Plausibility Check of an Electric Three-Phase System Description The present invention relates to a method of checking an operation of an electric three-phase system for plausibility. The system comprises a converter and a load, wherein the load is connected to the converter via an alternating current line having three phases. The load is symmetric with respect to the three phases and the converter is controlled by a controlling device which uses measured phase current values of the alternating current line for operating the system. Furthermore, the present invention relates to a corresponding arrangement. The converter may be, for example, an inverter connected to a second converter via a direct current intermediate circuit. In particular, the present invention relates to the field of electric high power applications, such as the delivery of electric energy to a driving motor of a railroad traction vehicle.

For many applications, a converter comprises a'direct current side and an alternating current side with three alternating current phases. The direct current side is connected to a direct current intermediate circuit. The electric load is connected to the three alternating current phases on the alternating current side. A controlling device controls the operation of the converter, wherein the phase currents of the three alternating current phases are used as input variables of the controlling process. For example, it is possible to use a controlling device, which is adapted to generate pulse width modulation (PWM) signals. Switching signals, which correspond to the PWM signals are transferred to the converter and effect the switching of electronic valves of the converter. However, other methods may be performed for controlling the converter.

There are several errors and malfunctions that might occur during the operation of the system: One or more of the three phases of the alternating current connection could be interrupted or the converter itself might malfunction. One or more of the three phases might be connected to ground. Furthermore, a sensor, which is used to detect a phase current of one of the phases, might be defect and/or an energy supply for operating the sensor could fail. A signal connection from the detector to the controlling device could be interrupted or not realised (possibly due to a loose connector). In addition, or alternatively, any device (such as an amplifier) which is used to process the signal from the detector could be defect or might not be adjusted correctly.

If there is a malfunction of measuring the phase currents (including a malfunction of processing the measuring signal), the controlling device will use a wrong input value and, as a consequence, the controlling device will try to adapt the phase current. In other words: the controlling device will output control signals to the converter in order to compensate the wrong input value. If the measured phase current is permanently too small (e. g. zero due to a failure of the detection), the converter can be damaged or even destroyed. Furthermore, if the load is a machine (e. g. an asynchronous machine), mechanical oscillations in the area of the machine are excited which may damage the system.

In many applications, the electric load is symmetric with respect to the three alternating current phases. In other words: the load is balanced. The expression"the load is symmetric with respect to the three alternating current phases"means that the root mean square (RMS) values of the electric currents carried by the three phases are equal if the operation is normal.

Examples are a star-connected load and a delta-connected load. In the case of the latter, the load comprises the same resistance and the same inductivity between each pair of phases.

However, there might be a small deviation from the balanced state, for example in the case of a delta-connected load. This deviation is acceptable, if a specified tolerance level is not exceeded. In the case of a star-connected load, the sum of the three phase currents of the converter output is zero if there is no failure, such as a connection to ground (earth).

If the sum of the three phase currents should be zero (e. g. star-connected load), it is possible to measure all three phase currents, to calculate the sum of the three phase currents and to compare the sum with a threshold value. If the threshold value is exceeded, it can be

concluded that there is a failure and appropriate actions can be taken, such as switching off the converter and thereby stopping the operation of the electromagnetic machine.

Furthermore, one of the three current sensors and other corresponding devices for processing the sensor signal have been omitted in order to save costs and space. As a result, the procedure described before cannot be applied. One possibility to check the function of the sensor signal processing in such an arrangement is to use an extra test coil in each sensor and to generate a signal current in the signal line by induction. The signal current can then be processed and the function of the corresponding processing devices can be checked.

It is an object of the present invention to provide a method of checking an operation of an electric three-phase system for plausibility, which allows for a reliable detection of errors during the operation of the system and which is sufficiently fast to detect the error before the system or parts of the system are damaged. Furthermore, it is desirable that the effort for the plausibility check is low and the plausibility check does not interfere with the normal operation of the system. A further object of the invention is to provide a corresponding arrangement.

In order to explain an important aspect of the invention a rotating vector or pointer is defined, wherein the pointer represents an operation state of the three-phase system. The pointer may be, in particular, a current space pointer of the load-current. It can be used to calculate other quantities that describe the operation state, e. g. a torque of an electromagnetic machine which is part of the load or which is the load. The pointer rotates at a frequency that is equal to the frequency of the alternating current carried by the phases. The length of the pointer (i. e. the root of the square of the pointer) may change during normal operation, for example as a result to a change of the load or a change of the demand. However, a variation of the length also contains information about input values (such as measured values of the phase currents of the system) which the pointer depends on. Consequently, absolute values, which are related to the length of the pointer, (e. g. the length, the square of the length and derived quantities such as the torque of the electromagnetic machine) depend on the measured phase currents of the three-phase system It is a basic idea of the invention to evaluate measurement information which is related to this pointer or-in other words-which describes properties of the pointer.

During the operation of a converter, any quantity describing the operation state of the three- phase system may be subject to oscillations and/or fluctuations. Thus, the length of the pointer may vary in a manner that makes it difficult to obtain useful information. To overcome this problem it is proposed to select specific operation states and/or times where the fluctuations are less significant.

In other words, the following method may be performed: An error in measuring phase currents of the system is detected by evaluating at least one absolute value which is related to the pointer, wherein the pointer rotates at a frequency that is identical to the frequency of the alternating current carried by the phases. A controlling device repeatedly generates identical or similar sequences of control signals for controlling the converter. Based on the timing of these sequences the time of determining (e. g. measuring) the at least one absolute value is selected or appropriate information is saved for later evaluation. I. e. the time corresponds to a rotational position pointer.

In particular, the following is proposed: A method of checking an operation of an electric three-phase system for plausibility, wherein the system comprises a converter and a load, in particular a driving motor of a railroad traction vehicle, and wherein: - the load is connected to the converter via an alternating current line having three phases, - the load is adapted to be symmetric with respect to the three phases, - phase currents carried by at least two of the three phases are measured, - the converter is controlled by a controlling device, - the controlling device uses values of the measured phase currents for operating the system, - the controlling device repeatedly generates identical or similar sequences of control signals for controlling the converter, wherein absolute values are calculated for operation times which correspond to the sequences, wherein the absolute values are related to a feature of a quantity describing an operation state of the system, wherein the quantity is a rotating pointer which rotates at a frequency that is equal to a frequency of the phase currents carried by the phases, wherein a length of the rotating pointer depends on the phase currents, wherein the absolute values depend on the

length of the rotating pointer, and wherein at least one of the absolute values is used to detect an error in measuring at least one of the phase currents.

In particular, the converter may be an inverter having electronic valves, wherein the converter is connected to a motor via the three phases. The electronic valves are repeatedly switched on and switched off by the controlling device in order to generate the alternating current and, thereby, a rotating magnetic field is produced which causes rotation of the motor's rotor.

Desired or predetermined figures of the magnetic flux of the motor as function of the angle of rotation can be realised by outputting corresponding control signals to the inverter. For example, the desired flux figure may be circular or may be a polygon having six or eighteen corners. However, the flux figures which are obtained in practice differ from the desired flux figures due to the switching of the electronic valves. Figures 8 and 11 of the accompanying drawing, which will be described in more detail later, show examples of flux figures that can be realised in practice. Especially at the corners of the flux figures, i. e. at specific rotating positions of the magnetic field produced by the stator, identical or similar sequences of the control signals are generated in each rotation cycle. For example, one specific valve is switched on and off during the sequence while the other valves remain switched off.

In practice, values that correspond to the switching actions may be stored in a data array for a whole rotation cycle or part of a rotation cycle. The entire controlling procedure may be performed by software and/or hardware. Particularly in this case, the calculation of the absolute values, which depend on the length of the rotating pointer (e. g. the current space pointer), can be triggered by the sequences. Alternatively or in addition, the absolute values may be calculated continuously and specific values, which correspond to the identical or similar sequences or to the corresponding rotational positions of the magnetic field, may be evaluated and/or may be saved. For example, if the rotating pointer is a current space pointer, measurement values of the at least two measured phase currents are saved and an evaluation procedure for checking the plausibility is performed on a regular basis, such as once at the end or after the end of each rotation cycle of the magnetic field.

As mentioned before, the rotating pointer may be a current space pointer, for example.

However, other pointers which contain the desired information about the system's behaviour may be defined, in particular pointers that are equivalent to or calculated using the current space pointer.

In particular when the rotating pointer is a current space pointer, which corresponds to phase shifts and amplitudes of the three phase currents, the absolute values are preferably calculated using the measured phase currents.

In particular when the absolute value is calculated from the rotating pointer and the magnetic flux of an electromagnetic machine (e. g. by calculating a product of the pointer and of the flux), the absolute value (e. g. the torque of the electromagnetic machine) may be calculated by the controlling device using the measured phase currents. For example, the controlling device may model operation behaviour of the load using a model that is implemented by software. The measured phase currents, further input quantities (such as an electric voltage and the rotational speed of the machine) and parameters of the machine are used by the model. E. g. , DE 195 31 771 Al (inventor: Depenbrock) describes a method and a device for determining a rotational speed of a rotating field machine. The document discloses that signal processing comprises a complete machine model including a converter, an alternating current side of which is connected to the machine. In particular, such a model can be used to calculate the rotating pointer and/or the absolute values.

As mentioned earlier, there are different possible reasons for a failure or malfunction of the phase current measurement. For example if there is a malfunction of a power supply used for the current measurement, this will usually result in an immediate response which can be detected by observing the current space pointer. In another case, the properties of a shunt resistance (which is used to convert a current signal into a voltage signal when measuring a phase current) may gradually change over time. In this case, the controlling device which uses the measured phase currents may try to compensate the corresponding measurement error. However, it is still possible to detect the measurement error, in particular if a model is used by the controlling device as mentioned before.

Preferably, the absolute value is a square value representing the square of the length of the rotating pointer. One advantage of using a square value resides in the fact that a length of a pointer can easily be calculated using square values of components of the pointer, e. g. of measured phase currents in the case of a current space pointer. Measurement time is saved and expenses are reduced.

Furthermore, it is preferred that the absolute values are calculated for a plurality of rotational positions of the pointer, each of which corresponds to one of the identical or similar sequences. A difference between the absolute values for at least two of the rotational positions or deviations of at least two of the rotational positions to a comparison value are used to determine a type of error in measuring at least one of the phase currents. In particular, gain and offset errors can be distinguished with this procedure. These two different types of errors show an individual characteristic behaviour with regard to fluctuations of the pointer's length during rotation, which will be described in more detail later. Therefore, it is possible to detect whether the measurement error is a gain error and/or an offset error.

One way to detect an error in measuring at least one of the phase currents is to perform a comparison: The absolute value is compared to a previous absolute value at a same rotational position of the pointer, or is compared to a comparison value derived from a plurality of previous absolute values. The comparison result is used to detect the error.

When the plausibility check is performed, it may include a special procedure for deciding whether a failure and/or malfunction exist. This special procedure is referred to as"Fuzzy- evaluation"in this description. It leads to particularly reliable decision results.

When the plausibility check is performed, it is proposed to repeatedly determine a first plausibility value, wherein the first plausibility value is an instantaneous measure of a degree of plausibility and/or non-plausibility. Further, a second plausibility value is derived from a plurality of the first plausibility values, wherein each of the first plausibility values can influence the second plausibility value correspondingly to its degree of plausibility and/or non-plausibility. An appropriate action is taken, if the second plausibility value fulfils a predetermined criterion.

In the Fuzzy-evaluation, the second plausibility value reflects the plausibility of a plurality of first plausibility values. Therefore, a faulty first plausibility value is less likely to result in an interruption of the converter operation, for example. The effect of the Fuzzy-evaluation can be compared with the effect of an intelligent filter. Furthermore, it can easily be implemented in hardware and/or software, for example by using a counter, wherein the counter value is the second plausibility value. The term"counter"is not limited to counting integer numbers. In the example, a first plausibility value with a lower degree of plausibility results in a greater

increase of the counter number than a first plausibility value with a higher degree of plausibility. Vice versa, the counter number can be decreased, if the degree of plausibility of the first plausibility value is high.

There are several features which can be combined with this basic approach independently from each other or in combination of at least some of the features. First, the degree of plausibility of the first plausibility value can be weighted and the second plausibility value is influenced correspondingly to the weighted first plausibility value. In the simplest case, a weight factor of 1 can be used for all degrees of plausibility of the first plausibility value.

However, it is also possible to decrease the weight of the first plausibility values according to an increase of the degree of plausibility.

Secondly, it is possible to decide whether the degree of plausibility of the first plausibility value matches a lower threshold value and/or is smaller than the threshold value. If this is the case, the second plausibility value is influenced to show the decreased plausibility. If this is not the case, the second plausibility value is not amended or is amended to show an increased total plausibility (e. g. the counter number is decreased).

Thirdly, it is possible that only the effect of a limited number of most recent first plausibility values influences the second plausibility value. This means that first plausibility values, which belong to incidents that took place too long ago, do not affect the second plausibility value. Alternatively, if a counter is used for the second plausibility value, both decreasing and increasing the counter number is possible, depending on the degree of plausibility of the first plausibility value. As a result, the counter number may be decreased to zero and, in this case, incidents that took place in the past do no longer affect the second plausibility value.

In particular, the first plausibility value is a difference between an instantaneous pointer length (or square of the length) and a comparison value, e. g. the average of the length values of one or more than one rotation cycle of the pointer. In addition or alternatively, a higher deviation to a normal, an expected or a desired result has a lower degree of plausibility and vice versa. In the case of the difference being the first plausibility value, the degree of plausibility is lower for higher differences. For example, the predetermined criterion may be that a threshold value of the second plausibility value is matched and/or exceeded. Generally, the Fuzzy-evaluation or one of its embodiments has the following advantages :

- Small and/or rare deviations to a normal, an expected or a desired state do not necessarily result in an interruption of the current converter operation.

- Large and/or many deviations to a normal, an expected or a desired state will result in a fast and reliable detection of the failure and/or malfunction.

- The procedure can easily be implemented. The parameters of the procedure, like the weight and the threshold values, can easily be adapted.

- A process, which might work at a small repetition frequency, but might be part of a higher-level control routine, can reliably check the result of the Fuzzy-evaluation, in particular check the counter number.

In particular, when an error in measuring at least one of the phase currents has been detected, the operation of the converter may be stopped and restarted and a detailed analysis of a reason for the error may be performed after stopping and before or during the restarting of the converter. For example, detailed checking routines may be performed when the converter operation is initialised.

Furthermore, it is proposed to provide an arrangement which is adapted to perform the method according to at least one of the embodiments described before. The arrangement may be part of the converter, in particular part of a controlling device of the converter. It may be realised by hardware and/or software.

Furthermore, the present invention includes: - a computer loadable data structure that is adapted to perform the method according to one of the embodiments described in this description while being executed on a computer, - a computer program, wherein the computer program is adapted to perform the method according to one of the embodiments described in this description while being executed on a computer,

a computer program comprising program means for performing the method according to one of the embodiments described in this description while the computer program is being executed on a computer or on a computer network, a computer program comprising program means according to the preceding item, wherein the program means are stored on a storage medium readable to a computer, a storage medium, wherein a data structure is stored on the storage medium and wherein the data structure is adapted to perform the method according to one of the embodiments described in this description after having been loaded into a main and/or working storage of a computer or of a computer network, and a computer program product having program code means, wherein the program code means can be stored or are stored on a storage medium in order to perform the method according to one of the embodiments described in this description, if the program code means are executed on a computer or on a computer network.

In particular, the computer or part of the computer network mentioned in one of the paragraphs before may be realised by using a processing unit, in particular a central processing unit of the controlling device. At least the processing of the absolute value and of the error detection can be performed by the computer or computer network. The computer program product is to be understood as a product that can be sold or traded as a good.

Specific embodiments of the present invention will be described in the following by way of example and with reference to the accompanying drawing. The figures of the drawing schematically show: Fig. 1 an arrangement with a converter, an alternating current side of which is connected to an electric load, in particular to an asynchronous motor; Fig. 2 devices and units used to process a sensor signal of a sensor shown in Fig. 1 ; Fig. 3 a flow chart illustrating a collection of measurement values used to perform a plausibility check; Fig. 4 a flow chart illustrating an evaluation of measurement values, in particular the measurement values collected in the procedure according to Fig. 3; Fig. 5 a flow chart illustrating the Fuzzy-evaluation; Fig. 6 diagrams illustrating the effect of an offset error; Fig. 7 diagrams illustrating the effect of a gain error;

Fig. 8 a polygon-shaped magnetic flux figure; Fig. 9 a graph illustrating the variation of the length of a rotating pointer during four consecutive rotation cycles, in case of the magnetic flux of Fig. 8 and in case of a gain error; Fig. 10 a graph illustrating the variation of the length of a rotating pointer during four consecutive rotation cycles, in case of the magnetic flux of Fig. 8 and in case of an offset error; Fig. 11 a circular-shaped magnetic flux figure; Fig. 12 a graph illustrating the variation of the length of a rotating pointer during four consecutive rotation cycles, in case of the magnetic flux of Fig. 11 and in case of an offset error; Fig. 13 a graph illustrating the variation of the length of a rotating pointer during four consecutive rotation cycles, in case of the magnetic flux of Fig. 11 and in case of a gain error; and Fig. 14 an arrangement of units and devices used for performing a plausibility check.

Fig. 1 shows a system 1 comprising a converter 11 and a machine 13. The converter 11 may be constructed as known from prior art. For example, the converter may be a DC/AC converter (inverter) and may comprise three parallel paths, which connect a first and a second DC connection line of a DC intermediate circuit (not shown in the figure). Each of the paths may comprise one bridge having two electronic valves (for example Gate Turn-Off thyristors or Insulated Gate Bipolar Transistors, IGBTs), which are connected in series to each other.

Each phase 5a, 5b, 5c of an AC connection to a machine 13 may be connected to a connecting point between the two electronic valves of one path.

Input signal lines of the electronic valves for receiving switching signals can be connected to a control signal input 10 of the converter. The control signal input 10 is connected to a control signal output of a controlling device 4 via a connection 9 of the arrangement shown in Fig. 1.

The DC side of the converter may be connected to a second converter, which is adapted to output a DC current to the converter. An input side of the second converter may be connected to a power supply network, for example a single-phase alternating current network of a railway system. However, other configurations and operations are possible, such as feeding back electric energy from the converter to the second converter.

As shown in Fig. 1 the three phases Sa, 5b, 5c of the AC connection connect the converter 11 to an electric load 13, in this case to an asynchronous motor, which may be the driving motor of a railway traction vehicle.

A first 5a and a second 5b of the phases Sa, Sb, 5c are combined, in each case with only one current sensor 3a, 3b for measuring the phase current. Since the electric load 13 is symmetric in the three phases 5a, 5b, 5c, the third phase 5c is not combined with a current sensor. The current sensors 3a, 3b are connected to the controlling device 4 via one sensor signal connection 7a, 7b for each sensor.

In high-power applications, such as in railway traction vehicles, the current sensor function may be based on the principle of detecting a current by evaluating the magnetic field produced by the current. An example of such a current sensor 3 and a corresponding arrangement for processing the sensor signal is shown in Fig. 2. The current sensor 3 is attached to one phase 5 and generates a current signal, which corresponds to the current carried by the phase 5. A signal line for outputting the sensor signal is connected to a current/voltage converter 41 (e. g. a shunt resistor) for converting the current signal to a voltage signal. The current/voltage converter 41 is optionally connected to a filter 43 for filtering the voltage signal in order to eliminate signal parts, which are the result of transient interferences, and in order to eliminate noise. An output of the filter 43 is connected to an input of an amplifier 45 for amplifying the voltage signal. If the filter 43 is not provided, the output of the current/voltage converter 41 may directly be connected to the amplifier 45.

A comparator may be provided between the current/voltage converter 41 and the amplifier 45, for example between the filter 43 and the amplifier 45. The comparator may be used to compare the measured phase current with a phase current measured by a redundant current sensor.

An output of the amplifier 45 is connected to an analogue to digital (A/D) converter 47 for digitising the voltage signal. The digitised signal can be used for digital data processing, in particular performed by a computer of the controlling device 4 shown in Fig. 1. For example, the devices 41,43, 45,47 of the arrangement shown in Fig. 2 may be arranged between the current sensor 3a or 3b and a signal input of the controlling device 4. Alternatively, at least some of the devices 41,43, 45,47 may be part of the controlling device 4.

During operation of the converter 11, the controlling device 4 for controlling the converter 11 uses phase current values, which are based on the measurement performed by the current sensors 3a, 3b. For example, model calculations for modelling the operation of the electric load 13 and the plausibility check can be performed by a central processing unit of the controlling device 4.

According to the flow chart shown in Fig. 3, a controlling device (such as the controlling device 4 of Fig. 1) may operate a converter as follows: in step S 10 the controlling device reads a switching state of controller valves and initiates corresponding switching actions in step S 11. For example, the switching state or corresponding electric potentials of the three phases may be represented using a set of three binary numbers saved in an array of consecutive switching actions. E. g. a set of numbers 1, 0,0 means that a first phase is to be switched to high potential, a second phase is to be switched to low potential and a third phase is to be switched to low potential. The high and the low potential may be the instantaneous potentials of a direct current intermediate circuit which is connected to the converter.

In the following step S 12 phase currents of at least two of the phases, which correspond to the time of the instantaneous switching state of the valves, are sampled. Typically, an asynchronous motor which is used as a driving motor of a railway traction vehicle rotates at a rotor frequency of 0 to 150 Hz or higher, the switching frequency of the converter is in the range of 250 Hz to 800 Hz and the sampling frequency of sampling the measured phase current values is even higher, namely in the range of some kHz, e. g. 2 to 4 kHz.

In the following step S 13 it is decided whether the sampled phase current values are to be evaluated in a plausibility check. For example, if a specific sequence of consecutive switching states is recognised (e. g. the switching states 0,0, 0 and 0,0, 1 alternate repeatedly at least 4 times), it is decided that the system is in a predetermined operation state and that the sampled phase current values are to be evaluated. As a result, the procedure continues with step S14, wherein the sampled phase current values are saved in a way which allows determining the corresponding operation state of the system later. For example, the values are saved in an additional data field which is assigned to the corresponding set of numbers of the instantaneous switching action. It is also possible that phase current values are saved for other switching states, in particular for every switching state, and that a marker information is

saved for marking the state and/or values for evaluation. Also, it is possible to evaluate the phase current values immediately.

After step S14, and if it is decided in step S13 that the sampled phase current values are not to be evaluated in the plausibility check, the procedure returns to step S 10.

As a result of the procedure, at least one set of measured phase current values, preferably a plurality sets, is/are available for evaluation. In a specific embodiment, six values of each measured phase current are used for evaluation in each rotation cycle of the magnetic stator flux vector of the machine, wherein the six values correspond to equidistant angles of rotation of the flux vector, e. g. to corners of a polygonal flux figure (see for example the flux figure shown in Fig. 8). Since the rotational speed of the machine is nearly constant during the same cycle of rotation, the six values correspond to six equidistant points of time as well.

According to the flow chart shown in Fig. 4, the evaluation may be performed by the controlling device as follows: In step S20 the controlling device reads the measured phase current values for at least one rotational position of the flux vector and calculates the length of the rotating pointer, in particular the current space pointer. For example, the measured phase current values of two of the phases are squared and the square of the length is calculated, wherein the angle of 120 degrees between the two phases and the rotational position of the pointer is taken into account.

In step S21, the length is compared to a mean value (e. g. the mean value of the point length values of the preceding three or four (or another number) rotation cycles of the flux vector or of the rotating pointer at the same rotational position. Since a variation of the length, which is caused by a change of the load power is comparatively small over three or four consecutive rotation cycles-if the load is an electric machine-the comparison will detect the effect of a measurement error.

In step S22 it is decided whether the difference between the length and the mean value exceeds a threshold value. If the threshold value is exceeded, the procedure continues with step S23. If the threshold value is not exceeded, the procedure returns to step S20. In step S23 an optional analysis of the type of error is performed, as will be described in more detail later with reference to Fig. 6 and Fig. 7. In another embodiment step S23 is omitted. In step

S24 an appropriate action is taken, such as interrupting or stopping the operation of the converter. For example, the threshold value may amount to 10 to 15 percent of the instantaneous length of the rotating pointer. It is possible to adapt the threshold value to the operation state of the system and/or to define the threshold value as a value which is referred to the mean value or to the instantaneous length.

The procedure which has been described with reference to Fig. 4 may be modified. In particular, it may not be appropriate to interrupt the operation of the converter, if the threshold value is exceeded just once. Therefore, the procedure according to Fig. 5 may be applied.

An exemplary way of performing the Fuzzy-evaluation is described in the following with reference to Fig. 5. In step S1 the length of the rotating pointer is calculated. In step Sla a corresponding comparison value of the length is obtained and/or processed. The calculated value and the comparison value are transferred to a comparator, which compares the two values in step S2. In particular, the comparator calculates the difference between the two values and, optionally, generates the absolute value of the difference. The difference or its absolute value is compared in step S3 with a threshold value, e. g. by the threshold comparing means. If the threshold value is exceeded, the procedure continues with step S4, wherein a counter value is increased by the amount by which the difference exceeds the threshold value.

If the threshold value is not exceeded, the procedure continues with step S5, wherein the counter value is decreased by a constant amount. After step S5, the procedure returns to the beginning and continues with step S 1.

After step S4, the procedure continues with step S6, wherein a decision is made whether the counter value matches a second threshold value or whether it matches or exceeds the second threshold value. If this is the case, it is decided that there is a failure and/or malfunction and an appropriate action is taken in step S7. If the second threshold value is not exceeded, the procedure returns to the beginning and continues with step S1.

The procedure described above can be implemented by software and/or hardware.

Furthermore, it is possible to modify the procedure. For example, the counter value may be decreased in step S5 by an amount that depends on the operation state of the system or of the load.

Fig. 6 and Fig. 7 show on the left-hand side figures of a rotating pointer. In each case two circular or nearly circular lines and a centre or origin M are shown. This representation is simplified, since the length of the pointer is usually subject to fluctuations as shown in Fig.

12, for example. The three straight, arrowed lines which pass through the origin M enclose an angle of 120 degrees between each pair of lines and define a non-rectangular coordinate system. They are not necessarily in line with the directions of the three phase currents, if the rotating pointer is a current space pointer of the three phases.

Six intersection points of in each figure one of the circular or nearly circular lines with the arrowed lines are marked with small circles and are denoted with tl to t6. The distances between the origin M and the intersection points correspond to the lengths of the rotating pointer in the corresponding rotational position. On the right-hand side of Fig. 6 and Fig. 7, a corresponding diagram of the length as a function of the rotational position or as a function of time is shown. If the rotational speed of the pointer is constant, both interpretations of the horizontal dimension are equivalent and tl to t6 can be interpreted as equidistant points in time.

In Fig. 6 a horizontal line representing the mean value of the length of one rotation cycle is drawn as a dashed line. The mean value is the same for both circular lines on the left-hand side of Fig. 6. In Fig. 7, a dashed line represents the mean value of the circular line. The mean value of the oval line on the left-hand side of Fig. 7 is drawn as a thin dotted line on the right-hand side of Fig. 7.

Fig. 6 shows the effect of an offset error in measuring phase currents, wherein the rotating pointer can be the current space pointer or a torque pointer of an electromagnetic machine, for example. The offset error results in a shift of the circular line (generally: of the pointer length figure) but does not affect the mean value of the length over one rotation cycle.

Consequently, there are two points of intersection between the length as a function of the rotational position or of time, as can be seen on the right-hand side of Fig. 6. As a result, an offset error can be detected by a corresponding procedure. For example if the pointer length is evaluated at six equidistant rotational positions (according to tl to t6), it is possible to decide that there is an offset error, when the differences between the mean value and the length at the first tl and at the fourth t4 rotational position or time have opposite signs, i. e. when the difference is negative at one of the positions (tl) and positive and the other position

(t4). Of course, an offset error in measuring another one of the phase currents or offset errors in measuring more than one of the phase currents might occur, so that the circular line is shifted in another direction. Consequently, it is preferred to evaluate the difference between the mean value and the pointer length at pairs of rotational positions or times which are separated by half of a rotation cycle or by half a rotation cycle duration. However, other methods of detecting the characteristic influence of an offset error as shown in Fig. 6 may be performed. In broader terms: the offset error results in an oscillation of the pointer length with the base frequency around the average pointer length value. Alternatively or in addition, this oscillation might be detected by other method.

Fig. 7 shows the effect of a gain error in measuring one of the phase currents. The length of the pointer has three intersection points with the mean value of the length of one rotation cycle as shown on the right-hand side of Fig. 7. Consequently, the gain error can be detected by the same method as described in the paragraph before, but the differences between the mean value and the length have the same sign (positive or negative) for the pair of rotational positions or times. In broader terms: the gain error results in an oscillation of the pointer length with two times the base frequency around the average pointer length value.

Alternatively or in addition, this oscillation might be detected by other method.

As mentioned earlier, Fig. 8 shows a six-cornered polygonal magnetic flux figure of an electromagnetic machine, in particular of an asynchronous motor. The corners correspond to six equidistant rotational positions or times tl to t6. Since the sequences of control signals generated by the controlling device in the area of the corners are similar or equal in every rotation cycle of the machine in static operation, these sequences or the information about these sequences can be used to determine the times tl to t6. Similarly, Fig. 11 shows a nearly circular magnetic flux figure. Corresponding centres or origins are denoted with reference letter M.

The goal of every control method for controlling a converter is a magnetic flux as near to a circle as possible. In practice, frequency control methods are chosen which generate approximations of the ideally circular magnetic flux figure (e. g. a 6-cornered or an 18- cornered flux figure). The number and position of the sampling points is determined by the number and position of the phases/phase-windings e. g. for the typical system with three phases. For the typical case of three phases with 120 degrees phase shift (and the

corresponding angular position), the six sampling points with 60 degree intervals between them may be used, as described before. It is preferred that the control system switches to one of the phases at one of the sampling points. All non-circular control methods typically switch to the next phase when the flux pointer crosses the angular phase position, thereby generating a"corner" (in flux as well as current, where the current under load may be phase shifted).

The graphs of a rotating pointer shown in Figures 9, 10, 12 and 13 belong to a current space pointer. From the graph in Fig. 9 it can be recognised that there are significant fluctuations of the pointer's length in consecutive rotation cycles. However, it can also be recognised that the fluctuations are comparatively small at the six corners of the graph. Consequently, it is possible to detect the gain error which has deformed the graph in the Figure by applying the procedure which has been described before with reference to Figures 6 and 7. In particular, it can be recognised from Fig. 9 that the distance between the corners at rotational angles or times t3 and t6 is smaller than the distances between the other pairs of opposite corners which indicates a gain error.

In Fig. 10, the graph is shifted towards the bottom (relative to the centre M) which indicates an offset error.

The graphs of a current space pointer shown in Figures 12 and 13 correspond to the flux figure of Fig. 11. In Fig. 12, a shift of the graph towards the left-hand side (relative to the centre M) can be recognised, which indicates an offset error. The graph shown in Fig. 13 is similarly deformed like the oval graph of Fig. 7, which indicates a gain error.

To avoid a faulty detection (e. g. due to variations of the electrical load) the deviations (which are the result of oscillations) mentioned above are to be detected for several periods of the base frequency of the phase currents. The detection does not have to be fast, since the deviations are typically small. Other methods for detecting more serious interferences may be applied at the same time. Consequently, it is at the same time safe, reliable and sufficient to apply the Fuzzy evaluation mentioned above. In addition, methods in order to compensate for load deviations might be applied (e. g. by using the torque instead of the current pointer length). This will increase the reliability of the detection.

The arrangement shown in Fig. 14 comprises units 51 to 59, which may be part of a device

50. The device 50 can be realised by software and/or hardware and may be part of the controlling device 4 of Fig. 1. An A/D converter 47 (in particular the converter 47 of Fig. 2) is connected to a measurement processing device 51 adapted to process measured values of phase currents carried by at least two of the three phases. The measurement processing device 51 is connected to a calculation device 53, adapted to calculate absolute values that depend on a length of a rotating pointer, wherein the rotating pointer rotates at a frequency that is equal to a base alternating current frequency of the phase currents. A selecting device 55 for selecting sequences of control signals is provided, wherein the sequences are either identical or similar sequences of control signals. These are repeatedly generated by the controlling device for controlling the converter. The selecting device 55 is adapted to effect the calculation of the absolute values for operation times of the selected sequences. In this particular embodiment the selecting device 55 is connected to a control unit 57, adapted to generate control signals for controlling the operation of the converter. The control unit 57 may include means for reading the sequences and for generating corresponding control signals such as PWM signals. The selecting device 55 is connected to the calculation device 53 and the calculation device 53 is connected to a detecting device 59 for detecting an error in measuring at least one of the phase currents. In an alternative embodiment, the calculation device may also be connected only indirectly to the selecting device.

During operation of the system, measurement values of the phase currents are transferred in digital form from the A/D converter 47 to the measurement processing device 51 which transfers measurement values to the calculation device 53, namely at least the measurement values which correspond to a selection procedure performed by the selecting device 57. A corresponding connection between the selecting device 55 and the measurement processing device 51 may be realised in a modified arrangement. Furthermore, the measurement processing device 51 may process the measurement values for other purposes such as for the use of the measurement values in controlling the operation of the system.

In the example illustrated by Fig. 14, the selecting device 55 detects when the identical or similar sequences of control signals occur during the operation of the system. It triggers and/or causes the calculation of the corresponding absolute values, so that the detecting device 59 can detect an error in measuring the phase currents.