DE102021212834A1 - mechanical switch - Google Patents

Abstract

A state determination for a mechanical switch is carried out by recording a course of the voltage across the switch or a course of the current through the switch during a switch-on process, one or more features of a switch-on chatter are determined from the course, from these features a status indicator for the switch is calculated.

Description

Mechanical switch for controllably closing and disconnecting an electrical connection.

In general, it is interesting for an electrical switch, in particular a low-voltage switch, to continuously provide information about its probable remaining service life. With such information about the switch, a predictive maintenance strategy can be pursued, for example. This is a proactive maintenance process that uses switch status information to predict maintenance needs. As a result, operational disruptions are counteracted preemptively and maintenance itself is more efficient.

This is opposed by the fact that, for economic and technical reasons, such switches are usually built very compactly. There is therefore little space available for a dedicated sensor system. In addition, the sensors would cause additional costs, which is not in economic relation to the costs of the switch itself, particularly in the case of inexpensive switches in the low-voltage range.

It is known to count the switching operations of a low-voltage switch in order to be able to make a statement about its remaining service life. In low-voltage switches, there is also the possibility of detecting the switching currents flowing through the switching arc when the switch is opened, which then enables a switching cycle to be weighted. In addition, and for more accurate weighting, the switching energy can be calculated. However, these methods only provide estimates of the remaining service life, but no direct statements about the current state of the switch.

It is the object of the invention to specify a mechanical switch in which an improved statement about the state is made possible.

This object is achieved by a mechanical switch having the features specified in claim 1.

The mechanical switch according to the invention for the controllable closing and disconnection of an electrical connection comprises a control device which is designed to record a voltage curve across the switch or a current curve through the switch. The control device is also designed to determine one or more features of a switch-on chatter from the voltage profile and to calculate a status indicator for the switch from these features.

In the method according to the invention for determining the state of a mechanical switch for controllably closing and disconnecting an electrical connection, a voltage curve across the switch or a current curve through the switch is recorded during a switch-on process. Furthermore, one or more features of a switch-on chatter are determined from the course and finally a status indicator for the switch is calculated from these features.

The design of the switch or the method advantageously achieves a status determination for the switch whose reliability always remains the same since it is based on current measured values. As a result, short-term faults can be detected and a warning given before a switching failure occurs, which is not possible with just counting switching operations, for example. Various influences on the contact chatter are advantageously recorded, for example also aging of the drive system.

• Die Merkmale für ein Einschaltprellen können eines oder mehrere der folgenden Merkmale umfassen:
  • - Eine Anzahl von auf Einschaltprellen zurückzuführenden Spannungspulsen. Dies entspricht der Anzahl von Rückprallvorgängen des Kontakts.
  • - Eine Dauer von auf Einschaltprellen zurückzuführenden Spannungspulsen.
  • - Eine Amplitude von auf Einschaltprellen zurückzuführenden Spannungspulsen.

Advantageous configurations of the switch according to the invention emerge from the dependent claims. The embodiment of the independent claims can be combined with the features of one of the subclaims or preferably also with those of several subclaims. In particular, the features from the subclaims can be combined with any of the independent claims. Accordingly, the following features can also be provided:

• The characteristics of power-on chatter may include one or more of the following characteristics:
  • - A number of voltage pulses due to power-on chatter. This corresponds to the number of times the contact has rebounded.
  • - A duration of voltage pulses due to switch-on chatter.
  • - An amplitude of voltage pulses due to power-on chatter.

These features allow the "severity" of the power-on chatter to be quantified. The features are not necessarily directly dependent on one another, so that more information can be obtained by considering more than one of the features.

The control device can be designed to determine a Fourier transformation, in particular a fast Fourier transformation, of the voltage profile in the area of the switch-on process. As a result, the voltage curve is transferred to the frequency domain. A separation of the two frequencies of the AC voltage and the bouncing process is advantageously achieved by the Fourier transformation. As a result, the bouncing process can be viewed independently of the voltage changes that can be traced back to the course of the AC voltage. In this way, an essentially uniform variable is also obtained from the potentially multiple bouncing processes, since the frequency of the bouncing processes is given by the mechanical components and is therefore essentially constant.

An increase in the power amplitude determined from the Fourier transform and/or a decrease in the peak frequency determined from the Fourier transform can be used as a status indicator. The peak frequency designates the frequency with the highest amplitude in the frequency range of the bouncing processes. If this frequency drops, the bouncing processes become slower, which indicates that the switch is aging.

The switch can be designed to also record a current profile in the area of a switch-on process. This can also be taken into account. For example, knowing the course of the current makes it easier to determine the start of the switch-on process.

If the current profile is known, an energy dissipated in the area of a switch-on process can be determined from the voltage profile and the current profile. For this purpose, the product of instantaneous voltage U(t) and instantaneous current I(t) is integrated over the period of the switch-on process.

A quotient of the energy and a current value when the switch is on can be used as a status indicator. The current value can be an RMS current value or a peak current value. The quotient corresponds to a product of an average bounce voltage U _Prell and the total duration of the bouncing process.

A neural network can be used to determine the condition indicator. In this case, it is advantageous if the neural network is trained with data from pre-aged switches. For this purpose, voltage curves from pre-aged switches and the status indicators resulting therefrom must be available. For example, the status indicators associated with a switch and voltage waveform can be manually associated. The neural network can receive information obtained, for example, by the Fourier transformation as input data, for example the position and/or amplitude of the bounce frequency.

The mechanical switch can be configured to controllably make and break a plurality of electrical connections.

In addition to evaluating the measured progression in the switch itself, values that represent the progression can also be transmitted from the switch to a cloud service, and the feature is determined and the status indicator is calculated in the cloud service.

• 1 eine einfache beispielhafte Schaltung mit einem Schalter,
• 2 den Aufbau eines Schützes als Teil des Schalters,
• 3 einen Spannungs- und Stromverlauf bei einem Einschaltvorgang des Schalters,
• 4 eine Darstellung des Spannungsverlaufs im Frequenzraum.

The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures. Show it:

• 1 a simple exemplary circuit with a switch,
• 2 the construction of a contactor as part of the switch,
• 3 a voltage and current profile when the switch is switched on,
• 4 a representation of the voltage curve in the frequency domain.

1 FIG. 12 shows an electrical circuit diagram for a circuit in which a contactor 10 is connected between a voltage supply 11 and a load 12. FIG. The contactor 10 includes only one contact 14. In real applications, however, contactors often include a plurality of contacts.

2 shows the basic structure of the contactor 10. The contactor 10 comprises a fixed magnetic core 102 and a movable armature 103 in a housing 101. The fixed magnetic core 102 and the movable armature 103 are designed, for example, in the shape of a letter E, with the yoke sides are turned away from each other. A coil 104 is arranged across the central arm of the two E-shapes, the coil 104 being stationary with the housing 101 together with the fixed magnetic core 102 . The coil 104 is connected to the outside with electrical control contacts with which the coil 104 can be energized.

The fixed magnetic core 102 and the movable armature 103 are mechanically connected by return springs 106 and are pushed apart by them. When the coil 104 is energized, the movable armature 103 is attracted to the fixed magnetic core 102. If the current through the coil 104 is switched off, there is no need the attractive force and the return springs 106 push the movable armature 103 away from the fixed magnetic core 102 again.

The movable armature 103 is connected via a mechanical holder 107 to a contact bridge 109 which is moved when the movable armature 103 is moved. Corresponding normally open contacts 110, 111 are arranged in such a way that the contact bridge 109 makes an electrical connection when the movable armature 103 is in the attracted state and is so spaced from the normally open contacts 110, 111 that no electrical connection is present.

Since the contact 14 is closed by the movement of the movable armature 103 by means of an accelerated mechanical movement, contact bouncing can occur, in which case the contact bridge 109 or parts thereof bounce off the make contacts 110, 111 or spring back. As a result, the mechanical contact is briefly interrupted once or several times. This does not completely break the electrical contact, since an arc maintains a current flow for the brief bouncing time.

This arc causes the contact system to age and, in the worst case, can even weld the contacts.

The force-displacement curve when the contact 14 closes is therefore designed in such a way that with a new contactor 10 there is practically no bouncing or the bouncing is reduced to a minimum. Contact erosion, however, means that bouncing occurs more frequently as switch 20 ages and bouncing events become more intense.

The switch-on chatter can be understood using the current and voltage values across or in the contact. The switch 20 is designed to record a voltage curve and a current curve over a switch-on process. An example of the resulting curves is in 3 shown.

Before a switch-on time t1, the voltage 31 shows a curve-like profile that corresponds to a section of a sine wave. Since contact 14 of contactor 10 is open at this time, the voltage across the contact corresponds to the AC voltage provided by power supply 11 . The current 32 before time t1 is 0 since no current can flow through the open contact 14.

At time t1, contact 14 is closed, ie switched on. Around this point in time, therefore, the voltage across contact 14 falls rapidly to 0V. During the same period, the current 32 increases; provided no switch-on chatter occurs, the current 32 rises to a value specified by the voltage supply 11 and the load 12 or—in the case of an AC voltage—to a course.

In this case, however, there is a switch-on chatter, which manifests itself in the fact that at a point in time t2 the voltage 31 rises again and the current 32 falls somewhat. Since the contact 14 is quickly closed again after rebounding, the voltage then falls again quickly and the current rises again. 3 then shows another bouncing, until the contact 14 finally closes at time t3.

A simple measure of contact chatter is the energy dissipated during the switching process. This results from the integral over the product of current 32 and voltage 31, i.e $$W_p={\displaystyle \int {}_{t1}^{t3}u\left(t\right)I\left(t\right)\mathrm{i.e}t}$$

Where U(t) is the instantaneous voltage 32 across contact 14 and I(t) is the instantaneous current 31 through contact 14. The integration limits t1 and t3 can be determined from the current and voltage 31, 32; However, they do not have to be determined very precisely, since the current is 0 A before switching on and the voltage is almost 0 V after switching on, so that there are hardly any contributions to the integral outside of the switching period.

The values of the current and voltage measurement should be recorded with a frequency of at least 1 kHz, preferably 10 kHz or more, i.e. with a time resolution of 0.1 ms or higher. Both multiplication and integration can be performed electronically, digitally, or a combination of both.

In a nearly bounce-free switch, the dissipated energy is not 0 J but has a finite value. This value can be determined for the contactor 10 when the contactor 10 is still new and therefore typically has little contact chatter. The value determined in this way can serve as a reference value in order to obtain an absolute measure of the state of the contactor 10 with later measurements by comparison with the reference value.

In addition to evaluating the switching energy, the bouncing can also be evaluated by examining the course of voltage or current 31, 32 in more detail. Both curves result as a mixture of the voltage curve in the open state, i.e. a sine curve in the case of an AC voltage, before the finally fast switch-on gang, in which the current 32 rises and the voltage 31 falls at a finite speed, and the oscillations that are caused by the switch-on chatter. Such a course is in 3 shown. In order to bring about reliable separation of the influences on the signal, a Fourier transformation is advantageously carried out, in particular a fast Fourier transformation (FFT). A possible resulting course of the amplitude 41 over the frequency is greatly simplified in 4 shown.

In addition to a first peak near the frequency f=0, there is a second peak at a frequency f2. The frequency f2 is the frequency of the AC voltage, for example 50 Hz. This portion of the signal is irrelevant for the evaluation of the contact chatter and is advantageously separated from the contact chatter by the transformation into the frequency domain. The contact bounce signal appears around a third frequency f3. This peak is washed out because the contact bounce differs significantly from a sine wave. This also results in significant frequency components at multiples of the third frequency f3, but in 4 are not shown. These do not have to be evaluated in addition to the peak at the third frequency f3, but they can be.

For example, the amplitude at the third frequency f3 or the maximum amplitude in the range around the third frequency f3 and/or the size of the frequency f3 itself can be used as a measure of the state of the contactor. An increase in amplitude and a decrease in frequency f3 both indicate increasing contact chatter. In addition to this relative consideration, both values can also be evaluated as absolute measures of the condition of the contactor 10 by making a comparison with the same values for contactors that have already been in service or have aged artificially and for which a condition assessment is available. In order to establish the connection between bounce signals and the state for a specific switch model, AI methods such as neural networks, which are trained with artificially aged switches, can also be used.

In order to be able to carry out the evaluations described, the contactor includes a device for measuring voltage and current via or through the contact 14. The evaluations described can be carried out locally on the contactor 10 itself, with the contactor 10 including a microcontroller for this purpose. In alternative configurations, the contactor or a switch comprising the contactor 10 can also have a connection to a cloud service and can transmit measured values for current 32 and/or voltage 31 to the cloud service, with the evaluation then being carried out at the cloud service.

Reference List

10

contactor

11

power supply

12

load

14

Contact

20

Switch

21

microcontroller

101

Housing

102

solid magnetic core

103

moving anchor

104

Kitchen sink

106

return springs

107

mechanical mount

109

contact bridge

110, 111

normally open contacts

t1, t2, t3

times

f2, f3

frequencies

Claims (11)

Mechanical switch (10) for controllably closing and separating an electrical connection, having a control device that is designed - record a course of the voltage (31) across the switch (10) or a course of the current (32) through the switch (10), - to determine one or more characteristics of a switch-on chatter from the course, - to calculate a status indicator for the switch from these characteristics. Mechanical switch (10) after claim 1 wherein the features are selected from the following features: - the number of voltage pulses due to power chatter, - the duration of voltage pulses due to power chatter, - the amplitude of voltage pulses due to power chatter. Mechanical switch (10) after claim 1 or 2 , in which the control device is designed to determine a Fourier transformation, in particular a fast Fourier transformation, of the voltage curve in the region of the switch-on process. Mechanical switch (10) after claim 3 , designed in such a way that an increase in the power amplitude determined from the Fourier transformation and/or a decrease in the peak frequency (f3) determined from the Fourier transformation is used as a status indicator. Mechanical switch (10) according to one of the preceding claims, in which a current profile in the region of a switch-on process is recorded. Mechanical switch (10) after claim 5 , in which an energy dissipated in the area of a switch-on process is determined from the voltage profile and the current profile. Mechanical switch (10) after claim 6 , designed in such a way that a quotient of the energy and a current value when the switch (10) is switched on is used as a status indicator. A mechanical switch (10) as claimed in any preceding claim, in which a neural network is used to determine the condition indicator, the neural network being trained on data from aged switches (10). Mechanical switch (10) according to one of the preceding claims for the controllable closing and breaking of a plurality of electrical connections, in particular a contactor (10). Method for determining the state of a mechanical switch (10) for controllably closing and separating an electrical connection, in which - During a switch-on process, a course of the voltage (31) across the switch (10) or a course of the current (32) through the switch (10) is recorded, - one or more characteristics of a switch-on chatter are determined from the course, - A status indicator for the switch (10) is calculated from these characteristics. procedure after claim 10 , in which values that represent the course are transmitted from the switch (10) to a cloud service and the determination of the feature and the calculation of the status indicator is carried out in the cloud service.

Images