EP4205155A1

EP4205155A1 - Contactor and method for observing a contactor

Info

Publication number: EP4205155A1
Application number: EP21762487.3A
Authority: EP
Inventors: Ingo Schaar
Original assignee: Eaton Intelligent Power Ltd
Current assignee: Eaton Intelligent Power Ltd
Priority date: 2020-08-28
Filing date: 2021-08-17
Publication date: 2023-07-05
Also published as: WO2022043139A1; GB202013507D0; GB2598358A

Abstract

A contactor (20) comprises a first input terminal (21), a first output terminal (22), first switching-contacts (23) which couple the first input terminal (21) to the first output terminal (22), a first measurement circuit (51) with an input (53) connected to the first input terminal (21) and an output (58) for providing a first measurement signal (SM1), a second measurement circuit (52) with an input (54) connected to the first output terminal (22) and an output (59) for providing a second measurement signal (SM1), and a signal processing circuit (55) which is connected on its input side to an output (58) of the first measurement circuit (51) and to an output (59) of the second measurement circuit (52) and is configured to provide an output signal (SOUT) depending on the first and the second measurement signal (SM1, SM2).

Description

Contactor and method for observing a contactor

The present disclosure refers to a contactor, an arrangement with a contactor and a method for observing a contactor .

A contactor is typically used to switch medium or high currents . The contactor includes first switching-contacts which comprise a first fixed contact and a first movable contact . To bring the contactor into an open state , the first movable contact is moved away from the first fixed contact . To bring the contactor into a closed state , the first movable contact is moved in the direction towards the fixed contact . Often the first switching-contacts comprises a first and a second fixed contact and a first and a second movable contact . The first and the second movable contact are electrically and mechanically connected to each other by a contact bridge of the contactor .

During a switching period of the contactor , the first switching-contacts may bounce . The bouncing occurs in the case that the contactor is brought into the closed state . Depending on the bouncing, the first switchingcontacts will lose some material . This is caused by small arcs which burn down the material at the surface of the movable and the fixed contacts . The status of the first switching-contacts can be investigated by opening the contactor . However, this is a very expensive procedure so that typically the contactor is replaced by a new contactor after a certain amount of time or is replaced when the contactor has failed .

It is an obj ect to provide a contactor , an arrangement with a contactor and a method for observing a contactor which allow information about the switching-contacts of a contactor to be received .

This obj ect is achieved by the subj ect-matter of the independent claims . Further developments are described in the dependent claims .

The definitions as described above also apply to the following description unless otherwise stated . There is provided a contactor comprising a first input terminal , a first output terminal , first switching-contacts , a first and a second measurement circuit and a signal processing circuit . The first input terminal is coupled via the first switching-contacts to the first output terminal . The first measurement circuit includes an input connected to the first input terminal and an output for providing a first measurement signal . The second measurement circuit comprises an input connected to the first output terminal and an output for providing a second measurement signal . The signal processing circuit is connected on its input side to an output of the first measurement circuit and to an output of the second measurement circuit . The signal processing circuit is configured to provide an output signal depending on the first and the second measurement signal .

Advantageously, a first input terminal voltage and a first output terminal voltage which are tapped at the first input terminal and at the first output terminal are provided to the first and the second measurement circuit . The first input terminal voltage and the first output terminal voltage depend on the bouncing and the state of the first switching-contacts . The first and the second measurement signal depend on the first input terminal voltage and the first output terminal voltage and thus provide information about the bouncing to the signal processing circuit . The output signal includes information about the state of the first switching-contacts .

In an embodiment of the contactor, the contactor comprises a first housing for housing at least the first switching-contacts . The first input terminal and the first output terminal are terminals of the first housing . Thus , the first input terminal and the first output terminal can be electrically and mechanically contacted from outside of the first housing and are electrically connected to the first switching-contacts inside of the first housing .

In an embodiment of the contactor, the first and the second measurement circuit and the signal processing circuit form a module . The module can be named plug-in module , add-on module or auxiliary module . The contactor comprises a second housing for housing the module . Thus , the first and the second measurement circuit and the signal processing circuit are housed by the second housing . The second housing is fabricated such that the second housing can be attached to the first housing and e . g . also be removed from the first housing . The second housing can be attached to an outside of the first housing . The second housing can be fixed to the first housing, e . g . by a screw, plug connection, snap-on connection, glue , spring , another fastening element or another locking device . Advantageously, the module can be provided separately from the parts of the conductor included by the first housing to a customer .

In an alternative embodiment of the contactor , the first and the second measurement circuit and the signal processing circuit are also housed by the first housing . The first and the second measurement circuit and the signal processing circuit may form a module arranged inside the first housing .

In an embodiment of the contactor, the signal processing circuit provides the output signal depending on a difference signal between the first and the second measurement signal . Advantageously, the difference signal is zero or approximately zero in case a steady state is reached after closing the first switching-contacts . The difference signal is different from zero in case the first switching-contacts are in an open state or during the bouncing . The first output terminal voltage is zero in case the contactor is in the open state . The first output terminal voltage deviates from zero during bouncing . Thus , bouncing occurs when the difference signal is different from zero and the first output terminal voltage is different from zero .

In an embodiment of the contactor, the signal processing circuit measures a time gap during which the difference signal is different from zero and the first output terminal voltage is different from zero and to generate the output signal depending on the time gap . The signal processing circuit may determine a time of the bouncing which corresponds to the time gap and is called bouncing time . In an embodiment of the contactor, the signal processing circuit provides the output signal depending on a time gap between a pulse start at a first point of time and a pulse end at a second point of time of at least one pulse of the difference signal . Typically, a start of a bouncing time is indicated by a start of a pulse of the difference signal and an end of the bouncing time is indicated by an end of a pulse of the difference signal . The bouncing time may comprise one or two pulses . The signal processing circuit acts as a time-to-digital converter, since the signal processing circuit determines the time gap between the first and the second point of time . The signal processing circuit is e . g . realized as microprocessor or microcontroller .

In an embodiment of the contactor, the first measurement circuit comprises a first voltage divider coupling the first input terminal to a reference potential terminal . The second measurement circuit comprises a second voltage divider coupling the first output terminal to a reference potential terminal . Advantageously, voltages are generated which have a lower voltage level than the first input terminal voltage and the first output terminal voltage .

In an embodiment of the contactor, the first measurement circuit comprises a first filter coupled to a first voltage divider tap of the first voltage divider and to the reference potential terminal . The second measurement circuit comprises a second filter coupled to a second voltage divider tap of the second voltage divider and to the reference potential terminal . The first and the second filter may be realized as low pass filter . The first and the second filter reduce noise , disturbances and bandwidth of the voltages provided by the first and the second voltage divider .

In an embodiment of the contactor, the first measurement circuit comprises a first analog-to-digital converter coupled to the first filter . The second measurement circuit comprises a second analog-to- digital converter coupled to the second filter . Advantageously, the first and the second measurement signals are implemented as digital signals . In an embodiment of the contactor, the first measurement circuit comprises a first protection circuit coupled to the first input terminal . The second measurement circuit comprises a second protection circuit coupled to the first output terminal . The first and the second protection circuit provide an overvoltage protection .

In an embodiment , the contactor comprises a second input terminal and a second output terminal , a third measurement circuit with an input connected to the second input terminal and an output for providing a third measurement signal , and a fourth measurement circuit with an input connected to the second output terminal and an output for providing a fourth measurement signal . The signal processing circuit is connected on its input side to the output of the third measurement circuit and to the output of the fourth measurement circuit and provides the output signal additionally depending on the third and the fourth measurement signal . The bouncing of the first switching-contacts and of the second switchingcontacts may be different . Advantageously, the output signal includes information gained from the first and the second switching-contacts .

In an embodiment , the contactor comprises a third input terminal , a third output terminal , third switching-contacts which couple the third input terminal to the third output terminal , a fifth measurement circuit with an input connected to the third input terminal and an output for providing a fifth measurement signal , and a sixth measurement circuit with an input connected to the third output terminal and an output for providing a sixth measurement signal . The signal processing circuit is connected on its input side to the output of the fifth measurement circuit and to the output of the sixth measurement circuit and provides the output signal additionally depending on the fifth and the sixth measurement signal . Advantageously, the output signal includes information gained from the first , second and third switching-contacts .

In an embodiment of the contactor, the signal processing circuit evaluates the first , the third and the fifth measurement signal and/or the second, the fourth and the sixth measurement signal and generates the output signal including a result of the evaluation in case the first , the second and the third switching-contacts are synchronized . The evaluation occurs at a point of time at a number of switching events . In case a sum, an average value or a range distribution of the measurement signals determined at one of the three switching-contacts deviates from the corresponding value determined at one of the other switching-contacts after a number of switching events larger than a predetermined value or percentage , the output signal is generated including the information that the first , the second and the third switching-contacts are synchronized .

A synchronization should be avoided to even distribute a deterioration on each of the three switching-contacts e . g . caused by arcs during lifetime .

There is provided an arrangement comprising a contactor and a load . The load is realized e . g . as a motor or a heater . The load includes a first phase terminal coupled to the first output terminal , a second phase terminal coupled to the second output terminal and a third phase terminal coupled to the third output terminal .

There is provided a method for observing a contactor . The contactor comprises first switching-contacts , a first input terminal , a first output terminal , a first and a second measurement circuit and a signal processing circuit . The method comprises : closing first switching-contacts between a first input terminal and a first output terminal , providing a first measurement signal by the first measurement circuit depending on a first input terminal voltage tapped at the first input terminal , providing a second measurement signal by a second measurement circuit depending on a first output terminal voltage tapped at the first output terminal , and providing an output signal depending on the first and the second measurement signal by a signal processing circuit .

Advantageously, the output signal provides an information about the first input terminal voltage and the first output terminal voltage which are influenced by the state of the first switching-contacts . The contactor is particularly suitable for the arrangement and for the method for observing a contactor . Features described in connection with the contactor can therefore be used for the arrangement and the method and vice versa . The method for observing a contactor could also be named method for operating a contactor .

In an embodiment , the method realizes an electrical lifespan investigation for a contactor . The contactor is used e . g . for motor control . The arrangement investigates the loss of material in the switching-contacts of the contactor by analyzing the voltage between the switching-contacts during the bouncing . With this information it is possible to give an estimation how long the contactor will work well before it is broken . In addition, an analysis of the phase-symmetry can be done .

Typically, during the switching period of a contactor the switchingcontacts will bounce . Depending on the bouncing the switching-contacts will lose some material . This is caused be little arcs which burns down the material .

In an embodiment , during the switching of the contactor the voltages above the switching-contacts are measured to record the bouncing voltage . The bouncing voltage depends on different parameters of the mechanical design of the contactor . Some of them will not change during the lifespan . Other will do and will have influence on the bouncing voltage . One of this is the thickness of the switching-contacts material . By analyzing this bouncing-voltage-signal conclusions on the status of the switching-contacts are drawn .

The following description of figures of embodiments shall further illustrate and explain aspects of the contactor and of the arrangement with the contactor . Parts , components and circuits with the same structure and the same effect , respectively, appear with equivalent reference symbols . Insofar as parts , components and circuits correspond to one another in terms of their function in different figures , the description thereof is not repeated for each of the following figures . Figures 1A and IB show an exemplary embodiment of an arrangement with a contactor;

Figures 2A and 2B show examples of signals of a contactor; and

Figure 3 shows a further exemplary embodiment of an arrangement with a contactor .

Figure 1A shows an exemplary embodiment of an arrangement 10 with a contactor 20 . The contactor 20 includes a first input terminal 21 and a first output terminal 22 . Moreover , the contactor 20 comprises first switching-contacts 23 which electrically couple the first input terminal 21 to the first output terminal 22 . The first switching-contacts 23 include a first and a second fixed contact 24 , 25 and a first and a second movable contact 26 , 27 . The first and the second movable contact 26 , 27 are mechanically and electrically connected to each other by a contact bridge 28 of the first switching-contacts 23 . The contactor 20 comprises a coil 29 and an armature 30 that couples the coil 29 to the contact bridge 28 . The contact bridge 28 is moved by the coil 29 via the armature 30 . The contactor 20 comprises a first and a second control terminal 31 , 32 that are coupled to the coil 29 . The first and the second control terminal 31 , 32 are typically named terminals Al and A2 . The control terminals 31 , 32 are used to receive a control signal for opening or closing the first switching-contacts 23 .

Moreover , the contactor 20 comprises a second input terminal 33 and a second output terminal 34 that are coupled to each other by second switching-contacts 35 of the contactor 20 . Additionally, the contactor 20 comprises a third input terminal 42 and a third output terminal 43 that are coupled to each other by third switching-contacts 41 of the contactor 20 . The second and the third switching-contacts 35 , 41 are realized such as the first switching-contacts 23 . The second switching-contacts 35 comprises a first and a second fixed contact 36 , 37 , a first and a second movable contact 38 , 39 and a contact bridge 41 . The third switchingcontacts 41 include a first and a second fixed contact 44 , 45 , a first and a second movable contact 46 , 47 and a contact bridge 48 . The second and the third switching-contacts 35 , 41 are mechanically connected to the first switching-contacts 23 such that the first , second and third switching-contacts 23 , 35 , 41 are moved parallel by the coil 29 . Thus , the first , second and third switching-contacts 23 , 35 , 41 are set in a closed state and are set in an open state at the same point of time in the ideal case .

The first , second and third input terminal 21 , 33 , 42 are connected to a grid 12 of the arrangement 10 . Thus , the grid 12 has three phases LI , L2 , L3 . Additionally, the arrangement 10 comprises a neutral conductor 50 that is connected to a reference potential terminal 49 . A neutral phase N is applied at the neutral conductor 50 . The first , second and third output terminal 22 , 34 , 43 are connected to a motor 11 of the arrangement 10 . Thus , the motor 11 has three phase terminals being connected to the three output terminals 22 , 34 , 43 . The motor 11 receives three phases .

The contactor 20 comprises a first and a second measurement circuit 51 , 52 . The first measurement circuit 51 includes an input 53 that is connected to the first input terminal 21 . Thus , the input 53 of the first measurement circuit 51 is connected to a connection line that connects the first input terminal 21 to the first fixed contact 24 . Similarly, the second measurement circuit 52 includes an input 54 that is connected to the first output terminal 22 . Thus , the input 54 of the second measurement circuit 52 is connected to a connection line that connects the first output terminal 22 to the second fixed contact 25 .

Additionally, the contactor 20 comprises a signal processing circuit 55 . The signal processing circuit 55 includes a first and a second input 56 , 57 . The first input 56 is connected to an output 58 of the first measurement circuit 51 . The second input 57 of the signal processing circuit 55 is connected to an output 59 of the second measurement circuit 52 .

Similarly, the contactor 20 comprises a third and a fourth measurement circuit 62 , 63 . The third measurement circuit 62 includes an input 64 that is connected to the second input terminal 33 . Similarly, the fourth measurement circuit 63 includes an input 65 that is connected to the second output terminal 34. A third input 68 of the signal processing circuit 55 is connected to an output 66 of the third measurement circuit 62. A fourth input 69 of the signal processing circuit 55 is connected to an output 67 of the fourth measurement circuit 63.

Additionally, the contactor 20 comprises a fifth and a sixth measurement circuit 76, 77. An input 79 of the fifth measurement circuit 76 is connected to the third input terminal 42. Similarly, an input 80 of the sixth measurement circuit 77 is connected to the third output terminal 43. A fifth input 83 of the signal processing circuit 55 is connected to an output 81 of the fifth measurement circuit 76. A sixth input 84 of the signal processing circuit 55 is connected to an output 82 of the sixth measurement circuit 77. The measurement circuits 51, 52, 62, 63, 76, 77 can also be named signal matching circuits. The six measurement circuits 51, 52, 62, 63, 76, 77 may be combined or integrated into one measurement circuit .

The signal processing circuit 55 may be realized as a microcontroller or microprocessor. The signal processing circuit 55 has a data output 95. The data output 95 is realized as a bus output. Additionally, the arrangement 10 comprises a field communication unit 96 that is connected to the data output 95 of the signal processing circuit 55. The field communication unit 96 is connected to a field bus 97. The signal processing circuit 55 includes a memory 98. The signal processing circuit 55 is connected to the reference potential terminal 49.

At the first, second and third input terminal 21, 33, 42, a first, second and third input terminal voltage u_L1, u_L2 , ^UL3 is tapped. At the first, second and third output terminal 22, 34, 43, a first, second and third output terminal voltage u_T1 , u_T2, ^UT3 tapped. A first measurement signal SMI is provided at the output 58 of the first measurement circuit 51. The first measurement signal SMI is generated as a function of the first input terminal voltage u_L1. Correspondingly, a second to a sixth measurement signal SM2 to SM6 is provided at the outputs 59, 66, 67, 81, 82 of the second to the sixth measurement circuit 52, 62, 63, 76, 77. An output signal SOUT is provided at the data output 95 of the signal processing circuit 55. Alternatively, the data output 95 of the signal processing circuit 55 is directly connected to the field bus 97.

The contactor 20 comprises a first housing 19. The first housing 19 encapsulates at least the first, second and third switching-contacts 23, 35, 41, the coil 29 and the armature 30. The first, second and third input terminal 21, 33, 42 and the first, second and third output terminal 22, 34, 43 are terminals of the first housing 19. The first to the sixth measurement circuit 51, 52, 62, 63, 76, 77 and the signal processing circuit 55 are also encapsulated by the first housing 19. The field communication unit 96 may also be encapsulated by the first housing 19. The first to the sixth measurement circuit 51, 52, 62, 63, 76, 77 and the signal processing circuit 55 may form a module. The field communication unit 96 may also be part of the module. Thus, the contactor 20 is realized with integrated self -surveillance function.

In an alternative embodiment, not shown, the contactor 20 comprises a second housing for housing the module. Thus, the first to the sixth measurement circuit 51, 52, 62, 63, 76, 77 and the signal processing circuit 55 are encapsulated by the second housing. The field communication unit 96 may also be encapsulated by the second housing. The second housing is designed such that the second housing can be attached to the first housing 19. Thus, the module is implemented for the surveillance of the switching parts of the contactor 20. A standard contactor can achieve an additional functionality by the module.

In an alternative, not shown embodiment, a heater or another load is connected to the first, second and third output terminal 22, 34, 43 (instead of the motor 11) .

Figure IB shows an exemplary embodiment of a first measurement circuit 51 that is a further development of the first measurement circuit shown in Figure 1A. The second to the sixth measurement circuit 52, 62, 63, 76, 77 may be realized such as the first measurement circuit 51. The first measurement circuit 51 receives the first input terminal voltage u_L1 at the input 53 and provides a first measurement signal SMI at the output 58 . The first measurement circuit 51 comprises a first voltage divider 100 that couples the input 53 and thus the first input terminal 21 to the reference potential terminal 49 . The first voltage divider 100 comprises a first and a second voltage divider resistor 102 , 103 which are connected in series . The first and the second voltage divider resistor 102 , 103 are arranged between the input 53 and the reference potential terminal 49 . The first voltage divider 100 may comprise a third voltage divider resistor 104 that is connected in series to the first and the second voltage divider resistor 102 , 103 .

Additionally, the first voltage divider 100 comprises a first voltage divider tap 105 that is arranged between the first and the second voltage divider resistor 102 , 103 . The first voltage divider resistor 102 couples the first voltage divider tap 105 to the reference potential terminal 49 .

The first measurement circuit 51 comprises a first filter 108 that is connected on its input side to the first voltage divider tap 105 . On its output side the first filter 108 is coupled to the output 58 of the first measurement circuit 51 . The first filter 108 is realized as a low pass filter . For example , the first filter 108 comprises a first capacitor 109 . The first capacitor 109 couples the first voltage divider tap 105 to the reference potential terminal 49 .

Additionally, the first measurement circuit 51 comprises a first analog- to-digital converter 115 , abbreviated as AD converter . An input 116 of the first AD converter 115 is connected to an output of the filter 108 . Thus , the input 116 of the first AD converter 115 is coupled to the first voltage divider tap 105 . The first AD converter 115 is connected to the reference potential terminal 49 . An output 117 of the first AD converter 115 is connected to the output 58 of the first measurement circuit 51 .

The output 58 of the first measurement circuit 51 is realized as data output . The first input 56 of the signal processing circuit 55 is realized as a data input .

Furthermore , the first measurement circuit 51 comprises a first protection circuit 120 that couples the input 53 to the reference potential terminal 49 . The first voltage divider 100 reduces a voltage level of the first input terminal voltage u_L1 . The reduced voltage u_L1 ' can be tapped at the first voltage divider tap 105 . The reduced voltage u_{L 1} ' is filtered by the first filter 108 . The filtered voltage u^^ ' ' is provided to the input 116 of the first AD converter 115 . The first AD converter 115 generates the first measurement signal SMI . The first measurement signal SMI represents a digitalized value of the first input terminal voltage u_{L 1} . Advantageously, the first filter 108 reduces noise and disturbances with higher frequencies in the first input terminal voltage u_L1 . Thus , the Nyquist theorem is fulfilled by the filtered voltage u_L1 ' ' that is provided to the input 116 of the first AD converter 115 .

In a corresponding manner , the second measurement circuit 52 generates the second measurement signal SM2 out of the first output terminal voltage u_Tg , the third measurement circuit 62 generates the third measurement signal SM3 out of the second input terminal voltage u_L2 , the fourth measurement circuit 63 generates the fourth measurement signal SM4 out of the second output terminal voltage u_T2 , the fifth measurement circuit 76 generates the fifth measurement signal SM5 out of the third input terminal voltage u^g , and the sixth measurement circuit 77 generates the sixth measurement signal SM6 out of the third output terminal voltage u^g . The voltage divider factors of the six voltage dividers 100 may be equal .

In an alternative , not shown, embodiment , the first measurement circuit 51 is free of the first AD converter 115 . Thus , the output 58 of the first measurement circuit 51 is directly connected to the output of the filter 108 and thus coupled to the first voltage divider tap 105 . In this case , the signal processing circuit 55 comprises an AD converter having an input connected to the first input 56 of the signal processing circuit 55 . Thus the step of converting the first input terminal voltage u_{L 1} or ^a voltage u_L1' , u_L1' ' derived from the first input terminal voltage u_L1 into a digitalized signal , namely the first measurement signal SMI , can be performed either by the first measurement circuit 51 or by the signal processing circuit 55. The second to the sixth measurement circuit 52, 62, 63, 76, 77 can be realized such as the first measurement circuit 51 and can also be free of an analog-to-digital converter. The signal processing circuit 55 performs the digitalization of the voltages at the output of the measurement circuits 51, 52, 62, 63, 76, 77.

Figure 2A shows an example of signals of the arrangement 10 as shown in Figures 1A and IB. The first input terminal voltage U_L1 and the first output terminal voltage u_T1 ^are shown as a function of a number N of samples. A distance between two samples corresponds to a time duration of e.g. 4 ps . A time t can be calculated according to the following equation : t N TA wherein N is a number of samples and TA is a sample time (e.g. TA = 4 ps) . The first AD converter 115 generates the first measurement signal SMI e.g. with this sample time TA. The arrangement 10 may use a system time which can be realized e.g. by a counter which counts in predetermined periods .

The grid 12 is realized as an alternating current grid. Thus, the first input terminal voltage u_L1 has a sinus form. In Figure 2A, a part of a period of a sinus of the first input terminal voltage u_L1_ is shown. At a start, the contactor 20 is set in an open state. Thus, the first output terminal voltage u_T1 has the value of zero or of approximately zero. Only in an ideal case, the first, second and third switching-contacts 23, 35, 41 close at the same point of time. In the example shown in Figure 2A, the first switching-contacts 23 close later than the second and the third switching-contacts 35, 41.

Thus, at a first point of time tla, the second switching-contacts 35 may close. Thus, the second input terminal voltage U_L2 is provided via the second switching-contacts 35 to the second output terminal 34. This value of the second output terminal voltage u-^2 generates via the motor 11 the first output terminal voltage u_T1 that can be measured at the first point of time tla . The values of the first output terminal voltage u_{T 1}_ show some oscillations after the first point of time tla and finally reach an approximately constant value . Thus , the first output terminal voltage u_{T 1}_ obtains a peak .

At a further point of time tic , the third switching-contacts 41 may close . Thus , the third input terminal voltage u_L3 is provided by the third switching-contacts 41 to the third output terminal 43 and generates the third output terminal voltage u_T3 which is provided to the motor 11 . The third output terminal voltage u_T3 generates a change of the first output terminal voltage u_T1 with some oscillations until the first output terminal voltage u_{T 1} reaches an approximately constant value . Thus , the first output terminal voltage u_{T 1} obtains a further peak . The first output terminal voltage u_{T 1} is a function of the second and the third output terminal voltages u_T2 , ^UT3 _and of the motor 11 .

At a second point of time tlb , the first switching-contacts 23 close . Thus , the first output terminal voltage u_T1 achieves the value of the first input terminal voltage u_{L 1}_ - The first output terminal voltage u_{T 1} has some smaller deviations from the first input terminal voltage u_{L 1} after the second point of time tlb .

A time gap At is a difference between the second point of time tlb and the first point of time tla . During this time gap At the first movable contact 26 moves towards the first fixed contact 24 ( or the first and the second movable contact 26 , 27 move towards the first and the second fixed contact 24 , 25 ) . This movement is performed at an approximately constant velocity Vk . Thus , a distance As can be calculated according to the following equation :

Δs = Vk • Δt with Δt = t1b - t1a , wherein As is a value of the distance , Vk is a value of the velocity, At is a value of the time gap and tla and tlb are values of the first and second points of time . The distance As corresponds to the distance or way of the first and the second movable contacts 26 , 27 before the first switching-contacts 23 are closed . Since arcs between the first and the second fixed contacts 24 , 25 and the first and the second movable contacts 26 , 27 reduce the material at the surface of these contacts , the distance As increases with the number of switching events . By detecting the time gap At the actual value of the distance As can be measured .

Figure 2B shows an example of signals of the arrangement 10 as shown in Figures 1A and IB . In Figure 2B, a first voltage difference u_{FF FF} is shown . The first voltage difference u_DIF1 can be calculated according to the following equation :

The first voltage difference u_DIF1 ^can compared with a first and a second threshold voltage uTHl , uTH2 . The first and the second threshold voltage uTHl , uTH2 may have the same amount but opposite signs .

For example , the first threshold voltage uTHl may have the value of 30 V and the second threshold voltage uTH2 may have the value of -30 V .

-30 V ≤ u_DIF1 = u_L1 - u_{T 1} ≤ 30 V

In the arrangement 10 shown in Figures 1A and IB, the signal processing circuit 55 generates a first difference signal SDIF1 according to the following equation : wherein SMI and SM2 are the first and the second measurement signal . The first difference signal SDI F1 thus represents a digitized value of the first difference voltage u_DIF1. The first point of time tla and the second point of time tlb are determined by the signal processing circuit 55 using a first and a second digital threshold STI , ST2 which correspond to the first and the second threshold voltage uTHl , uTH2 . Thus , the first point of time tla is detected, when the first difference signal SDIF1 crosses the first or the second digital threshold STI , ST2 . The second point of time tlb is detected when the first difference signal SDI F1 crosses the first or the second digital threshold STI , ST2 . Since there are oscillations inside the first output terminal voltage u_Tj_ , several crossings of the first and the second threshold voltage uTHl , uTH2 occur which do not indicate the second point of time tlb . The signal processing circuit 55 determines the second point of time tlb out of the several crossings .

The signal processing circuit 55 is configured to determine the time gap At . The signal processing circuit 55 is configured to calculate the distance As . The signal processing circuit 55 generates the output signal SOUT that is provided at the data output 95 . The output signal SOUT can include at least one of the following alternatives :

The output signal SOUT is a function of the time gap At or of the distance As . The output signal SOUT includes a digital value of the time gap At or of the distance As . The digital value may have 1 bit , 2 bit , 3 bit , 4 bit or more than 4 bit . Thus , the further procedure in using the output signal SOUT is performed by a central unit , not shown, connected to the field bus 97 .

The signal SOUT is a function of a sum of the time gaps At or of a sum of the distances As , for example from the start of operation of the contactor 20 . The operation of the contactor 20 starts with the first use after leaving the factory . Thus , after setting the contactor 20 in a conducting state , the time gap At or the distance As is determined and added to the previous sum stored in the memory 98 of the signal processing circuit 55 . The signal SOUT is equal to the sum of the previous time gaps At or of the previous distances As or is a function of the sum of the previous time gaps At or the sum of the previous distances As .

Alternatively, a predetermined value such as a maintenance value or alarm value is stored in the memory 98 of the signal processing circuit 55. Each contactor 20 may be tested in the factory and an individual predetermined value may be calculated and stored in the factory in each contactor 20, e.g. in the memory 98. Alternatively, the individual predetermined value determined in the factory can be stored later in the memory 98. The individual predetermined value can be provided e.g. via the internet. Thus, the signal processing circuit 55 compares the sum of the time gaps At or the sum of the distances As with the predetermined value. In the case that the sum of the time gaps At or the sum of the distances As is larger than the predetermined value, the output signal SOUT includes a status signal. The status signal may be named maintenance or alarm signal. The status signal indicates that the contactor 20 should be replaced by a new contactor.

The signal processing circuit 55 generates a second and a third difference signal SDIF2, SDIF3 as a function of a second and a third voltage difference u_DFF2, ^UDIF3 in ^a similar manner. Thus:

SDIF2 = SM3 - SM4 = f (u_DIF2) ,

^UDIF2 = ^UL2 ” ^UT2,

SDIF3 = SM5 - SM6 = f (u_DIF3) ,

^UDIF3 ^{= U}L3 ^{- U}T3

The signal processing circuit 55 detects the time gaps At for the first, the second and the third switching-contacts 23, 35, 41 using the first, second and third difference signal SDIF1, SDIF2, SDIF3. Since one of the three switching-contacts is the fastest, one time gap of the three time gaps detected at a switching event may be zero or approximately zero. A second time gap of the three time gaps may have a small value and a third time gap of the three time gaps may have a larger value. In the example shown in Figures 2A and 2B, the first switching-contacts 23 obtain the largest value of the time gaps At.

In an example, the signal processing circuit 55 adds the three time gaps At measured at the first, the second and the third switching-contacts 23, 35, 41 at a switching event of the contactor 20 and may use this sum instead of a single time gap, as discussed above. The signal processing circuit 55 may sum up these sums, for example from a start of the use of the contactor 20.

Alternatively, the signal processing circuit 55 only sums up the largest time gap At of the three time gaps measured at the first, the second and the third switching-contacts 23, 35, 41.

By analyzing the signals, it is possible to detect the first contact of the still "best" contact. This is given when:

Thus, the first point of time t_1a is determined (t_1a = t_1a) . The next contacts will come a little bit later at the further point of time t_1c and at the second point of time tib with the same conditions (t_1c = t_1c, t_1b = t_1b) .

If an actor has a defined velocity v_actor(t1) at the first point of time t_1a, the parameters for the calculation of the difference of the distance are (^vactor(t1) vk) :

The actor includes the movable contacts 26, 27, the contact bridge 28 and the armature 30. The velocity of the actor is e.g. the velocity of the contact bridge 28. The distance As can be named gap between the first and the following contacts. If the distance As has reached a contactor specific value, the possibility of a failure will rise. For example, a contactor may fail at value of the distance As of 0, 6 mm to 0,8 mm.

If the control terminals 31, 32 which are also named terminals A1/A2 of the contactor 20 are observed, a start time t₀ can be determined. The start time t₀ is the point where the actor starts to close the main contacts of the contactor 20. This can be detected by analyzing the voltage at the control terminals 31, 32. This is the easiest way to get to the start time to. At a contactor 20 with a desynchronization the start time t₀ is detected e.g. by observing the current into the control terminals 31 , 32 . If the current rises to 10% of a maximum current I_max, the start time to is reached . At the beginning of the lifespan the difference of t₁ - to = Δt_start can be used as initialization of the distance S Q of the movable contacts 26 , 27 . The value so is known from the end-test of the contactor 20 . During the lifespan, the first point of time t_1a will rise . So , the rising value At can be used to calculate the rising distance :

These equations can be used to calculate the distance or way to close e . g . the first switching-contacts 23 . During the condition of the second point of time tlb the voltage u_L1 at the first switching-contacts 23 can be recorded . So it is possible to get a statistical overview to provide information about possible synchronization .

If the three switching-contacts 23 , 35 , 41 are always switched under the same load conditions , the switching-contacts with the largest load will wear out fastest . Therefore , if there is no equal distribution of load conditions , this can be communicated via the fieldbus 97 and an operator may react .

For example , the signal processing circuit 55 determines the second, the fourth and the sixth measurement signal SM2 , SM4 , SM6 ( or the first , the third and the fifth measurement signal SMI , SM3 , SM5 or the first to the sixth measurement signal SMI to SM6 ) e . g . at the second point of time tlb ( alternatively at the first point of time tla , the further point of time tic or another predetermined point of time ) for a number of switching events . The number may be predetermined . The signal processing circuit 55 adds the values of the second measurement signal SM2 and generates a first sum or calculates a first average value , it adds the values of the fourth measurement signal SM4 and generates a second sum or calculates a second average value ; it also adds the values of the sixth measurement signal SM6 and generates a third sum or calculates a third average value .

The signal processing circuit 55 or a central unit connected to the signal processing circuit 55 evaluates the first , second and third sum or the first , second and third average . The output signal SOUT may include a result of the evaluation . In case the three sums deviate from each other or the three averages deviate from each other more than a predetermined percentage ( e . g . the smallest sum or average is less than 80% , 60 % or 40% of the largest sum or average ) , then the contactor 20 has obtained a synchronization in the number of switching events . Synchronization means that one of the three switching-contacts 23 , 35 , 41 is much more often closed at a zero-crossing of its output terminal voltage in comparison to the other two-switching-contacts . Thus , the other two switching-contacts obtain a higher deterioration due to arcs .

Alternatively, the signal processing circuit 55 registers how often the value of the second measurement signal SM2 is inside at least one voltage range at the second point of time tlb ( alternatively at the first point of time tla, the further point of time tic or another predetermined point of time ) at a number of switching events . The number of voltage ranges may be 1 , 2 , 3 or more than 3 . Similarly, the signal processing circuit 55 registers how often the value of the fourth measurement signal SM4 and the value of the sixth measurement signal SM6 is inside at least one voltage range at said point of time at the number of switching events . Thus , the signal processing circuit 55 determines a range distribution of the second, fourth and sixth measurement signal SM2 , SM4 , SM6 . The signal processing circuit 55 or the central unit evaluate the numbers in the registers . The output signal SOUT may include a result of the evaluation . In case the numbers in the registers of the second measurement signal SM2 , the numbers in the registers of the fourth measurement signal SM4 and the numbers in the registers of the sixth measurement signal SM6 deviate from each other significantly, then the contactor 20 has obtained a synchronization in the number of switching events . Alternatively, the first , the third and the fifth measurement signal SMI , SM3 , SM5 or the first to the sixth measurement signal SMI to SM6 are evaluated instead of the second, the fourth and the sixth measurement signal SM2 , SM4 , SM6 .

An operator of the arrangement 10 may react in case of a synchronization . The first , second and third output terminal voltage u_T1, u_T2 , ^UT3 ( or/and the first , second and third input terminal voltage U_{L 1} , ^UL2 ' ^UL3 ) ^are observed by the six measurement circuits 51, 52, 62, 63, 76, 77 and the signal processing circuit 55 to check for the presence or absence of a synchronization .

The following definitions or terms are used in the text below:

Complete stroke CS : This is the distance the armature 30 moves from a start position to an end position.

Empty stroke ES: This is the distance the armature 30 moves from the start position to a first contact. This value can be specified for each of the three switching-contacts 23, 35, 41. In addition to the armature 30, also the first contact bridge 28 and the first and the second movable contact 26, 27 of the first switching-contacts 23 and the corresponding parts of the second and the third switching-contacts 35, 41 move the distance of the empty stroke. The armature 30 is coupled to the first contact bridge 28 by a coil or spring. Thus, the armature 30 is able to move even in case the movable contacts 26, 27 have been stopped by the fixed contacts 24, 25.

Through stroke TS: The distance the armature 30 moves from the first contact to the end position. This is specified for each of the switchingcontacts 23, 35, 41 or for each movable contact 26, 27, 38, 39, 46, 47.

CS = ES + TS [0.1]

Examples of contactors were switched with a defined load with high currents, e.g. occurring at starts of the motor 11. In order to be able to observe the contactors and their contact erosion during the test, the contactors were quasi-dismantled every 100,000 switching cycles and the through stroke of each individual switching-contacts was determined with an optical method.

In order to avoid the disassembly, bridge voltages u_Bx over the three switching-contacts 23, 35, 41 per contactor 20 can be examined. The bridge voltages u_Bx comprise the voltages u_B1 ^UB2 ' ^UB3 which correspond to the first, second and third voltage difference U_DIF1 ^UDIF3- The index x stands for 1, 2 and three and refers to the first, second and third switching-contacts 23, 35, 41. The bridge voltages u_Bx can be calculated using the equations:

The high amplitudes in the first voltage difference u_DIF1 = u_B1 are largely dependent on the amplitude of the first input terminal voltage ^UL1 • Therefore, the first difference voltage u_B1 is also dependent on the time of contact. In addition, the high amplitudes in the first output terminal voltage u_T1 are dependent on the other switching-contacts 35, 41 (L2 - T2 and L3 - T3) . Therefore, there is also a dependency on the contacting of the other switching-contacts. In addition, the high amplitudes on the first output terminal voltage u_T1 mean that the first switching-contacts 23 has not yet closed because the material on the fixed and movable contacts 24 to 27 has already been reduced. This is seen e.g. by measuring the through strokes (optical method) . Thus, these contacts have a longer distance s_switching contacts to move until the switching-contacts closed. The empty stroke becomes larger:

Advantageously, operation or measurements in the time domain are able to gain useful information.

Determination of the stroke difference: A measuring device monitors the behavior at the output terminals 22, 34, 43 which can be also named contacts Tl, T2 and T3. More specifically, the second, fourth and sixth measurement circuit 52, 63, 77 determine the second, fourth and sixth measurement signal SM2, SM4 , SM6 as a function of the second, fourth and sixth output terminal voltage u_T1, ^UT2, ^UT3 • ^{As soon as} the amount of one of the second, fourth and sixth output terminal voltage u_T1 , u_T2, ^UT3 rises above a certain threshold voltage uTHl, the measurement is started. The threshold voltage uTHl can be named trigger value. This point in time is referred as the first point of time tla.

The time at which the bridge voltage u_Bx (=voltage difference

^UDIF2' ^UDIF3^ remains below the threshold voltage uTHl - e.g. 30V - is called tit x :

The index x stands for 1, 2 and 3 and refers to the first, second and third switching-contacts 23, 35, 41. The three switching-contacts 23, 35, 41 may have three different points of time t_lb t_{lb 2}, t_{lb 3}. If high-quality contactors with electronics are used, a constant velocity v_k can be assumed for the contact making. This is ensured by the reasonably constant conditions that occur with electronic control, e.g. constant pick-up voltage for the coil 29 through control, temperature compensation and/or constant mounting position.

Thus, the differences between individual switching-contacts 23, 35, 41 can be determined from equation [2.1] . In this case, the contact with the most material, i.e. the contact with the largest through stroke, is used as a reference. The velocity v_k is equal to v(t)_{contact bridge} and v_actor(t1) . At the "best" switching-contacts, the first and the second point of time tla, tlb coincide, resulting in tla = tlb, resulting in the difference being zero (tla - tlb = 0) . From this information it is already possible to make a prediction for the lifetime of the contactor 20. For example a type of contactors fail with a difference or distance of approximately 0.6 mm to 0.8 mm. These values are initially valid for this type of contactors . For other contactors the values may still deviate.

In an example, the fault pattern in this condition mainly refers to poor contact making. Welding occurs less frequently. In addition, the entire burn-up should be examined. If the contacts lose material evenly due to the switching energy, it is possible that an imminent failure cannot be detected only with the upper method. Therefore, the empty stroke can be determined, e.g. in addition.

Determination of the empty stroke: With the same aspects as discussed above, a constant behavior of the contactor 20 is assumed. Thus, it is assumed that the drive is always accelerated with the same forces. Thus, the time from the first excitation to the point of time of the first contact closure (for new contactors) is constant. So the start time tO is the time at which the drive of the contactor 20 starts to move. The start time tO can be detected using various criteria: Applying a minimum voltage to the control terminals 31, 32 of the contactor 20 and/or detecting a minimum current in the control terminals 31, 32 (e.g. 10% of the maximum current) and/or opening of an auxiliary switch.

Thus, during a first switching, a time period is determined which lasts from the start-up until the closing (in new condition) . Thus: t_init = t_2new - t₀ [4.1] tinit stands for the time period of the empty stroke until the first contact is made in the new state s_new of the contactor 20. New state is the state before the first use of the contactor 20. This can be determined during the final test at the factory. The parameters could be loaded from the Internet using the QR code or the serial number. If the contacts change over time, t₂ will always take place later. Therefore, the equation [2.1] can also be written differently:

Insertion of the equations will result in: ^sburn-~up is a thickness of material on the fixed and movable contacts 24 to 27 which has been reduced since the start of operation of the contactor 20. s_burn-up is a thickness of lost material. This allows to calculate or estimate the remaining contact materials and thus provides early warning of contactor failure. The output signal SOUT may include an information about the thickness s_burn~_up . The output signal SOUT may include a status signal indicating when the thickness s_burn-up is equal or higher than a threshold. The value of the threshold is stored e.g. in the memory 98. Equations [4.1] to [5.5] can be applied to the first, second and third switching-contacts 23, 35, 41.

Figure 3 shows a further exemplary embodiment of an arrangement 10 with a contactor 20 which is a further development of the above shown embodiments. The contactor 20 comprises a first and a second rectifier 131, 132. The first rectifier 131 couples the first input terminal 21 to the input 53 of the first measuring circuit 51. The second rectifier 132 couples the first output terminal 22 to the input 54 of the second measuring circuit 52. Correspondingly, the contactor 20 comprises a third to a sixth rectifier 133 to 136 which couple the input and output terminals 33, 34, 42, 43 to the inputs 64, 65, 79, 80 of the third to the sixth measurement circuits 62, 63, 76, 77, as shown in Figure 3. The first to the sixth rectifier 131 to 136 are realized as full-bridge rectifiers. The first to the sixth rectifier 131 to 136 are each connected to the reference potential terminal 49 and to the neutral conductor 50. Thus, only positive voltages are applied to the inputs of the measurement circuits 51, 52, 62, 63, 76, 77.

Alternatively, the first to the sixth rectifier 131 to 136 are realized as half-bridge rectifiers.

In an alternative, not-shown, embodiment, the contactor 20 is free of the second fixed contact 25 and free of the second movable contact 27. The switching occurs by opening or closing the contact between the first movable contact 26 and the first fixed contact 24 only.

The contactor 20 may be implemented as switch.

In an alternative, not-shown, embodiment, the contactor 20 is free of the second and the third switching-contacts 35, 41. The time gap At or the distance As is calculated e.g. using the start time tO and the second point of time tlb (At = tlb - tO) . The contactor 20 is used for switching a single phase.

The embodiments shown in Figures 1A to 3 as stated represent examples of the improved contactor and arrangement; therefore, they do not constitute a complete list of all embodiments according to the improved contactor and arrangement. Actual contactors and arrangements may vary from the embodiments shown in terms of parts, structures, shape and circuits, for example .

Reference Numerals

10 arrangement

11 motor

12 grid

19 first housing

20 contactor

21 first input terminal

22 first output terminal

23 first switching-contacts

24 , 25 fixed contact

26 , 27 movable contact

28 contact bridge

29 coil

30 armature

31 , 32 control terminal

33 second input terminal

34 second output terminal

35 second switching-contacts

36 , 37 fixed contacts

38 , 39 movable contacts

40 contact bridge

41 third switching-contacts

42 third input terminal

43 third output terminal

44 , 45 fixed contacts

46 , 47 movable contacts

48 contact bridge

49 reference potential terminal

50 neutral conductor

51 , 52 measurement circuit

53 , 54 input

55 signal processing circuit

56 , 57 input

58 , 59 output

62 , 63 measurement circuit

64 , 65 input 66 , 67 output

68 , 69 input

76 , 77 measurement circuit

79 , 80 input

81 , 82 output

83 , 84 input

95 data output

96 field communication unit

97 field bus

98 memory

100 first voltage divider

102 to 104 voltage divider resistor

105 first voltage divider tap

108 first filter

109 first capacitor

115 first analog-to-digital converter

116 input

117 output

120 first protection circuit

131 to 136 rectifier

N number of sample

SDI F1 to SDI F3 difference signal

SMI to SM6 measurement signal

SOUT output signal tla first point of time tlb second point of time tic further point of time

^UDIF1 ^{to U}DIF3 voltage difference

^UL1 ' ^UL1 ' ' ^UL1" first input terminal voltage

^UL2 ' ^UL3 input terminal voltage

^UT1 ' ^UT2 ' ^UT3 output terminal voltage

At time gap

As distance

Claims

Claims :

1. Contactor (20) , comprising a first input terminal (21) and a first output terminal (22) , first switching-contacts (23) which couple the first input terminal (21) to the first output terminal (22) , a first measurement circuit (51) with an input (53) connected to the first input terminal (21) and an output (58) for providing a first measurement signal (SMI) , a second measurement circuit (52) with an input (54) connected to the first output terminal (22) and an output (59) for providing a second measurement signal (SM2) , and a signal processing circuit (55) which is connected on its input side to the output (58) of the first measurement circuit

(51) and to the output (59) of the second measurement circuit

(52) and is configured to provide an output signal (SOUT) depending on a time gap (At) between a pulse start at a first point of time (tla) and a pulse end at a second point of time (tlb) of at least one pulse of a first difference signal (SDIF1) between the first measurement signal (SMI) and the second measurement signal (SM2) .

2. Contactor (20) of claim 1, wherein the first measurement circuit (51) comprises a first voltage divider (100) coupling the first input terminal (21) to a reference potential terminal (49) and wherein the second measurement circuit (52) comprises a second voltage divider coupling the first output terminal (22) to the reference potential terminal (49) .

3. Contactor (20) of claim 2, wherein the first measurement circuit (51) comprises a first filter (108) coupled to a first voltage divider tap (105) of the first voltage divider (100) and to the reference potential terminal (49) , and wherein the second measurement circuit (52) comprises a second filter coupled to a second voltage divider tap of the second voltage divider and to the reference potential terminal (49) . ontactor (20) of claim 3, wherein the first measurement circuit (51) comprises a first analog-to-digital converter (115) coupled to the first filter (108) and to the output (58) of the first measurement circuit (51) , and wherein the second measurement circuit (52) comprises a second analog-to-digital converter coupled to the second filter and to the output (59) of the second measurement circuit (52) . ontactor (20) of one of claims 1 to 4, wherein the first measurement circuit (51) comprises a first protection circuit (120) coupled to the first input terminal (21) and wherein the second measurement circuit (52) comprises a second protection circuit coupled to the first output terminal (22) . ontactor (20) of one of claims 1 to 5, further comprising a second input terminal (33) and a second output terminal (34) , second switching-contacts (35) which couple the second input terminal (33) to the second output terminal (34) , a third measurement circuit (62) with an input (64) connected to the second input terminal (33) and an output (66) for providing a third measurement signal (SM3) , and a fourth measurement circuit (63) with an input (65) connected to the second output terminal (34) and an output (67) for providing a fourth measurement signal (SM4) , wherein the signal processing circuit (55) is connected on its input side to the output (66) of the third measurement circuit

(64) and to the output (67) of the fourth measurement circuit

(65) and is configured to provide the output signal (SOUT) additionally depending on the third and the fourth measurement signal (SM3, SM4 ) . Contactor (20) of claim 6, further comprising a third input terminal (42) and a third output terminal (43) , third switching-contacts (41) which couple the third input terminal (42) to the third output terminal (43) , a fifth measurement circuit (76) with an input (79) connected to the third input terminal (42) and an output (81) for providing a fifth measurement signal (SM5) , and a sixth measurement circuit (77) with an input (80) connected to the third output terminal (43) and an output (82) for providing a sixth measurement signal (SM6) , wherein the signal processing circuit (55) is connected on its input side to the output (81) of the fifth measurement circuit

(76) and to the output (82) of the sixth measurement circuit

(77) and is configured to provide the output signal (SOUT) additionally depending on the fifth and the sixth measurement signal (SM5, SM6) . Contactor (20) of claim 7, wherein the signal processing circuit (55) is configured to evaluate the first, the third and the fifth measurement signal (SMI, SM3, SM5 ) and/or the second, the fourth and the sixth measurement signal (SM2, SM4, SM6) and to generate the output signal (SOUT) including a result of the evaluation in case the first, the second and the third switching-contacts (23, 35, 41) are synchronized. rrangement (10) , comprising a contactor (20) of one of claims 1 to 8, and a load which is realized as a motor (11) or a heater, wherein the load includes a first phase terminal coupled to the first output terminal (22) , a second phase terminal coupled to the second output terminal (34) and a third phase terminal coupled to the third output terminal (43) . Method for observing a contactor (20) wherein the contactor (20) comprises a first input terminal (21) , a first output terminal (22) , first switching-contacts (23) , a first and a second measurement circuit (51, 52) and a signal processing circuit (55) , the method comprising: closing the first switching-contacts (23) between the first input terminal (21) and the first output terminal (22) , providing a first measurement signal (SMI) by the first measurement circuit (51) depending on a first input terminal voltage (u_L1 ) tapped at the first input terminal (21) , providing a second measurement signal (SM2) by a second measurement circuit (52) depending on a first output terminal voltage (u_T1 ) tapped at the first output terminal (22) , and providing an output signal (SOUT) depending on a time gap (At) between a pulse start at a first point of time (tla) and a pulse end at a second point of time (tlb) of at least one pulse of a first difference signal (SDIF1) between the first measurement signal (SMI) and the second measurement signal (SM2) by the signal processing circuit (55) .