Abstract:
The invention inter alia relates to a method for monitoring the bearing current of an electrical machine ( 10 ). An electrode ( 100 ) arranged at a distance (d) to a shaft and the shaft—due to the gap (S) between the electrode and the shaft—produce a measurement capacitance (C) and an electric shift current (i) which flows through the measurement capacitance when there is a temporal change of the voltage (Ug) applied between the shaft and the housing is measured. A measurement signal (Ms) indicating a bearing current flow is generated when the shift current or a measurement variable produced by the shift current meets a predetermined trigger criterion. Preferably, the electrode has a circular inner contour so that the gap is annular. The annular inner contour results in an error compensation in the case of a balance error of the shaft because the factor dC/dt remains at least substantially constant. Due to the contactless measurement of the shift current, no contact brushes for contacting the shaft are required. The method can be used irrespective of whether the bearings are insulated from the machine housing or not.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2009/059341, filed Jul. 21, 2009, which designated the United States and has been published as International Publication No. WO 2010/010081 and which claims the priority of German Patent Application, Serial No. 10 2008 035 613.1, filed Jul. 25, 2008, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method and an arrangement for monitoring the bearing currents in an electrical machine. 
     Methods of this type are known, for example, from the international patent application WO 2006/134068. One of the methods described in this document provides for the electrical voltage which is present between a shaft in the electrical machine and the housing to be measured with the aid of electrical contact brushes, which rest mechanically on the shaft and make electrical contact with it. The voltage measured in this way is evaluated in an evaluation device, and a measurement signal is produced when a bearing current occurs. 
     Furthermore, the cited document discloses another method for monitoring the bearing current; in this other method, a bearing which bears the shaft of the machine is electrically isolated from the machine housing. The electrical voltage between the bearing and the housing is measured, and evaluated, in order to detect a bearing current event. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of specifying a method by means of which a bearing current can be monitored particularly easily, and with as little complexity as possible. 
     According to one aspect of the invention, this object is achieved by a method for monitoring the bearing current in an electrical machine, in particular an electric motor or an electrical generator, which has a shaft which is mounted by means of at least one bearing in a housing such that it can rotate, wherein a measurement capacitance is formed by an electrode, which is arranged at a distance from the shaft, and by the shaft—on the basis of the gap between the electrode and the shaft—and an electrical displacement current is measured which flows through this measurement capacitance when the voltage which is present between the shaft and the housing rate of change, and a measurement signal which indicates a bearing current flow is produced when the displacement current or a measurement variable formed by this displacement current complies with a predetermined initiation criterion. 
     Advantageous refinements of the method according to the invention are specified in the dependent claims. 
     The invention accordingly provides that a measurement capacitance is formed by an electrode, which is arranged at a distance from the shaft, and by the shaft—on the basis of the gap between the electrode and the shaft—and an electrical displacement current is measured which flows through this measurement capacitance when the voltage which is present between the shaft and the housing rate of change, and a measurement signal which indicates a bearing current flow is produced when the displacement current or a measurement variable formed by this displacement current complies with a predetermined initiation criterion. 
     One major advantage of the method according to the invention is that this method does not require any contact brushes in order to allow a voltage change in the shaft-housing voltage which is present on the shaft to be measured. Specifically, in contrast to the prior art, the method according to the invention does not, for example, involve measuring the voltage which is present on the shaft per se, but of evaluating the capacitive electric displacement current flowing away from the shaft when a voltage change occurs, and of evaluating this displacement current. The displacement current is measured by a measurement electrode which is arranged at a distance from the shaft, that is to say without touching it, and forms a capacitance with the shaft. Because the electric displacement current is measured, there is therefore no need to make electrical contact with the shaft, as a result of which there is no need for contact brushes, which are dependent on wear, and require continuous maintenance of the machine. 
     A further major advantage of the invention is that it can be used with any desired electrical machines; for example, bearing current events can be measured independently of whether the bearings are or are not isolated from the housing. 
     The measurement capacitance is particularly preferably formed by an electrode which has a circular hole, through which the shaft is passed and which is arranged concentrically with respect to the shaft such that the gap is annular. A circular internal contour and a circular gap make it possible to avoid measurement errors when the shaft is unbalanced. When unbalanced, the distance between the shaft and the electrode will change as a function of position during the rotation of the shaft, as a result of which a current component which has nothing to do with the discharging process and is caused solely as a result of the variation of the gap over time because of the unbalance of the shaft additionally occurs, in addition to the electric displacement current when the shaft is electrically discharged, in accordance with the relationship i≈C*dUg/dt+Ug*dC/dt (C: measurement capacitance, Ug: shaft-housing voltage, is current). This current component If=Ug*dC/dt has no noticeable disturbance effect if the gap is annular, because, although some ring sections may be closer to the shaft at some times than at other times during the rotation of the shaft, the opposite ring sections in opposition will assume a greater distance from the shaft, in such a way that compensation occurs because the factor dC/dt remains small. 
     The measurement capacitance is preferably formed by an annular electrode, that is to say an electrode which has both a circular internal contour and a circular external contour. 
     According to one preferred refinement of the method, the initiation criterion is considered to be satisfied when the displacement current or a measurement variable formed by this displacement current reaches or exceeds a predetermined limit current. 
     According to another preferred refinement of the method, the displacement current is passed through a resistance, and the initiation criterion is considered to be satisfied when the measurement voltage dropped across the resistance, or a variable derived from it, satisfies a predetermined voltage initiation criterion. Preferably, the voltage initiation criterion is considered to be satisfied when the measurement voltage dropped across the resistance, or a variable derived from it, reaches or exceeds a predetermined limit voltage. 
     In order to suppress external disturbance influences, it is considered to be advantageous if the measurement voltage dropped across the resistance is subjected to high-pass and/or bandpass filtering, and the measurement signal is produced when the filtered measurement voltage, or a variable derived from it, satisfies the predetermined voltage initiation criterion. 
     Furthermore, the invention relates to an arrangement having an electrical machine, in particular an electric motor or an electrical generator, which has a shaft which is mounted by means of at least one bearing in a housing such that it can rotate, and having a measurement device for monitoring the bearing current. 
     The object of this aspect of the invention is to specify an arrangement which is as independent of wear as possible and allows monitoring of the bearing current irrespective of whether bearings which bear the shaft of the machine are or are not electrically isolated from the housing of the machine. 
     According to another aspect of the invention, this object is achieved by an arrangement having an electric machine, in particular an electric motor or a generator, which has a shaft which is mounted by means of at least one bearing in a housing such that it can rotate, and having a measurement device for monitoring the bearing current, wherein the measurement device has an electrode which is arranged at a distance from the shaft and, together with the shaft, forms a measurement capacitance on the basis of the gap between the shaft and the electrode, and the measurement device has an evaluation device which is connected to the electrode and detects an electric displacement current which flows through the measurement capacitance when the voltage which is present between the shaft and the housing rate of change, and produces a measurement signal, which indicates a bearing current flow, when the displacement current or a measurement variable formed by this displacement current complies with a predetermined initiation criterion. 
     Advantageous refinements of the arrangement according to the invention are specified in dependent claims. 
     With regard to the advantages of the arrangement according to the invention, reference should be made to the above statements in conjunction with the method according to the invention, since the advantages of the arrangement correspond substantially to those of the method according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will be explained in more detail in the following text with reference to exemplary embodiments; in this case, by way of example: 
         FIG. 1  shows a first exemplary embodiment of an arrangement having an electrical machine, and having a measurement device for monitoring the bearing current, in the form of a longitudinal section; 
         FIG. 2  shows the bearing for a shaft in the arrangement as shown in  FIG. 1 , in the form of a cross section; 
         FIG. 3  shows the positioning of an electrode of the arrangement as shown in  FIG. 1 , in the form of a cross section; 
         FIG. 4  shows an electrical equivalent circuit of a first exemplary embodiment of an evaluation device for the arrangement as shown in  FIG. 1 ; 
         FIG. 5  shows a second exemplary embodiment of an evaluation device for the arrangement as shown in  FIG. 1 ; 
         FIG. 6  shows a second exemplary embodiment of an arrangement having an electrical machine and having a measurement device for measuring the bearing current, in which an electrode of the arrangement has an annular internal and external contour; 
         FIG. 7  shows the bearing for a shaft in the arrangement as shown in  FIG. 6 , and 
         FIG. 8  shows a third exemplary embodiment of an arrangement having an electrical machine and having a measurement device for monitoring the bearing current, in which an electrode of the arrangement has an annular internal contour and an external contour of any other design shape. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For the sake of clarity, the same reference symbols are always used for identical or comparable components in the figures. 
       FIG. 1  shows an exemplary embodiment of an arrangement which has an electrical machine  10  which, by way of example, may be an electric motor or an electrical generator, as well as a measurement device  20  for monitoring the bearing current. 
     As can be seen in  FIG. 1 , the machine  10  is equipped with a housing  30  which is formed by an upper housing part  40  and a lower housing part  50 . 
     A shaft  60  of the machine  10  is held by two bearings  70  and  80  such that it can rotate. The two bearings  70  and  80  are illustrated only schematically in  FIG. 1 . There is generally a lubricant film between the shaft  60  and the two bearings  70  and  80 , or within the two bearings  70  and  80 , although this is not shown in any more detail in  FIG. 1 , and is merely indicated by the reference symbol  90 . 
     The bearing  70  and the bearing  80  may be electrically conductively connected to the two housing parts  40  and  50 ; instead of this, both bearings  70  and  80  may also be electrically isolated from the housing  30  by means of an insulator, which is not illustrated in  FIG. 1 . It is irrelevant to the operation of the measurement device  20  whether electrical isolation is or is not provided between the bearings  70  and  80  on the one hand and the housing  30  on the other hand. 
     As can also be seen in  FIG. 1 , the measurement device  20  is equipped with an electrode  100  which is arranged at a distance d from the shaft  60 . A gap is therefore formed between the electrode  100  and the shaft  60 , and is annotated with the reference symbol S in  FIG. 1 . The gap S results in an electrical measurement capacitance C being formed between the shaft  60  and the electrode  100 , and this is illustrated by a dotted line in  FIG. 1 . 
     The electrode  100  is electrically connected to one measurement input  110  of an evaluation device  120 . Another measurement input  130  of the evaluation device  120  is connected to the housing ground  140 , which is the electrical potential of the housing  30  of the machine  10 . 
       FIG. 2  shows one exemplary embodiment of a bearing for the shaft  60  on the basis of the bearing  70 , in the form of a longitudinal section. As can be seen, the bearing  70  has an outer bearing ring  200 , which is indirectly or directly connected to the housing  30 , as well as a multiplicity of roller bodies  210 . The roller bodies  210  are arranged between the outer bearing ring  200  and an inner bearing ring  220 , which is connected to the shaft  60 . The inner bearing ring  220  may be a component of the bearing  70  or may be part of the shaft  60 ; however, the latter is irrelevant to the operation of the bearing. 
     The lubricant film  90  is located between the roller bodies  210  and the two bearing rings  200  and  220 . As already stated, it is irrelevant whether the outer bearing ring  200  is electrically isolated from the housing  30 , or is electrically connected to it. 
     When the machine  10  is stationary, the roller bodies  210  are located in the inner and outer bearing rings. The roller bodies may be balls, cylinders, needles or may have other physical shapes. In general, particularly in the case of very large machines, the roller bodies and the bearing shells are composed of steel. This results in the shaft  60  being electrically shorted to the housing  30  when the machine is stationary. When the shaft  60  is rotated, the roller bodies  210  run on the lubricant film  90 ; the latter is fed from a wedge-shaped lubricant supply in front of the respective roller body. The lubricant film leads to electrical isolation, which depends on the viscosity of the lubricant (which changes with age, and with the temperature of the bearing, etc.). Depending on the machine size, a voltage may now build up between the shaft  60  and the housing  30 , which voltage may depend on the characteristics of the bearing, on the motor control (for example frequency converter and its termination at the motor, etc.) and on the quality of the bearing itself (grooves, roughness, etc.). At a certain voltage, the isolation of the lubricant film  90  breaks down (about 10 . . . 12 V in the case of small motors, and 70 . . . 100 V in the case of large motors) and a bearing current occurs. By way of example, the measurement of the bearing current will be explained in more detail further below. 
       FIG. 3  illustrates the shaft  60  in the form of a cross section, to be precise along a section B-B as shown in  FIG. 1 . The figure shows the electrode  100 , which is arranged at a distance from the shaft  60 , such that the gap S is formed. The evaluation device, which is connected to the electrode  100 , cannot be seen in this section. 
       FIG. 4  will now be used to explain how the evaluation device  120  as shown in  FIG. 1  operates. For this purpose,  FIG. 4  shows the electrical equivalent circuit of the evaluation device  120 , by way of example.  FIG. 4  shows the measurement capacitance C to which an electrical resistance R is connected. The resistance R is followed by an electrical comparator  300 . 
     When a change now occurs in the electrical potential which is present on the shaft  60 , for example because an electric discharge current is flowing, then the electrical voltage Ug between the shaft  60  and the housing ground  140  will accordingly change. This voltage change results in an electric displacement current i, which flows through the resistance R and leads to a voltage drop across this resistance R. The current i is given approximately by:
 
 i≈C*dUg/dt  
 
     The voltage drop across the resistance R is also used as the measurement voltage Um, and is fed into the comparator  300 . The comparator  300  compares the measurement voltage Um with a predetermined minimum voltage, and produces a measurement signal Ms, which indicates a bearing current flow, if the measurement voltage Um exceeds the predetermined minimum voltage. 
     The values for R and C are preferably in each case matched to the type of machine (motor/generator, physical size, design, power); by way of example, test measurements are carried out for this purpose with different values for R and C, until an R/C value pair have been determined with optimum measurement characteristics for the respective machine. 
     By way of example, the comparator  300  produces a binary measurement signal, for example with a logic 1, when the predetermined minimum voltage is exceeded. If, in contrast, the comparator  300  finds that the voltage Um does not reach the predetermined minimum voltage, then, for example, it produces a binary output signal with a logic 0 on its output side, as the measurement signal Ms. 
     The evaluation device  120  makes use of the fact that a discharge current from the shaft  60  via the bearing  70  or  80  to the housing  30  always leads to a relatively large voltage change dUg/dt, and that, accordingly, a bearing current flow leads to a correspondingly large voltage Um, which can be detected by the comparator  300 . In contrast, other current flows, which are not based on a discharge of a potential which is present on the shaft  60 , will generally have lower rates of change, as a result of which they are not detected by the evaluation device  120 , specifically because the quotient dUg/dt for these other current flows will be too low. 
       FIG. 5  shows an alternative embodiment of the evaluation device  120  as shown in  FIG. 1 . In this exemplary embodiment, in addition to the comparator  300 , the evaluation device  120  has a bandpass filter  310 , which is arranged electrically between the electrical resistance R and the comparator  300 . 
     The function of the bandpass filter  310  is to filter out of the measurement voltage Un disturbance frequencies which do not result from a discharge process on the shaft  60 , and to form a filtered measurement voltage Um′ in order that disturbance frequencies do not adversely affect the method of operation of the comparator  300 . The bandpass filter will preferably have only a small amount of attenuation in a frequency range between 1 MHz and 100 MHz, since the discharge-current frequencies to be detected by the evaluation device  120  will lie in this frequency range. The signal Um is suppressed outside said frequency range between 1 MHz and 100 MHz. 
       FIG. 6  shows a second exemplary embodiment of an arrangement having a machine  10  and a measurement device  20 . In contrast to the exemplary embodiment shown in  FIG. 1 , the electrode  100  in the exemplary embodiment shown in  FIG. 6  is chosen to have a different design. Therefore, as can be seen from  FIG. 6 , the electrode  100  does not approach the shaft  60  like a rod, as is shown by way of example in  FIG. 3 , but, instead of this, has an annular opening  400  through which the shaft  60  is passed. The mechanical design of the electrode  100  and the arrangement of the shaft  60  relative to the electrode  100  are illustrated once again in  FIG. 7 , in the form of a cross section along the cross-section line B-B shown in  FIG. 6 . As can be seen, the electrode  100  has an annular shape, and both its internal contour and its external contour are circular. Because of the circular internal contour, an annular gap S is formed between the shaft  60  and the electrode  100 . 
     One major advantage of the annular configuration of the electrode  100  is that, if the shaft  60  is unbalanced, a considerably smaller error current will occur than in the case of the exemplary embodiment shown in  FIG. 1 . This will be explained in more detail briefly in the following text: 
     If the shaft  60  in the exemplary embodiment shown in  FIG. 1  is not borne ideally and an unbalance occurs, then the distance d between the shaft  60  and the electrode  100  will vary during the rotation of the shaft  60 , as a result of which, on the basis of the relationship:
 
 i≈C*dUg/dt+Ug*dC/dt  
 
     in addition to the electric displacement current, a current component will also additionally occur during an electrical discharge from the shaft  60 , which has nothing to do with the discharge process and is caused solely by the variation of the gap S over time, because of the unbalance of the shaft. This current component or error current can therefore be calculated to be:
 
 If=Ug*dC/dt  
 
     The error current If will not cause any significant disturbance in the exemplary embodiment shown in  FIG. 6 , specifically because, although any unbalance during rotation of the shaft will result in some ring sections being closer to the shaft at some times than at other times, the opposite ring sections will, however, in opposition be at a greater distance from the shaft  60 . In other words, this therefore leads to compensation, and to the error current being largely eliminated, since those ring sections which would lead to a high capacitance at times because of the shorter distance between the shaft  60  and the annular electrode  100  are compensated for by other ring sections whose capacitance component is actually less because of the greater distance between the shaft  60  and the annular electrode  100 . 
     In summary, in the case of the exemplary embodiment shown in  FIG. 6 , the annular internal contour of the electrode  100  and therefore the annular shape of the gap S result in better measurement accuracy than in the case of the electrode configuration shown in  FIG. 1 . 
       FIG. 8  shows a third exemplary embodiment of the configuration of the electrode  100  as shown in  FIG. 1  and  FIG. 6 .  FIG. 8  shows the electrode  100  in the form of a cross section, as well as the arrangement of the shaft  60  which is passed through the electrode  100 . As can be seen from  FIG. 8 , the electrode  100  has an annular internal contour, as a result of which an annular gap S is formed between the electrode  100  and the shaft  60 . 
     In contrast to the exemplary embodiment shown in  FIG. 6 , the external contour  410  of the electrode  100  is, however, not annular, but is of any other desired shape. 
     The annular internal contour and the annular shape of the gap S once again results in the error compensation, as already explained in conjunction with  FIGS. 6 and 7 , in the event of mechanical unbalancing of the shaft  60 , specifically because, if the distance d between individual sections of the electrode  100  and the shaft  60  varies over time, other electrode sections, in general the respectively opposite electrode sections, will be further away, thus resulting in compensation for the error current overall
 
 If=Ug*dC/dt  
 
     because the factor dC/dt is very small.