Detecting insufficiently compressed laminated cores of a rotor by means of a structure-borne-sound resonance detection using an electrodynamic structure-borne-sound excitation

Abstract

The invention relates to a method for detecting the connection state of laminated cores (P), which are compressed together, of a rotor (R) of an electric machine (EM). According to the invention, structure-borne sound is introduced into the shaft (W) or into the rotor (R) by generating a magnetic field which acts on the rotor (R). The sound, which propagates within the rotor (R), is detected while the structure-borne sound is being introduced and / or within a period of time which immediately follows the introduction of the structure-borne sound. It is ascertained whether the detected sound exhibits a resonance behavior (RV) within a frequency band (FB) which characterizes the vibration of an individual laminated core (P). A signal which represents laminated cores (P) that are insufficiently compressed together is output if a resonance behavior has been ascertained in the frequency band (FB), otherwise a signal is output which represents laminated cores (P) that are sufficiently compressed together.

Description

[0001] Detection of insufficiently pressed laminated cores of a rotor by structure-borne sound resonance detection with electrodynamic structure-borne sound excitation

[0002] Description

[0003] Electrical machines have a rotor that rotates relative to a stator. The stator can, for example, generate a rotating magnetic field that the rotor follows or that causes the rotor to rotate. For this to happen, the rotor must also be magnetic (at least in synchronous and DC machines), for which purpose permanent magnets, for example, are mounted in the rotor. Soft iron material is also used to guide the magnetic flux within the rotor, usually in the form of rotor laminations. These are stacked on top of each other. By using individual, electrically insulated soft iron laminations, eddy current losses within the soft iron material of the rotor can be greatly reduced.

[0004] The laminations can be provided as laminated cores, with the cores being pressed together. Pressing them together in the axial direction provides stiffening, particularly to prevent torsion and axial bending of the rotor. This is particularly relevant for power motors, which can be set into vibration due to excitation. This, in addition to bearing damage, can lead to unpleasant noises and interference signals in the electrical control system. With sufficiently pressed cores, the cores are firmly connected to one another by the pressing, so that the increased stiffness caused by the pressing leads to resonance effects only at very high frequencies and with low amplitudes, which are less detrimental.Such low resonance effects are associated with a compression that is sufficient to result in a high degree of rigidity in the connection of the laminated cores to one another.

[0005] It has been recognized that insufficient compression can lead to undesirable, strong resonance effects, which also occur at unfavorable, lower frequencies. The object of the invention is to provide a method for easily detecting insufficient compression between the laminated cores.

[0006] It is therefore proposed to detect insufficient compression between the laminations of a rotor through a vibration test, in which a vibration exciter (shaker) causes the shaft or rotor to vibrate at different frequencies in order to detect resonance effects. Insufficiently compressed laminations will vibrate individually and relative to each other, whereas sufficiently compressed laminations will behave like a continuous body with regard to vibrations or, due to the stiffening (created by the compression), vibrate as a single body.

[0007] If the laminations are insufficiently compressed, resonance effects occur in a frequency range that characterizes laminations vibrating relative to each other (and differs from the resonance frequencies of sufficiently stiffened or compressed laminations). The invention detects these resonance effects, particularly based on the frequency and the amplitude present there (or the frequency dependence of the amplitude), in order to identify a rotor as insufficiently compressed. The term "resonance effect" is synonymous with the term "resonance behavior."

[0008] The frequency range in which such resonance effects occurred, which are associated with insufficiently sealed laminations, is significantly below that of the resonance frequencies of a rotor stiffened by compression, which is why faulty (insufficiently sealed) laminations can be identified by a simple spectral analysis of the vibration test results.

[0009] It was discovered that the resonance effects that arise from insufficiently pressed laminated cores can be reliably detected even when the rotor is installed in the electric machine, or even when a gear or belt pulley is located on the rotor shaft, or even when the rotor is connected to other (external) components to transmit motion. The procedure described here therefore makes it possible to detect insufficiently pressed laminated cores within the rotor without disassembling the electric machine. This allows, in particular, a test of the pressing of the laminated cores during / after completion of the electric machine or during maintenance. In particular, an insufficiently pressed laminated core can also be detected when the electric machine is already installed in a vehicle, for example as a starter generator or as a traction electric motor.

[0010] A method is therefore proposed for detecting the connection state of laminated cores that are crimped together. The laminated cores are the laminated cores of a rotor of an electrical machine and, in particular, form the soft magnetic material in which permanent magnets can be embedded. The rotor can be a rotor of a permanently excited electrical machine or a separately excited electrical machine. The connection state refers, in particular, to the press connection (axial connection) between the various laminated cores that follow one another axially. In this case, the laminated cores are arranged on a shaft of the electrical machine and crimped together there. The connection state indicates whether the press connection between the laminated cores is rigid or whether movement between the laminated cores is possible, leading to vibration (movement of the laminated cores among one another).The connection state reflects the mechanical stiffness of the connection between (adjacent) end faces of the lamination stacks. The lamination stacks are stamped, aligned sheets of soft magnetic material, which are stacked on top of each other, preferably over insulating layers (e.g., varnish). The lamination stacks are torsionally connected to the rotor shaft. The shaft extends through the centers of the laminations.

[0011] To check the connection between the laminated cores (not just indirectly via the shaft, but the connection directly between the individual laminated cores), the rotor or shaft is set into vibration. To make this simple, structure-borne sound is introduced into the rotor (and thus also into the shaft) by exciting the stator using an electrical stator signal. In other words, structure-borne sound is generated electrodynamically and thus introduced into the rotor. At least one winding already present in the electrical machine is used here (stator winding, excitation winding of the rotor), which is also used to generate power during rotational operation (generation of a torque at a speed greater than zero) of the electrical machine. At least one of the stator windings of the stator is (electrically) excited using the stator signal.The stator signal is in particular an alternating current or an alternating voltage that is impressed on at least one stator winding or applied to at least one of the stator windings. The stator signal is an alternating signal. The electrical stator signal generates an alternating magnetic field in the stator. The magnetic field generates a force or a movement in the rotor. The magnetic field, which is generated by exciting the stator, acts on the rotor. The magnetic flux, which is generated by exciting the stator, is transferred to the rotor. The magnetic field generates a force through magnetostriction, reluctance and / or magnetostatics, and thus a movement or deformation (bending, torsion, etc.) of the rotor. The magnetic field generates structure-borne noise in the rotor (and also in its shaft due to the transmission of structure-borne noise from the rotor to the shaft) through magnetostriction, reluctance and / or magnetostatics.Its excitation by the stator signal thus functions as a vibration exciter, causing the rotor to vibrate. The introduction of structure-borne sound involves generating the structure-borne sound in the rotor or generating the structure-borne sound in the stator and transmitting it to the rotor.

[0012] The magnetic field acting on the rotor, which is generated by the stator signal from the stator, causes the rotor to oscillate, so that the rotor as a whole is excited to oscillate. Exciting the stator using the stator signal in particular causes the rotor or shaft to oscillate. Due to the physical connection between the laminated cores and the shaft of the rotor, every part of the rotor is usually set into vibration when one of these parts is excited. As a result, structure-borne sound or vibration is introduced into the shaft or rotor or generated there by the stator (in particular at least one stator winding) being excited by the stator signal. By operating the stator in this way, namely exciting the stator using the stator signal, the rotor is excited to oscillate, in particular the shaft or the laminated cores.

[0013] By generating the magnetic field, an acoustic signal is generated in the rotor. The acoustic signal comprises various frequencies that occur either simultaneously (e.g., as noise, particularly as white noise or as spectrally matched noise) or sequentially (e.g., in a sweep signal). The structure-borne sound generated in the rotor thus comprises various frequencies. Furthermore, by exciting the stator, structure-borne sound is generated in the stator itself. This structure-borne sound propagates particularly to the rotor, so that by generating the magnetic field, structure-borne sound (emanating from the stator) is also introduced into the rotor.

[0014] Furthermore, the sound, particularly structure-borne sound, propagating within the rotor is recorded. This is done while the structure-borne sound is being introduced or while the rotor is being set into vibration, or within a period immediately following the end of the structure-borne sound introduction. During the latter period, reverberation can thus be recorded, the spectrum of which is also characteristic of the connection state between the laminated cores.

[0015] Sound detection generally involves determining the vibration resulting from the introduction of structure-borne sound into the rotor. This determines the rotor's response to the introduced structure-borne sound, from which conclusions can be drawn about the connection status. The sound can be detected directly using structure-borne sound sensors, laser microphones, or airborne sound microphones, or it can be detected via the induced signal in a winding of the rotor and / or stator, which results from the structure-borne sound and the correspondingly changing magnetic ratio within the respective winding. In a sense, a winding of the electrical machine itself can thus be used as a (magnetodynamic) structure-borne sound microphone.

[0016] A structure-borne sound sensor can be used that is sensitive in all three spatial directions. The individual signals from each spatial direction can be evaluated individually or processed in combination. In general, at least one structure-borne sound sensor can be used. The at least one structure-borne sound sensor can be arranged on a peripheral surface (or end face) of the shaft; in general, the structure-borne sound detectable on the peripheral surface (or end face) of the shaft can be detected by a structure-borne sound sensor provided there. This also applies to the end face or outer surface of the rotor. Several structure-borne sound sensors can be provided on the peripheral surface of the shaft (or rotor) at a distance from one another (in particular evenly spaced).Several groups of structure-borne sound sensors can be provided, with the structure-borne sound sensors of a group being provided at the same longitudinal position on the shaft (or rotor) and being circumferentially (in particular evenly) spaced from one another. The groups can be arranged at different longitudinal positions (= axial positions) on the shaft (or rotor), i.e., they can be offset from one another in the axial direction of the shaft. In general, the structure-borne sound can be recorded at the locations where a structure-borne sound sensor can be provided, in particular by means of a laser microphone that replaces the structure-borne sound sensor.

[0017] The sound can be detected by detecting the structure-borne sound within the rotor or shaft, and can also be detected by detecting the vibration of a surface of the rotor or shaft, and can additionally be detected by detecting the airborne sound caused by the vibration of the rotor or shaft. When detecting sound, different frequencies are also detected, in particular frequencies that correspond to the introduced frequencies. The detection of the sound provides for the frequency dependence of the amplitude within the sound to be determined. The detection of the sound can provide for the measurement of a sound power, a sound velocity or a sound pressure, or for the detection of a signal that is induced in the rotor and / or the stator (for example in an excitation winding of the rotor and / or in a stator winding of the stator) by the structure-borne sound.In particular, a characteristic for the amplitude of the vibration of the rotor or shaft can be determined to detect the sound.

[0018] The recorded sound is spectrally analyzed. It is determined whether the recorded sound exhibits resonance behavior within a frequency band. This frequency band is characteristic of the vibration of an individual laminated core. In particular, the frequency band includes components that occur when individual laminated cores, but not sufficiently pressed together, are set into vibration. The frequency band covers a range in which the resonance frequencies of individual laminated cores fall that are not mechanically rigidly connected to other laminated cores at the end (but only via a shaft, for example). This frequency band covers in particular a range in which the resonance behavior or resonance effects of a rotor fall whose laminated cores are not or only insufficiently pressed together. The frequency band therefore covers frequencies that correspond to the mechanical resonance frequency of individual laminated cores (mounted on the shaft).

[0019] The resonance behavior can be represented by certain amplitude curves (depending on the frequency), by certain amplitude levels or strengths of the sound signal that are exceeded, or by a strong increase or decrease in the strength of the sound depending on the frequency, whereby the latter is typical for signals whose frequency is close to a resonance frequency of a system.

[0020] A signal is emitted that indicates the connection status, whereby the signal depends on whether or not significant resonance behavior has been detected in the frequency band. The signal can, for example, be an error signal that specifically identifies the electrical machine in question or the specific rotor. The signal can lead to the rotor or electrical machine in question being rejected or sorted out, or being pressed again, or at least being marked as faulty. A signal is emitted that indicates insufficiently pressed laminated cores if resonance behavior has been detected in the frequency band. Otherwise, a signal is emitted that indicates sufficiently pressed laminated cores. Laminated cores that cannot vibrate individually due to the way they are pressed together are referred to as sufficiently pressed laminated cores.Insufficiently pressed laminated cores are those that can vibrate individually, at least partially, due to their insufficient compression. The terms "sufficient" and "insufficient" thus reflect the stiffness of the connection, the connection condition of which is being recorded. Above a certain level of resonance, the rotor (or its laminated cores) is categorized as insufficiently pressed.

[0021] The sound can be detected by detecting the structure-borne sound propagating within the rotor. A structure-borne sound sensor can be used for this purpose. This sensor is preferably attached to the rotor in a sound-transmitting manner. In particular, it can be mounted at a different location on the rotor (or shaft) than the vibration exciter. Both can be attached to the same end of the rotor, in particular to the same end or end face of the shaft, but preferably radially and / or axially offset from each other.

[0022] The structure-borne sound sensor converts the vibration of the rotor or shaft into an electrical signal. The device used to record the sound, like the vibration exciter, can be part of a vibration testing system. The structure-borne sound sensor can be connected to the rotor or shaft directly or via a sound-transmitting connection. Sound can also be recorded by detecting vibrations of the surface of the rotor or shaft. This can be done contactlessly, for example, using a laser microphone. A laser microphone scans the surface, and by changing the interference of the emitted laser light with the reflected laser light, the vibration of the surface of the rotor or shaft can be recorded based on the (time-)varying brightness. Furthermore, sound can be recorded by detecting the (airborne) sound emitted by the rotor or shaft, or generally by the electrical machine.For this purpose, a microphone can be used that is located near the electrical machine and that is specifically aimed at it.

[0023] In addition, the structure-borne noise can be detected by detecting an induced signal generated by induction in at least one stator winding and / or in an excitation winding of the electric machine. The induced signal reproduces the structure-borne noise, so that the structure-borne noise propagating in the rotor is detected by detecting the induced signal. If the same stator winding is used to generate the magnetic field as for detecting the induced signal, or if different windings are used, the induced signal can be detected after the stator is excited, thus detecting the ringing.

[0024] A different winding can be used to generate the magnetic field than to detect the induced signal, for example by exciting the stator and detecting the induced signal at the rotor, by exciting the rotor and detecting the induced signal at the stator, or by exciting the stator at a first stator winding and detecting the induced signal at a second stator winding. In this case, the excitation signal (or the associated signal component) can be removed from the detected (induced) signal, in particular by subtracting the (amplitude-adjusted) excitation signal from the detected signal or by subtracting the signal component in the detected signal that correlates with the excitation signal (i.e.exceeds a predetermined correlation threshold), or by high-pass filtering (by means of which the excitation signal is strongly attenuated, while the signal intended for detecting the sound is less attenuated in comparison), or a combination of these. By detecting the induced signal, the sound propagating within the rotor is detected (among other things). The structure-borne sound can be introduced into the rotor (or into the shaft) as a longitudinal wave. Its (main) propagation direction can correspond to the axial extension direction of the rotor below the shaft. The propagation direction of the structure-borne sound or the longitudinal wave can also be in the tangential direction of the rotor or the shaft. Furthermore, the propagation direction of the longitudinal wave that reproduces the sound can be aligned in the radial direction of the rotor or the shaft.The orientation of the force resulting from the magnetic field can thus be radial, tangential, or axial with respect to the rotor or shaft. The terms radial, axial, and tangential refer to the circular-cylindrical shape of the shaft or rotor. The magnetic field can also generate an axial force in the rotor or shaft. The magnetic field can also generate a torsional force (acting in the circumferential direction of the rotor). Opposing stator windings of the stator can be excited with the same excitation signal or with complementary signals, in particular signals that are phase-shifted, for example by 90° or 180°.Furthermore, at least one stator winding of the stator and one excitation winding of the rotor can be excited with the same excitation signal or with mutually complementary signals, in particular signals that are phase-shifted from one another, for example signals that are phase-shifted by 90° or 180°. Furthermore, different stator windings arranged at different angles (<180°) in the stator can be excited by the electrical excitation signal, in order to generate, in particular, torsional vibrations. The different stator windings can be excited with the same excitation signal or can be excited with excitation signals that are phase-shifted or time-shifted from one another. The time offset can be selected such that the rotor executes a rotary movement with alternating directions of rotation. The deflection of the rotary movement can be very small (<10 pm <10 nm circumferential movement) and can, in particular, correspond to a desired sound velocity of the structure-borne sound.In particular, the excitation signal is such that a rotational movement of the rotor results that is less than 0.1°, 0.01° or 0.001°. This can be referred to as "essentially no" rotational movement. The angular velocity of the rotational movement is preferably above 500 or 1000 2*Pi / s, where 2*Pi denotes the full angle of the cross-section of the shaft / rotor. The electrical excitation signal is in particular an alternating signal with periodically changing polarity. The frequency components of the excitation signal in particular have a frequency of at least 100 Hz, 1000 Hz or 2000 Hz. The excitation signal is in particular such that the stator excited by the excitation signal does not generate a magnetic rotating field, preferably no rotating field that the rotor can follow (for example due to the excessive angular velocity differences between rotor and stator).The excitation signal is preferably not a multi-phase three-phase current, in particular not a multi-phase three-phase current designed to excite the rotor to a significant rotational movement (for example, by more than 0.01° or 0.001° or even 0.1° or 1°). The excitation signal is configured such that it does not generate a jerking movement in the rotor, but rather a continuous rotational movement with alternating directions of rotation and preferably with no significant deflection of the rotor, i.e., with a deflection of no more than 0.01° or 0.001° or 0.1° or 1°.

[0025] The frequency of the alternating signal can be variable. The excitation signal is, in particular, a sweep signal, i.e., an alternating signal with a (preferably continuously) variable frequency, preferably based on a sinusoidal signal of variable frequency. The excitation signal can also be a noise signal, for example, a signal that has a plurality of components of different frequencies at any given time. The excitation signal is, in particular, white noise or spectrally matched noise. The excitation signal thus has components of different frequencies that occur simultaneously (in the case of noise) or sequentially (in the case of a sweep signal). In particular, the excitation signal is not a signal that generates a significant rotational movement of the rotor, preferably neither starting nor braking. This allows increased reliability when carrying out the method.

[0026] In addition, the structure-borne sound can be introduced into the rotor at different positions relative to the stator of the electric machine. In this case, a pole piece surface of the rotor can be positioned centrally opposite the armature surface of the stator, or it can be positioned centrally opposite a slot in the stator. It can be provided that the stator is moved into different positions relative to the rotor during the introduction of the structure-borne sound. Preferably, the frequency range is scanned in each of these positions (or the sound is introduced for the frequencies of the frequency range and also recorded).

[0027] In the step of introducing the structure-borne sound, the stator is excited to generate a sound wave with a (main) propagation direction that is perpendicular or parallel to a longitudinal axis of the rotor. In particular, the vibration exciter can have multiple sound emission surfaces, with one sound emission surface having a normal that corresponds to the (main) propagation direction, and another sound emission surface that is perpendicular or at least inclined thereto.

[0028] To determine whether or not the detected sound within the frequency band exhibits resonance behavior, it can be determined whether the intensity of the detected sound within the frequency band lies above a predetermined threshold. Alternatively, it can be determined whether the intensity of the detected sound is greater than the intensity of the detected sound in a neighboring frequency band. The frequency band can be directly or indirectly adjacent to the first-mentioned frequency band. On the one hand, a predetermined threshold value can be used as a comparative standard, or the intensity of the sound in a neighboring frequency band can be considered as a standard for comparison. The neighboring frequency band is no more than a factor of 2, 5 or 10 away from the first-mentioned frequency band if the neighboring frequency band lies above the first-mentioned frequency band.The first-mentioned frequency band is separated from the adjacent frequency band by no more than a factor of 2, 5, or 10 if the adjacent frequency band lies below the first-mentioned frequency band. Furthermore, the resonance behavior can be captured by a dependence of the strength on frequency corresponding to the resonance behavior.

[0029] A (significant) resonance behavior can be detected as follows. If the intensity decreases particularly sharply with frequency within the frequency band, or if the intensity increases particularly sharply with frequency, or if there is a frequency-selective decrease in intensity with frequency or an increase in intensity with frequency within the frequency band, then this is characteristic of resonance behavior within the frequency band. It can thus be determined whether the absolute value of the derivative of the sound intensity (derivative of the sound intensity with respect to frequency) lies above a threshold value within the frequency band characteristic of individually vibrating laminated cores. The intensity of the sound can be determined from its amplitude, power, sound velocity or sound pressure, or electrical quantities derived from these, which are generated by a device for detecting structure-borne sound in the rotor (or in the shaft).

[0030] In addition to detecting whether the connection between the sheet stacks to be pressed is inadequate or not, it can also be determined whether there is bearing damage. This also makes it possible to determine whether the detected sound lies in a frequency band that is indicative of bearing damage. The same detected sound can be used for this purpose that is also used to detect resonance behavior. Only the frequency band is different, so that in addition to the first frequency band, another frequency band that is indicative of bearing damage is also considered in order to be able to detect bearing damage in addition to inadequate connection conditions. The frequency band that is indicative of bearing damage preferably does not overlap with the adjacent frequency band.

[0031] The introduction of structure-borne sound and / or the recording of sound can be carried out on an electrical machine that is installed, that is connected to other parts of a vehicle drive or output in a motion-transmitting manner, or whose housing has not been removed, and / or on whose shaft a gear or transmission wheel is mounted. In other words, the method can also be carried out on an electrical machine or rotor that is already fully assembled, and in particular that is already installed. It is not necessary to remove the housing or wheels (gears, pulleys, general output wheels) attached to the shaft, since the housing and the wheels have only a minor effect on the vibration behavior that arises when the laminated cores are not sufficiently stiffened (apart from a common attachment to a shaft).

[0032] The frequency band characteristic of the vibration of an individual laminated core extends from a lower limit frequency to an upper limit frequency. The frequency band is thus defined by its upper and lower limit frequencies. The lower frequency is not greater than 3000 Hz or 2500 Hz. The upper limit frequency is preferably not less than 4000 Hz or 4500 Hz. For example, the lower limit frequency is 3100 Hz, 2750 Hz or 2600 Hz. Furthermore, the upper limit frequency can be 3750 Hz or 4000 Hz. In particular, the excitation signal has no significant components (i.e. power component < 5% of the total signal power) with a frequency below 2500 Hz, 2000 Hz, 1500 Hz or even 1000 Hz or 500 Hz or 100 Hz.

[0033] The electrical machine is, in particular, a separately excited or self-excited synchronous machine. The excitation winding EW serves to excite the rotor for the rotational operation of the electrical machine (electric motor, generator) and is thus connected or connectable to an excitation signal generator (direct current, optionally adjustable) and can be connected or connectable to a detection circuit for detecting structure-borne noise. A signal conditioning circuit can be connected downstream of the detection circuit (e.g., an amplifier), which is configured to remove the excitation signal (or the associated signal component) from the detected (induced) signal, as described above. An evaluation circuit is connected downstream of the detection circuit and, if applicable, the signal conditioning circuit, which is configured to determine the resonance behavior or the frequency behavior due to bearing damage.The circuits described here can be implemented as analog circuits, as partially analog and partially digital circuits, or as digital circuits. In particular, the circuits or their functions can be implemented by at least one software module that implements the respective function. Figure 1 serves to further explain the functionality of the method described here, particularly using a symbolic representation of an electrical machine.

[0034] Figure 2 serves to illustrate vibration processes or resonance effects that can occur when laminated cores are inadequately connected.

[0035] Figure 3 serves as an example of resonance effects within the recorded sound.

[0036] Figure 1 symbolically shows an electric machine EM with a rotor R (inside the machine) that is surrounded by the stator of the electric machine. The stator has a stator yoke SJ and several pole cores PK, each of which is surrounded by a stator winding SW. The pole cores point toward the rotor R in a radial direction, with the longitudinal axis of the stator windings corresponding to this radial direction. When the windings SW are energized, a magnetic field is created that acts on the electric machine EM from the pole cores.

[0037] The rotor R has a magnetic polarization (magnetization), the direction RP of which is shown by the arrow in Fig. 1. The rotor can have at least one permanent magnet to represent the polarization. An embodiment is shown in which the rotor is magnetized by means of an excitation winding EW. For the rotational operation of the electric machine, the excitation winding EW is magnetized with a direct current in order to achieve a constant (possibly controllable) magnetization; to carry out the method described here, the excitation winding can be supplied with the excitation signal in order to introduce or generate structure-borne sound in the rotor. Alternatively or additionally, one or more stator windings SW can be supplied with the excitation signal in order to introduce or generate the structure-borne sound in the rotor. The stator yoke serves to guide the magnetic flux between the individual pole cores PK or stator teeth.In the rotor, the excitation winding EW is wound on the symbolically represented armature (diagonally hatched).

[0038] Fig. 1 shows a plan view of the end face of an electrical machine. Fig. 2 shows a rotor R from which a shaft W (or a section of this shaft) protrudes. The rotor shaft is not shown in Fig. 1 for clarity. The excitation signal generates an alternating magnetic field in the rotor and / or stator, which generates structure-borne noise and thus introduces it (directly or indirectly) into the rotor. Figure 2 provides a more detailed explanation of the vibrations resulting from structure-borne noise, which may occur depending on the connection between the core packs.

[0039] Figure 2 symbolically shows a rotor R with three laminated cores P mounted on a shaft W. For ease of understanding, the distances between the laminated cores P are clearly shown in order to illustrate the vibrations that occur when the connection between the cores P is insufficient and when it is sufficient. The upper double arrow SA symbolizes vibration when the connection is sufficient, i.e. a state in which the laminated cores P are firmly connected to one another (in particular via their end faces and not just via the common shaft W). In this case, the laminated cores P together form a body that can vibrate radially over the entire length of all the laminated cores P, as shown.

[0040] In contrast to the double arrow SA shown above in Fig. 2, if the connection between the lamination stacks P is insufficient, there is a possibility that the stacks P will vibrate independently. This is represented by the lower double arrows SU. The stiffness is not provided by a direct connection between the lamination stacks, but only by the fastening of the stacks P to a common shaft W.

[0041] It is easy to see that with sufficient stiffening, i.e. with a sufficient connection directly between laminated cores P (i.e. via their end faces or by a rigid connection on the outer surface of the laminated cores), these together form a single body, whereas with insufficient stiffening, i.e. with insufficient connection between the laminated cores P, they can vibrate individually. In the latter case, there is not the strong stiffening as in the former case, but only a connection via the shaft W, so that the laminated cores P can vibrate individually, with the resonance frequency being characterized by the stiffness of the shaft (and not by the stiffness of the connection between the cores P).

[0042] The individual vibration of the laminated cores P relative to each other is shown in Figure 3. There, the intensity ST of the recorded / detected sound is shown along the abscissa, while the frequency is shown along the ordinate. As mentioned, the structure-borne sound can be detected using a structure-borne sound sensor attached to the rotor (on the front side of the shaft, on the rotor, or on the circumferential surface), or by detecting the signal induced by the structure-borne sound in the rotor and / or stator (in particular, in a rotor winding such as the excitation winding EW or in at least one stator winding SW).

[0043] The diagram in Figure 3 shows the spectral behavior of the strength ST of the recorded sound. A first curve A shows the frequency behavior with sufficiently interconnected lamination stacks P. It is clearly evident that, particularly in the frequency band FB, there is a steady decrease in strength ST with frequency f. In contrast, curve U shows the behavior of the same rotor, but with insufficiently interconnected lamination stacks P. There, in the frequency band FB, it is evident that the individual lamination stacks can oscillate individually, with RV representing the resonance behavior. Thus, precisely in the frequency band FB, curve U shows a resonance-related, strong reduction in strength (depending on the frequency), followed by a strong resonance-related increase. This results in both a negative and a positive peak, which characterizes the resonance behavior that is characteristic of insufficiently interconnected lamination stacks P.Such behavior is not evident in curve A. From this, the spectral behavior in the frequency band FB can easily be used to determine the connection state.

[0044] On the one hand, the resonance behavior RV can be recorded because the strength of the sound ST exceeds a certain threshold value that is not exceeded by curve A. If the threshold value is -10 dB, for example, then the resonance behavior RV can be clearly recorded based on the high amplitude and can be differentiated from the (insignificant) resonance behavior in curve A. Furthermore, it can be seen that in curve U, compared to curve A, the strength ST changes very strongly with frequency, especially in the frequency band FB, both in the sense of a strong decrease and a strong increase depending on the frequency. This strong frequency dependence (amount of change in strength with frequency) offers a further possibility of recording resonance effects RV.Furthermore, it is possible, for example, to determine a strength in the adjacent frequency band X, such as the average strength across the frequency band X, in order to compare this (average) strength with the maximum strength of the sound in the frequency band FB. The adjacent frequency band X forms the norm, or threshold, against which the strength in the frequency band FB is compared. For example, the frequency band X lies between 2600 Hz and 2800 Hz. The frequency band X is designed in such a way that resonance effects caused by insufficiently connected laminated cores have only a minor impact in this frequency band X.

[0045] The slight frequency offset between the curves U and A in the range below 2750 Hz may be due to the insufficiently connected laminated cores P, because the connection state between laminated cores obviously also has an impact on the overall frequency behavior of the rotor.

[0046] Also shown is a frequency band LS, in which frequency dependencies of the sound intensity can be found, which are characteristic of bearing damage. This frequency band can therefore also be considered during sound analysis or recording to additionally identify bearing damage.

Claims

Patent claims 1 . Method for detecting the connection state of pressed-together laminated cores (P) of a rotor (R) of an electrical machine (EM), comprising the steps: Introducing structure-borne sound into the shaft (W) or into the rotor (R) by generating a magnetic field acting on the rotor (R) by exciting the stator (SW, PK, SJ) and / or the rotor using an electrical excitation signal; Detecting sound propagating within the rotor (R) during the introduction of the structure-borne sound and / or within a period of time immediately following the introduction of the structure-borne sound; Determining whether the detected sound exhibits a resonance behavior (RV) within a frequency band (FB) characteristic of the vibration of an individual laminated core (P); Emitting a signal representing insufficiently pressed laminated cores (P) if a resonance behavior in the frequency band (FB) has been determined, and, otherwise, emitting a signal representing sufficiently pressed laminated cores (P).

2. The method according to claim 1, wherein the detection of sound is carried out by detecting the structure-borne sound propagating within the rotor by means of a structure-borne sound sensor (A) which is attached to the rotor (R) in a sound-transmitting manner, by detecting vibrations of the surface (O) of the rotor (R) or of the shaft (W) by means of a laser microphone or by detecting sound which is emitted by the rotor (R) or of the shaft (W) by means of a microphone or by detecting a signal which is induced by the structure-borne sound in the rotor and / or in the stator.

3. Method according to claim 1 or 2, wherein the structure-borne sound is introduced into the rotor (R) as a longitudinal wave, the propagation direction of which corresponds to an axial extension direction (a) of the rotor (R) or the shaft (W), the propagation direction of which corresponds to a tangential direction (t) of the rotor (R) or the shaft (W), or whose direction of propagation corresponds to a radial direction (r) of the rotor or the shaft.

4. The method according to claim 1, 2 or 3, wherein the stator (SW, PK, SJ) has stator windings (SW), wherein the stator (SW, PK, SJ) is excited with the excitation signal by impressing an alternating electrical current into at least one stator winding (SW) of the stator or by applying an alternating electrical voltage to at least one stator winding (SW) of the stator or wherein the rotor has an excitation winding (EW), wherein the rotor is excited with the excitation signal by impressing an alternating electrical current into the excitation winding (EW) or by applying an alternating electrical voltage to the excitation winding (EW).

5. Method according to one of the preceding claims, wherein the stator (SW, PK, SJ) is excited with the electrical excitation signal which is fed to at least one stator winding of the stator via at least one ohmic load resistor, or by connecting a secondary winding of a transformer to at least one stator winding of the stator, and an alternating signal is applied to a primary winding of the transformer, or wherein the rotor is excited with the electrical excitation signal which is fed to an excitation winding (EW) of the rotor via an ohmic load resistor, or by connecting a secondary winding of a transformer to an excitation winding (EW) of the rotor, and an alternating signal is applied to a primary winding of the transformer.

6. Method according to one of the preceding claims, wherein the determination of whether the detected sound within the frequency band (FB) has a resonance behavior (RV) has a frequency dependence which is characteristic of a bearing damage, is carried out by determining whether the intensity (ST) of the detected sound within the frequency band (FB) is above a predetermined threshold value or is above the intensity (ST) of the detected sound in an adjacent frequency band (X), or whether, within the frequency band (FB) characteristic of the vibration of an individual laminated core, the magnitude of the derivative of the intensity (ST) of the sound with variable frequency is above a threshold value.

7. Method according to one of the preceding claims, wherein it is further determined whether the detected sound is in a frequency band (LS) which is characteristic of a bearing damage, or whether the detected sound has a frequency dependence which is characteristic of a bearing damage.

8. Method according to one of the preceding claims, wherein the steps of introducing structure-borne sound and detecting sound are carried out on an electrical machine (EM) which is installed in a vehicle or which is connected in a motion-transmitting manner to other parts of a vehicle drive or vehicle output, or whose housing is not removed or on whose shaft a gear or transmission wheel is mounted.

9. Method according to one of the preceding claims, wherein the frequency band (FB) which is characteristic of the vibration of an individual laminated core (P) extends from a lower limit frequency to an upper limit frequency, the lower limit frequency is not greater than 3000 Hz or 2500 Hz and the upper limit frequency is not less than 4000 Hz or 4500 Hz.

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