Device for protecting a braking resistor for locomotives

Abstract: A device for protecting a braking resistor for locomotives supplied with AC power obtained by chopping the DC voltage from the power grid comprising at least one pack of resistive components of the resistor which has at least one reference resistive component which is made of a different material than the other resistive components of the pack but is suitable to react thermally in the same manner as the other resistive components of the pack when it is crossed by the same current, and a control circuit which is associated with the pack of resistive components and is suitable to detect their temperature by detecting the variations in the electrical characteristics of the reference resistive component.

Description: The present invention relates to a device and a method for protecting a braking resistor for locomotives. It is known that the temperature reached in a resistor by its resistive components is currently generally detected by thermostats arranged proximate to, or in contact with, one or more of the resistive components. The temperature of the resistive components of a resistor in fact tends to rise when the components are crossed by a preset current, regardless of whether the resistive components through which the current flows are series- or parallel-connected and are cooled with natural or forced ventilation. The use of thermostats arranged in contact with, or proximate to, one or more of the resistive components entails the fact that the temperature that is detected can be influenced by many external factors and accordingly does not correspond to the temperature actually reached by the resistive components, consequently leading to their failure. If a thermostat is directly associated with one or more of the resistive components, the technical operating characteristics of the components might in fact be modified, since their structure and configuration are altered. If instead a thermostat is simply arranged proximate to one or more of the resistive components, it does not detect the exact temperature reached by the resistive components, also due to an offset between the time when the resistive components reach the preset temperature and the time when this temperature is reported. In this case, the delay in reporting that the temperature has been reached may allow the resistive components to reach a higher temperature, which can be fatal to their structure, causing them to bum out and accordingly fail. Alternative techniques of temperature measurement, in which the temperature-dependent resistance of the braking resistor is monitored, are known from US 4 035 692A and DE 25 41 249 A. The aim of the present invention is to eliminate the above-mentioned drawbacks of the prior art. Within the scope of this aim, an important object of the invention is to provide a device for protecting a braking resistor for locomotives which is capable of detecting the temperature of the resistive components that are present in the resistor instantaneously and constantly over time, i. e., without offsets between when the actual temperature is detected and when it is reported. Another object of the invention is to provide a device for protecting a braking resistor for locomotives which allows to report an abnormal overtemperature condition on the resistor regardless of its cause, for example insufficient ventilation, abnormal current on the resistor, etcetera. Another object of the invention is to provide a device for protecting a braking resistor for locomotives which allows to detect a smoothed voltage, so that the voltage is a function of the temperature alone and is not affected by the current that circulates in the resistive components of the resistor. Another object of the invention is to provide a device for protecting a braking resistor for locomotives which has a system for self-checking its efficiency in order to check for any operating anomalies. This aim, these objects and others which will become apparent hereinafter are achieved by a device for protecting a braking resistor for locomotives according to claim 1. Further characteristics and advantages of the present invention will become apparent from the following description of a preferred but not exclusive embodiment of the device according to the invention, illustrated only by way of non-limitative example in the accompanying drawing, wherein: the only figure is a schematic view of the circuit portion of the device which is suitable to detect a preset temperature in a plurality of resistive components of a resistor according to the present invention. With reference to the above figure, the device for protecting a braking resistor for locomotives supplied with AC current produced by chopping the DC voltage of the power grid, generally designated by the reference numeral 1, comprises a plurality of packs of resistive components, only one of which will be considered for the sake of simplicity. The pack of resistive components of the resistor has a plurality of resistive components, each designated by the reference numeral 3, between which at least one reference resistance component 4 is arranged in a position which can vary according to requirements; the reference component is advantageously made of a different material than the other resistive components 3 of the pack but is suitable to react thermally in the same manner as the resistive components 3 of the pack when crossed by the same current that flows through them. In other words, the reference resistive component 4 has the same thermal behavior as each one of the resistive components 3 that compose the pack, since both are subjected to the same heat exchange rules, such as conduction, convection and radiation. However, although the reference resistive component 4 has the same thermal behavior as each one of the resistive components 3, its temperature coefficient α, which is the temperature coefficient of resistance that expresses the variation in resistivity with temperature, is different from the coefficient of the resistive components 3 of the pack. In particular, α designates the temperature coefficient of resistance that expresses the variation in resistivity as the temperature varies, according to the known formula R=RO(1+αδT). Clearly, the number of reference resistive components 4 introduced in a pack of resistive components 3 varies from a minimum of 1 upward, according to design requirements and to the intended industrial use of the resistive components, arranged in series or in parallel with natural or forced ventilation. By virtue of this solution, as will become apparent hereinafter, it is possible to detect the temperature of the resistive components 3 by using for this purpose a control circuit 5 which is associated with the pack of the resistive components by detecting the variations in the electrical characteristics of the reference resistive component 4 when the same electric current flows through the reference resistive component 4 and the resistive components 3. The control circuit 5 has a first means 6 and a second means 7 for smoothing the voltage across the terminals m1 and m2 of the device. Moreover, the control circuit 5 has a third means 8 for determining a discharging time which is longer than the charging time of the second means 7, so as to have a voltage which is a function of the temperature alone and is not affected by the current that circulates in the resistive components 3. Advantageously the control circuit 5 is self-powered, so that it causes no problems during its operation, and that the first voltage smoothing means includes an inductor 10. In particular, the second voltage smoothing means can include a capacitor 11 or a free-wheeling diode, according to requirements. The third means that determines a discharging time which is longer than the charging time of the inductor 10 and of the capacitor 11 includes a blocking diode 12. As shown in the only figure, the control circuit 5 further comprises a vacuum contact or reed relay 20 and can be provided with a test or checking coil 14 which is suitable to check the operating efficiency of the control circuit before using it. In particular, the device, as shown in the figure, is constituted by a plurality of resistive components 3 and by one or more reference resistive components 4, crossed by the same current but made of materials having a different thermal coefficient of resistivity or temperature coefficient α. The terminals A, B and C are between the resistive components 3 and the reference resistive component or components 4; A is the negative terminal and C is the positive terminal. The circuit further comprises measurement and setting resistors which are external to the braking resistor and are designated RA, RA1, RB and RB1; the control terminal D is arranged between them. The direction of the current is shown by the arrows I in the figure. In the figure, the reference numeral 20 designates the vacuum contact or reed relay and the reference numeral 21 designates for example an indicator located inside the locomotive and connected to the control coil 14. In particular, the control circuit, by virtue of the capacitor 11 and the inductor 10, is capable of smoothing the voltage across the terminals m1 and m2 while the blocking diode 12 allows to have different time constants T1 and T2 for charging and discharging the capacitor 11, in which T1, i.e., the charging time constant, must be as small as possible, while T2, the discharging time coefficient, must be as large as possible. The present invention also relates to a method for protecting a braking resistor for locomotives supplied with AC current obtained by chopping the DC voltage of the power grid according to claim 7. The method consists in arranging, in at least one pack of resistive components of the resistor, at least one reference resistive component which is suitable to react thermally in the same manner as the other resistive components of the pack when it is crossed by the same current that flows through the pack of resistive components. The reference resistive component is made of a different material than the other resistive components of the pack, so that in the reference resistive component the temperature coefficient of resistence that expresses the variation in resistivity with temperature is different from the coefficient of the resistive components of the pack. In this manner it is possible to detect the temperature of the resistive components of the pack simply by detecting the variations in the electrical characteristics of the reference resistive component by means of a control circuit. Conveniently, the control circuit allows to read a voltage which is a function of the temperature alone and is not affected by the current that circulates in the resistive components, by virtue of the presence of an inductor, a capacitor and a blocking diode in the circuit. Moreover, the control circuit is self-powered, so as to be extremely simple and reliable. In practice it has been observed that the device according to the present invention is particularly advantageous, in that it allows to know at all times the value of the temperature of the plurality of resistive components and therefore to prevent them from reaching or exceeding a preset temperature which would cause them to possibly bum out. This temperature detection method, in addition to being extremely simple and functional, is fully immune to external or random influences. The invention thus conceived is susceptible of numerous modifications and variations, within the scope of the appended claims. In practice, the materials used, as well as the dimensions, may be any according to requirements and to the state of the art. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each feature identified by way of example by such reference signs.

Claims: A device (1) for protecting a braking resistor for locomotives supplied with AC power obtained by chopping the DC voltage from the power grid, characterized in that it comprises at least one pack of resistive components (3) of the resistor which is connected in series with at least one reference resistive component (4) which is made of a different material than the other resistive components (3) of the pack but is suitable to react thermally in the same manner as the other resistive components (3) of the pack when it is crossed by the same current that flows through the pack of resistive components, so that in the reference resistive component (4) the temperature coefficient of resistance is different from the coefficient of said resistive components (3) of the pack, said device further comprising measure mend and setting resistors (RA, RA1, RB, RB1) connected across the series combination of the resistive components (3) and the reference resistive component(s) (4) and a control terminal (D) arranged between them and a control circuit (5) which is associated with said pack of resistive components (3) and is suitable to detect their temperature by detecting the variations in the electrical characteristics of said reference resistive component (4), said control circuit (5) having a first means (6) and a second means (7) for smoothing the voltage between the control point (D) and the junction (B) between the reference resistive component (4) and the at least one pack resistive components (3) and a third means (8) for determining a discharging time which is longer than the charging time of said second means (7), so as to have a voltage which is a function of the temperature alone and is not affected by the current that circulates through said resistive components (3). The device (1) according to claim 1, characterized in that said control circuit (5) is self-powered. The device (1) according to one or more of the preceding claims, characterized in that said first voltage smoothing means (6) comprises an inductor (10). The device (1) according to one or more of the preceding claims, characterized in that said second voltage smoothing means (7) comprises a capacitor (11). The device (1) according to one or more of the preceding claims, characterized in that said third means (8) comprises a blocking diode (12). The device (1) according to one or more of the preceding claims; characterized in that it comprises at least one control coil (14) for checking its operating efficiency before using it. A method for protecting a braking resistor for locomotives powered with AC current obtained by chopping the DC voltage of the power grid, characterized in that it consists in: connecting in series at least one pack of resistive components (3) of the resistor with at least one reference resistive component (4) which is suitable to react thermally in the same manner as the other resistive components (3) of the pack when it is crossed by the same current that flows through the pack of resistive components (3), said reference resistive component (4) being made of a different material than the other resistive components (3) of the pack, so that in said reference resistive component (4) the temperature coefficient of resistance is different from the coefficient of the resistive components (3) of the pack; detecting the temperature of said resistive components (3) of the pack by detecting the variations in the electrical characteristics of the reference resistive component (4) by means of a control circuit (5) which has a first means (6) and a second means (7) for smoothing the voltage between a control point (D) arranged between measurement and setting resistors (RA, RA1, RB, RB1) and a junction (B) between the reference resistive component (4) and the at least one pack of resistive components (3), whereby the measurement and setting resistors (RA, RA1, RB, RB1) are connected across the series combination of the resistive components (3) and the reference resistive component(s) (4), and a third means (8) for determining a discharging time which is longer than the charging time of said second means, so as to have a voltage which is a function of the temperature alone and is not affected by the current that circulates in said resistive components (3). The method according to claim 7, characterized in that when a preset temperature is reached, the power supply of the resistor with a chopped constant voltage is disconnected. The method according to one or more of the preceding claims, characterized in that said control circuit (5) is self-powered. The method according to one or more of the preceding claims, characterized in that said first voltage smoothing means (6) comprises an inductor (10). The method according to one or more of the preceding claims, characterized in that said second voltage leveling smoothing means (7) comprises a capacitor (11). The method according to one or more of the preceding claims, characterized in that said third means (8) comprises a blocking diode (12).

Inventors: FORNARI MANUELA, FORNARI, MANUELA
Applicants: FORNARI MANUELA, FORNARI, MANUELA
Application year: 1999
Main classification: B60L3
IPC codes: H02H7, B60L3