WO2008116433A1

WO2008116433A1 - Method and device for suppressing a bearing current in an electrical machine

Info

Publication number: WO2008116433A1
Application number: PCT/DE2007/000557
Authority: WO
Inventors: Michael Brauer; Jörg HASSEL; Carsten Probol
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-03-27
Filing date: 2007-03-27
Publication date: 2008-10-02
Also published as: DE112007003524A5

Abstract

The invention relates to a method and device (1) for suppressing a bearing current (I<SUB>b</SUB>) flowing across a bearing (2) of an electric machine. At least one electric voltage present at a winding of the electric machine is measured and a common-mode voltage (U <SUB>gl</SUB>

Description

description

Method and device for suppressing a bearing current on an electrical machine

The invention relates to a method and a device for suppressing a bearing current flowing through a bearing of an electrical machine.

In the bearings of electrical machines, such as an electric generator or an electric motor, there may be an undesirable current flow as a result of electrostatic charge or when powered by a power electronic actuator. Some of these bearing currents are so-called EDM (Electric Discharge Machin) currents, in which arc discharges occur in the bearing. It comes in particular in the lubricating film, which is located between the rolling elements and the races of each camp, rollovers and discharges. As a result, premature wear of the lubricant and the bearing can be adjusted altogether. Premature bearing failure is also a potential consequence.

In order to avoid these damaging bearing currents, grounding brushes between the rotor and the housing are currently used in three-phase drives with power electronic feed. This achieves a grounding of the runner. However, the grounding brushes are subject to wear, so that the maintenance and repair costs increase. In addition, the contact safety of the grounding brushes is not always given, especially in difficult environmental conditions, so that it can lead to the formation of bearing currents and increased bearing wear.

An object of the invention is therefore to provide a method of the type described, which ensures a long life of the bearings used in the electrical machine. This object is achieved by the features of independent claim 1. The method according to the invention is one in which at least one electrical voltage applied to the electrical machine is measured, from the result of the voltage measurement a common-mode voltage is determined the common-mode voltage, a compensation voltage is determined and a component of the electric machine, which is electrically connected to the bearing, is acted upon by the compensation voltage, so that a voltage drop across the bearing bearing voltage is at least partially compensated.

The inventive method is characterized by the fact that the bearing currents operating point and plant specific, so in particular state-oriented, are suppressed. The application of the determined in particular on the basis of a state detection compensation voltage to the bearing leads to a substantial compensation of the bearing voltages that would otherwise cause the arc discharges and thus the bearing currents to large values. The remaining residual bearing stresses are too low to still cause arc discharges to any detrimental extent. Ideally, the bearing voltages, which are preferably also measured, even disappear completely due to the compensation.

The determination of the compensation voltage according to the invention in a simple, but still very efficient way. It is derived from the common mode voltage (= common mode voltage). The latter, for example, can be easily measured by means of a voltage measurement on at least one winding of the electrical machine, for example by adding the strand voltages detected at the terminals of the winding strands, or by direct measurement of the voltage between the star point of the winding strands and the housing of the electrical machine , or by means of a measurement of the increase of the wave potential of the electric machine, eg via the electric field. Especially with a power electronic fed and grounded AC drive Such a common-mode voltage can form, which causes the dangerous load currents due to the parasitic capacitances of the three-phase drive and in conjunction with the sudden voltage changes at the input terminals of the phase windings. It has been recognized that the common mode voltage which is easy to measure or to be detected can be used as a measure of the bearing voltages dropping at the bearings and thus also for their compensation. The compensation thus depends, in particular, on a detected current state of the electric machine, for example, an electric motor, namely the current common-mode voltage.

Advantageous embodiments of the method according to the invention emerge from the features of the claims dependent on claim 1.

Favorable is a variant in which the common mode voltage is inverted to determine the compensation voltage and multiplied by a constant factor. The compensation voltage can thus be particularly easily and in particular also electronically, e.g. using a microprocessor or microcontroller. At the same time a very good compensating effect is achieved with a compensation voltage determined in this way.

According to a favorable embodiment, the constant factor or, in particular, also another process parameter during the execution of the method, that is to say in particular online, is determined or at least adapted. As a result, changes in the behavior of the electrical machine and / or in the coupling of the compensation voltage can be reacted directly. In particular, a feedback is provided for this purpose, so that a control circuit is formed.

However, there is also an alternative and likewise advantageous embodiment in which the constant .Factor or in particular also a different process parameter is determined in advance. Then the compensation method is very fast. The Predetermination of the constant factor can be done, for example, during an initial learning phase. This learning phase can also be carried out during the commissioning of the electrical machine.

Preferably, it is further provided that the loading of the component of the electric machine with the compensation voltage by means of a capacitive coupling takes place. This type of coupling is particularly well suited for feeding voltage to a moving component of the electrical machine, such as a motor. a rotor or a motor shaft. The capacitive coupling is in particular contactless.

In addition, the bearing voltage prevailing over the bearing or the bearing current flowing through the bearing may be detected and taken into account when determining the compensation voltage. As a result, the quality of the compensation can be further improved. In contrast to the common-mode voltage, which is an indirect measure, the bearing current and the bearing voltage are direct measurement variables which allow immediate monitoring of the conditions in the respective bearing. The detection and, in particular, feedback of these direct measures allow a very rapid response to changes in the state of the bearing.

In another favorable embodiment, a time curve of the compensation voltage is adapted to a time course of the bearing voltage or the bearing current. This advantageous measure also serves to further improve the quality of compensation. It allows, for example, a reduction of voltage overshoots. In addition, special types of bearing current can be prevented particularly efficiently. In particular, a frequency-selective compensation can also take place.

Another object of the invention is therefore to provide a device of the type described, the ensures a long life of the bearings used in the electrical machine.

To solve this problem, a device according to the features of claim 8 is given. The device according to the invention comprises a voltage detection unit for measuring at least one voltage applied to a winding of the electric machine and for determining a common-mode voltage based on the measured voltage, an evaluation and control unit connected to the voltage detection unit for determining a compensation voltage based on the common-mode voltage and the evaluation and control unit connected coupling means for acting on a bearing electrically connected to the bearing component of the electric machine with the compensation voltage, so that at least partially compensates a falling over the bearing bearing voltage.

Advantageous embodiments of the device according to the invention emerge from the claims dependent on claim 8. The device according to the invention and its embodiments essentially offer the same advantages that have already been described in connection with the method according to the invention and its variants.

Further features, advantages and details of the invention will become apparent from the following description of exemplary embodiments with reference to the drawing. It shows:

1 shows an embodiment of a device for compensating a bearing voltage occurring in an electrical machine in a block diagram representation and

2 shows a further embodiment of a device for bearing voltage compensation in a schematic equivalent circuit diagram representation. Corresponding parts are provided in FIG 1 and 2 with the same reference numerals.

FIG. 1 shows a block diagram of a device 1 for compensating a bearing voltage Ub present at a bearing 2 of an electrical machine. Of the electric machine designed as an inverter-fed variable-speed electric motor, only a greatly simplified section is shown, namely a motor shaft 3, which is rotatably mounted in an engine casing 4 grounded, for example, by means of the bearing 2 designed as a roller bearing. In the exemplary embodiment, no bearing insulation is provided. It can however be used optionally.

The device 1 contains a voltage detection unit 5, an evaluation and control unit 6 connected thereto with a compensation voltage generator 7 and a coupling unit 8 fed by the compensation voltage generator 7 and coupled to the motor shaft 3.

The voltage detection unit 5 is used for metrological determination of a common mode voltage U _g i, which is present at a winding of the electric machine not shown in detail. The common-mode voltage U _g χ can be measured either directly as one between a star point of the example three-strand winding and the motor housing 4 dropping voltage. In this variant, the voltage detection unit 5 contains only a voltmeter. Alternatively, however, the phase voltages U _u , U _v and U _w can be measured at the terminals of the three winding strands. From the measured three voltages, the common-mode voltage U _g i can then be determined in a downstream processing step in accordance with the relationship:

U_ _gl - (1) determine where the voltage values given are each vectors. This is indicated by the underlining of the voltage symbols. In this alternative, the voltage detection unit 5 thus contains three voltage meters and a processing subunit, which may also be designed electronically. According to a further embodiment not shown in detail, the functionality of this processing subunit is taken over by the evaluation and control unit 6.

The main function of the evaluation and control unit 6 is to determine from the directly measured or indirectly determined common mode voltage U _g i a compensation _voltage U _kO mp so that they largely compensated for the coupling of the motor shaft 3, the bearing voltage U _b and thus the Generation of bearing currents Ib prevented. In contrast to the other voltage variables, the bearing voltage U _b and also the bearing current I _{b are} not vector variables, but variables which vary with time. They are therefore presented without underlining. In order to reliably suppress the EDM (Electric Discharge Machinig) bearing currents which are particularly detrimental to the bearing life, the compensation _voltage U _koπ tp in the _exemplary embodiment shown corresponds to the equation:

Hto _n p

-k _{gmM -U} _gl ⁽ 2)

determined, with

^e i ⁿ coupling factor of the coupling unit 8, with BVR (= Bearing Voltage Ratio) a bearing _stress factor and k is _gesa mt a weighting factor is designated.

The coupling factor k _co pp _e i is either known on the basis of the specification of the coupling unit 8, which is designed capacitively, for example, or it can be estimated taking into account the parasitic components, in particular the parasitic capacitances, of the electrical machine. The bearing stress factor BVR (= Bearing Voltage Ratio) is essentially determined by the properties of the electrical machine. It depends on various influencing factors, such as the temperature, the thickness of a lubricating film provided in the bearing 2, an optionally provided bearing insulation and the grounding conditions. It typically assumes values in the range between 2% and 10%.

The weighting factor kg _tal is composed of the coupling factor k _ko PPEI and the bearing stress factor BVR. It indicates by how much the inverted common-mode voltage U _g i is to be amplified in order to obtain the compensation voltage U _ko iηp required under the current conditions. The weighting factor kg _total can therefore also be understood as a gain or damping factor. It can be easily determined for an electrical machine equipped with the device 1 and stored in the evaluation and control unit 6.

The coupling unit 8 serves omp to their intended target, ie at the bearing 2 to bring the corresponding compensation voltage generated by the compensation voltage generator 7 to the specifications determined in the evaluation and control unit 6 U _k. This is done by means of a coupling to the motor shaft 3, which in turn has a low-resistance, good electrical connection to an inner bearing ring of the bearing 2. The coupling unit 8 is formed capacitive in the embodiment, so that it is able to couple the compensation voltage U _kom p contactlessly on the rotatable Motorwel- Ie 3.

If the weighting factor k _totally according to equation (2) can not be determined mathematically in the individual case due to a lack of knowledge of the submodules coupling factor k k _o pp _e i and bearing stress factor BVR, a determination can also be made on the basis of a prior and optionally iterative learning phase. This determination takes place before the start of the actual compensation operation. It is therefore an offline process. The device 1 according to FIG. 1 also comprises means for online adaptation of the calculation rule for the compensation voltage Ukomp. There is provided a sensor unit 9 for the metrological detection of the bearing voltage U _b and / or the bearing current Ib, which is connected by means of a feedback branch 10 to the evaluation and control unit 6. The sensor unit 9, the structure and mode of action of which is described, for example, in DE 10 2005 027 670 _A1 , supplies a measured value of the respectively detected variable to the evaluation and control unit 6. In this way, in particular, the weighting factor kg can be _determined during the current process Operating to be adapted to the current conditions in the camp 2.

In this respect, the device 1 with the sensor unit 9, the feedback branch 10, the evaluation and control unit 6, the coupling unit 8 and the electrical connection via the motor shaft 3 to the bearing 2 also includes a closed loop 11, the control of the detected common mode voltage Ugi is superimposed. The control voltage components 5, evaluation and control unit 6, coupling unit 8 and electrical connection via the motor shaft 3 to the bearing 2 form a control circuit 12. The detection of the common-mode voltage U _g χ serves, for example, for "triggering" and coarse setting of the amplitude The additional measured value detection by means of the sensor unit 9 on the other hand serves for fine adjustment.

The relationship given in equation (2) describes a very simple method for determining the compensation voltage Uk _om p • Only a small effort is required for implementation. Instead of the simple relationship according to equation (2), even more complex formula relationships with more influencing parameters can be used. Thus, in another embodiment, an adaptation of the time course of the compensation voltage U _ko mp in addition to the determined bearing current I _b or the determined bearing voltage U _b provided. FIG. 2 shows a further exemplary embodiment of a device 13 for bearing voltage compensation in a schematic equivalent circuit diagram. Compared with the block diagram representation according to FIG. 1, in the substitute circuit diagram according to FIG. 2 simplified schematization is undertaken elsewhere. Thus, the voltage detection unit 5 and partly also the evaluation and control unit 6 are not included in the equivalent circuit diagram according to FIG. Only the compensation voltage generator 7, which is a part of the evaluation and control unit 6, is considered in the equivalent circuit diagram as a voltage source.

For the equivalent circuit diagram representation of FIG 2 but more details of the electric machine contains. Thus, a stator winding 14 with three each of the phase conductors u, v and w associated winding strands 15, 16 and 17 given again. The inductances contained in the winding strands 15 to 17 symbolize the respective partial windings. The winding strands 15 to 17 are, for example, electrically connected together in a common star point 18. In another embodiment, not shown, the winding strands 15 to 17 may also be connected in delta.

Between this neutral point 18 and the motor housing 4, which is only indicated symbolically in FIG. 2, the common mode voltage U _g χ drops, which can be measured directly at this point by means of the voltage detection unit 5. Alternatively, the line voltages U ₁₁ , U _v and U _w present at the phase terminals 19, 20 and 21 of the winding strands 15, 16 and 17 can also be measured and used according to equation (1) for determining the common-mode voltage Ugi.

Furthermore, the equivalent circuit diagram of FIG 2 includes parasitic capacitances of the electric machine, which play a role in the determination of the coupling factor and the bearing voltage factor BVR and thus the weighting factor k _gesaint . These parasitic capacities are Winding housing capacitances C _WF between the respective winding strand 15, 16 and 17 and the housing 4 to a winding rotor capacitance C _m between the neutral point 18 of the winding 14 and the motor shaft 3, a rotor housing capacitance C _RF between the motor shaft 3 and the housing 4, for example, caused by a lubricating film bearing capacity C _B and caused by a bearing insulation bearing insulation capacitances C _BI .

Regardless of the different types of representation in FIGS. 1 and 2, the device 13 effects a similar bearing _voltage compensation and thus bearing _{current suppression} as the device 1 according to FIG. 1. The compensation _voltage U _kOI p is again determined on the basis of the detected common-mode voltage U _g i and, accordingly, from the compensation voltage _generator. Tor 7 generated. The coupling of the compensation voltage U _ko mp on the motor shaft 3 is also capacitive in the device 13 by means of the coupling unit eighth

Basically, the bearing stress compensation can also be applied to another electrical machine, such as a motor. a generator, are used. The method has been described as an example of a three-phase machine. In principle, however, it may also be provided in a single-phase machine or in a DC machine.

Claims

claims

1. A method for suppressing a bearing current (I _b ) flowing through a bearing (2) of an electrical machine, in which a) at least one electrical voltage applied to the electrical machine (U _{u /} U _{v /} U _w ; U _g i) is measured b) from the result of the voltage measurement, a common mode voltage (Ugi) is determined c) based on the common mode voltage (U _g i) a compensation voltage (Ukoπp) is determined and d) a component (3) of the electric machine with the bearing (2) is electrically connected, with the compensation voltage (U _k o _m p) is acted upon, so that over the bearing (2) sloping bearing voltage (U _b ) is at least partially compensated.

2. Method according to claim 1, characterized in that the common-mode voltage (U _g i) is inverted to determine the compensation voltage (U _kom p) and is multiplied by a constant factor (k _total ; BVR,

is multiplied.

3. Method according to claim 2, characterized in that the constant factor (k _total , BVR, kkoppei) is determined or at least adapted during the execution of the method.

4. Method according to claim 2, characterized in that the constant factor (k _total ; BVR, kk _Oppel ) is determined in advance.

5. The method according to claim 1, characterized in that the loading of the component (3) of the electric machine with the compensation voltage (U _comp ) by means of a capacitive coupling.

6. The method according to claim 1, characterized in that over the bearing (2) pending bearing voltage (Ub) or over the bearing (2) flowing bearing current (I _b ) detected and taken into account in the determination of the compensation voltage (U _ko inp) with.

7. The method according to claim 6, characterized marked, that a time course of the compensation voltage (U _kom p) to a time course of the bearing voltage (Ub) or the bearing current (I _b ) is adjusted.

8. A device for suppressing a bearing current (I _b ) flowing via a bearing (2) of an electrical machine comprising a) a voltage detection unit (5) for measuring at least one voltage (U ₁₁ , U ₁₁ , ₁₁ ) present on a winding (14) of the electrical machine. U _v , U _w ; U _g i) and for determining a common-mode voltage (Ugi) on the basis of the measured voltage (U _u , U _v , U _w ; Ugi), b) an evaluation and (5) connected to the voltage detection unit (5) Control unit (6) for determining a compensation voltage (Ukomp) on the basis of the common mode voltage (Ugi) and c) coupling means (8) connected to the evaluation and control unit (6) for acting on a component (3) electrically connected to the bearing (2) ) of the electric machine with the compensation voltage (Ukoinp), so that at least partially compensates for a over the bearing (2) sloping bearing voltage (U _b ).

9. The device according to claim 8, characterized marked, that the evaluation and control unit (6) for determining the compensation voltage (U _k omp) is designed to invert the common mode voltage (Ugi) and with a constant factor (k _gesain t BVR, kkoppei)

10. The device according to claim 9, characterized in that the evaluation and control unit (6) is adapted to determine the constant factor (kg _it amt; BVR, k _co pp _e i) during operation or at least adapt.

11. The device according to claim 9, characterized in that the constant factor (k _gesaint ; BVR, k _kop p _e i) is deposited as a predetermined size in the evaluation and control unit (6).

12. The device according to claim 8, characterized in that capacitive coupling means (8) for applying the compensation voltage (Ukoiπp) are provided on a particular movable component (8) of the electric machine.

13. The apparatus according to claim 8, characterized in that a sensor (9) for detecting the above the bearing (2) pending bearing voltage (U _b ) or of the bearing (2) flowing bearing current (Ib) is provided.

14. The apparatus according to claim 13, characterized in that a feedback branch (10) between the sensor (9) and the evaluation and control unit (6) is provided.

15. The apparatus of claim 13, characterized in that the evaluation and control unit (6) is adapted to adapt a time course of the compensation voltage (U _k omp) to a time course of the detected bearing voltage (Ub) or the detected bearing current (Ib).