WO2011103883A1

WO2011103883A1 - Method for detecting plasma bearing currents

Info

Publication number: WO2011103883A1
Application number: PCT/EP2010/001150
Authority: WO
Inventors: Sven Gattermann; Jörg HASSEL; Ralf Kowalewski; Reinhard Maier; Carsten Probol; Arno Steckenborn; Hans Tischmacher
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-02-24
Filing date: 2010-02-24
Publication date: 2011-09-01
Also published as: EP2513642A1; CN102770757A; CN102770757B

Abstract

In electric machines, electric currents that significantly reduce the service life of the bearings can occur. Bearing currents are electric currents that occur in rolling or plain bearings of electric machines. The invention relates to a method for the early detection of the development of damage in a bearing, in particular crater formation, owing to bearing currents as a result of a lack of heat dissipation in the case of a short discharge time, plasma formation being detected on the basis of the evaluation of an electromagnetic spectrum of the field strength that arises during the discharge. The solution is based on the following basic concepts: The discharges due to the bearing currents must be considered as a chronologically dynamic process. A discharge must occur within a short time, e.g., in the three-digit picosecond range, in order to produce craters that lead to groove formation. If the discharge occurs slowly, only melting and solidification with crater formation and low material removal occur. Said craters do not lead to groove formation. In contrast, if a faster discharge occurs, the heat dissipation is less.

Description

description

Method for the detection of plasma bearing currents Field of the invention

In electric machines, electric currents (bearing currents, English: Bearing Currents) occur, which significantly reduce the life of the bearings. Bearing currents are electrical currents that take place in rolling or plain bearings of electrical machines.

They are caused by electrical voltages (bearing stresses) that occur due to electrical or magnetic stray fields within the machine or due to external currents that flow from the outside through the machine. Once the bearing voltage above the breakdown voltage

(Frittspannung) of the lubricating film, the current flow takes place.

The negative effects of bearing currents are, for example

- Fat burning (reduction of residual lubricity)

- Crater formation in the raceway and the rolling elements

- and in extreme cases: corrugation in the raceways.

The corrugations are aligned transversely to the raceway.

These bearing currents have been a well-known phenomenon for decades and result in considerable expense for users and possibly high warranty costs for manufacturers. There is therefore great interest in a measuring method or on sensors that can measure bearing currents and evaluate them meaningfully.

State of the art

There are established measuring methods and sensors for bearing currents, which, however, are expensive to perform and at the assessment of the bearing currents has considerable uncertainties.

The assessment of whether bearing currents in electrical machines can already reach a detrimental extent or can still be tolerated is only possible in the prior art at very high or very low bearing currents. In between, there is a large gray area, which comprises a considerable proportion of propulsion systems, in which no information is currently available.

Remedies against bearing currents are often associated with considerable costs and, because of the large gray area, make it difficult to assess whether they will be sufficient. In the past, however, cost-intensive measures sometimes did not lead to the desired goal.

The object of the invention is to provide a solution to the above problems. A measuring method should be specified which offers a better evaluation of the bearing currents with regard to a detrimental effect on the life of the bearing.

Presentation of the invention

This object is achieved by a method for the early detection of the formation of damage in a bearing, in particular cratering, by bearing currents due to a lack of heat dissipation during a short discharge time, wherein a detection of plasma formation takes place on the basis of the evaluation evaluation of a characteristic of the energy or discharge time Size, in particular an electromagnetic spectrum of the field strength which arises during the discharge, according to patent claim 1. The object is further achieved according to claim 7 by a device for early detection of the formation of damage, especially cratering, in a warehouse by bearing flows due to lack of heat discharge at short discharge time, with means for measuring at least one evaluation of a characteristic of the energy or discharge time Size, in particular the field strength in the discharge depending on the frequency are provided and with means for evaluating an electromagnetic spectrum of the field strength which arises during the discharge.

The solution is based on the following basic idea: The discharges caused by the bearing currents must be regarded as a temporally dynamic process. Previous considerations do not do this. A discharge must be within a short time, z. B. in the three-digit picosecond range, done to produce craters, which lead to rippling.

If the discharge is too slow, z. B. in the nanosecond range, only a Aufschmelzung and solidification takes place with a crater formation with low material removal. These craters do not cause rippling.

If, on the other hand, a faster discharge occurs, the heat emission is less.

There are a number of different types of bearing currents. It was recognized that especially the bearing currents have a particularly damaging effect, which lead to the so-called corrugation, which are associated with the plasma formation and a particularly short discharge time. By contrast, slower discharges do not cause any damage. The processes were explained with a new perspective, taking into account the theory of heat dissipation.

The plasma discharges can be easily distinguished from other discharges by their electromagnetic interference spectrum. the. This makes it relatively easy to detect especially the particularly harmful plasma discharges.

The new method enables the simple evaluation of load currents, eg. B. during commissioning or conversion by a service technician or by accompanying measurements by a sensor during operation. A damage occurring in the future can therefore be determined in advance. A remedy can be assessed immediately with particularly high informative value. This reduces warranty costs, increases plant availability and reduces the effort for the plant operator.

Background is a flow of heat energy in the material at speeds in the range of about 2 pm / ns depending on the material. This concentrates the energy of the discharge in a smaller volume. Due to the higher energy per unit volume, the enthalpy for evaporation or plasma formation is exceeded. In particular, by the plasma discharge takes place a significantly higher material removal. A faster discharge does not allow the heat energy to flow away. This leads locally to a particularly high temperature, which leads to evaporation or plasma formation with high material removal.

Especially this type of discharge can be recognized by its electromagnetic interference spectrum. There is a radiation of electromagnetic energy in the microwave range, in a particularly advantageous manner this is observed in the frequency range 300 MHz to 3 GHz, partly even at even higher frequencies (up to 10 GHz). A detection of the plasma discharges on the basis of the characteristic frequency range enables a simple, immediate and particularly meaningful evaluation of the bearing currents.

Particularly advantageous embodiments are specified in the subclaims. Description of the preferred embodiments

Brief description of the drawings

In the following, the invention will be explained with reference to exemplary embodiments. Show

1 shows a schematic representation of the formation of corrugations during the discharge of bearing current,

FIG. 2 is a block diagram of the method for detection in an embodiment;

Figure 3 shows the microwave emission by plasma discharge at two different operating conditions, and

FIG. 4 shows an exemplary measuring arrangement. FIG. 1 shows a schematic representation of the plasma discharge in the time window tO-tl with crater formation and corrugations at t2 in the left-hand case and cratering without corrugation in the right-hand case of the illustration. The heated area is calculated according to the formula radius r = v * ti.

The theoretical crater diameter r is as follows: r = V _th * T * F

V _th is the propagation velocity of the thermal

Shockwave (speed of sound)

T is the pulse duration F is a shape factor for the current pulse time course and the temperature profile during propagation.

E denotes in FIG. 3 the electric field strength at a distance of about 5 cm from the bearing. Frequency is the frequency of the electric field strength spectrum.

"Ambient noise" refers to the ambient field strength, which is also measured when the inverter is switched off Tips in the curve show z. As telecommunications networks such as the D network at 950 MHz, the e-network at 1.85 GHz, or UMTS. It can be seen from the curve that at 200 revolutions per minute (200 rpm), the bearing currents show a clearly higher-frequency behavior than at 1500 revolutions per minute. The discharges through the bearing currents thus have a much shorter time constant (eg in the range of a few 100 ps). The short discharge in cooperation with the heat propagation only a very limited area is warmed ER, so that the material in the crater is herausge ^¬ hurls than plasma. This effect leads to the formation of corrugations, which represent damage to the bearing.

In the case of FIG. 3, for example, the damaging bearing currents could be reduced by modifying the motor / converter system for 200 revolutions per minute (rpm). The behavior at 1500 rpm, however, is acceptable, although bearing current events are present. An evaluation can likewise advantageously take place via an analysis of several spectral lines. In practice, measurements in the time domain may be performed at different frequencies 31 simultaneously or sequentially. A measurement in the time domain with sufficient sample rate and subsequent data processing z. B. by FFT 32 (English Fast Fourier Transformation) is also possible. Alternatively, similar to a heterodyne receiver (spectrum analyzer), a measurement may be performed. In all cases, by measuring at several frequencies, the drop in amplitude (eg field-bounded electric, magnetic, electromagnetic field, radiated power density or conducted voltage, current, power) can be determined with frequency. From this, the time constant of the discharge can be estimated. Alternatively, other parameters may be determined that aim to be a measure of the time in which the discharge occurs. A combination of amplitude evaluation and evaluation of the time constant may also be advantageous. An evaluation of the frequency of the discharges in combination with the frequency range and / or the amplitude for different frequencies also increases the significance. The evaluation can be done with statistical tools, for example in histograms.

Especially the detection via microwave radiation is advantageous. A line-bound measurement in the microwave frequency range is theoretically possible, but in practice more complex.

If frequencies are evaluated in which usual broadcasting services such as GSM and UMTS mobile radio, DECT and WLAN are omitted (ambient noise, see above), the signal-to-noise ratio improves. Even by combining the measurement at different frequencies, the statement ^¬ force can be increased. Furthermore, by gating converter converters, the signal-to-noise ratio can be increased if future converters with faster switching times (silicon carbide SiC technologies) are used.

FIG. 2 shows an exemplary block diagram of the method according to the invention. The evaluation of the Ma ^¬ machine with bearing currents 1 can also be limited to parts of the described evaluations. The evaluation and

Measured value processing 3 can also take place in a control unit of an inverter 5. Evaluation is carried out in a separate from the control units of the inverter hardware characteristics advantageously to control units of the inverter by radio, fiber optic or wired be ^¬ averages and forwarded from there to the system to initiate ei ^¬ ner message or action may or when loading may be readable on call. Also, an indicator may represent the bearing load, e.g. In the traffic light colors green / yellow / red. The following steps therefore comprise the method according to the invention:

- Detection especially of discharges caused by bearing currents in electrical machines, which are associated with particularly high material removal and thus have a particularly damaging effect. These discharges lead to rippling. The detection is based on the characteristic fingerprint in the frequency domain: In combination with the fast discharges required to prevent local heat dissipation for plasma discharges, especially high frequency components in electromagnetic interference spectrum and detection of these high frequency components in the microwave frequency range by a sensor arise

- If necessary, analog or digital signal processing for the suppression of interference signals, z. B. by mobile phones by taking into account the time course and the frequency ^¬ spectrum of the discharges. Here several ^¬ re frequencies can for example be evaluated and considered only when accompanied as plasma discharge.

- Evaluation of the measurements based on the recognized relationships between heat conduction, discharge time and material removal. A high frequency spectrum in combination with a high amplitude leads to a high material removal. High material removal leads to a reduction in the service life of the bearing due to corrugation.

- A supplementary measurement at frequencies below 300 MHz can also increase the significance if it is concluded, for example, on the amplitude or time constant. FIG. 4 shows an exemplary measurement setup. Darge ^¬ represents is a motor with two motor shafts W right and left of the rotor R and in each case a bearing L in the side view. The bearing current is a bearing voltage U _Lag he preceded. The engine mount L is completely or partially discharged by the bearing current with sparking. This creates a "voltage jump" that could be measured directly, eg. B. via a contact brush, which grinds on the motor shaft. However, it is particularly advantageous to measure the emission with an antenna A, since this is wear-free and emits the high frequencies of interest well. Contact brushes usually wipe off over time. Although carbon fiber contact brushes have a particularly low wear, but are expensive and can get contact problems with contamination.

Claims

claims

Method for early detection of the occurrence of damage in a bearing (bearing), in particular cratering, by bearing currents due to lack of heat dissipation in a short discharge time, characterized in that a detection of plasma formation takes place on the basis ^¬ evaluation of a characteristic of the energy or discharge time size, in particular the electromagnetic spectrum of the field strength which arises during the discharge.

Method according to claim 1, characterized in that

for measuring the bearing is set in rotation in a certain speed of rotation and

the measurement of the spectrum takes place within a certain frequency range, in particular in the frequency range between 300 MHz and 3 GHz.

Method according to one of the preceding claims, characterized in that

after the measurement, a filtering of the measured values takes place, in particular for the suppression of interfering signals of the environment.

Method according to one of the preceding claims, characterized in that

the evaluation takes place via an analysis of at least two frequencies and the measurement of the frequency to be evaluated takes place simultaneously or successively.

Method according to one of the preceding claims, characterized in that

the measurement is carried out in the time domain and in a further step, in particular by means of Fast Fourier Trans formation, a conversion takes place.

6. The method according to claim 4 or 5, characterized in that

an evaluation of at least two frequencies takes place and a determination is made of the drop in the amplitude as a function of the frequency.

7. A device for the early detection of the formation of damage, especially cratering, in a warehouse by bearing currents due to lack of heat flow in a short

Discharge time, characterized in that

Means are provided for measuring at least one characteristic of the energy or discharge time size, in particular the field strength in the discharge depending on the frequency, and

Means for evaluating the size, in particular an electromagnetic spectrum of the field strength which arises during the discharge.

8. The device according to claim 7, characterized by a bearing which is offset for measurement in rotation in a certain rotational speed and the measurement of the spectrum is carried out within a certain frequency range, in particular in the frequency range between 300 MHz and 3 GHz.

9. Device according to claim 7 or 8, characterized by

further means for filtering the measured values are provided, in particular for suppressing interference signals of the

Surroundings .

10. Device according to one of the claims 11 or 12, characterized by

Means for converting measured values, in particular by means of

Fast Fourier Transformation, where the measurement is performed in the time domain.