GB217924A - Improvements in and relating to elastic fluid turbine rotors and methods of avoiding tangential bucket vibration therein - Google Patents

Abstract

217,924. British Thomson-Houston Co., Ltd., (Assignees of Campbell, W.). June 22, 1923, [Convention date]. Vibrations, indicating, recording, and registering.-For the purpose of determining the natural frequency of the blading in high speed turbine rotors, a rotor disc 25, Fig. 1, is mounted side by side with a solid disc 23 on a shaft 22 which can be driven at different speeds. The solid disc 23 carries an electromagnet 34, the core 33 of which stands near to an iron plate 43, Fig. 3, secured to a group of blades 30 on the turbine disc. The electromagnet is energized by a continuous current and any relative movement between plate 43 and the core 33 due to vibration of the blades on the rotor will cause a periodic variation of the energizing current. This current passes through the primary of a transformer and the periodic current produced in the secondary is passed to a valve amplifier, the anode circuit of which is used to provide current for an oscillograph. A second oscillograph is subjected to an oscillating current of known periodicity is used to provide a second curve on the visual screen or photographic record as a time standard. An electromagnet 74 placed near to the path of the rotating magnet 34 causes a sharp variation in the amplified current at each revolution and provides thereby a record of the speed of the rotors. The rotating magnet 34 is secured in the end of a brass tube 37 passed through a hole in the rotor 23 and secured on either side by plates 39 fastened on the rotor and against which nuts 40 on the tube 37 are tightly screwed. If desired, vibration of the blades may be promoted by means of a steam jet from a stationary nozzle 85 which impinges, once in each revolution of the rotor, upon a projection 83 secured to the blade group. An equation is given expressing the change in the natural frequency of a blade group as the speed of revolution increases in terms of the natural frequency of the group when stationary and when a constant in the equation had been determined experimentally for blade groups of a particular character, it is not necessary to determine experimentally, the critical speeds of each rotor, if the calculated natural running frequencies give critical speeds removed 20, 15 and 10 per cent from the normal operating speed. The natural frequency of a blade group at rest may be found by subjecting the group to the pull of an electromagnet and varying the frequency of the alternating current energizing the magnet until the group vibrates.