Method for determining a contact wear of a switching device, and switching device

Abstract

The invention relates to a method for determining a contact wear of a switching device (1) comprising an anchor (2) that is moved by means of a magnetic drive (3) in a tripping event, at least one fixed contact (7) and at least one moveable contact (8) mechanically connected to the anchor (2). The method comprises the steps of: - determining a contact time (10) when fixed contact (7) touches the moveable contact (8), - determining an inductance (11) of the magnetic drive (3) at the contact time (10), - determining the contact wear by comparing an initial inductance (12) with the inductance (11) at the contact time (10). Furthermore, a switching device (1) is specified.

Description

[0001] Description

[0002] Method for determining a contact wear of a switching device, and switching device

[0003] The present disclosure relates to a method for determining a contact wear of a switching device. The present disclosure further relates to a switching device.

[0004] One object to be achieved is, inter alia, to specify an improved method for determining a contact wear of a switching device which in particular allows monitoring a condition of the contacts over time. A further object to be achieved is to specify an improved switching device which allows to monitor its contact wear over time.

[0005] These objects are achieved, inter alia, by a method comprising the features of independent claim 1 and by a switching device comprising the features of claim 11, respectively .

[0006] In at least one embodiment of the method for determining a contact wear of a switching device, the switching device comprises an anchor that is moved by means of a magnetic drive in a tripping event. The anchor preferably comprises a magnetic material such as iron or ferrite.

[0007] For example, the switching device comprises at least one moveable contact and at least one fixed contact. In particular, the switching device comprises two states. For example, in a first state the fixed contact is separated from the moveable contact and in the second state the fixed contact is in contact with the moveable contact. In a tripping event, in particular, the switching device switches from the first to the second state.

[0008] Preferably, the moveable contact is mechanically connected to the anchor. Thus, the moveable contact is in particular moveably by means of the magnetic drive. In particular, the moveable contact is connected to the anchor by at least one contact spring.

[0009] In particular at a tripping event, the anchor is moved such that the moveable contact touches the fixed contact. It is possible that the anchor moves further after the fixed contact and the moveable contact touch each other. This movement of the anchor may increase a pressure applied to the contacts and preferably ensures a stable connection between the contacts. In particular, the contact spring connecting the moveable contact to the anchor is compressed when the anchor travels further after the fixed and moveable contact touch.

[0010] For example, when switching from the first state to the second state, the distance the anchor travels until the moveable contact and the fixed contact touch is referred to as "free stroke". In this case, the total distance the anchor can travel is referred to as "full stroke". That is, the full stroke is the free stroke plus a distance the anchor travels after fixed and moveable contact touch each other in order to increase a pressure on contacts, which is referred to as "contact stroke" in the following.

[0011] The method comprises a step in which a contact time is determined. The contact time is a point in time when the fixed contact touches the moveable contact. In particular, the contact time is a period of time between a start of the movement of the anchor, i.e. the occurrence of the tripping event, and the fixed contact and moveable contact touching each other. The corresponding tripping event is in particular a transition between the first and the second state. Hence during the tripping event the switching device is "closed".

[0012] In particular, the contact time determines the period of time the anchor takes to travel the free stroke.

[0013] The method further comprises a step of determining an inductance of the magnetic drive at the contact time.

[0014] The method further comprises a step of determining the contact wear by comparing an initial inductance with the inductance at the contact time. For example, the initial inductance is divided by the inductance at the contact time for determining the contact wear.

[0015] The initial inductance is, for example, an inductance that is determined and stored during or immediately after production of the switching device. Hence, the initial inductance is in particular determined in a state of the switching when no or essentially no contact wear is present. The contact wear may be given as a relative value or as an absolute value. In particular, the inductance at the contact time is greater than the initial inductance.

[0016] Preferably, the switching device comprises two fixed contacts and two moveable contacts. The fixed contacts are in particular connected to a power line. The moveable contacts are preferably arranged on a contact bridge comprising an electrically conductive material such as copper and / or steel. The fixed and / or moveable contacts preferably comprise an electrically conductive material such as copper. The contact bridge is preferably connected, i.e. mechanically connected, to the anchor. Preferably the contact bridge is connected to the anchor via the contact spring.

[0017] In the second state an electrical connection is established between the fixed contact by the moveable contacts as parts of the contact bridge. This means that current flows from the power line to a first fixed contact, to the moveable contacts, to a second fixed contact, to the power line. In the first state this current path is interrupted since the fixed contacts and the moveable contacts are separated.

[0018] In a tripping event, for example from the first state to the second state, the magnetic drive is preferably supplied with current, generating a magnetic field. The magnetic drive comprises preferably a solenoid. The magnetic field in particular applies a force on the anchor attracting the anchor to a core of the magnetic drive. The solenoid is preferably wound around the core. The anchor subsequently moves such that the fixed and moveable contacts touch at the contact time. That is, the anchor travels to free stroke.

[0019] When the fixed and moveable contact touch, a gap between the anchor and the core may be present. This gap corresponds to the contact stroke. Subsequently, the anchor may travel the contact stroke until the anchor and the core touch each other. Thus, the anchor may travel the full stroke in total.

[0020] Since the anchor is mechanically coupled to the contact bridge, the contact bridge preferably moves to an end position. At the end position the moveable contact and the fixed contact are connected with a pressure that is applied via the contact spring. Hence, via applying a drive current to the magnetic drive, the switching device may be switched.

[0021] The switching device is preferably a contactor.

[0022] The method described here is based on the following technical considerations. Switching devices such as contactors that switch electrical energy are subject to wearing or contact wear. The wearing may be caused by electric arcs occurring when the contacts are opened or when the contacts bounce during a closing process. Thereby, a material of the contacts is heated and may melt. Furthermore, material of the contacts may be mechanically removed. Hence, the contacts may degenerate over time and switching operations, respectively. Therefore, switching devices such as contactors are generally regarded as wearing parts. Thus, if a switching is particularly expensive, installed in a place that is difficult to access in an application, or a failure is particularly critical, it is advantageous to know the condition of the switching device.

[0023] In order to close the contacts of a switching device described here such as a contactor, the magnetic drive is supplied with a drive current as a function of time, introducing a magnetic flux in the magnetic drive. The magnetic flux generates a magnetic force by which the anchor is moved to the end position and the contacts are closed. An inductance of the magnetic drive may be calculated as a function of time when the magnetic flux and the drive current of the magnetic drive are known. This means that the inductance of the magnetic drive is in particular given by N times the magnetic flux divided by the drive current. Hence, where L denotes the inductance of the magnetic drive, N is a number of turns of the magnetic drive, is the magnetic flux through the magnetic drive, and i is the drive current.

[0024] If a contact time is known, which is in particular the point in time where the moveable and fixed contact touch each other, the inductance at this contact time can be evaluated.

[0025] Furthermore, the inductance of the magnetic drive may also be formulated as a function of stroke. The inductance as a function may read as follows: where Acoredenotes an area of the anchor, lCOre isalength of the anchor, pg is the vacuum magnetic permeability, pris the relative permeability of the anchor, and xg is the full stroke. Since prhas preferably a comparably high value and the factor may be negligible, the inductance of the llrmagnetic drive may be reciprocally proportional to the stroke in good approximation.

[0026] If the contacts of the switching device undergo wearing, an actual stroke at a certain switching operation increases since the anchor has to travel a further distance if material of the contacts wears off. Hence, a ratio of an actual contact stroke and an initial contact stroke can be a relative measure for the contact wear. Further, since the inductance of the magnetic drive is in good approximation reciprocally proportional to the contact stroke, and at the contact time the anchor has traveled the free stroke, the ratio of the actual contact stroke and the initial contact stroke equals a ratio of the initial inductance (for example determined during production) at closed contacts and the inductance at the contact time (for example during operation after a plurality of switching operations). Hence, where xac|-uaj_ denotes the actual free stroke at a specific switching operation at which the contact wear may be determined, Xinitial isthe initial free stroke (for example determined during production), xg denotes the full stroke (which is considered constant after a plurality of switching operations), Linj_tj_aiis the initial inductance of the magnetic drive at closed contacts (for example during operation after a plurality of switching operations), and Lactual is the inductance at the contact time at the specific switching operation. The initial free stroke (and thus the initial contact stroke) and the initial inductance are preferably determined when no contact wear has yet occurred, for example during or immediately after production of the switching device.

[0027] Therefore, the ratio of the initial inductance and the inductance at the contact time can be advantageously used to determine or quantize the contact wear.

[0028] According to at least one embodiment of the method, an inductive voltage signal of the magnetic drive is measured by a measurement coil and an inflection point of the inductive voltage signal is determined for determining the contact time, wherein the inflection point corresponds to the contact time. The measurement coil may be a solenoid.

[0029] The measurement coil is preferably arranged in an influence region of the magnetic field generated by the magnetic drive when being supplied with the drive current. For example, the measurement coil is wound around the core of the magnetic drive. Hence, the measurement coil and the magnetic drive may be coupled, and an approximately identical magnetic flux emerges in the magnetic drive and the measurement coil. Due to the magnetic field generated by the magnetic drive, a voltage may be induced in the measurement coil. The inductive voltage signal is preferably the induced voltage as a function of time. The inductive voltage signal may be measured by a voltage measurement device such as a voltmeter.

[0030] After the anchor travels the free stroke, i.e. at the contact time, the anchor and hence the optional contact bridge and moveable contact have a certain velocity. Thus, a momentum can be assigned to the moveable parts of the switching device, which is at least partially transferred to fixed parts of the switching device such as the fixed contact. This momentum transfer has an effect on the magnetic flux and thus the inductive voltage signal. This means that the inductive voltage may comprise an inflection point at the contact time.

[0031] For example, the inflection point and hence the contact time can be determined by calculating a derivative of the inductive voltage signal, which subsequently may be evaluated with respect to horizontal tangents indicating the inflection point.

[0032] According to at least one embodiment, the method comprises the following steps for determining the inductance at the contact time. In a first step of the method, a drive current signal for the magnetic drive may be measured. The drive current signal is in particular a current through the magnetic drive as a function of time. The drive current signal is measured, for example, by a current measurement device such as an amperemeter.

[0033] In a further step, a magnetic flux signal of the magnetic drive is calculated by integrating the inductive voltage signal. Hence, the magnetic flux signal t>(t) is obtained by where u-j_nc(T) denotes the inductive voltage signal, tg is a point in time at which the movement of the anchor begins, i is the integration variable, and t is the time.

[0034] The magnetic flux signal gives in particular the magnetic flux generated by the magnetic drive as a function of time.

[0035] In a subsequent step, an inductance signal is obtained by dividing the magnetic flux signal by the drive current signal. The inductance signal gives in particular the inductance of the magnetic drive as a function of time. The inductance at the contact time may then be obtained by evaluating the inductance signal at the contact time.

[0036] Thus, by providing a measurement coil magnetically coupled to the magnetic drive for measuring the inductance voltage signal and measuring the drive current signal through the magnetic drive, the contact time and the inductance at the contact time can be obtained. This allows to determine the contact wear.

[0037] According to at least one embodiment, the contact wear is a ratio of the contact stoke of the anchor and the initial contact stroke, as in particular discussed above, for example in connection with Eq. (3). For example, the contact stroke according to this embodiment is the actual contact stroke at the tripping event or switching operation during which the contact wear is determined. If the initial contact stroke is known, it is also possible to express the contact wear as a number of times of the initial contact stroke. For example, the inductance at the contact time is 120% of the initial inductance. In this case, the contact wear may be approximately 80%.

[0038] According to an alternative embodiment, the contact wear is a difference between the actual contact stroke and the initial contact stroke. In particular, the actual contact stroke is given by xg- xac|-ua]_, and the initial contact stroke is given by xg- xinj_tia]_. Hence, Eq. (3) may be rearranged to read as wherein the left-hand side determines the contact wear according to the present embodiment. For example, if the inductance at the contact time is 120% of the initial inductance, the contact wear is approximately 20% of the initial contact stroke.

[0039] According to at least one embodiment, the steps of the method described herein are carried out by an evaluation unit. The evaluation unit may be a microcontroller, an applicationspecific integrated circuit (ASIC), field-programmable gate array (FPGA) or the like.

[0040] The evaluation unit is preferably coupled to the current measurement device for obtaining the drive current signal and the measurement coil for obtaining the inductance voltage signal. The evaluation unit further preferably comprises a data line port or a communication interface to output the contact wear. According to at least one embodiment, the initial inductance, and optionally the initial contact stroke and / or initial free stroke, is / are determined upon production of the switching device and is / are stored in a storage of the evaluation unit. At the time of production no contact wear is present. Hence, a total contact wear can be determined if the initial inductance and / or the initial stroke are determined during production or immediately after production.

[0041] According to at least one embodiment, the switching device is a contactor, and the magnetic drive comprises a solenoid. The solenoid has N turns, for example.

[0042] According to at least one embodiment, the anchor is connected to at least one moveable contact of the switching device. In particular, the anchor is mechanically connected to the moveable contact by a contact spring. At the contact time the moveable contact contacts at least one corresponding fixed contact of the switching device. Thereby, an electrical path may be closed and current may flow through the switching device. Hence, by opening and closing the moveable and fixed contacts, a current or power can be controlled.

[0043] According to at least one embodiment of the method, the contact wear is stored after every tripping event or after every hundredth tripping event or after every thousandth tripping event or after every ten thousandth tripping event. The storage interval may depend on the application of the switching device. The stored contact wear may be output for external evaluation. For example, external evaluation can be carried out by stochastic methods or a method using artificial intelligence. By storing and evaluating the contact wear over time, the contact wear may be monitored, thus facilitating maintenance of the switching device. Furthermore, a typical lifetime of the switching device can be determined or predicted. This may be advantageous for maintenance of the switching device. For example, if the lifetime may be predicted, the switching device may be replaced at a favorable time in the application. A favorable time may be, for example, if the application is not in use or the switching device is not required in the application at a specific time or at a regular maintenance interval.

[0044] Furthermore, a switching device is specified. The switching device may use the method described herein in accordance with one or more embodiments during operation. This means that all features disclosed for the method are also disclosed for the switching device and vice versa.

[0045] The switching device comprises an anchor and a magnetic drive configured to move the anchor in a tripping event. For example, the magnetic drive comprises a solenoid that generates a magnetic field when being supplied with current. The anchor comprises a magnetic material such as iron or ferrite and is, for example, pulled towards a core of the magnetic drive when the magnetic drive is supplied with current due to the emerging magnetic field. The solenoid is preferably wound around the core.

[0046] The switching device further comprises a measurement coil configured to measure an inductive voltage signal at the tripping event. The measurement coil comprises, for example, a solenoid and is preferably magnetically coupled to the magnetic drive. For example, the measurement coil is wound around the core of the magnetic drive. This means that if a magnetic field emerges from the magnetic drive, a voltage is induced in the measurement coil. The induced voltage may be measured over time, for example by a voltmeter, to obtain the inductive voltage signal.

[0047] The switching device further comprises a current measurement device configured to measure a drive current signal for the magnetic drive at the tripping event. The current measurement device may be an amperemeter. The drive current signal is in particular the drive current for the magnetic drive as a function of time.

[0048] The switching device further comprises an evaluation unit that is configured to perform a method described herein, in particular as discussed in the context of one or more embodiments above.

[0049] According to at least one embodiment, the switching device further comprises at least one fixed contact and at least one moveable contact. In a first state of the switching device the fixed contact is separated from the moveable contact, and in a second state of the switching device the fixed contact and the moveable contact are in electrical contact with each other. At a tripping event the switching device switches from the first state to the second state.

[0050] For example, the switching device comprises two fixed contacts separated from one another, and two moveable contacts arranged on a contact bridge and electrically connected via the contact bridge. At the tripping event the moveable contacts are preferably brought into contact with the corresponding fixed contacts, thereby forming an electrical connection between the fixed contacts. The fixed contacts may each be connected to a power line. In the first state the power line may be interrupted and in the second state the power line may be closed. This means that the switching device is in particular configured to control a current in the power line. The switching device is, for example, a contactor.

[0051] According to at least one embodiment of the switching device, the anchor is mechanically coupled to the moveable contact, and the anchor is moveable with respect to the fixed contact. For example, the anchor is coupled to the contact bridge. At a tripping event, the anchor moves due to the magnetic drive. This may cause the contact bridge to move such that the moveable and fixed contacts touch each other, i.e. the switching device is closed. Hence, by moving the anchor, in particular by means of the magnetic drive, the switching device can switch states.

[0052] According to at least one embodiment of the switching device, the evaluation unit is integrated in a common housing of the switching device. For example, all parts or a majority of the parts of the switching device are arranged in the housing. In particular, the anchor, the magnetic drive, the measurement coil, the current measurement device, moveable and fixed contacts and the contact bridge are arranged in the housing. In the present embodiment, the evaluation unit is also arranged in the housing.

[0053] In the present embodiment, for example, the power line and a data line of the evaluation unit for inputting control instructions and / or outputting data such as the contact wear are ports of the housing.

[0054] According to at least one embodiment of the switching device, the evaluation unit is at least partially arranged outside the housing of the switching device and is connected to the measurement coil and / or the current measurement device via at least one data line. In particular, the anchor, the magnetic drive, the measurement coil, the current measurement device, moveable and fixed contacts and the contact bridge are arranged in the housing.

[0055] The evaluation unit may be integrated in an external computer or server. For example, the data line is at least partially formed by an internet connection. It is possible that measurement data from the measurement coil and the current measurement device, in particular the inductance current signal and the drive current signal, are output from the switching device via the data line and evaluated externally by the evaluation unit. This may reduce the complexity of the switching device, and the evaluation unit may evaluate data from a plurality of switching devices in parallel or simultaneously .

[0056] The switching device and the external evaluation unit may form a switching arrangement.

[0057] According to at least one embodiment, the switching device or the switching arrangement comprises an external evaluation unit, the switching device further comprising a communication interface for wireless communication. For example, the communication interface comprises an antenna. The communication interface is preferably arranged in the housing of the switching device or a part of an outer surface of the housing. The data line connecting an external evaluation unit to the switching device may be at least partially formed by a wireless connection. An at least partial wireless connection between the evaluation unit and the switching device advantageously reduces the complexity when installing the switching device in an application.

[0058] Further advantages and advantageous embodiments and further developments of the method and the switching device described herein will become apparent from the following exemplary embodiments shown in connection with schematic drawings. Identical elements, elements of the same kind or elements having the same effect are provided with the same reference signs in the figures. The figures and the proportions of the elements shown in the figures are not to be regarded as true to scale. Rather, individual elements may be shown exaggeratedly large for better representability and / or for better comprehensibility.

[0059] In the figures:

[0060] Figure 1 shows a schematic representation of a switching device described here according to a first exemplary embodiment in a first state;

[0061] Figure 2 shows a schematic representation of a switching device described here according to the first exemplary embodiment in an intermediate state;

[0062] Figure 3 shows a schematic representation of a switching device described here according to the first exemplary embodiment in a second state; Figure 4 shows a schematic representation of a switching device described here according to a second exemplary embodiment in a first state;

[0063] Figure 5 shows a schematic representation of a switching device described here according to a third exemplary embodiment in a first state;

[0064] Figure 6 shows a block diagram illustrating a method for determining a contact wear described herein according to an exemplary embodiment;

[0065] Figure 7 illustrates an inductive voltage signal obtained during performance of the method according to the exemplary embodiment;

[0066] Figure 8 illustrates a derivative of the inductive voltage signal obtained during performance of the method according to the exemplary embodiment;

[0067] Figure 9 illustrates a magnetic flux signal obtained during performance of the method according to the exemplary embodiment;

[0068] Figure 10 illustrates a drive current signal obtained during performance of the method according to the exemplary embodiment; and

[0069] Figure 11 illustrates an inductance current signal obtained during performance of the method according to the exemplary embodiment. Figure 1 illustrates a switching device 1 described herein according to a first exemplary embodiment in a first state la. Figure 2 illustrates the switching device 1 according to the first exemplary embodiment in an intermediate state 1c between the first state la and a second state lb. Figure 2 illustrates the switching device 1 according to the first exemplary embodiment in a second state lb.

[0070] The switching device 1 comprises fixed contacts 7 and corresponding moveable contacts 8. The fixed contacts 7 and moveable contacts 8 comprise an electrically conductive material such as copper. The fixed contacts 7 are connected to a power line 70 that connects, for example, a power source on one end to a load on an opposing end.

[0071] The moveable contacts 8 are arranged on a contact bridge 80 of an electrically conductive material such as copper and / or steel. The contact bridge 80 is mechanically coupled to an anchor 2 by a contact spring 16.

[0072] The anchor 2 comprises a magnetic material such as iron or ferrite and is arranged at least partially in a magnetic drive 3. The magnetic drive 3 comprises a solenoid 19 wound around a core 18. The core 18 comprises a magnetic material.

[0073] In the first state la, the moveable contacts 8 and the fixed contacts 7 are separated from another. This means that the power line 70 is interrupted and the switching device 1 is "open". Furthermore, the anchor 2 is arranged a distance from the core 18. The anchor 2 is held in position by magnetic drive springs 17. At a tripping event from the first state la to a second state lb, the fixed contacts 7 and moveable contacts 8 are connected such that the power line 70 is closed via the contacts 7, 8 and the contact bridge 80. The switching device 1 is "closed".

[0074] At the tripping event, the magnetic drive 3 is supplied with a drive current. The drive current generates a magnetic field that acts on the anchor 2, causing the anchor 2 to be pulled towards the core 18 of the the magnetic drive 3 (cf. Figures 2 and 3). Thereby, the contact bridge 80 moves towards the fixed contacts 7, closing the contacts 7, 8. In particular, the anchor 2 and the contact bridge 80 are moved by a free stroke 13 until the contacts 7, 8 touch each other (Figure 2).

[0075] At the time the contacts 7, 8 touch each other, also referred to as contact time 10, the switching device 1 is in the intermediate state 1c as illustrated in Figure 2. In the intermediate state 1c, the anchor 2 travelled the free stroke 13. A gap is present between the anchor 2 and the core 18. This gap corresponds to a contact stroke 13a.

[0076] Subsequently to the intermediate state 1c, the switching device 1 reaches the second state lb. In contrast to the intermediate state 1c, the anchor 2 travels the contact stroke 13a in the second state lb (Figure 3). This causes the contact spring 16 to be compressed. Thus, a pressure is applied to the moveable contacts 8 such that a reliable connection between the moveable contacts 8 and the fixed contacts 7 can be established. During the tripping event, i.e., the switching from the first state la to the second state lb, the anchor 2 travelled a full stroke 13b, which is the sum of the free stroke 13 and the contact stroke 13a. The full stroke 13b corresponds to a distance between the anchor 2 and the core 18 in the first state la as illustrated in Figure 1. In the second state lb the magnetic drive springs 17 are compressed.

[0077] It is also possible to switch the switching device 1 from the second state lb to the first state la by terminating a current supply for the solenoid 19. This causes the magnetic drive springs 17 to decompress and consequently pushing the anchor 2 to its position in the first state la.

[0078] The switching device 1 further comprises a measurement coil

[0079] 4, which is magnetically coupled to the magnetic drive 3. In particular, the measurement coil 4 is wound around the core 18 of the magnetic drive 3. This means that the magnetic field of the magnetic drive 3 induces a voltage in the measurement coil 4. This voltage can be measured over time by a voltmeter of the measurement coil 4 in order to calculate a magnetic flux of the magnetic drive 3.

[0080] The switching device 1 further comprises a current measurement device 6. The current measurement device 6 is an amperemeter and is configured to measure the drive current of the magnetic drive 3 over time.

[0081] The switching device 1 further comprises an evaluation unit

[0082] 5, which is configured to perform a method for calculating a contact wear that may occur due to arcs emerging upon opening the contacts 7, 8, bouncing of the contacts 7, 8 when closing the contacts 7, 8, and / or other mechanical erosion of a material of the contacts 7, 8. The method will be described in detail below. The evaluation unit 5 is a microcontroller.

[0083] The evaluation unit 5 comprises a storage 50, in which parameters for the method, in particular an initial inductance 12 and an initial stroke 14 are stored. Via a data line 51, the parameters may be input to the evaluation unit 5, and / or the calculated contact wear may be output.

[0084] In contrast to the switching device 1 according to the first exemplary embodiment, the evaluation unit 5 of the switching device 1 according to Figure 4 is arranged outside a housing 15 for the contacts 7, 8, the magnetic drive 3 and the anchor 2. The evaluation unit 5 is thus an external evaluation unit 5. The evaluation unit 5 is connected to the current measurement device 6 and the measurement coil 4 by data lines 51. In other aspects, the second exemplary embodiment comprises the same features as the first exemplary embodiment .

[0085] In contrast to the switching device 1 according to the second exemplary embodiment, the switching device 1 according to Figure 5 comprises a communication interface 52 arranged in the housing 15. The communication interface 52 is configured to wirelessly commute with the external evaluation unit 5. The communication interface 52 may comprise an antenna.

[0086] Hence, the data line 51 may at least partially be formed by a wireless connection. In other aspects, the third exemplary embodiment comprises the same features as the first exemplary embodiment .

[0087] In the second exemplary embodiment and the third exemplary embodiment, the external evaluation unit 5 is, for example, a central computer or a server, which is configured to carry out the method. An external evaluation unit 5 allows for a reduction of complexity of the switching device 1.

[0088] Figure 6 shows a block diagram of a method described herein according to an exemplary embodiment. The method is configured to detect a contact wear of a switching device 1, in the present embodiment of the switching device 1 according to the first exemplary embodiment (cf. Figures 1 to 3).

[0089] In a step 101 or immediately before step 101 a tripping event occurs that lets the switching device 1 switch from the first state la to the second state lb. At the beginning of the tripping event, in step 101 an inductive voltage signal 21 is obtained by the measurement coil 4, and a drive voltage signal 22 is obtained by the current measurement device 6.

[0090] The inductive voltage signal 21 gives the inductive voltage 21a induced in the measurement coil 4 due to the magnetic field emerging from the magnetic drive 3 as a function of time and is shown in Figure 7. In Figure 7 the inductive voltage 21a in V is shown as a function of time 20 in s, where at a time 20 of 0 s the tripping event starts. At times 20 smaller than approximately 0.025 s, the voltage 21a moderately decreases before it moderately increases until approximately 0.04 s. For times 20 greater than 0.04 s, the voltage 21a increases faster.

[0091] The drive current signal 22 gives the drive current 22a for the magnetic drive 3 in A as a function of time 20 in s and is shown in Figure 10. As can be seen, from the start of the tripping event the drive current 22a increases until approximately 0.04 s. At approximately 0.04 s the drive current signal 22 decreases to a local minimum at about 0.05 s. At this time, the anchor 2 has travelled the full stroke 13b such that the anchor 2 touches the core 18 (Figure 3).

[0092] A duration of step 101 covers the whole tipping event. The following steps in particular concern the evaluation of the inductive voltage signal 21 and the drive current signal 22 and do not have to be carried out during the tripping event.

[0093] In step 102 a magnetic flux signal 23 is calculated by integrating the inductive voltage signal 21 as shown in equation (4) above. The magnetic flux signal 23 gives a magnetic flux 23a in mV-s through the magnetic drive 3 as a function of time 20 in s and is shown in Figure 9. The magnetic flux 23a increases monotonously with increasing time 20.

[0094] In step 103 a derivate 25 of the inductive voltage signal 21 is calculated. From the derivative a contact time 10 may be obtained in step 105. The derivative 25 is given in V / s as a function of time 20 in s and is shown in Figure 8. The derivative 25 is approximately constant or increases only slightly until 0.04 s. At about 0.045 s the derivative has a horizontal tangent. For times 20 greater than 0.045 s the derivative 25 increases rapidly.

[0095] In step 104 an inductance signal 24 is calculated by dividing the magnetic flux signal 23 by the drive current signal 22 as discussed in equation (1) above. The inductance signal 24 gives an inductance 24a of the magnetic drive 3 in mH as a function of time 20 in s and is shown in Figure 11. The inductance 24a is approximately constant or increases only slightly until 0.04 s. For times 20 greater than 0.04 s the inductance 24a increases rapidly.

[0096] In step 105 the contact time 10 is obtained. The contact time 10 gives the point in time when the fixed contacts 7 and moveable contacts 8 touch each other (Figure 2). In other words, the contact time 10 represents the point in time when the moveable contacts 8 directly contact the fixed contacts 7 for the first time. This means that at the contact time 10 the anchor 2, i.e. the contact bridge 80, has traveled the free stroke 13. For times larger than the contact time 10, the anchor 2 travels the contact stroke 13a, thereby increasing a force applied to the contacts 7, 8 to establish a good mechanical connection between the contacts 7, 8 by increasing force on the contact bridge 80.

[0097] When reaching the contact position, i.e. at the contact time, the anchor 2 and hence the contact bridge 80 and moveable contacts 8 have a certain velocity. Thus, a momentum can be assigned to the moveable parts of the switching device 1, which is at least partially transferred to fixed parts of the switching device 1 such as the fixed contacts 7. This momentum transfer has an effect on the magnetic flux and thus the inductive voltage signal 21. This means that the inductive voltage 21a may comprise an inflection point at the contact time. The inflection point and hence the contact time 10 are determined by calculating a derivative 25 of the inductive voltage signal 21 in step 103, which is evaluated in step 105 with respect to horizontal tangents to obtain the contact time 10.

[0098] In step 106 the contact wear is determined. This is achieved by evaluating the inductance signal 24 at the contact time 10 as illustrated in Figure 10 to obtain the inductance 11 at the contact time 10.

[0099] If the contacts of the switching device 1 undergo wearing, the contact stroke 13a decreases since the anchor 2 has to travel a further distance if material of the contacts 7, 8 wears off. Hence, a ratio of the contact stroke 13a and an initial contact stroke 14 can be a relative measure for the contact wear. The initial contact stroke 14 is a distance the anchor 2 has to travel if no contact wear is present, for example directly after production of the switching device 1.

[0100] Further, since the inductance 24a of the magnetic drive 3 is in good approximation reciprocally proportional to the contact stroke 13, the ratio of the stroke 13 and the initial contact stroke 14 equals a ratio of the initial inductance 12 at closed contacts and the inductance 11 at the contact time 10 as demonstrated above in connection with equation (3).

[0101] The initial inductance 12 and the initial contact stroke 14 are determined upon production of the switching device 1 and stored in the storage 50 of the evaluation unit 5. At the time of production no contact wear is present. Hence, a total contact wear can be determined if the initial inductance 12 and the initial contact stroke 14 are determined during production or immediately after production.

[0102] The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments. Rather, the invention encompasses any new feature and also any combination of features, which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

[0103] List of references

[0104] 1 electric switching device la, lb first state, second state of switching device

[0105] 1c intermediate state of switching device

[0106] 2 anchor

[0107] 3 magnetic drive

[0108] 4 measurement coil

[0109] 5 evaluation unit

[0110] 6 current measurement device

[0111] 7 fixed contact

[0112] 8 moveable contact

[0113] 10 contact time

[0114] 11 inductance at contact time

[0115] 12 initial inductance

[0116] 13 free stroke

[0117] 13a contact stroke

[0118] 13b full stroke

[0119] 14 initial contact stroke

[0120] 15 housing of switching device

[0121] 16 contact spring

[0122] 17 magnetic drive spring

[0123] 18 core

[0124] 19 magnetic drive solenoid

[0125] 20 time

[0126] 21 inductive voltage signal

[0127] 21a inductive voltage

[0128] 22 drive current signal

[0129] 22a drive current

[0130] 23 magnetic flux signal

[0131] 23a magnetic flux

[0132] 24 inductance signal

[0133] 24a inductance 25 derivative of inductive voltage signal

[0134] 50 storage

[0135] 51 data line

[0136] 52 communication interface 70 power line

[0137] 80 contact bridge

[0138] 101...106 method steps

Claims

Claims1. Method for determining a contact wear of a switching device (1) comprising an anchor (2) that is moved by means of a magnetic drive (3) in a tripping event, at least one fixed contact (7) and at least one moveable contact (8) mechanically connected to the anchor (2); the method comprising the steps of:- determining a contact time (10) when the fixed contact (7) touches the moveable contact (8),- determining an inductance (11) of the magnetic drive (3) at the contact time (10),- determining the contact wear by comparing an initial inductance (12) with the inductance (11) at the contact time (10).

2. Method according to claim 1, wherein, for determining the contact time (10),- an inductive voltage signal (21) of the magnetic drive (3) is measured by a measurement coil (4), and- an inflection point of the inductive voltage signal (21) is determined, the inflection point corresponding to the contact time (10).

3. Method according to claim 1 or 2, wherein, for determining the inductance (11) at the contact time (10),- a drive current signal (22) for the magnetic drive (3) is measured,- a magnetic flux signal (23) of the magnetic drive (3) is calculated by integrating the inductive voltage signal (21), and- obtaining an inductance signal (24) by dividing the magnetic flux signal (23) by the drive current signal (22),and- evaluating the inductance signal (24) at the contact time(10).

4. Method according to one of the preceding claims, wherein the contact wear is a ratio of a contact stroke (13) of the anchor (2) and an initial contact stroke (14).

5. Method according to one of claims 1 to 3, wherein the contact wear is a difference of a contact stroke (13) and an initial contact stroke (14).

6. Method according to one of the preceding claims, wherein the steps of the method are carried out by an evaluation unit (5).

7. Method according to one of the preceding claims, wherein the initial inductance (12) is determined upon production of the switching device (1) and is stored in a storage (50) of the evaluation unit (5).

8. Method according to one of the preceding claims, wherein the switching device (1) is a contactor, and the magnetic drive (3) comprises a solenoid.

9. Method according to one of the preceding claims, wherein the anchor (2) is connected to at least one moveable contact (8) of the switching device (1) by a contact spring.

10. Method according to one of the preceding claims, wherein the contact wear is stored after every thousandth tripping event.

11. Switching device (1) comprising- an anchor (2),- a magnetic drive (3) configured to move the anchor (2) in a tripping event,- a measurement coil (4) configured to measure an inductive voltage signal (21) at the tripping event,- a current measurement device (6) configured to measure a drive current signal (22) for the magnetic drive (3) at the tripping event, and- an evaluation unit (5) configured to perform the method according to one of the preceding claims.

12. Switching device (1) according to claim 11, further comprising at least one fixed contact (7) and at least one moveable contact (8), wherein- in a first state (la) of the switching device (1) the fixed contact (7) is separated from the moveable contact (8), and in a second state (lb) the fixed contact (7) and the moveable contact (8) are in electrical contact with each other, and- wherein at a tripping event the switching device (1) switches from the first state (la) to the second state (lb).

13. Switching device (1) according to claim 11 or 12, wherein the evaluation unit (5) is integrated in a common housing (15) of the switching device (1).

14. Switching device (1) according to claim 11 or 12, wherein the evaluation unit (5) is at least partially arranged outside a housing (15) of the switching device (1) and is connected to the measurement coil (4) and / or the current measurement device (6) via at least one data line (51).

15. Switching device (1) according to claim 14, further comprising a communication interface (52) for wireless communication, wherein the communication interface (52) is arranged in the housing (15) of the switching device (1) and the data line (51) is formed by a wireless connection.

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