Patent ID: 7064559

Claim:
A method for measuring the vibrations of a stator bar of an electric generator while in use, said vibrations corresponding to a distance to be measured over a period, said electrical generator having a layer of compressible dielectric substance wherein said layer of dielectric substance is sandwiched anywhere between several parallel layers of solid and fixed dielectric substances having each a constant but not necessarily identical permittivity, said layers being closely but variably spaced from a grounded conductive or semi-conductive surface, said stator bar acting as said grounded conductive or semi-conductive surface, said distance is an air gap between a stator-side surface of a tightening wedge of said stator bar and a rotor-side surface of said stator bar, the method comprising: using a capacitive sensor installed from the outside, and without modifying the existing arrangement of parallel dielectric layers, on a rotor-side surface of said tightening wedge which is adjacent or forms part of an external surface of the first layer of said solid dielectric substances which is furthest from the grounded conductive or semi-conductive surface, said sensor being composed of a first conductive or semi-conductive plate which is parallel to the parallel layers of dielectric substances, said first plate being furthest from said first layer, and a second parallel superimposed conductive or semi-conductive plate which is nearest said first layer and electrically insulated from the first plate, and both plates being parallel to the parallel layers of dielectric substances; feeding said first plate of said sensor a high frequency signal; measuring over time the values of the current generated by said high frequency signal between the second plate and the grounded conductive or semi-conductive surface which faces the dielectric layers; wherein the measured current varies as an inverse function of the thickness of said compressible dielectric substance, thereby enabling the determination of the amplitude of the variations of thickness of said compressible dielectric substance and hence said distance.