Abstract:
The present disclosure relates to a measuring device for measuring vibrations of a rotor and to a method for measuring the vibrations of a rotor. Disclosed is a measuring device for measuring vibrations of a rotor of an electric machine having a guiding rod with a plate at one end of the guiding rod, the plate to abut at the rotor in operation, an actuator for inducing a shock to the rotor via the guiding rod and via the plate, and a monitor device for measuring the vibrations at the rotor caused by the induced shock.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application No. 14178750.7 filed Jul. 28, 2014, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a measuring device for measuring vibrations of a rotor and to a method for measuring the vibrations of a rotor. 
     BACKGROUND 
     The electric machine is in particular a rotating electric machine such as a synchronous generator to be connected to a gas or steam turbine (turbogenerator) or a synchronous generator to be connected to a hydro turbine (hydro generator) or an asynchronous generator or a synchronous or asynchronous electric motor or also other types of electric machines. 
     In the technical field of generators, especially hydrogenerators, a rotor rotates within a stator with a certain speed. The mechanical system is designed for best operation in certain speed ranges. Further, the excess of a critical speed of the rotor is to be avoided. The critical speed is a quantity depending on several influences and is thus a unique quantity for each rotor. Furthermore, the critical speed can change over time and is not a constant quantity. The critical speed is a machine data which is of interest for the operator. The document JPH 0991044 describes a calculation method for calculating the critical speed of a rotor. Based on a mere mathematical approach in this document the eigenfrequency of the respective rotor is calculated from a variety of quantities as tool weight, static load, bending moment, rotor material, shear force; and further the critical speed of the rotor is determined from this calculated eigenfrequency. 
     SUMMARY 
     It is an object of the invention to provide a measuring device and a measuring method to provide the critical speed of a rotor of an electric machine. 
     The object is solved with a measuring device as set forth herein and with a measuring method as set forth herein. Further examples of the invention are disclosed in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages will be more apparent from the description of a preferred but non-exclusive embodiment of the measuring device and measuring method, illustrated by way of non-limiting example in the accompanying drawings. 
         FIG. 1  is a schematic cross-section of a rotor and a measuring device for measuring vibrations of the rotor with two plates held by a guiding rod, an actuator at the guiding rod to induce a shock to the rotor, and a monitor device for measuring the vibrations caused by the induced shock. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the FIGURE, this shows a schematic side view of a rotor, wherein like reference numerals designate identical or corresponding parts throughout the several views. 
       FIG. 1  is a schematic cross-section side view of a rotor  5  of an electric machine which rotates in a direction into the image plane.  FIG. 1  shows the outer face of the rotor  5  at the left side and the inner face with the shaft  20  at the right side. Shown in a schematic way is a commonly used rotor  5  of a high power hydro generator for generating electric power. The rotor  5  is usually operated in a stator housing of a corresponding stator (not shown) of the hydro generator. On the left side of  FIG. 1  a cross-section of a bearing  51  of the rotor  5  is shown to support the rotor  5  and to allow the free rotation of the rotor  5 . The middle and the right part of the rotor  5  are designed commonly in an empty fashion, i.e. not as a laminated metal core, with the main parts being the inner shaft  20  of the rotor  5  and the outer sheath. The rotor  5  has rather the structure known as spider in the art. The herewith described measuring method and measuring device  1  can be applied to the rotor  5  in operation, i.e. with a rotating rotor  5  creating a centrifugal force. In the centre of the shown rotor  5  a measuring device  1  according to an example of the invention is shown. The measuring device  1  comprises a guiding rod  6  projecting through parts of the rotor  5  essentially radial to the rotor  5 , i.e. perpendicular to the axis of the rotor  5 . The guiding rod  6  is installed in the free space of the rotor  5  in between arms (not shown) of a rotor spider of the hydro generator. At the left end of the guiding rod  6  a plate  7  is releasably fixed to the guiding rod  6 , also referred to as shock plate. The plate  7  is from metal as the guiding rod  6  and has a rectangular or circular shape. The plate  7  has a certain mass making it suitable for transferring an impulse or momentum to the rotor bearing  51  to which the plate  7  abuts in operation of the measuring device  1 . The plate  7  at the left end of the guiding rod  6  is pressed against the bearing  51  of the rotor  5  at the outer part of the rotor  5 . At the right end of the guiding rod  6  a holding plate  8  is fixed to the guiding rod  6  near the centre of the rotor  5 . The holding plate  8  is from metal and has a rectangular or circular shape. The holding plate  8  abuts the frame near the shaft  20  of the rotor  5  at the right side. The function of the holding plate  8  is mainly to support the guiding rod  6  and to exert a counter force to the plate  7  at the other end of the guiding rod  6 . An actuator  9  is arranged at the guiding rod  6  which encompasses the guiding rod  6  and which has a cylindrical shape. The actuator  9  is made from metal in this example and has a mass suitable to exert a proper impulse or momentum to the plate  7  sufficient to cause an effect at the material of the plate  7  and the rotor  5 . The actuator  9  is designed here as a piston and is movable along the whole length of the guiding rod  6 . Adjacent to the guiding rod  6 , in the side view of  FIG. 1  above the guiding rod  6 , a locking system  90  is installed as part of the measuring device  1 . The locking system  90  comprises an arrangement of levers  93  which is fixed to the wall  81  at the rotor  5  near the inner end. The locking system  90  further comprises a spring  91 , a pivot  92 , and adjustment sleeves  98  as described below. A first lever  94  of the arrangement of levers  93  projects essentially parallel to the guiding rod  6 . A second lever  95  and a third lever  96  of the arrangement of levers  93  project perpendicularly from the first lever  94  in the direction of the guiding rod  6 . Between these two levers, the second lever  95  and the third lever  96 , a spring  91  is arranged which stores a retention force in the unstretched position which exerts on the second lever  95  and the third lever  96  to each other. Thus, the second lever  94  and the third lever  95  are held together by the spring force of the spring  91 . For mechanical realization and as a mounting for the spring  91  the adjustment sleeves  94  are mounted at the second lever  95  and the third lever  96 . The spring  91  engages the adjustment sleeves  98  at both levers  95 ,  96 . The spring  91  is appropriate to exert a mechanical force which has a sufficient magnitude to withstand the centrifugal force acting on the actuator  9  when the rotor  5  rotates up to a certain rotation speed. In the position of  FIG. 1  the actuator  9  or piston is blocked by the second lever  95  of the arrangement of levers  93  which engages the edge of the actuator  9  and blocks the centrifugal force exerted at the actuator  9 . The pivot  92  connecting the first lever  94  with the second lever  95  holds the second lever  95  in the direction perpendicular to the guiding rod  6  in this view of  FIG. 1 . To operate the measuring device  1  the rotation speed of the rotor  5  is enhanced to a speed at which the centrifugal force on the actuator  9  excels the holding force of the spring  91 . The second lever  95  is then swivelled around the pivot  92  releasing the actuator  9 . Hence, the actuator  9  or piston accelerates along the guiding rod  6  assumed that the rotor  5  further rotates. The actuator  9  is accelerated to the left in the view of  FIG. 1 , which is the radial direction of the rotor  5  to the outside of the rotor  5 . At the outer end of the guiding rod  6  the actuator  9  knocks against the plate  7  or shock plate at which the movement of the actuator  9  is stopped. The collision of the actuator  9  with the plate  7  creates a vibration in the area of the bearing  51  of the rotor  5 . This vibration effect is measured by the measuring device  1 . To this end the measuring device  1  comprises a monitor device  10  which is attached smoothly to the bearing  51 . The monitor device  10  can be attached to the bearing  51  by clamping to the bearing  51  for example. The monitor device  10  is suitable for measuring the vibrations transmitted in the material of the bearing  51 . In an example the monitor device  10  comprises an induction sensor which commonly measures vibrations by means of generating a magnetic field caused by the mechanical shifts of the surface of the bearing  51 . This magnetic field is measured, amplified, and from the amplified signal the amplitude and frequency can be determined. Notably, the eigenfrequency of the bearing  51  or rotor  5  can be determined. The monitor device  10  has a display for showing the results or can optionally be read out from remote. The measuring device  1  and method as described can thus determine the system specific and time variable eigenfrequency in real time and with the rotor  5  rotating. The measuring method can also be repeated with different rotation speeds of the rotor  5  to enhance the sensitivity of the measurement. The monitoring device  10  can further include a calculator for calculating a system specific critical speed of the corresponding rotor  5 . Here, the calculator determines the critical speed of the rotor  5  by multiplying the eigenfrequency with the number sixty (eigenfrequency f×60). The operator of the measuring device  1  receives the critical speed from the monitoring device  10  comprised by the measuring device  1 . Disclosed here is a simple and efficient device and method to obtain the important quantity of the critical rotation speed of a rotor  5  which is of interest for the operator of the generator. 
     While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the aforementioned documents is incorporated by reference herein.