The motor-generator is spinning its web again
© 2012 EPFL
In the beginning of this year the TCV tokamak underwent impressive inner and outer maintenance works. Particularly, the facility supplying the electricity needed to run the tokamak – the motor-generator and its flywheel – has been dismantled. The rotor of the motor-generator, which is the rotating part, was sent to the factory for a check and everything finally reassembled.
First of all, the flywheel was separated from the rest of the motor-generator and put aside for inspection, a delicate manipulation for this steel cylinder with a radius of 0.7 meters and a weight of 20 tons. Then, the most impressive part of the dismantling of the motor-generator consisted in extracting its rotor. A cylinder of 10 meters long with a diameter of 1.1 meters and a weight of 40 tons had to be pulled out of its housing, while the distance between the fixed part - called stator - and the rotor is only 65 millimetres. This operation was possible thanks to the use of air cushions placed between the two pieces.
Once removed, the rotor was transported to the manufacturer Alstom in Birr (AG) where many checks and tests did not revealed any single failure. The overspeed test at 4200 RPM showed a perfect mechanical stability and excellent balancing. On the other hand, the bearings supporting the rotor and allowing its rotation almost without any friction showed some signs of wear and needed remanufacturing.
The reassembly started by the insertion of the rotor, the reassembly of its two bearings followed by the refitting of the flywheel and its bearing. The two elements as well as the three bearings were then aligned to the hundredth of a millimetre. Once this alignment was finished and the absence of radial run-out was verified, the flywheel and the rotor have been coupled again. After some last fittings of the alignment of the bearings and a check of the whole balance, the facility passed with satisfaction the tests in real operating conditions and can now be used again by the physicists for their experiments.
The use of the tokamak TCV requires a powerful electricity supply but for pulse lasting a few seconds only. The motor-generator and its flywheel are an ideal system for this kind of situation. The spinning masses (60 tons) are accelerated during a few minutes (up to 3600 RPM) using the electricity from the grid, after which the motor-generator functions as generator and supplies, in a few seconds, the accumulated energy allowing to achieve powers that wouldn't have been possible to take directly from the network.
