IMPROVED METHOD FOR DETERMINING THE LOSS CURRENT OF A ROTOR ELECTRIC MACHINE

Description

[0001] The invention relates to a method for determining a loss torque of an electric machine with a rotor, wherein the electric machine is mounted on a measuring device in such a way that an axial torque on the machine can be measured.

[0002] Document DE 102013109415 A1 states that it is known to determine the loss torque of a transmission component by measuring two torques and calculating the difference between them.

[0003] The resistance or load torque Mw applied to an electric machine during motor operation consists of a driving torque Ma and a loss torque Mv, which opposes the driving torque. The loss torque is caused primarily by air friction, bearing friction, and, depending on the electric machine, by brush friction and / or electrical eddy currents.

[0004] Document WO 2018 / 046296 A1 discloses a method for characterizing an electromechanical actuator unit, wherein a value for a loss torque of the electromechanical actuator unit is determined based on an idle current, the difference between the first value and the second value for the voltage induced in the electric motor and the inertia of the electromechanical actuator unit.

[0005] Document WO 2019 / 144171 A1 further discloses a measuring system for determining a force and / or a torque on a torque-transmitting shaft, wherein the measuring system has at least three, in particular at least four, piezoelectric elements, each with a preferred direction, which are arranged at different positions around an axis of rotation of the shaft in a force flow transmitted via the shaft, in such a way that a force of the force flow is applied, in particular exclusively, to the piezoelectric elements, wherein the preferred directions are each parallel to or in a single plane which is intersected by the axis of rotation, and wherein the preferred direction of at least two, in particular at least three, of the piezoelectric elements is neither parallel nor antiparallel to each other.

[0006] Document WO 2019 / 144172 A1 further discloses a measuring device for determining a force and / or a torque on a torque-transmitting shaft, which is supported by a bearing device, in particular on a machine whose output and / or input shaft is formed by the torque-transmitting shaft, wherein the measuring device has at least two, preferably three or four, piezoelectric elements in a fixing device, wherein the fixing device supports the piezoelectric elements and is designed in such a way that a force, in particular shear force, between the bearing device and the support device for supporting the bearing device can be measured by means of the piezoelectric elements.

[0007] It is an object of the invention to provide an improved method for determining the loss torque of an electric machine. In particular, it is an object of the invention to be able to determine the loss torque Mv without measuring electrical parameters of the electric machine.

[0008] This problem is solved by the doctrine of independent claims. Advantageous embodiments are claimed in dependent claims.

[0009] A first aspect of the invention relates to a method for determining a loss torque of an electric machine with a rotor, wherein the electric machine is mounted on a measuring device in such a way that an axial torque on the machine can be measured and wherein the rotor is freely rotating, comprising the following steps: First, operating the machine in such a way that the rotor reaches a predefined speed; second, operating the machine at idle when the predefined speed is reached, monitoring a signal from the measuring device representing the axial torque on the machine; identifying a jump in the signal; and determining the magnitude of the jump, the magnitude indicating the torque loss of the electric machine.

[0010] A second aspect of the invention relates to a system for determining a loss torque of an electric machine with a rotor, wherein the machine is mounted on a measuring device in such a way that an axial torque on the machine can be measured and wherein the rotor is freely rotating, comprising: Means for controlling the operation of the machine, configured to first operate the machine in such a way that the rotor reaches a predefined speed, and then to operate the machine at idle; means for monitoring a signal from the measuring device representing the axial torque of a machine; means for identifying a jump in the signal; and means for determining the magnitude of the jump, the magnitude being the loss torque of the electrical machine.

[0011] Freely rotating within the meaning of the invention preferably means not subjected to an external load.

[0012] A piezoelectric element according to the invention comprises at least one piezoelectric crystal and contacts for its electrical connection. Preferably, the piezoelectric element is designed as a sensor with further components such as a housing. More preferably, the piezoelectric crystal exhibits a piezoelectric effect.

[0013] A preferred direction within the meaning of the invention indicates in which direction of loading of the piezoelectric element, in particular by means of a shear force on its end face, the strongest stress is generated in the piezoelectric crystal of the piezoelectric element. The preferred direction is also referred to as the polarization direction.

[0014] An axial torque within the meaning of the invention is preferably the torque applied to the rotor of a machine.

[0015] In an idle state as defined by the invention, the control current preferably produces neither a drive nor a braking torque. More preferably, the control current is switched off.

[0016] The invention is based on the approach of determining the loss torque of an electric machine via a reaction torque to support the electric machine by setting the drive torque M a of the electric machine to zero.

[0017] According to the invention, this is achieved by first bringing the machine to a predefined rotational speed while it is freely rotating, and then abruptly switching to an idle mode of the electric machine while maintaining a constant rotational speed. As a result, there is a jump in the torque applied to the rotor 4, which corresponds to the axial reaction torque of the motor.

[0018] The method according to the invention makes it possible to directly measure the torque loss of an electric machine without having to apply a load to the rotor. In other words, the moment of inertia of the rotor is not distorted by a measuring device rotating with it. This allows the torque loss of the electric machine to be determined much more accurately.

[0019] According to the invention, electrical parameters such as induced voltages and currents of the electric motor do not need to be measured to determine the torque loss. In this respect, the invention has the advantage that the torque loss cannot be affected by any measurement. Furthermore, there is no need to first calculate the torque loss from electrical measurements using physical relationships. Compared to such a calculation, the inventive method for determining the torque loss is also significantly more accurate.

[0020] Furthermore, the method according to the invention can be carried out on test benches that are significantly simpler in design than, for example, those using a torque flange based on strain gauges. Also, according to the invention, no load machine is required. Moreover, no speed ramps need to be performed.

[0021] In an advantageous embodiment, the method further includes the following step: Filtering a signal using a low-pass filter, which preferably has a cutoff frequency of about 500 Hz.

[0022] Accordingly, the system for determining a loss moment preferably includes means for filtering the signal by means of a low-pass filter, which preferably has a cutoff frequency of about 500 Hz.

[0023] In a further advantageous embodiment, the method also includes the following step: First fitting of at least a first section of the signal before the identified jump and second fitting of at least a second section of the signal after the identified jump, wherein the magnitude of the jump is determined on the basis of the first and second fittings.

[0024] Furthermore, the system for determining a loss torque preferably includes means for first fitting the at least one first section of the signal before the identified jump and for second fitting the at least one second section of the signal after the identified jump, wherein the magnitude of the jump is determined by the means based on the first and second fittings. The torque jump in the measurement signal is a low-frequency signal with a small amplitude. Therefore, it is advantageous to filter the measurement signal to detect the jump.

[0025] Fitting allows for a more precise determination of the signal value in each section. A linear or quadratic fit is preferably used.

[0026] Fitting, as defined in the invention, is preferably a technique for optimally adapting a given mathematical model function to data points. More preferably, fitting is also called curve fitting. Preferably, fitting is achieved by minimizing the sum of the squared distances.

[0027] In a further advantageous embodiment of the method, the measuring device comprises a fixing device and at least one piezoelectric element, preferably three, more preferably four piezoelectric elements, each with a preferred direction, wherein the fixing device carries the at least one piezoelectric element and mounts the electrical machine over the at least one piezoelectric element in such a way that forces between the electrical machine and the fixing device can be measured by means of at least one piezoelectric element.

[0028] The use of piezoelectric elements as measuring elements in the measuring device enables a particularly precise determination of the step in the signal. Piezoelectric elements allow for highly dynamic measurement, so that they accurately depict the change in the signal from the value before the step to the value after the step.

[0029] Furthermore, by mounting the electric machine on at least one piezoelectric element, a reaction torque to the axial torque acting on the rotor can be measured – without taking measurements on the rotor itself. Piezoelectric elements are particularly well-suited for this purpose because they exhibit very high stiffness and sensitivity to highly dynamic vibrations.

[0030] In principle, it is possible to support the electric machine using only a single piezoelectric element and to support other support points using other support elements. However, part of the force flow then does not pass through the piezoelectric elements, but rather through the other support elements. Therefore, it is advantageous to use more piezoelectric elements to support the electric machine. In particular, it is advantageous to use two, three, or even four piezoelectric elements and then support the electric machine exclusively via the piezoelectric elements, so that the entire force flow generated by the electric machine passes through the piezoelectric elements. This results in particularly high measurement accuracy.

[0031] In a further advantageous embodiment of the method, the preferred direction or directions are each parallel to or in a single plane and the axis of rotation of the rotor intersects the plane at an angle between 45° and 135°, preferably between 85° and 95°, most preferably at least substantially perpendicular.

[0032] In this advantageous embodiment, the motor is supported at its front face by the fixing device. This arrangement allows the loss torque to be determined particularly easily using piezoelectric shear elements. In particular, the measurement signals from individual piezoelectric elements can be easily combined to determine force components.

[0033] In this context, "at least substantially in accordance with the invention" preferably means an angular range of + / - 1°.

[0034] In a further advantageous embodiment of the method, the piezoelectric elements are each arranged at different positions around a rotational axis of the rotor of the electric machine. This allows the electric machine to be supported particularly advantageously on the fixing device.

[0035] In a further advantageous embodiment of the method, the at least one piezoelectric element comprises a first sub-element and a second sub-element, each of which shear forces can be measured, wherein a preferred direction of the first sub-element is oriented at least substantially perpendicular to a preferred direction of the second sub-element, wherein the sub-elements are each arranged along their end faces relative to each other, and wherein a measurement of the first sub-element and a measurement of the second sub-element are taken into account in signals of the measuring device.

[0036] This makes the measuring device insensitive to the orientation of the preferred direction of the two sub-elements. In other words, as long as their relative orientation corresponds to the defined one, the two sub-elements can be arranged on the fixing device in any way, while a full measurement signal is always available. This significantly simplifies the assembly of the measuring device and avoids errors in determining a loss torque due to misalignment of one or more preferred directions.

[0037] In a further advantageous embodiment of the method, the preferred direction of the at least one piezoelectric element is aligned at least substantially tangentially to a direction of rotation of the rotor. This also allows for a simple calculation of the loss torque of the electric machine.

[0038] In a further advantageous embodiment of the method, the axial torque signal takes into account the respective angle between the preferred direction of the at least one piezoelectric element and the tangents to the direction of rotation at the locations of the at least one piezoelectric element. By considering the orientation of the preferred direction, the torque loss can be calculated with particular accuracy.

[0039] In a further advantageous embodiment, the preferred direction or directions are each parallel to or in a single plane, or the preferred direction or directions are each perpendicular to a single plane, wherein an axis of rotation of the rotor is aligned at least substantially parallel to the plane, in particular in a horizontal direction.

[0040] In this context, "at least substantially in accordance with the invention" preferably means an angular range of + / - 1°.

[0041] This allows the torque loss to be determined with the rotor's axis of rotation arranged horizontally, while the motor is in a horizontal position. This is particularly advantageous when the motor is mounted on a test bench where it will also be connected to a load machine in another measuring configuration. Furthermore, in this advantageous embodiment, the torque loss can be determined using piezoelectric pressure elements.

[0042] In a further advantageous embodiment of the method, the piezo elements are arranged at different positions and two piezo elements are arranged on one of the two sides of the fixing device with respect to the axis of rotation.

[0043] This symmetrical arrangement makes it particularly easy to determine the torque when the rotor's axis of rotation is aligned "horizontally".

[0044] In a further advantageous embodiment of the method, the piezoelectric elements comprise a first sub-element and / or a second sub-element, by means of which a shear force can be measured, and a third sub-element, by means of which a compressive force can be measured, wherein the sub-elements are arranged relative to each other along their end faces, and wherein a measurement of the first sub-element and / or the second sub-element and a measurement of the third sub-element are included in the signal of the measuring device. With this type of piezoelectric element, both shear forces and compressive forces can be measured.

[0045] In a further advantageous embodiment of the method, the measuring device further comprises a pre-tensioning device and first pre-tensioning elements, wherein the at least one piezoelectric element between the fixing device and the pre-tensioning device can be pre-tensioned or pre-tensioned by means of the first pre-tensioning elements in such a way that the at least one piezoelectric element is fixed by friction, and wherein the electric machine is fixed to the pre-tensioning device in a rotationally fixed manner.

[0046] Applying a preload using a preloading device allows for the exchange of electrical motors on the measuring device without affecting its calibration. Piezoelectric elements generally require preloading to exhibit the desired linear behavior. However, each preloading and unloading can alter the force flow and thus the calibration of the measuring device. With the preloading device, the system only needs to be preloaded once, and the calculations remain consistent. This allows for particularly rapid exchange of electrical motors.

[0047] If no pre-tensioning device is available, the piezo elements are fixed directly between the motor 2, in particular its housing, and the fixing device 5 by means of a force-fit connection.

[0048] In a further advantageous embodiment of the method, the electric machine is attached to the fixing device and is supported by the at least one piezoelectric element in such a way that the at least one piezoelectric element is fixed by friction.

[0049] This keeps the piezo elements in their position and eliminates the need for calibration when changing machines.

[0050] In a further advantageous embodiment of the method, the fixing device or the electrical machine is supported exclusively by the at least one piezoelectric element.

[0051] This means that the entire relevant force flow passes through the piezoelectric element(s). This results in particularly high measurement accuracy.

[0052] In a further advantageous embodiment, the signal of the axial moment is determined by means of a system of equations, in particular a linear system, based on measurements of the at least one piezoelectric element.

[0053] The loss torque can be determined with particular accuracy by using a system of equations. In particular, the preferred directions of the piezoelectric elements do not need to be precisely aligned, as the respective contribution of the piezoelectric elements to different force components can be taken into account in the linear system of equations.

[0054] Further advantages and features will become apparent from the following description in conjunction with the figures. They show, at least partially schematically: Figure 1 A partially transparent top view of a first embodiment of a measuring arrangement for determining a loss torque of an electric machine; Figure 2a partially transparent side view of the measuring arrangement according to Figure 1 ; Figure 3 a partially transparent top view of a second embodiment of the measuring arrangement for determining a loss torque of an electric machine; Figure 4 a partially transparent top view of a third embodiment of the measuring arrangement for determining a loss torque of an electric machine; Figure 5 a partially transparent side view of the measuring arrangement according to Figure 4 ; Figure 6 a partially transparent top view of a fourth embodiment of the measuring arrangement for determining a loss torque of an electric machine; Figure 7 a partially transparent side view of the measuring arrangement according to Figure 4 perpendicular to the axis of rotation; Figure 8 another partially transparent side view of the measuring arrangement according to Figure 4 parallel to the axis of rotation; Figure 9 an alternative embodiment of a piezoelectric element; Figure 10 another alternative embodiment of a piezoelectric element; Figure 11 a diagram showing the time course of the angular velocity of the rotor, the torque applied to the rotor and the control current as a function of time; Figure 12 another diagram, which again shows the control current and the torque Mz applied to the rotor as a function of time; Figure 13 a block diagram of a method for determining a loss torque of an electric machine; and Figure 14 An exemplary embodiment of a system for determining a loss moment.

[0055] Based on the Figures 1 and 2 A first embodiment of a measuring arrangement 1 for determining a loss torque of an electric machine 2 is explained. Figure 2 This is a partially transparent side view of the measuring arrangement 3. Figure 1 .

[0056] The electric machine 2 is preferably a motor with a rotating shaft 9, which has a rotor 4 that rotates on the rotating shaft 9 about an axis of rotation 7.

[0057] The measuring arrangement 1 includes a measuring device 3 for measuring torques. The measuring device 3 preferably includes a fixing device 5 and a preloading device 8. Piezoelectric elements 6i, 6ii, 6iii, and 6iv are arranged between the fixing device 5 and the preloading device 8, and are fixed and preloaded by means of preloading elements, in particular screws (not shown). The piezoelectric elements 6i to 6iv are preferably piezoelectric measuring elements that utilize the piezoelectric shear effect. This means that the piezoelectric elements measure shear forces FS,i to FS,iv that act in a plane in which the piezoelectric elements 6i to 6iv are arranged.

[0058] Motor 2 is in the one in the Figures 1 and 2 In the illustrated embodiment, the preload device 8 is fixed in a rotationally fixed manner. Thus, torques acting on the shaft 9 or the rotor 4 are transmitted via the housing of the motor 2 to the preloading element 8. The fixing device 5 is preferably mounted in a rotationally fixed manner, so that these torques induce the shear forces FS,i to FS,iv at the piezoelectric elements 6i to 6iv.

[0059] The electric motor 2 is freely suspended. This means that the rotor 4 rotates freely, i.e., it is not subjected to an external load. This is common to all embodiments of the measuring arrangement 1.

[0060] As from Figure 2 As can be seen, the measuring elements 6i to 6iv are at least partially incorporated into recesses of the fixing plate 5.

[0061] Preferably the pre-tensioning device 8, which is located in Figure 2The pre-tensioning plate has a recess through which the shaft 9 of the motor 2 can be guided. Furthermore, the fixing device 5, which is designed in Figure 2 It is designed as a fixing plate and has such a recess.

[0062] The force components measured by the piezoelectric elements 6i to 6iv are in Figure 1 shown. These are the force component in the Y-direction Fy, the force component in the X-direction Fx, and the axial torque about the axis of rotation 7 Mz. As shown in Figure 1 The preferred directions of the piezo elements, indicated by the vector arrows FS,i to FS,iv of the measured shear forces, are shown to be tangential to a direction of rotation of the rotor 4 about the axis of rotation 7.

[0063] In this case, the axial torque Mz, which is applied to motor 2, can be easily calculated using the following equation: Mz = − F S , i + F S , ii − F S , iii + F S , iv ∗ R where R is the distance of the piezo elements 6i to 6iv from the axis of rotation 7 of the rotor 4.

[0064] The signs preceding the measured shear forces result from the respective orientation of the preferred direction of the piezoelectric elements 6i to 6iv. The measured shear forces FS,i to FS,iv are calculated from the measurement signals S i , S ii , S iii , S iv, which are preferably specified in picocoulombs, and a respective sensor sensitivity, which is preferably specified in N / pC.

[0065] In the equation above, the respective angle β between the preferred directions of the piezoelectric elements 6i to 6iv and the respective tangent at the location of the piezoelectric element 6i to 6iv to the direction of rotation of the rotor 4 plays an important role. This angle β determines what proportion of a measured shear force FS is attributable to the tangential direction and thus to the torque Mz.

[0066] The tangential force Ftaus of the shear force FS is calculated according to the following equation: F t = F S ⋅ cos β

[0067] In the Figures 1 and 2 In the first embodiment of the measuring arrangement 1 shown, the angle β = 0° for the piezoelectric elements 6ii and 6iv and β = 180° for the piezoelectric elements 6i and 6iii. However, the individual piezoelectric elements 6i to 6iv can also be positioned arbitrarily. As described above, this means that not the entire force in the tangential direction is measured, but only a partial force Ft according to the equation above. In order to calculate the torque Mz according to the equation given above, the measured tangential component of the force F must be recalculated to 100% using a weighting factor. For example, for β = 45°, Ft would be 0.707 - FS. Therefore, the measured tangential force Ft would have to be multiplied by 1.293 to compensate for missing signal components.

[0068] To ensure a full measurement signal is always available, piezoelectric elements 6i can also be used, each comprising two sub-elements 6i-1 and 6i-2, arranged abutting each other at their end faces. For this purpose, the preferred orientation of the individual sub-elements 6i-1 and 6i-2 should be perpendicular, as shown in Figure 9 This is indicated by the vectors of the measurable shear forces FSi-1 and FSi-2 shown. The magnitude of the shear force in the tangential direction is given by the projected length of the sum vector onto the tangential direction.

[0069] This configuration of the piezoelectric element 6i to 6iv therefore always generates a measurement signal Si that corresponds to that of a single piezoelectric element whose preferred direction would be oriented tangentially to the direction of rotation of the rotor 4.

[0070] As an alternative to the equation mentioned above for calculating the torque Mz, a system of equations, particularly a linear one, can also be used to calculate the force components Fx, Fy and the torque component Mz in the plane formed by the position of the piezoelectric elements 6i, 6ii, 6iii, 6iv. This linear system of equations can be summarized in matrix notation and represented as follows: Mz Fx Fy = C 1 i ⋯ C 1 iv ⋮ ⋱ ⋮ C 3 i ⋯ C 3 iv Si Sii Siii Siv

[0071] The individual coefficients of the matrix can be determined by calibration measurements using the measuring device 3, which are preferably carried out after pre-tensioning of the piezo elements 6i to 6iv.

[0072] Figure 3 Figure 1 shows a second embodiment of a measuring arrangement 1. In contrast to the first embodiment of the measuring arrangement 1, the second embodiment only has the fixing device 5, which is designed as a fixing plate.

[0073] In the second embodiment, a preloading device in the form of an additional plate is not present. Instead, the housing of the motor or the motor 2 itself is arranged directly on the piezoelectric elements 6i to 6iv. The housing of the motor or the motor 2 is preloaded with the fixing plate 5 in such a way that a force-fit is formed between the housing of the motor or the motor 2 itself on the one hand, and between the piezoelectric elements 6i to 6iv and the fixing plate 5 on the other. In this way, shear forces can also be applied to the piezoelectric elements 6i to 6iv in this embodiment.

[0074] A third embodiment of a measuring arrangement for determining a loss torque of an electrical machine is described with regard to the Figures 4 and 5Explained: In this embodiment, both the fixing devices 5a, 5b and the pre-tensioning device 8a, 8b are designed in two parts. The fixing device has a first fixing plate 5a and a second fixing plate 5b, which are spaced apart from each other by a gap 10. The pre-tensioning device has a first half-shell 8a and a second half-shell 8b. The first half-shell 8a is arranged on the first fixing plate 5a via two piezoelectric elements 6iii, 6iv and is attached to the first fixing plate 5a by means of pre-tensioning elements (not shown). The second half-shell 8b is also arranged on the second fixing plate 5b via two piezoelectric elements 6i, 6ii. These are also attached to each other by clamping elements (not shown), so that shear forces can be introduced onto the piezoelectric elements 6i, 6ii via a frictional connection.

[0075] Figure 5 shows a side view of the measuring arrangement 1 according to Figure 4 As from Figure 5As can be seen, the motor 2 can be fixed using the two half-shells 8a, 8b.

[0076] The half-shells 8a, 8b are preferably designed to fix the electric motor 2 by means of a frictional connection. For this purpose, the half-shells 8a, 8b each have a shoulder 11a, 11b by means of which the motor 2 can be clamped. For clamping, the measuring device 3 preferably has further clamping means (not shown) with which the first fixing plate 5a and the second fixing plate 5b, and thus also the first half-shell 8a and the second half-shell 8b, can be pre-tensioned against each other. These clamping elements are also preferably designed as screw elements.

[0077] A gap (no reference symbol) between the two half-shells 8a, 8b, but also the gap 10 between the first fixing plate 5a and the second fixing plate 5b can be designed in such a way that the shaft 9 can also be accommodated here.

[0078] The one in the Figures 1 to 5 The three embodiments shown have in common that the motor 2 is arranged with one of its end faces against the measuring device 3. However, the measuring principle according to the invention, which is explained further below, also works when the motor 2 is arranged with its other end face towards the measuring device 3.

[0079] The preloading device 8 can also be designed as a motor-specific adapter plate. Furthermore, additional plates, designed as motor-specific adapter plates, can be mounted on the preloading plate 8. This reduces setup times for the measuring arrangement 1, in which the motor 2 is installed on the measuring device.

[0080] Even if, with regard to all three embodiments of the Figures 1 to 5As shown, the axis of rotation 7 of the rotor 4 is oriented perpendicular to a plane spanned by the piezoelectric elements 6i to 6iv or their preferred directions. However, the axis of rotation 7 can also be oriented obliquely to this plane, as long as it intersects the spanned plane. Preferably, the axis of rotation 7 intersects the plane at an angle between 45° and 135°. If the axis of rotation is not oriented perpendicular to the plane, the orientation must be taken into account in the equation mentioned above for calculating the torque Mz by means of appropriate geometric factors. Accordingly, an oblique orientation of the axis of rotation 7 with respect to the aforementioned plane also necessitates a recalculation of the coefficients of the calibration matrix or the coefficients of the aforementioned system of equations.

[0081] Regarding the Figures 6 to 8A fourth training example of a measuring arrangement 1 for determining a loss torque of an electrical machine 2 is explained below.

[0082] Figure 6 Figure 1 shows a top view of the measuring arrangement 1 according to the fourth embodiment. In contrast to the first, second and third embodiments, the shaft 9 of the electric motor 2 and thus also the axis of rotation 7 of the rotor 4 is arranged parallel to a plane which is defined by the position of the piezoelectric elements and / or their preferred directions 6i to 6iv.

[0083] Also in the Figures 6 to 8 The force components shown are those that can be measured using the measuring arrangement 1 according to the fourth embodiment. These are, firstly, the force component in the X-direction Fx, the force component in the Y-direction Fy, and the axial torque Mz of shaft 9.

[0084] Figure 7 shows a side view of measuring arrangement 1 from Figure 6, where the measuring setup is shown partially semi-transparently.

[0085] As from Figure 7 As can be seen, in the fourth embodiment the piezo elements 6i to 6iv are also arranged between the fixing plate 5 and the prestressing plate 8.

[0086] As in the first three embodiments of the measuring arrangement 1, in the fourth embodiment of the measuring arrangement 1 the piezoelectric elements 6i to 6iv are also arranged at least partially in recesses of the fixing plate 5. In this fourth embodiment as well, preloading elements (not shown) are provided which preload the preloading plate 8 with the fixing plate 5 and thus force-fit the piezoelectric elements 6i to 6iv between the preloading plate 8 and the fixing plate 5.

[0087] This is also from Figure 8 visible, which shows a side view in the direction of the front of the electric motor 2 of the measuring arrangement 1.

[0088] To determine the force components shown, the piezoelectric elements 6i to 6iv each have two sub-elements 6ii-1, 6ii-3; 6iii-1, 6iii-3; 6i-1, 6i-3; 6iv-1, 6iv-3. The two sub-elements of a piezoelectric element 6i to 6iv each have different preferred directions, as shown in the Figures 6 to 8 is shown.

[0089] Whereas those in the Figures 7 and 8 The piezoelectric sub-elements 6i-1 to 6iv-1 arranged above each exhibit a preferred orientation in a plane formed by the position of the piezoelectric elements or parallel to this plane, represented by the vectors of the measured forces FSi, FSii, FSiii, FSiv or by the vectors shown in Figure 6The extensions of the piezo elements 6i to 6iv shown below exhibit the piezo sub-elements 6i-3 to 6iv-3 having orientations of the preferred directions which are oriented perpendicular to this plane, represented by the vectors of the measured forces FN,i , FN,ii , FN,iii , FN,iv .

[0090] Based on a measurement of these forces in Figure 8 Given the geometric quantities shown, lever H in relation to the shear forces FS and lever B in relation to the normal forces FN, the torque Mz about the axis of rotation 7 can be determined according to the following equation: M z = H ⋅ + F S , i − F S , ii − F S , iii + F S , iv + B ⋅ + F N , i − F N , ii − F N , iii + F N , iv

[0091] In this equation, too, the signs of the forces depend on the installation direction of the individual sub-elements 6ii-1, 6ii-3; 6iii-1, 6iii-3; 6i-1, 6i-3; 6iv-1, 6iv-3 of the piezoelectric elements 6i to 6iv. Here, too, the measured shear forces FS and normal forces FN are derived from the measurement signals Si, Sii, Siii, Siv, Ni, Nii, Niii, Niv, which are preferably measured in picocoulombs.

[0092] In this embodiment as well, manufacturing tolerances lead to deviations in practice, which result in errors when using the equation given above. Therefore, in the fourth embodiment, a system of equations, particularly a linear one, is preferably used, the coefficients of which are determined by calibration measurements after preloading the measuring device 3.

[0093] Such a system of equations can be summarized in matrix notation as follows, where S1 to S4 are the signals Si to Siv, the signals of the shear force measurement FS,i to FS,iv, and the signals N i to N iv are the measurement signals of the normal force components FN,i to FN,iv. Mz Fx Fy Mx My = C 1 Si ⋯ C 1 Niv ⋮ ⋱ ⋮ C 5 Si ⋯ C 5 Niv Si ⋮ Siv Ni ⋮ Niv

[0094] To compensate for a misalignment of the preferred directions of the sub-elements 6i-1 to 6iv-1, the fourth embodiment of the measuring arrangement 1 can also provide that the shear forces are determined additively with two superimposed sub-elements 6ii-1, 6ii-2; 6iii-1, 6iii-2; 6i-1, 6i-2; 6iv-1, 6iv-2, as shown in Figure 10 as shown. The superimposed sub-elements 6ii-1, 6ii-2; 6iii-1, 6iii-2; 6i-1, 6i-2; 6iv-1, 6iv-2 must each have preferred directions that are arranged at right angles to each other.

[0095] In this case too, the shear force Ft in the tangential direction to the direction of rotation of the rotor 4 results from the projected length of the sum vector onto the tangential direction.

[0096] In order to determine the normal force nonetheless, a third sub-element 6i-3 is available, which can measure a normal force FN,i.

[0097] In all illustrated embodiments of the measuring arrangement, any number of piezoelectric elements 6i to 6iv can be installed. However, it is essential to note that the preload plate and / or the electric motor 2 must be supported at at least three points to achieve stability. Therefore, if fewer than three piezoelectric elements 6i to 6iv are used, one or two additional support elements must be provided. These, however, lead to force shunts, which impair the measuring accuracy of the measuring device 2. In particular, they proportionally reduce the force flow through the existing piezoelectric element(s) 6i to 6iv. Furthermore, the additional support elements can introduce nonlinearities.

[0098] Regarding the fourth embodiment of the measuring arrangement 1, it can also be provided that the sub-elements 6i-1, 6ii-1, 6iii-1, 6iv-1, which measure the shear force, are rotated by 45°. While this reduces the proportion of the measured force component in the tangential direction to the direction of rotation of the rotor 4 Ft, it allows for an additional measurement of the force component Fz. This can also be done using a system of equations, which in this case is slightly modified: Mz Fx Fy Mx My Fz = C 1 Si ⋯ C 1 Niv ⋮ ⋱ ⋮ C 6 Si ⋯ C 6 Niv Si ⋮ Siv Ni ⋮ Niv

[0099] Based on the Figures 11 to 13 An exemplary embodiment of a method for determining a loss torque of an electric machine 2 with rotor 4 is described below.

[0100] This method can utilize measurement setups such as those used in relation to the Figures 1 to 10have been described. However, it is also possible to use a different measuring arrangement 1 and a different measuring device 3, with which the loss torque can also be determined using method 100. In particular, measuring devices 3 can be used which have measuring elements other than piezoelectric elements. Furthermore, other geometric arrangements are also possible, both with regard to the measuring device 3 and with regard to the measuring arrangement 1 as a whole.

[0101] As described at the beginning, the rotor 4 of an electric motor 2 is accelerated by interaction in a magnetic field. This generates the driving torque M a. Simultaneously, this driving torque M a is opposed by loss torques M v.

[0102] When rotor 4 is accelerated, a torque opposes the acceleration, which is generated by the moment of inertia J of rotor 4. Here, Mz = J φ̈ : M z = M a − M v = J φ ¨

[0103] According to the equation, during an acceleration or braking process of the rotor 4, the drive torque M a cannot be distinguished from the loss torque M v.

[0104] For constant angular velocity (φ̈ = 0), the following applies: M a − M v = 0

[0105] If motor 2 is now operated at idle, so that rotor 4 comes to a stop, a so-called "coast-down" operation, the drive torque M a = 0. Accordingly, the measured axial torque M z = - M v .

[0106] Idle operation can be achieved by disconnecting the motor from the power supply or interrupting the control current.

[0107] Accordingly, the axial torque Mz of rotor 4 reacts with a step response because, at the moment of the start of no-load operation, only the loss torque Mv is present. This step response is also shown in the diagram of Figure 11The graph also shows the course of the control current I over time.

[0108] The step response of the axial torque Mz can be measured as a reaction torque at the motor 2, in particular by means of one of the illustrated embodiments of a measuring arrangement 1.

[0109] The moment the driving torque M a becomes zero, the rotational speed φ̇ of the rotor 4 begins to decrease, as shown in the diagram according to Figure 11 depicted.

[0110] The use of piezoelectric elements is particularly well suited for this measurement, as piezoelectric elements are especially good at measuring dynamic forces.

[0111] Figure 12 Figure 1 shows another diagram in which real measurements of the control current I and the axial torque Mz of a real measurement on a measuring arrangement 1 according to the first embodiment are shown.

[0112] The measured braking torque Mv is 0.5 Nm. The area shown in the diagram as control current I corresponds to three periodic phase currents with amplitudes of approximately 40 A. These drop to 0 at the time of coast-down. The axial torque signal Mz is filtered by a low-pass filter at approximately 500 Hz. The step response of the axial torque Mz is derived from the linear fits of the torque signal Mz before and after coast-down, shown as lines. A block diagram of the exemplary embodiment of method 100 for determining a loss torque of an electric machine 2 with a rotating shaft 4 is shown in Figure 11 depicted.

[0113] As explained above, the electric motor 2 is mounted freely rotating on the measuring device 1 and, in particular, no load machine is connected to the freely rotating rotor 4 or its shaft 9.

[0114] In a first step 101, the motor 2 is operated by means of a control current I such that the rotor 4 reaches a predefined speed. The motor 2 is accelerated to the predefined speed independently, i.e., by the acceleration torque M a generated in the motor 2.

[0115] Three-phase sinusoidal currents are preferably used as control currents, depending on the type of motor 2.

[0116] Once the predefined speed is reached, motor 2 is preferably operated at a constant speed.

[0117] In a second step 102, the motor 2 is then operated at idle. Preferably, the control current I is switched off, in particular interrupted. Preferably a switch or relay is used for this purpose.

[0118] In a third step 103, the signal is filtered using a low-pass filter, which preferably has a cutoff frequency of 500 Hz. This allows a low-frequency signal to be detected from a highly dynamic measurement signal, such as that generated by piezoelectric elements 6i to 6iv.

[0119] In a fourth step 104, a jump in the filtered signal is identified.

[0120] Depending on the position of the jump, the signal is fitted in a first section before the identified jump and in a second section after the jump in a fifth work step 105.

[0121] Based on the signal segments fitted before and after the jump, the magnitude of the jump is determined in a sixth step 106. This magnitude of the jump corresponds to the axial torque Mz of the rotor 4 at the time of coast-down, i.e., when the motor 2 switches to idle operation, and thus to the negative loss torque Mv at this moment. Preferably, the loss torque Mv is output to an interface.

[0122] Figure 14Figure 1000 shows a system 1000 for determining the torque loss of an electric machine 2 with a rotor, using one of the exemplary embodiments of the measuring arrangements. The system 1000 includes means 1001 for controlling the operation of the machine 2, configured to first operate the machine 2 in such a way that the rotor 4 reaches a predefined speed, and then to operate the machine at idle. Furthermore, the system 1000 includes means 1002 for monitoring a signal from the measuring device 3, which represents the axial torque at the machine 2. The system 1000 also includes means 1004 for identifying a discontinuity in the signal and means 1006 for determining the magnitude of the discontinuity, the magnitude of which indicates the torque loss of the electric machine 2. Further, optional means 1003 for filtering the signal and means 1005 for fitting the signal may be present.

[0123] It should be noted that the embodiments described are merely examples and do not in any way restrict the scope of protection, the applications, or the structure. Rather, the preceding description provides the person skilled in the art with a guideline for implementing at least one embodiment, whereby various modifications, particularly with regard to the function and arrangement of the described components, can be made without departing from the scope of protection as defined by the claims and these equivalent combinations of features.

Claims

1. Method (100) for determining a loss torque of an electric machine (2) with a rotor (4), wherein the machine (2) is mounted on a measuring device (3) in such a way that an axial torque can be measured on the machine (1) and wherein the rotor (4) is freely rotating, comprising the following steps: first operating (101) of the machine (2) in such a way that the rotor (4) reaches a predefined rotational speed; second operating (102) of the machine (2) at idle speed when the predefined rotational speed is reached, whereas a signal from the measuring device (3), which represents the axial torque on the machine (2), is monitored; identifying (104) a jump in the signal; and determining (106) the magnitude of the jump, wherein the magnitude indicates the loss torque of the electric machine (2).

2. Method (100) according to claim 1, further comprising the step of: filtering (103) the signal by means of a low-pass filter, which preferably has a cut-off frequency of approximately 500 Hz.

3. Method (100) according to claim 1 or 2, further comprising the step of: first fitting (105) of at least a first portion of the signal before the identified jump and second fitting of at least a second portion of the signal after the identified jump, wherein the magnitude of the jump is determined based on the first fitting and the second fitting.

4. Method (100) according to one of claims 1 to 3, wherein the measuring device (3) has a fixing device (5) and at least one piezo element, preferably three, even more preferably four piezo elements (6i, 6ii, 6iii, 6iv), each with a preferred direction, wherein the fixing device (5) carries the at least one piezo element (6i, 6ii, 6iii, 6iv) and supports the electric machine (2) via the at least one piezo element (6i, 6ii, 6iii, 6iv) in such a way that forces between the electric machine (2) and the fixing device (5) can be measured by means of the at least one piezo element (6i, 6ii, 6iii, 6iv).

5. Method (100) according to claim 4, wherein the preferred direction or directions are each parallel to or lie in a single plane, and wherein the axis of rotation (7) of the rotor (4) intersects the plane at an angle between 45° and 135°, preferably between 85° and 95°, most preferably at least substantially perpendicular.

6. Method (100) according to claim 5, with at least two piezo elements (6i, 6ii, 6iii, 6iv), wherein the piezo elements (6i, 6ii, 6iii, 6iv) are each arranged at different positions around a rotational axis (7) of the rotor (4) of the electric machine (2).

7. Method (100) according to one of claims 4 to 6, wherein the at least one piezoelectric element (6i, 6ii, 6iii, 6iv) comprises a first sub-element (6i-1, 6ii-1, 6iii-1, 6iv-1) and a second sub-element (6i-2, 6ii-2, 6iii-2, 6iv-2) by means of which shear forces can be measured, wherein a preferred direction of the first sub-element (6i-1, 6ii-1, 6iii-1, 6iv-1) is aligned at least substantially perpendicular to a preferred direction of the second sub-element, wherein both sub-elements (6i-1, 6i-2; 6ii-1; 6ii-2; 6iii-1, 6iii-2; 6iv-1, 6iv-2) are arranged along their end faces relative to each other, wherein a measurement of the first component (6i-1, 6ii-1, 6iii-1, 6iv-1) and a measurement of the second sub-element (6i-2, 6ii-2, 6iii-2, 6iv-2) are taken into account in the signal of the measuring device (2).

8. Method (100) according to one of claims 4 to 7, wherein the preferred directions of the at least one piezo element (6i, 6ii, 6iii, 6iv) are each aligned at least substantially tangentially to a direction of rotation of the rotor (4).

9. Method (100) according to one of claims 5 to 8, wherein a respective angle between the preferred directions of the at least one piezo element (6i, 6ii, 6iii, 6iv) and the tangents to the direction of rotation of the rotor (4) at the locations of the at least one piezo element (6i, 6ii, 6iii, 6iv) is taken into account in the axial torque signal.

10. Method (100) according to claim 4, wherein the preferred direction or directions are each parallel to or lie in a single plane, or wherein the preferred direction or directions are each perpendicular to a single plane, and wherein an axis of rotation (7) of the rotor (4) is aligned at least substantially parallel to the plane, in particular in a horizontal direction.

11. Method (100) according to claim 10 with at least two piezo elements (6i, 6ii, 6iii, 6iv), wherein the piezo elements (6i, 6ii, 6iii, 6iv) are each arranged at different positions and two piezo elements (6i, 6iv; 6ii, 6iii) are arranged on one of the two sides of the fixing device with respect to the axis of rotation (7).

12. Method according to claim 10 or 11, wherein the at least one piezo element (6i, 6ii, 6iii, 6iv) comprises a first sub-element (6i-1, 6ii-1, 6iii-1, 6iv-1) and / or a second sub-element (6i-2, 6ii-2, 6iii-2, 6iv-2), by means of which a shear force can be measured, and a third sub-element (6i-3, 6ii-3, 6iii-3, 6iv-3) by means of which a compressive force can be measured, wherein the sub-elements (6i-1, 6i-2, 6i-3; 6ii-1; 6ii-2, 6ii-3; 6iii-1, 6iii-2, 6iii-3; 6iv-1, 6iv-2, 6iv-3) are arranged along their front sides relative to each other, wherein a measurement of the first sub-element (6i-1, 6ii-1, 6iii-1, 6iv-1) and / or the second sub-element (6i-2, 6ii-2, 6iii-2, 6iv-2) and a measurement of the third sub-element (6i-3, 6ii-3, 6iii-3, 6iv-3) are taken into account in the signal of the measuring device.

13. Method (100) according to one of claims 4 to 12, wherein the measuring device (3) further comprises a preloading device (8) and first preloading elements, wherein the at least one piezo element (6i, 6ii, 6iii, 6iv) can be preloaded or are preloaded between the fixing device (5) and the preloading device (8) by means of the first preloading elements in such a way that the at least one piezo element (6i, 6ii, 6iii, 6iv) is fixed by force, and wherein the electric machine (2) is fixed to the preloading device (8) in such a way that it cannot rotate.

14. Method (100) according to one of claims 4 to 13, wherein the electric machine (2) is attached to the fixing device (5) and is supported by the at least one piezo element (6i, 6ii, 6iii, 6iv) in such a way that the at least one piezo element (6i, 6ii, 6iii, 6iv) is fixed by force.

15. Method (100) according to one of claims 4 to 14, wherein the fixing device (5) or the electric machine (2) is supported exclusively by the at least one piezo element (6i, 6ii, 6iii, 6iv).

16. System (1000) for determining a loss torque of an electric machine (2) with a rotor (4), wherein the machine (2) is mounted on a measuring device (3) in such a way that an axial torque can be measured on the machine (1) and wherein the rotor (4) is freely rotating, comprising: means (1001) for controlling operation of the machine (2), set up to initially operate the machine (2) in such a way that the rotor (4) reaches a predefined rotational speed, and thereafter to operate the machine at idle speed; means (1002) for monitoring a signal of the measuring device (3) representing the axial torque on the machine (2); means (1004) for identifying a jump in the signal; and means (1006) for determining the magnitude of the jump, the magnitude indicating the loss torque of the electric machine (2).