Bad vibes – solving a problem for research and industry

An invention from IPP for damping vibrational disturbance / universally applicable in driving machines

November 06, 2003

An inexpensive method of damping harmful torsional vibration in rotating drive systems has been developed at Max Planck Institute of Plasma Physics (IPP) in Garching and submitted for patent. The method was devised in the context of fusion research but is universally applicable – wherever machines in energy technology and process engineering are subjected to pulse-like loads causing destructive torsional vibration in the generator shaft or drive shaft.

One of the flywheel generators at IPP: on the left the big flywheel; under the casing on the right the generator. The arrow shows where the sensor measures the torsional vibration on the axis.

It all began when a routine check of the flywheel generators powering the plasma experiments at IPP revealed damage to the rotary axis. For Germany’s largest fusion experiment, ASDEX Upgrade, the generators supply powers of one to three hundred megawatts in pulses lasting about ten seconds. At first it could not be explained why traces of wear were observed on the coupling between a 76-ton flywheel and the generator driven by it. A thorough investigation revealed the cause: fast variation of the load on the generator from which IPP scientists extract different high electric energies in rapid succession during an experiment can react on the complete system as an excitation and cause the axis of the entire system to vibrate. The rotor of the generator then vibrates against the big flywheel and thus twists the steel rotary axis. This was measured with a special sensor operating without contact. Although only minimal twisting by hundredths to tenths of a degree was found, the resulting loads on the axis are very high. This alternating loading incurs damage to the couplings which in the long term can lead to destruction of the shaft.

Once the generator was repaired, the scientists resorted for the time being to switching off fusion experiments whenever the measuring equipment indicated excessive vibration. Since, however, this resulted in too many plasma discharges being prematurely terminated, it was decided to develop a method of actively combating vibration: controlled by elaborate electronics, a magnet coil in counter-phase with the measured angular velocity of the vibration provides the system with just enough energy to ensure optimum damping of the vibration. This does not require a lot of power: Whereas the generator supplies over a hundred megawatts of electric power, about one megawatt is sufficient to damp the vibration. The two damping circuits used on the flywheel generators at IPP have meanwhile given successful performance in more than a thousand plasma discharges.

The new method was submitted for patent through Garching Innovation GmbH. The inventor, Dr.-Ing. Christof Sihler, is confident that it will attract interest and find licensees: “The method developed by us needs little power, is demonstrably reliable, and can be used not only on large generators but also for damping torsional vibration in rotating drive shafts.”

The method would thus be universally suitable for suppressing torsional vibration in rotating drives, e.g. turbogenerators in power plants, industrial machinery, wind power plants, marine diesel engines, and pumps. Torsional vibration is, admittedly, rare in uniformly operating drives; but if it does in fact occur, e.g. when machines are started up or when they are triggered by dynamic control or loading, the ensuing damage can be severe and incur appreciable expense for repairs. The precaution of switching off machines to avoid vibration can also entail expense. “Whenever switching operations or dynamic processes in energy technology, process engineering or plasma physics involve pulse-like loads in energy production or driving machinery, thus causing vibration in the generator shaft or drive shaft”, explains Dr. Sihler, “we can provide an effective method of damping it. Its advantage: neither the process nor the generator nor the driving machine has to be modified; the necessary components can subsequently be simply and quickly installed and are also inexpensive since little power is needed.”

Isabella Milch



Background:
The aim of fusion research is to develop a power plant deriving energy from fusion of atomic nuclei. The fuel is an ionised low-density gas, a hydrogen plasma, which has to be confined in magnetic fields and heated to very high temperatures to ignite the fusion fire. The electric power required in plasma experiments for the magnetic field and heating is provided at IPP in Garching by big flywheel generators.

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