AT527343A1 - Control procedure for test specimen with a multiphase alternating current on a test bench - Google Patents

Abstract

The present invention relates to a control method and a corresponding control device (110) for controlling a multi-phase alternating current for a test object (300) during operation on a test bench (200), characterized by the following steps: detecting (S10) individual phase currents (Ip1, Ip2, Ip3) of a multi-phase alternating current to the test object (300); determining (S11) harmonic frequency components of the phase currents (Ip1, Ip2, Ip3) based on a fast Fourier transformation of the detected phase currents (Ip1, Ip2, Ip3); determining (S21) a shape index of a waveform of the phase currents (Ip1, Ip2, Ip3) from a ratio of the determined harmonic frequency components of the phase currents (Ip1, Ip2, Ip3) to an amplitude of a fundamental frequency; Determining (S22, S23) at least one symmetry index of the phase currents (Ip1, Ip2, Ip3) from a ratio of the phase currents (Ip1, Ip2, Ip3) to one another; and outputting (S30) a control signal based on the determined shape index and/or the at least one determined symmetry index, for intervening in the operation of the test object (300).

Description

Control procedure for test specimen with a multiphase alternating current on a test bench

The present invention relates to a control method for controlling the operation of a test object with a multiphase alternating current on a test bench, a computer program product comprising commands for carrying out such a control method and a control device for carrying out

such a control procedure.

It is known that electric motors are brought into different performance states and tested on test benches during development. Such test bench tests often run over a longer period of time and run the electric motors through different operating states and different operating times. For this purpose, such very expensive and complexly equipped test benches are usually used to the maximum possible capacity in order to test electric motors with high efficiency.

In addition, the test objects are often pushed to the limit of their load capacity during operation on the test bench. In the case of non-linear loads, such as electric motors as consumers of a multi-phase alternating current, this can lead to destabilizing operating modes, which can reach critical states. Such critical states can lead to reversible but also irreversible damage to the electric motor as the test object and the connected power electronics. This can lead to the demagnetization of electric motor components. Therefore, complex sensor systems are available in such test benches in order to detect such critical situations as quickly as possible.

recognize and avoid.

The disadvantage of a conventional test bench system is that in such a case the test object is shut down and stopped and the associated test bench test is discarded. If this happens overnight during a test operation, it leads to hours of downtime because the test bench cannot be restarted manually. In addition, critical situations are also possible which are detected by the sensors too late to prevent damage to the test object or connected electrical components in time.

It is therefore the object of the present invention to at least partially remedy the disadvantages described above. In particular, it is the object of the present invention to be able to ensure at least partially automated operation of an electric motor on a test bench with the highest possible level of safety in a cost-effective and simple manner.

The inventors of the present invention have previously developed a method in which phase currents are recorded and average phase currents and trend phase currents are determined from them. In addition, current indices are determined from a ratio of the average phase currents to the trend phase currents and assigned to an index corridor. This analysis can be used to identify a critical development of the current phases that indicates an unstable operating state.

However, this requires calculations of the root mean square for the effective current Irms (rms=root main square) for each phase current, which places high demands on computing capacity for real-time operations. On the other hand, the inventors found in practice that there is still room for improvement in the detection of critical developments and unstable states for a corresponding control procedure for test operation on the test bench.

It is a further object of the invention to at least partially eliminate the disadvantages described above. In particular, it is the object of the present invention to create a method and a corresponding technology that allows improved detection of destabilizations in operating states and greater safety in an at least partially automated operation of a

multiphase alternating current test object on a test bench.

The above object is achieved by a control method having the features of claim 1, a computer program product having the features of claim 14 and a control device having the features of claim 15. Further features and details of the invention emerge from the subclaims, the description and the drawings. Features and details that are described in connection with the control method according to the invention naturally also apply in connection with the

inventive computer program product and the inventive

Control device and vice versa, so that with regard to the disclosure to the

individual aspects of the invention are always referred to each other

or can become.

According to the invention, a control method is used to control the operation of a test object with a multiphase alternating current on a test bench. Such a control method is characterized by the following steps: detecting individual phase currents of a multiphase alternating current to the test object; determining harmonic frequency components of the phase currents based on a fast Fourier transformation of the detected phase currents; determining a shape index of a waveform of the phase currents from a ratio of the determined harmonic frequency components of the phase currents to an amplitude of a fundamental frequency; determining at least one symmetry index of the phase currents from a ratio of the phase currents to one another; and outputting a control signal based on the determined shape index and/or the at least one determined symmetry index, for intervening in the operation of the test object.

According to this disclosure, the term control or controlling includes both control and regulation and accordingly, for example, the term control signal also includes a control or regulation signal.

According to this disclosure, the term frequency components refers to parts of harmonics, i.e. the overtones with an integer multiple of the frequency of a fundamental wave or fundamental oscillation, from which a waveform of the phase current is superimposed. The determination of the frequency components includes a value ratio that determines a distribution of the phase current

to the individual harmonics with different intensities.

According to this disclosure, the term fundamental harmonic frequency refers to a sinusoidal waveform having the measured amplitude of the sinusoid and having the measured frequency of the alternating current.

The control method controls at least partially in an interventional, overriding manner the operation of the test object with a multiphase alternating current, i.e. in particular a power electronics such as an inverter for

be referred to.

The aim of the control method is, in contrast to the known solutions, to detect or even predict potentially critical situations of the test object, such as an electric motor, much earlier in order to improve the protection of the electric motor. In particular, this makes it possible to react more quickly and earlier than in the prior art and to initiate steps for the stabilization and/or protection of the electric motor and the connected electrical components. In order to achieve this aim, the control method according to the invention involves recording the phase currents of the individual phases of the electric motor. In other words, current sensors are provided for each phase, which allow the phase currents for each phase of the

electric motor.

The invention provides for the first time to define target values or key performance indicators for the stability of the alternating current, which are derived from the waveform of the measured current values of the individual phase currents using a fast Fourier transformation (FFT). The fast Fourier transformation (FFT) allows a transformation of the recording of time-related measured values into a representation of frequency-related values, in particular of the aforementioned value ratio, which measures a distribution of a recorded waveform of the phase current to the harmonics, i.e. the individual harmonic oscillations with different intensities.

One of the target variables according to the invention is a form index. The form index describes the dimension of the harmonics that cannot be assigned to the fundamental oscillation. In other words, the form index describes how strongly the phase current is distorted in comparison to the fundamental oscillation, i.e. the first harmonic, and forms a measure of congruence or conformity in the sense of a

Superimposing the waveforms of the phase currents, which individual

Deviations from an averaged waveform of the phase currents

highlights.

Another target value according to the invention relates to two possible variants of a symmetry index. The first variant of the symmetry index is indicative of a relative value of the amplitude of the individual phase currents and is formed from an individual value of the amplitude of the first harmonic of a phase current compared to an average value of the amplitude of the other phase currents. In other words, an individual proportion or intensity is measured in each phase current in relation to a common, averaged fundamental frequency from the phase currents.

The second variant of the symmetry index is indicative of a relative value of the phase shift of the individual phase currents and is formed from an individual phase shift between two consecutive phases compared to an average value of the phase shifts between the other phase currents. The measured values of the phase currents are used directly as the basis for calculating the second variant of the symmetry index in relation to the phase shift.

Compared to the above-mentioned method, which is based on the calculation of the root mean square of the effective current Irms of the phase currents, which, from a purely mathematical point of view, is formed from the integrated area under the waveform of the phase currents, the analysis according to the invention based on the shape index and symmetry index as defined above has the following advantages:

As a first advantage, these target quantities provide a more precise insight and more accurate prediction of possible instabilities in the phase currents, since the analysis takes into account the waveform, which contains more information about a trend than the calculated integral of the area under the waveform.

As a second advantage, these target values require less computing capacity, since the effective current does not have to be calculated for all phase currents, and since the calculations for analyzing the waveform are limited to the few target values mentioned.

are reduced.

As a result, there is on the one hand the advantage in the practice of test bench operation that an intervention of the control method according to the invention in the operation of the test object is triggered even more accurately in borderline cases, or, expressed in contrast, can be avoided with greater certainty in borderline cases, thereby reducing aborts of test operation.

On the other hand, the advantage in practice of test operation is that the intervention of the control method according to the invention in the operation of the test object is even faster and more precise, thus reducing the risk of consequential damage to the test object or to the test bench.

It may also be advantageous if the method step of determining at least one symmetry index comprises determining a symmetry index with respect to the amplitude of the phase currents, based on a ratio of individual amplitudes of a specific harmonic frequency component of the phase currents to an average value of the amplitudes of the same harmonic frequency component of the phase currents. As mentioned above, as a possible variant, the symmetry between the phase currents can thus be determined using

a characteristic feature of the waveform.

It may also be advantageous if the method step of determining at least one symmetry index comprises determining a symmetry index with respect to the phase of the phase currents, based on a ratio of individual phase shifts between the phase currents to an average value of the phase shifts between the phase currents. As mentioned previously, as a possible variant, the symmetry between the phase currents can thus be checked using a further characteristic feature of the waveform.

7127

Preferably, in the control method according to the invention, both

Variants of the symmetry index, both in terms of amplitude and

Phase shift determined and compared.

Furthermore, it is advantageous if the control method comprises further steps, namely comparing the shape index with a distortion threshold value; and comparing the at least one symmetry index with at least one asymmetry threshold value; wherein the method step of outputting the control signal is carried out on the basis of a condition that the shape index exceeds the distortion threshold value and/or the at least one symmetry index exceeds the at least one asymmetry threshold value. Thus, conditions and findings for outputting the control signal

be chosen in a more differentiated manner.

Further advantages are achieved if the control method includes a further step of calibrating the distortion threshold value and the at least one asymmetry threshold value in relation to a test object type. The test operation can thus be individually adapted to the test object.

It is also advantageous if the multiphase alternating current for the test object comprises more than three phase currents, whereby the test object is preferably an electric motor with more than three phases. The control method is characterized by effective use of the computing capacity, so that a large number of phases can also be monitored in real time.

It is also advantageous if the control signal output for intervening in the operation of the test object comprises a control barometer whose value is formed in relation to a value of the shape index or a value of at least one symmetry index. Information on the evaluation and magnitude of the deviation of a phase current can thus flow directly into a controlled countermeasure.

Further advantages are achieved if the control signal output causes at least one of the following control interventions on the test bench to intervene in the operation of the test object: stabilizing an unstable phase; limiting a load on the test object; temporarily switching off the test object, or completely switching off the test object. Thus, various types of

Countermeasures to safety-relevant intervention in the operation of the test bench

available.

In addition, it can be advantageous if the process step of recording individual phase currents is carried out continuously and for all phase currents of the multiphase alternating current. The control method is characterized by an effective use of the computing capacity, so that all phases

can be permanently monitored in real time.

It is also advantageous if the process step of determining harmonic frequency components is repeated continuously for a rolling time window and is carried out for all phase currents of the multiphase alternating current. This provides a frequency-oriented and therefore processing-effective selection of the time periods.

In addition, it can be advantageous if the method steps of determining the shape index and determining the at least one symmetry index are carried out in parallel and continuously for the same rolling time window of the determined harmonic frequency components. This simplifies the processing of the measurement data.

Further advantages are achieved if the time window correlates with the period duration of the phases of the test object. Period duration can be easily defined using the current values through zero crossings.

It is also advantageous if the time window has a variable length, which is adjusted in particular on the basis of the control signal. In this way, the processing of the measurement data is adjusted or optimized to a frequency of the alternating current or a speed of an electric motor as the test object.

The present invention also relates to a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of a control method according to the invention. A computer program product according to the invention therefore also brings with it the same advantages as have been explained in detail with reference to a control method according to the invention.

Further advantages, features and details of the invention emerge from the following description, in which embodiments of the invention are described in detail with reference to the drawings.

They show schematically:

Fig. 1a is a graphical representation of the course of three phase currents,

which are recorded in a graph over a time axis;

Fig. 1b is a graphical representation of a superposition of the three phase currents from Fig. 1a;

Fig. 2 shows a schematic test bench structure with an electric motor as test object and a control device for carrying out a control method according to the invention;

Fig. 3 is a block diagram showing the sequence of method steps in an embodiment of a control method according to the invention;

Fig. 4 shows diagrams with measured values and determined target values during a stable operating state; and

Fig. 5 is a representation of the same diagrams from Fig. 3 with measured values and determined target values during an unstable

operating state.

Figures 1a and 1b provide a brief introduction to the facts, which

the control procedure and its monitoring function.

Fig. 1a shows a sinusoidal curve of current values from three phase currents Ip1, Ip2, Ip3, which are tapped at the terminals of a stator of an electric motor, which in the present embodiment is operated as a test object 300 on a test bench 200, or at an output of an inverter 220 connected upstream of the stator for controlling the electric motor. In stable operating states in which a non-linear load such as the electric motor generates variable speeds and torques, the phase currents Ip1, Ip2, Ip3 have a largely uniform curve shape.

In a constant operating state of the electric motor, the waveforms of the phase currents 1p1, Ip2, Ib3 are approximately congruent, as shown in Fig. 1b. In this representation, the phase shifts between the phase currents Ip1, Ip2, Ip3 are calculated out and the waveforms of the phase currents 1p1, Ip2, Ip3 are graphically superimposed. In practice, this allows operating states recorded after a test operation has been aborted or parallel to the ongoing test operation, but with a time delay, i.e. offline, to be visually reproduced and, if necessary, to obtain information about instability of the alternating current phases afterwards. However, real-time monitoring, i.e. online during test operation, is not possible with such a manual check. It therefore does not offer a method or basis for obtaining information in good time and deciding on a countermeasure in the event of instabilities occurring.

Fig. 2 shows a structure of a test bench 200 in which a control method according to the invention is carried out on a control device 100, while an electric motor (M) 300 is operated as a test object in a test run. For this purpose, the test bench 200 comprises a current source 210 for power supply, which in the embodiment shown provides a direct voltage (DC), and an inverter (DC/AC) 220, which is controlled by a control unit (CPU) 240. The inverter 220 provides a power conversion into a multi-phase alternating current with the three phase currents 1p1, Ip2, Ip3 shown, and controls stator coils under the control of the control unit 240.

before.

Deviating from the described embodiment, the electric motor 300

and the inverter 220 has more than 3 phases, in particular 6 phases or any

other plurality of phases.

The control unit 240 controls the operation of the electric motor 300, for example, depending on the execution of a previously stored test protocol, which is specified by the test bench 200. During operation, the phase currents 1p1, Ip2, Ip3 are each detected by current sensors (S) 230. The current sensors 230 are connected to measuring inputs of the control device 100. The control device 100 comprises a detection module 10, a detection module 20, a determination module 30, a comparison module 40 and an output module 50, the function of which is explained in more detail below in connection with the control method.

is explained.

In Fig. 3, the method steps are shown in a sequence of embodiments of the control method described in more detail. The method steps from Fig. 3 are divided into the individual modules of the control device 100 from Fig. 2 as follows. The detection module 10 carries out measurements S10 of the phase currents 1p1, Ip2, Ip3. The determination module 20 carries out a determination S20 of frequency components from the measured values by calculating a fast Fourier transformation. The determination module 30 carries out several determinations S31, S32 and S33 of indices as target values. The comparison module 40 carries out comparisons S41, 42 and S$43 of the indices with assigned threshold values. The output module 50 carries out an output S50 of a control signal based on the indices, in particular depending on the comparisons or in relation to the values of the indices.

Embodiments of the control method shown in Fig. 3 are described in more detail below.

In step S10, the phase currents Ip1, Ip2, Ip3, i.e. preferably all phase currents of all phases of the test object 300 are measured by current sensors 230 and recorded over time, resulting in waveforms of the measured current values. In step S20, online, i.e. in real time, a power analyzer calculates a Fast Fourier Transformation (FFT) of the measured current values for predetermined time windows, such as a period of a

fundamental frequency of alternating current, a frequency spectrum is created that

phase current.

In step S31, a shape index of each phase current in relation to the fundamental frequency of the alternating current is determined from the decomposition of the waveform into the harmonic frequency components. The shape index is formed from a sum of proportions of the frequency components, e.g. the sum of the current values of the second, third, fifth and zeroth harmonics, divided by the value of the amplitude of a sine wave with the fundamental frequency.

In step S32, a symmetry index is determined from the decomposition of the waveform into the harmonic frequency components with respect to the amplitudes of the phase currents 1p1, Ip2, Ip3 in a relationship to one another. The amplitude symmetry index is formed from an amount of the maximum values of the amplitudes of the first harmonics of all phase currents 1p1, Ip2, Ib3 minus the minimum values of the same amplitudes of the first harmonics of all phase currents Ip1, Ip2, Ip3, divided by an average value of the same amplitudes of the first harmonics of all phase currents I1p1, Ip2, Ip3.

In step S33, a symmetry index relating to the phase shift of the phase currents Ip1, Ip2, Ip3 in relation to one another is determined from the measured waveforms along the time axis. The phase symmetry index is formed from an amount of the maximum values of the phase differences between the successive phase currents Ip1, Ip2, Ip3 minus the minimum values of the same phase differences between the successive phase currents Ip1, Ip2, Ip3, divided by an average value of the same phase differences between the successive phase currents.

Subsequently, in step S41, the shape index is compared with a distortion threshold value that is predetermined with respect to operational reliability of the test bench 200 or is pre-calibrated and set to a type of test object 300. This

The value can be, for example, 1 or less, preferably 0.3.

A comparison result is then made as to whether the form index

distortion threshold is exceeded or not, and optionally how large the

excess.

Furthermore, in step S42, the amplitude symmetry index is compared with an amplitude symmetry threshold value that is predetermined or calibrated beforehand. This value can also be, for example, 1 or less, preferably 0.3. A comparison result is then made as to whether the amplitude symmetry index exceeds the amplitude symmetry threshold value or not, and optionally how large the excess is.

Optionally, preferably additionally, the phase symmetry index is compared in step S43 with a phase symmetry threshold value that is predetermined or calibrated beforehand. This value can also be, for example, 1 or less, preferably 0.3. A comparison result is then made as to whether the phase symmetry index exceeds the phase symmetry threshold value or not, and optionally how large the excess is.

In step S50, a control signal is then output depending on the comparison results, which is processed by the control device 240 of the test bench 200.

Among other things, two embodiments of the control method shown in Fig. 3 can be distinguished by the type and generation of the output control signal.

In a first embodiment, the comparison steps S41, S42 and S43 serve to inform whether the respective threshold value has been exceeded, for example in the form of an output of two states or a trigger flag. In the event of a threshold value being exceeded, or alternatively after a cumulative exceedance of the threshold values in all comparisons, a control signal is output in step S50, which always triggers a similar intervention on the control device 240, such as an emergency shutdown or a load reduction in

predetermined extent.

In a second embodiment, the steps of comparing S41, S42 and S43 serve to output a comparison value which represents the original index value or

In another variant of the embodiments of the control method shown in Fig. 3, only the step S32 for determining the amplitude symmetry index and then the step S42 of comparing with an amplitude asymmetry threshold value are carried out. In a complementary variant of the embodiments of the control method shown in Fig. 3, only the step S33 for determining the phase symmetry index and then the step S43 of comparing with a phase asymmetry threshold value are carried out.

Deviating from the embodiments of the control method shown in Fig. 3, in an alternative embodiment the steps S41, S42 and S43 of comparing the determined indices with the corresponding threshold values can be omitted. In this alternative embodiment, a value of the indices output after the steps of determining S31, S32 and S33 flows directly or with a computational conversion as a control parameter into the control signal output in step S50, as described for the second embodiment in Fig. 3.

Fig. 4 shows a measurement protocol of the recorded waveforms of the three phase currents Ip1, Ip2, Ip3 in a stable operating state. A continuous measurement is shown in the upper line. Below this is a time window for which the Fast Fourier Transformation was carried out. The diagram below shows the frequency spectrum with the harmonic frequency components from the phase currents Ip1, Ip2, Ip3. The shape index (shape index) has low values of less than 0.15 and the waveforms are sinusoidal.

Fig. 5 shows a measurement protocol in an unstable operating state. The three phase currents I1p1, Ip2, lb3 do not show sinusoidal waveforms in the upper line and are not uniform. The frequency spectrum diagram shows a much broader distribution over a higher number of

of less weighted portions of harmonic frequency components, whereby the distribution of the phase currents 1p1, Ip2, Ib3 is also much less uniform. The shape index (shapel index) shows higher values between 0.5 and 1.

The above explanation of the embodiments describes the present invention exclusively by way of examples.

list of reference symbols

10 recording module

20 Investigation Module

30 Determination module 40 Comparison module

50 output modules

100 Control device 200 Test bench

210 power source

220 inverter

230 current sensors

240 Control device 300 Test object / electric motor

Ip1, 1p2, Ip3 phase currents

Claims (1)

patent claims 1. Control method for controlling an operation of a test object (300) with a multiphase alternating current on a test bench (200), characterized by the following steps: Detecting (S10) individual phase currents (Ip1, Ip2, Ip3) of a multiphase alternating current to the test object (300); Determining (S20) harmonic frequency components of the phase currents (Ip1, Ip2, I1p3) based on a Fast Fourier Transformation (FFT) of the detected phase currents (Ip1, Ip2, I1p3); Determining (S31) a shape index of a waveform of the phase currents (Ip1, Ip2, Ip3) from a ratio of the determined harmonic frequency components of the phase currents (Ip1, Ip2, Ip3) to an amplitude of a fundamental frequency; Determining (S32, S33) at least one symmetry index of the phase currents (Ip1, Ip2, Ip3) from a ratio of the phase currents (Ip1, Ip2, Ip3) to one another; Outputting (S50) a control signal based on the determined shape index and/or the at least one determined symmetry index, to intervene in the operation of the test object (300). 2. Control method according to claim 1, characterized in that the method step of determining (S32, S33) at least one symmetry index comprises determining (S32) a symmetry index with respect to the amplitude of the phase currents (Ip1, Ip2, 1p3) based on a ratio of individual amplitudes of a certain harmonic frequency component of the phase currents (Ip1, Ip2, 1p3) to an average value of the amplitudes of the same harmonic frequency component of the phase currents (Ip1, 1p2, 1p3). 3. Control method according to claim 1 or 2, characterized in that the method step of determining (S32, S33) at least one symmetry index comprises determining (S33) a symmetry index with respect to the phase of the phase currents (Ip1, Ip2, 1p3) based on a ratio of individual phase shifts between the phase currents (Ip1, Ip2, 1p3) to an average value of the phase shifts between the phase currents (Ip1, Ip2, 1p3). 4. Control method according to one of the preceding claims, further characterized by the steps: Comparing (S41) the shape index with a distortion threshold; and Comparing (S42, S43) the at least one symmetry index with at least one asymmetry threshold; where the method step of outputting (S50) the control signal is carried out on the basis of a condition that the shape index exceeds the distortion threshold and/or the at least one symmetry index exceeds the at least one asymmetry threshold. 5. Control method according to claim 8, further characterized by the step: Calibrating the distortion threshold and the at least one asymmetry threshold with respect to a test object type. 6. Control method according to one of the preceding claims, characterized in that the multiphase alternating current for the test object (300) comprises more than three phase currents (Ip1, Ip2, 1p3), and wherein preferably the test object (300) is an electric motor with more than three phases. 7. Control method according to one of the preceding claims, characterized in that the output control signal for intervening in the operation of the test object (300) comprises a control barmeter whose value is in a ratio to a value of the form index or a value of the at least one symmetry index is formed. - Stabilizing an unstable phase current (Ip1, Ip2, 1p3) - Limiting a load of the test object (300) - temporary shutdown of the test object (300) - complete switch-off of the test object (300). 9. Control method according to one of the preceding claims, characterized in that the method step of detecting (S10) individual phase currents (Ip1, Ip2, 1p3) is carried out continuously and for all phase currents of the multiphase alternating current. 10. Control method according to one of the preceding claims, characterized in that the method step of determining (S20) harmonic frequency components is repeated continuously for a rolling time window and is carried out for all phase currents of the multiphase alternating current. 11. Control method according to claim 5, characterized in that the method steps of determining (S31) the shape index and determining (S32, S33) the at least one symmetry index are carried out in parallel and continuously for the same rolling time window of the determined harmonic frequency components. 12. Control method according to one of the preceding claims, characterized in that the time window correlates with the period duration of the phase currents (Ip1, Ip2, I1p3) of the test object (300). 13. Control method according to one of the preceding claims, characterized in that the time window has a variable length, which is adapted in particular on the basis of the control signal. 15. Control device (100) for controlling operation of a test object (300) with a multi-phase alternating current on a test bench (200), comprising a detection module (10) for detecting the phase current (Ip1, Ip2, Ip3) for each phase of the test object (300), a determination module (20) for determining harmonic frequency components of the phase currents (Ip1, Ip2, Ip3) based on a fast Fourier transformation of the detected phase currents (Ip1, Ip2, Ip3), a determination module (30) for determining a shape index of a waveform of the phase currents (Ip1, Ip2, Ip3) from a ratio of the determined harmonic frequency components of the phase currents (Ip1, Ip2, Ip3) to a harmonic fundamental frequency, and for determining at least one symmetry index of the phase currents (Ip1, Ip2, Ip3) from a Relationship of the phase currents (Ip1, Ip2, I1p3) to one another; and an output module (50) for outputting a control signal on the basis of the determined shape index and/or the at least one determined symmetry index, for intervening in the operation of the test object (300), characterized in that the detection module (10), the determination module (20), the determination module (30), the comparison module (40) and the output module (50) are designed for carrying out a control method with the features of one of claims 1 to 13.