VSD System
High lateral vibrations were observed on the gearbox during a back-to-back test of a VSD system consisting of a synchronous motor controlled by a frequency converter. The nonsynchronous vibrations occurred at two operating speeds. In conjunction with the compressor manufacturer, ØDS determined that the problem was caused by resonance between the 1st torsional natural frequency and the slip frequency component 6| fL-fM|. Due to low bearing loads, a considerable coupling to lateral vibrations was present.

Case Story II: Excitation by Electrical Grid
The LS coupling shear pins of an offshore turbo-generator unit repeatedly failed due to fatigue. Torsional measurements showed periods of very high torsional vibrations at the 1st natural frequency. The system was found to be very lightly damped with some self-excitation.

ØDS designed a system for long term data acquisition in order to assess the torque variation with time. Subsequently, ØDS demonstrated that the torsional vibrations were excited through the electrical grid by the thyristor controls of two glycol heaters on the platform (see reverse side).