Patent Application
20220157504
The present invention relates to a method for closing the contacts of an electrical switching device during a switch-on process according to the preamble of independent claim 1. The electrical switching device has an electromechanical drive with a coil and with an armature, which can be moved between an open position and a closed position, wherein the coil is energized in order to close the contacts of the electrical switching device. The armature of the electromechanical drive is connected with a movable contact of the electrical switching device. The present invention further relates to an electrical switching device according to the preamble of independent claim 10.
Electrical switching devices, in particular high-power contactors, are subjected to sometimes high thermal fluctuations in many fields of application. This is true, for example, for high-power contactors which are employed in railway vehicles, motor vehicles, or in outdoor installations. The coil of the electromagnetic drive can moreover be subjected to very high thermal fluctuations in operation just due to self-heating.
In railway applications, the temperature range reaches from about −40° C. in Siberia to 110° C. in certain desert areas. The electric resistance of the coil changes here by the factor 1.8. If no compensation is accomplished, the pickup current—the current which flows in the coil when the contacts are being closed—and the switching behavior of the switching device change correspondingly. In a cold state, the pickup is faster due to the lower resistance, which may lead to an increased bouncing of the contacts of the electrical switching device during closing, and which quite basically results in an increased mechanical load of the components. At very high temperatures, the contacts are possibly not closed quickly enough, so that fluttering events and an increased wear due to arcing may result.
If there is no temperature compensation, the drive thus has to be designed to be more robust and therefore larger. This leads to comparatively heavy and expensive switching devices.
If, however, a temperature compensation is to be accomplished, a lower voltage must be applied to the coil at low temperatures, and a higher voltage at higher temperatures to be able to ensure a uniform switching behavior or a uniform switch-on time or pickup time, respectively, over the complete temperature range. To this end, the temperature prevailing in the coil or the coil resistance depending thereon must be detected. This can be done, for example, by means of a temperature sensor. An additional temperature sensor, however, results in a more cumbersome construction and renders the manufacture of the electrical switching device more expensive.
However, there are already also methods for measuring the coil inductance and the coil resistance without any direct determination of the coil temperature. Such a method is known, for example, from US 20180174786 A1. Such methods, however, require a comparatively high computing power and therefore demand the use of expensive microprocessors.
It is therefore the object of the present invention to indicate a method of the type mentioned in the beginning which permits a simple temperature compensation with low hardware demands and in particular without the requirement of a temperature sensor, and which does not disadvantageously extend the pickup process.
The object is achieved by the features of independent claim 1.
Accordingly, in a method according to the preamble of independent claim 1, the solution of the problem is according to the invention if first a constant first voltage U1 is applied to the coil during a first time period T1 and a measurement value is determined, wherein
The idea of the present invention is based on the following known equation for the current through a coil upon the application of a voltage (this applies as long as the armature does not move):
If the quantities L, I, U and t are known, the coil resistance R can be calculated therefrom which in turn depends on the temperature. The actual calculation of the coil resistance, however, is not necessary according to the invention. Only a measurement value depending on the coil resistance and thus on the temperature is determined.
If the first time period T1 is fixed, this measurement value is the current value IMess which appears at the end of the first time period T1. In accordance with this current measurement value IMess, the voltage U2 which is finally applied to the coil to pick up the armature, that means to move the armature from the open position to the closed position and thereby close the contacts is thereupon defined. The optimal pickup voltage U2 at a certain current measurement value IMess can be previously determined, for example, experimentally by corresponding series of measurements and stored in a memory of a control unit of the switching device.
The first time period T1 must be selected such that the armature does not already move during the first time period. Otherwise, the armature's reaction occurring in the magnetic field during the movement of the armature would falsify the current measurement at the end of the first time period, and the above-mentioned equation would no longer apply. The first time period must be long enough for the end values of the current measurement to be far away from each other at the upper and lower temperature limits—caused by the change of resistance of the coil due to the influence of temperature—such that a sufficiently large measuring range is achieved. The measuring accuracy and the resolution of the measuring device for the coil current must be taken into consideration here. The first voltage U1 applied to the coil during the first time period T1 should be selected preferably high so that the current flowing in the coil will become preferably high in the course of the first time period in such a way that during the first time period, at the lowermost operating temperature and taking into consideration tolerances, the armature does not yet move.
On the other hand, the first time period should be preferably short, so that the switch-on process is not unnecessarily delayed.
As an alternative to the above-described determination of the measurement value with a fixed first time period T1, a fixed current limit ISoll to be achieved can also be defined. In this case, the measurement value depending on the temperature and thus on the coil resistance is the first time period T1 which passes until the current limit ISoll is reached. Compared to the first alternative, this second alternative, however, is somewhat more complicated to realize since the coil current must be measured during the total first time period T1. It will be appreciated that in this second alternative, too, firstly the first voltage U1 must be kept constant until the predetermined current value ISoll is reached, and secondly, the first voltage U1 or the current value ISoll to be reached must be defined such that the armature is not set into motion until the current limit ISoll is reached.
In both above-mentioned cases, the current will increase during the total first time period T1. This means that the first time period T1 does not last long enough for a stationary final current to appear in the coil. With R=U/I, the resistance can be quite easily determined in this case, however, the measuring time required for this would be clearly longer than the total common pickup process of the switching device and would therefore be inacceptable. A great advantage of the method according to the invention is thus that the pickup process is not remarkably extended.
During the first time period T1, a constant first voltage U1 is applied to the coil according to the invention. This means that there will be no closed-loop control of the current flowing in the coil. The constant voltage is applied to the coil over the total first time period T1.
The present invention permits a simple temperature compensation without any complicated and expensive hardware. In particular, no temperature sensor is required to carry out the method according to the invention. Only a corresponding current measuring means is required to be able to measure the current flowing in the coil. In electrical switching devices with a closed-loop control of the holding current after the switch-on process, such a current measuring means is present anyway. To carry out the method, a small and inexpensive microcontroller can be used.
The present invention is in particular suited for electrical contactors.
Advantageous embodiments of the method according to the invention are the subject matter of subclaims.
According to a preferred embodiment of the present invention, the first time period T1 is fixed, the measurement value being a current value IMess determined at the end of the first time period T1 by measuring a current flowing in the coil, the first time period T1 and the first voltage U1 being selected in such a way that the armature is not set into motion during the first time period T1. As already described above, this embodiment is easier to realize than the alternative with a fixed current limit ISoll.
According to a further preferred embodiment of the present invention, the second time period directly follows the first time period. This ensures a short switch-on time. In the determination or defining of the second voltage U2 which is applied to the coil after the first time period T1 has lapsed to move the armature from the open position to the closed position and thereby close the contacts, the current value for the coil current must be considered in the process, which is already reached at the end of the first time period and thereby forms the starting value for the pickup phase during the second time period T2.
According to a further preferred embodiment of the present invention, the second voltage U2 is constant during the second time period T2. This substantially facilitates the method according to the invention. In the abstract, however, it is conceivable to impress a certain voltage characteristic during the second time period whose parameters are defined by means of the determined measurement value. A constant voltage in the sense of this embodiment is also understood as a mean voltage during the second time period adjusted by means of pulse width modulation.
According to a further embodiment of the present invention, the second voltage is defined in accordance with the measurement value such that the armature always reaches the same speed during the closing of the contacts independent of the temperature of the coil. The pickup voltage U2 required for this at a certain temperature-dependent measurement value can be experimentally determined by corresponding series of measurements. To this end, the switching device can be, for example, correspondingly heated or cooled, respectively, wherein subsequently both the current measurement value IMess at the end of the first time period T1 and the switching behavior in different pickup voltages during the second time period T2 are measured.
In an alternative embodiment, the second voltage is defined in accordance with the measurement value such that the armature is moved into the closed position always within the same time period during the closing of the contacts independent of the temperature of the coil. This means that the time period until the contacts are closed is to be always equal. In this embodiment, too, the required pickup voltage U2 can be experimentally determined at a certain temperature-dependent measurement value.
According to a further preferred embodiment of the method according to the invention, the definition of the second voltage U2 is accomplished by means of the measurement value by reading out a default value from a table stored in a memory. Thereby, no complicated calculations are required during the switch-on process. An inexpensive and simple microcontroller can be used for controlling. The mentioned table is furthermore preferably stored in the memory of the microcontroller used for controlling. In the table, for example, the concrete values for the pickup voltage (second voltage U2), or else other default values suited for controlling can be stored. For example, instead of the concrete voltage values, pulse width modulation default values can be stored. For the voltage values U1 and U2 are preferably adjusted by means of pulse width modulation. Possible fluctuations of the supply voltage are here preferably compensated by corresponding changes of the pulse width modulation. For the method according to the invention, it is not necessary to determine concrete values for the resistance and/or the temperature of the coil in operation. Only the correlation between the measurement value and the default value or voltage value U2 derived from the resistance or temperature, respectively, is relevant.
As an alternative, an approximation function for calculating the default value can be derived from the concretely determined default values or from the values for the second voltage U2 on the basis of the measurement value, so that instead of a complete table, only the parameters of a calculation specification must be transferred to the memory of the microcontroller used for controlling. While this requires a somewhat higher computing power, it needs less disk space. In this exemplified embodiment, too, possible fluctuations of the supply voltage are preferably compensated by corresponding changes of the pulse width modulation.
The values for the pickup voltage U2 corresponding to a certain measurement value or the above-discussed default values are preferably determined for a larger temperature range, for example for a temperature range of maximally 0° C. to at least 50° C., further preferred for a temperature range of maximally −20° C. to at least 80° C., further preferred for a temperature range of maximally −40° C. to at least 110° C., and particularly preferred for a temperature range of maximally −60° C. to at least 130° C. The values are stored in a table, and either the table itself or the calculation specification derived therefrom is transferred to the memory of the microcontroller. For a satisfactory temperature compensation, it is sufficient for the values to be determined for discrete temperatures with a delta of, for example, 1° C., or also with greater differences of, for example, 5° C. Since the concrete temperatures are not relevant after all for the method, the input quantity into the table is, however, the measurement value. Therefore, for the table, measurement values with a constant delta are preferably used, which does not reflect in a constant delta of the temperature.
After the second time period has lapsed, the control unit can pass over into a holding mode. Since for holding the armature in the closed position, less power is required than for picking up the armature, the performance can be reduced. According to a further embodiment of the method according to the invention, the second time period T2 is fixed, thus further facilitating the method. Preferably, the second time period T2 may alternatively end when a suited sensory mechanism or evaluation recognizes that the armature is in the closed position. In this embodiment of the method according to the invention, too, the control unit can subsequently pass over into the holding mode.
The invention furthermore provides an electrical switching device according to the preamble of independent claim 10 whose control unit is designed and configured to carry out the method according to the invention.
According to a preferred embodiment of the electrical switching device, the control unit comprises a microcontroller in which a table with possible measurement values and corresponding default values or, according to an alternative embodiment, a calculation specification for calculating a default value by means of the measurement value, is stored.
The invention will be illustrated more in detail below with reference to drawings.
In the drawing:
In the following illustrations, equal parts are designated by equal reference numerals. If a drawing contains reference numerals which are not explicitly discussed in the pertaining description of the figures, reference is made to previous or following descriptions of the figures.
The switching-on of the supply voltage is effected via the supply voltage switch 14.
In accordance with the measured current value IMess depending on the temperature of the coil, a suited second voltage U2 is defined thereupon which is larger than the first voltage U1 and which is applied to the coil 2 during a second time period T2 directly following the first time period T1 to move the armature 3 from the open position into the closed position and thereby close the contacts. The second time period T2 thus represents the second phase of the switch-on process. The second voltage U2 corresponding to a certain current measurement value IMess is read out, for example, from a table stored in the microcontroller.
Upon completion of the switch-on process, the control unit of the contactor passes over into a holding mode. The holding mode is maintained during the third time period T3.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 109 176.4 | Apr 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/060022 | 4/8/2020 | WO | 00 |
