Detection of insufficiently pressed lamination stacks of a rotor by structure-borne sound resonance detection with electrodynamic structure-borne sound excitation

Abstract

A method for determining the contact state of pressed-together laminated cores (P) of a rotor (R) of an electric machine (EM) involves introducing structure-borne sound into the shaft (W) or into the rotor (R) by generating a magnetic field acting on the rotor (R). The sound propagating within the rotor (R) is detected during the introduction of the structure-borne sound and / or for a period immediately following its introduction. It is determined whether the detected sound exhibits resonance behavior (RV) within a frequency band (FB) characteristic of the vibration of a single laminated core (P). A signal representing insufficiently pressed-together laminated cores (P) is output if resonance behavior is detected in the frequency band (FB); otherwise, a signal representing sufficiently pressed-together laminated cores (P) is output.

Description

[0001] Electric 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 sets the rotor in motion. For this to work, the rotor must also be magnetic (at least in synchronous and DC machines), for which permanent magnets are typically mounted within the rotor. Soft iron material, usually in the form of rotor laminations, is used to guide the magnetic flux within the rotor. These laminations are stacked on top of each other. By using individual, electrically insulated soft iron laminations, eddy current losses within the rotor's soft iron material can be significantly reduced.

[0002] The laminations can be arranged as stacks, with the stacks being pressed together. Pressing them together in the axial direction results in stiffening, particularly to prevent torsion and axial bending of the rotor. This is especially relevant for power motors, which can be set into vibrations by excitation. These vibrations can lead to bearing damage and, in particular, unpleasant noises and interference signals in the electrical control system. With sufficiently pressed stacks, the laminations are firmly bonded together by the pressing process, so that the increased stiffness only leads to resonance effects at very high frequencies and with low amplitude, which are less detrimental.Such low resonance effects are associated with a compression that is sufficient to result in high stiffness in the connection of the sheet metal stacks to each other.

[0003] It has been recognized that insufficient compression can lead to undesirable, strong resonance effects, which also occur at unfavorable, lower frequencies. An object of the invention is to provide a means by which insufficient compression between the laminated cores can be easily detected.

[0004] It is therefore proposed that insufficient compression between the laminated cores of a rotor be detected by a vibration test, in which a vibration exciter (shaker) sets the shaft or rotor into vibrations of different frequencies in order to detect resonance effects. With insufficiently compressed laminated cores, these vibrate individually and in relation to each other, whereas with sufficiently compressed laminated cores, they behave like a single, continuous body with regard to vibrations, or, due to the stiffening (created by the compression), vibrate as a unified body.

[0005] If the laminated cores are insufficiently pressed, resonance effects occur in a frequency range that characterizes lamination stacks vibrating relative to each other (and differs from the resonance frequencies of sufficiently stiffened or pressed laminated cores). According to the invention, these resonance effects are detected, in particular based on the frequency and the amplitude present therein (or the frequency dependence of the amplitude), in order to identify a rotor as insufficiently pressed. The term resonance effect is synonymous with the term resonance behavior.

[0006] The frequency range in which such resonance effects occur, which are associated with insufficiently pressed laminated cores, lies significantly below that of the resonance frequencies of a rotor stiffened by pressing, which is why faulty (insufficiently pressed) laminated cores can be identified by a simple spectral analysis of the result of the vibration test.

[0007] It has been recognized that the resonance effects resulting from insufficiently pressed laminated cores can be reliably detected even when the rotor is installed in the electric machine, even when a gear or pulley is located on the rotor shaft, or even when the rotor is connected to other (external) components for motion transmission. Therefore, the procedure described here makes it possible to identify insufficiently pressed laminated cores within the rotor without disassembling the electric machine. This allows, in particular, testing the lamination of the cores during / after the electric machine's assembly or during maintenance. Specifically, an insufficiently pressed laminated core can be detected even when the electric machine is already installed in a vehicle, for example, as a starter generator or traction electric motor.

[0008] A method for determining the contact state of pressed-together laminated cores is therefore proposed. These laminated cores are the cores of a rotor in an electric machine and, in particular, form the soft magnetic material in which permanent magnets can be embedded. The rotor can be that of a permanent magnet electric machine or a separately excited electric machine. The contact state refers specifically to the press fit (axial connection) between the various laminated cores that follow one another axially. Here, the laminated cores are arranged on a shaft of the electric machine and pressed together. The contact state indicates whether the press fit between the laminated cores is rigid or whether movement between the cores is possible, leading to vibration (movement of the laminated cores relative to one another).The connection state describes the mechanical stiffness of the connection between (adjacent) end faces of the laminated cores. The laminated cores are stamped, aligned sheets of soft magnetic material, preferably stacked on top of each other over insulating layers (e.g., varnish). The laminated cores are rotationally fixed to the rotor shaft. The shaft extends through the centers of the laminated cores.

[0009] To check the connections between the laminated cores (not just indirectly via the shaft, but the direct connections between the individual cores), the rotor or shaft is set into vibration. To simplify this process, structure-borne sound is introduced into the rotor (and thus also into the shaft) by exciting the stator with 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 electric machine is used (stator winding, rotor excitation winding), which is also used to generate power during rotation (generating torque at a speed greater than zero). The stator signal electrically excites at least one of the stator windings.The stator signal is, in particular, an alternating current or an alternating voltage that is impressed into or applied to at least one stator winding. 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 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 and thus a movement or deformation (bending, torsion, etc.) of the rotor through magnetostriction, reluctance, and / or magnetostatics. The magnetic field generates structure-borne sound in the rotor (and also in its shaft due to the transmission of structure-borne sound from the rotor to the shaft) through magnetostriction, reluctance, and / or magnetostatics. The stator, or rather...Its excitation by the stator signal thus functions as a vibration exciter, setting the rotor into vibration. Introducing structure-borne sound involves either generating the structure-borne sound within the rotor or generating it within the stator and transmitting it to the rotor.

[0010] The magnetic field acting on the rotor, generated by the stator signal, causes the rotor to vibrate, thus setting the entire rotor into vibration. Specifically, exciting the stator via the stator signal results in the rotor and / or shaft being set into vibration. Due to the physical connection between the rotor's laminated core and shaft, every part of the rotor is typically set into vibration when any one of these parts is excited.

[0011] This introduces or generates structure-borne sound or vibration into the shaft or rotor by exciting the stator (in particular at least one stator winding) using the stator signal. By operating the stator in this way, namely by exciting it with the stator signal, the rotor is set into vibration, in particular the shaft or the laminated cores.

[0012] Generating the magnetic field thus creates an acoustic signal in the rotor. This acoustic signal comprises various frequencies, which occur either simultaneously (e.g., as noise, particularly white noise or spectrally matched noise) or sequentially (e.g., in a sweep signal). The structure-borne sound generated in the rotor therefore includes various frequencies. Furthermore, exciting the stator generates structure-borne sound within the stator itself. This structure-borne sound propagates, in particular, towards the rotor, so that generating the magnetic field also introduces structure-borne sound (originating from the stator) into the rotor.

[0013] Furthermore, the sound is recorded, in particular structure-borne sound propagating within the rotor. This is carried out while the structure-borne sound is being introduced, or while the rotor is being set into vibration, or within a period of time immediately following the end of the structure-borne sound introduction. During the latter period, a reverberation can thus be recorded, the spectrum of which is also characteristic of the connection state between the laminated cores.

[0014] The detection of sound generally involves determining the vibration resulting from the introduction of structure-borne sound into the rotor. This allows the rotor's acoustic response to the introduced structure-borne sound to be determined, from which the connection status can be inferred. The sound can be detected directly using structure-borne sound transducers, 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 conditions within the winding. In a sense, a winding of the electric machine itself can thus be used as a (magnetodynamic) structure-borne sound microphone.

[0015] A structure-borne sound sensor sensitive in all three spatial directions can be used. The individual signals from each direction can be evaluated separately or combined and processed together. Generally, at least one structure-borne sound sensor can be used. This sensor can be located on a lateral surface (or end face) of the shaft; generally, the structure-borne sound detectable on the lateral surface (or end face) of the shaft can be detected by a sensor located there. This also applies to the end face or lateral surface of the rotor. Several structure-borne sound sensors can be spaced apart (especially uniformly spaced) on the lateral surface of the shaft (or rotor).Several groups of structure-borne sound sensors can be provided, with the sensors of each group being located at the same longitudinal position on the shaft (or rotor) and spaced circumferentially (in particular, uniformly) apart 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, structure-borne sound can be detected 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.

[0016] Sound can be determined by detecting structure-borne sound within the rotor or shaft, and furthermore by detecting the vibration of a surface of the rotor or shaft, and also by detecting the airborne sound generated by the vibration of the rotor or shaft. Various frequencies are also detected during sound detection, particularly frequencies corresponding to the input frequencies. Sound detection also includes determining the frequency dependence of the amplitude within the sound. Sound detection can involve measuring sound power, particle velocity, or sound pressure, or detecting a signal induced in the rotor and / or stator (e.g., in a rotor excitation winding and / or a stator winding) by structure-borne sound.In particular, a characteristic for the amplitude of the vibration of the rotor or shaft can be determined for the purpose of sound detection.

[0017] The recorded sound is analyzed spectrally. It is determined whether the recorded sound exhibits resonance behavior within a specific frequency band. This frequency band is characteristic of the vibration of a single 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 encompassing the resonance frequencies of individual laminated cores that are not rigidly connected to other laminated cores at their ends (but only, for example, via a shaft). This frequency band also covers a range encompassing the resonance behavior or resonance effects of a rotor whose laminated cores are not, or only insufficiently, pressed together. The frequency band thus covers frequencies corresponding to the mechanical resonance frequency of individual laminated cores (mounted on the shaft).

[0018] The resonance behavior can be represented by certain amplitude profiles (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, the latter being particularly typical for signals whose frequency is close to a system's resonance frequency.

[0019] A signal is emitted indicating the connection status, the nature of which depends on whether or not significant resonance behavior has been detected in the frequency band. This signal can, for example, be a fault signal, specifically identifying the electric machine or rotor in question. The signal may lead to the rotor or electric machine being discarded, sorted out, re-pressed, or at least marked as defective. A signal indicating insufficiently pressed laminated cores is emitted if resonance behavior has been detected in the frequency band. Otherwise, a signal indicating sufficiently pressed laminated cores is emitted. Sufficiently pressed laminated cores are defined as those that, due to their compression, cannot vibrate independently of one another.Laminated cores are defined as those that, due to insufficient compression, can vibrate independently, at least partially. The terms "sufficient" and "insufficient" thus describe the stiffness of the connection, the condition of which is being assessed. Above a certain degree of resonance, the rotor (or its laminated cores) is categorized as insufficiently compressed.

[0020] Sound can be detected by capturing the structure-borne sound propagating within the rotor. A structure-borne sound transducer can be used for this purpose. This transducer is preferably mounted on 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, especially the same end or face of the shaft, but preferably offset radially and / or axially from each other.

[0021] The structure-borne sound sensor converts the vibration of the rotor or shaft into an electrical signal. The device used to detect the sound, like the vibration exciter, can be part of a vibration testing system. The structure-borne sound sensor can be connected directly to the rotor or shaft or via a sound-transmitting connection. Alternatively, sound can be detected by sensing vibrations of the rotor or shaft surface. This can be done without contact, for example, using a laser microphone. In this case, a laser microphone scans the surface, and the vibration of the rotor or shaft surface can be detected by measuring the changes in the interference between the emitted and reflected laser light based on the (time-dependent) variation in brightness.Furthermore, sound can be detected by capturing the (airborne) sound emitted by the rotor or shaft, or more generally by the electrical machine. For this purpose, a microphone can be used, positioned near the electrical machine and specifically aimed at it.

[0022] Furthermore, structure-borne sound can be detected by capturing an induced signal generated by induction in at least one stator winding and / or in an excitation winding of the electrical machine. This induced signal reproduces the structure-borne sound, thus capturing the structure-borne sound propagated by the rotor. If the same stator winding is used to generate the magnetic field as for capturing the induced signal, or if different windings are used, the induced signal can be captured after excitation of the stator in order to detect the reverberation.

[0023] 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 process, the excitation signal (or the corresponding signal component) can be removed from the detected (induced) signal, in particular by subtracting the (amplitude-matched) excitation signal from the detected signal or by subtracting the signal component in the detected signal that correlates with the excitation signal (i.e.,a predefined correlation threshold is exceeded), or by high-pass filtering (which strongly attenuates the excitation signal while the signal intended for sound detection is attenuated less), or a combination thereof. By capturing the induced signal, the sound propagating within the rotor is detected (among other things).

[0024] Structure-borne sound can be introduced into the rotor (or shaft) as a longitudinal wave. Its (primary) propagation direction can correspond to the axial direction of the rotor beneath the shaft. The propagation direction of the structure-borne sound or the longitudinal wave can also be tangential to the rotor or shaft. Furthermore, the propagation direction of the longitudinal wave, which reproduces the sound, can be radial to the rotor or shaft. The orientation of the force resulting from the magnetic field can therefore 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. Furthermore, the magnetic field can generate a torsional force (acting in the circumferential direction of the rotor).Opposite stator windings can be excited with the same excitation signal or with complementary signals, in particular signals that are out of phase with each other, for example by 90° or 180°. Furthermore, at least one stator winding and one rotor excitation winding can be excited with the same excitation signal or with complementary signals, in particular signals that are out of phase with each other, for example by 90° or 180°. In addition, different stator windings arranged at different angles (< 180°) within the stator can be excited by the electrical excitation signal, in order to generate torsional vibrations in particular. Here, the different stator windings can be excited with the same excitation signal or with excitation signals that are out of phase with each other or out of time.The time offset can be selected such that the rotor performs a rotational movement with alternating directions. The displacement of the rotational movement can be very small (< 10 µm < 10 nm circumferential movement) and can, in particular, correspond to a desired particle velocity of the structure-borne sound.

[0025] Furthermore, structure-borne sound can be introduced into the rotor in different positions relative to the stator of the electric machine. In this case, a pole shoe surface of the rotor can be positioned centrally opposite the armature surface of the stator, or centrally opposite a slot of 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 covered in each of these positions (i.e., the sound is introduced and recorded for the frequencies of the frequency range).

[0026] In the step of introducing 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 several sound emission surfaces, one of which has a normal corresponding to the (main) propagation direction, and another which is perpendicular or at least inclined to this normal.

[0027] To determine whether the detected sound exhibits resonance within the frequency band, it can be ascertained whether the intensity of the detected sound within the frequency band exceeds a predefined threshold. Alternatively, it can be determined whether the intensity of the detected sound exceeds the intensity of the detected sound in an adjacent frequency band. The adjacent frequency band can be directly or indirectly adjacent to the first-mentioned frequency band. Thus, a predefined threshold can be used as a comparative standard, or the intensity of the sound in an adjacent frequency band can be considered the standard for comparison. The adjacent frequency band is no more than a factor of 2, 5, or 10 away from the first-mentioned frequency band if the adjacent frequency band lies above the first-mentioned frequency band.The first-mentioned frequency band is no more than a factor of 2, 5, or 10 apart from the adjacent frequency band if the adjacent frequency band lies below the first-mentioned frequency band. Furthermore, the resonance behavior can be determined by a dependence of the strength on the frequency corresponding to the resonance behavior.

[0028] A (significant) resonance behavior can be detected as follows. If the sound intensity decreases particularly sharply with frequency within the frequency band, or if the sound intensity increases particularly sharply with frequency, or if there is a frequency-selective decrease or increase in intensity with frequency within the frequency band, then this is characteristic of a resonance behavior within the frequency band. It can thus be determined whether, within the frequency band that characterizes individually vibrating laminated cores, the magnitude of the derivative of the sound intensity (derivative of sound intensity with respect to frequency) exceeds a threshold value. The sound intensity can be defined as its amplitude, power, particle velocity, or sound pressure, or as derived electrical quantities generated by a device for detecting structure-borne sound in the rotor (or shaft).

[0029] In addition to detecting whether the connection between the sheet metal stacks to be pressed is insufficient, it is possible to detect whether bearing damage is present. Furthermore, it can be determined whether the detected sound lies within a frequency band characteristic of bearing damage. The same sound sample used to detect resonance behavior can be used for this purpose. Only the frequency band differs, so that in addition to the first frequency band, another frequency band characteristic of bearing damage is also considered in order to detect bearing damage as well as insufficient connection conditions. The frequency band characteristic of bearing damage preferably does not overlap with the adjacent frequency band.

[0030] The introduction and / or detection of structure-borne sound can be performed on an electrical machine that is installed, connected to other parts of a vehicle drive or output system, 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 performed on electrical machines or rotors that are already fully assembled, and in particular, already installed. It is not necessary to remove the housing or gears (gears, pulleys, or output gears in general) attached to the shaft, since the housing and gears have only a negligible effect on the vibration behavior that results when the laminated cores are not sufficiently stiffened to one another (apart from a common mounting on a shaft).

[0031] The frequency band characterized for the vibration of a single laminated core extends from a lower cutoff frequency to an upper cutoff frequency. The frequency band is thus defined by its upper and lower cutoff frequencies. The lower cutoff frequency is not greater than 3000 Hz or 2500 Hz. The upper cutoff frequency is preferably not less than 4000 Hz or 4500 Hz. For example, the lower cutoff frequency is 3100 Hz, 2750 Hz, or 2600 Hz. Furthermore, the upper cutoff frequency can be 3750 Hz or 4000 Hz.

[0032] The electric 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 electric machine (electric motor, generator) and is therefore connected to, or can be connected to, an excitation signal generation (direct current, possibly adjustable) and can be connected to, or can be connected to, a detection circuit for structure-borne sound. A signal conditioning circuit may be connected downstream of the detection circuit (e.g., an amplifier), which is designed to remove the excitation signal (or the corresponding 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 designed to determine the resonance behavior or the frequency response caused by bearing damage.The circuits described here can be implemented as analog circuits, as partly analog and partly digital circuits, and as digital circuits. In particular, the circuits or their functions can be realized by at least one software module that implements the respective function.

[0033] The Fig. Figure 1 serves to explain in more detail the functioning of the method described here, in particular by means of a symbolically represented electrical machine.

[0034] The Fig. Figure 2 serves to illustrate vibration processes or resonance effects that can occur with insufficiently connected laminated cores.

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

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

[0037] The rotor R has a magnetic polarization (magnetization), the direction of which RP is indicated by the arrow of the Fig. Figure 1 shows the rotor. 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 rotational operation of the electric machine, the excitation winding EW is magnetized with a direct current to achieve a constant (possibly controllable) magnetization. To carry out the method described here, the excitation winding can be supplied with the excitation signal to introduce or generate structure-borne sound in the rotor. Alternatively or additionally, a stator winding SW or several stator windings SW can be supplied with the excitation signal to introduce or generate structure-borne sound in the rotor.

[0038] 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 armature (shown diagonally hatched), which is represented symbolically.

[0039] The Fig. Figure 1 shows a top view of an end face of an electric machine. Fig. Figure 2 shows a rotor R from which a shaft W (or a section of this shaft) protrudes. For clarity, the rotor shaft is shown in the Fig. 1 not shown. The excitation signal generates an alternating magnetic field in the rotor and / or stator, which in turn generates structure-borne sound and thus introduces it (directly or indirectly) into the rotor. Fig. Section 2 serves to explain in more detail the vibrations resulting from structure-borne sound, which vary depending on the connection between bleach packages.

[0040] Fig. Figure 2 symbolically shows a rotor R with three laminated cores P mounted on a shaft W. For clarity, the distances between the laminated cores P are clearly shown to illustrate the vibrations that occur with insufficient and sufficient connection between the cores P. The upper double arrow SA symbolizes a vibration with sufficient connection, i.e., a state in which the laminated cores P are firmly connected to each other (specifically via their end faces and not just via the common shaft W). In this case, the laminated cores P together form a single body that, as shown, can vibrate radially along the entire length of all laminated cores P.

[0041] In contrast to these in Fig. As shown in the double arrow SA above, if the connection between the laminated cores P is insufficient, the cores P may vibrate independently. This is illustrated by the double arrows SU below. In this case, stiffness is not achieved through a direct connection between the laminated cores, but only through the mounting of the cores P on a common shaft W.

[0042] It is readily apparent that with sufficient stiffening, i.e., with sufficient direct connection between the laminated cores P (i.e., via their end faces or through a rigid connection on the lateral surface of the laminated cores), they together form a single body, whereas with insufficient stiffening, i.e., with insufficient connection between the laminated cores P, they can oscillate independently of each other. In the latter case, the strong stiffening of the former is not achieved, but only a connection via the shaft W, allowing the laminated cores P to oscillate individually, with the resonant frequency being characterized by the stiffness of the shaft (and not by the stiffness of the connection between the cores P).

[0043] The individual oscillation of the lamination stacks P relative to each other is from Fig. Figure 3 shows the intensity ST of the recorded / detected sound on the abscissa, while the frequency is shown on the ordinate. As mentioned, structure-borne sound can be detected using a structure-borne sound sensor attached to the rotor (at the end 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 (especially in a rotor winding such as the excitation winding EW or in at least one stator winding SW).

[0044] The diagram of Fig.Figure 3 shows the spectral behavior of the intensity ST of the recorded sound. A first curve A shows the frequency response with sufficiently interconnected laminated cores P. It is clearly evident that, particularly in the frequency band FB, there is a continuous decrease in the intensity ST with frequency f. In contrast, curve U shows the behavior of the same rotor, but with insufficiently interconnected laminated cores P. In the frequency band FB, it is evident that the individual laminated cores can vibrate independently, with RV representing the resonance behavior. Curve U thus exhibits a strong resonance-induced decrease in intensity (dependent on frequency) in the frequency band FB, followed by a strong resonance-induced increase. This results in both a negative and a positive peak, which characterizes the resonance behavior typical of insufficiently interconnected laminated cores P.Such behavior is not evident in curve A. Therefore, the connection state can readily be deduced from the spectral behavior in frequency band FB.

[0045] Firstly, the resonance behavior RV can be detected because the sound intensity ST exceeds a certain threshold that curve A does not. If the threshold is, for example, -10 dB, then the resonance behavior RV can be clearly detected based on the high amplitude and distinguished from the (non-significant) resonance behavior in curve A. Furthermore, it is evident that in curve U, compared to curve A, the intensity ST changes very strongly with frequency, particularly in the frequency band FB, exhibiting both a sharp decrease and a sharp increase depending on the frequency. This strong frequency dependence (the magnitude of the change in intensity with frequency) offers another way to detect resonance effects RV.Furthermore, it is possible, for example, to determine a level in the adjacent frequency band X, such as the average level across frequency band X, in order to compare this (average) level with the maximum level of the sound in frequency band FB. Here, the adjacent frequency band X forms the norm, or threshold, against which the level in frequency band FB is compared. Frequency band X lies, for example, between 2600 Hz and 2800 Hz. Frequency band X is designed in such a way that resonance effects caused by insufficiently connected lamination stacks have only a minor impact in this frequency band X.

[0046] The slight frequency offset between curves U and A, approximately in the range below 2750 Hz, may result from the insufficiently connected laminated cores P, because the connection state between laminated cores apparently also has an effect on the overall frequency behavior of the rotor.

[0047] Also shown is a frequency band LS in which frequency dependencies of the sound intensity are found that are characteristic of bearing damage. This frequency band can therefore also be considered during the evaluation or recording of the sound in order to additionally identify bearing damage.

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 that represents insufficiently pressed together laminated cores (P) if a resonance behavior in the frequency band (FB) has been determined, and, otherwise, emitting a signal that represents sufficiently pressed together laminated cores (P). [2] 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 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 whose propagation direction corresponds to an axial extension direction (a) of the rotor (R) or the shaft (W), whose propagation direction corresponds to a tangential direction (t) of the rotor (R) or the shaft (W), or whose propagation direction corresponds to a radial direction (r) of the rotor or the shaft. [4] 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 via at least one ohmic load resistor to at least one stator winding of the stator, 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 via an ohmic load resistor to an excitation winding (EW) of the rotor, 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 determining whether the detected sound within the frequency band (FB) has a resonance behavior (RV) has a frequency dependence which is indicative 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 above the intensity (ST) of the detected sound in an adjacent frequency band (X), or whether within the frequency band (FB) which is indicative of the vibration of an individual laminated core, the amount of the derivative of the intensity (ST) of the sound with a 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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