METHOD OF MEASURING A DC CURENT OR A LOW-FREQUENCY AC CURRENT

Abstract: A method of measuring a DC current or a low-frequency AC current with the aid of a transformer having a first and a second winding (2 and 3), where the currrent is intended to be supplied to the first winding (2). In accordance with the invention, a modulating voltage from a modulating voltage source (5) is applied to the second winding (3) via a resistor (4). The separation in phase between the modulating voltage and the voltage across the second winding (3) is measured in two consecutive zero passes with the aid of a phase comparator (6). The latter is adapted to a generate control signal in response to the difference between these two separations in phase. This control signal is caused to control a compensating current generator (7) adapted to feed a compensating current to the second winding (3) for compensating the flux in the transformer core (1) caused by the current in the first winding (2) until said difference is equal to zero. The compensating current then constitutes a copy of the current in the first winding (2) transformed over with the transforming ratio of the transformer.

Description: METHODOFMEASURINGADCCURENTORALOW-FREQUENCYACCURRENT TECHNICAL FIELD The present invention relates to a method of transforming a DC current or a low-frequency AC current with the aid of a transformer having a first and a second winding where the current is supplied to the first winding, and an apparatus for carrying out the method. BACKGROUND ART In applications such as safety systems for railway signals controlled by computerised switchgear, there is a need of galvanically insulated measurement of the effective value of the lamp currents. Known solutions for enabling this with the aid of a transformer are both complicated and expensive, however. DISCLOSURE OF INVENTION The object of the present invention consists in providing a simple and cheap method of transforming a DC current or a low-frequency AC current with the aid of a transformer, and an apparatus for carrying out the method. This object is achieved in accordance with the invention by the method and apparatus being given the characterising features disclosed in the claims. DESCRIPTION OF FIGURE The invention will be described below with reference to the accompany¬ ing drawing, the sole figure on which illustrates an embodiment of an apparatus in accordance with the invention. OMH_ PREFERRED EMBODIMENT An embodiment of an apparatus in accordance with the invention is il¬ lustrated in the Figure, and is intended for transforming a DC or low-frequency AC current with the aid of a transformer, the core 1 of which is assumed to be a toroidally shaped ferπ ' te core in the il- lustrated embodiment, through the central hole of whichthere 1S ^ n ~ serted the primary winding in the shape of a wire 2, this core carry¬ ing a secondary winding 3 having a predetermined number of turns for obtaining a pretermined transformation ratio in the transformer. The DC or low-frequency AC current which is to be transformed is ap- plied to the transformator primary winding 2 in a manner not more closely illustrated. Amodulat.fng voltage source 5 is connected, across the secondary winding 3 of the transformer, in series with a phase-shifting network, which is a resistor 4 in the illustrated embodiment. Via the resistor 4 the sour- ce 5 is adapted for applying a modulating voltage across the secondary winding 3 with an amplitude such that the transformator core 1 is satu¬ rated. By "low-frequency current" is intended in this context a current having a frequency amounting to at most about 1/10 of that of the mo¬ dulating voltage source 5. This in turn amounts to between 5 and 10 kHz fore a core diameter of about 8 mm. The current through the primary winding 2 causes a magnetic flux in the transformer core 1, and according to the invention this flux is compensated by a current being supplied to the secondary winding 3 until the flux in the core 1 becomes equal to zero, the compensating current through the secondary winding 3 then constituting a copy of the current in the primary winding 2 transformed over at the trans¬ forming ratio of the transformer. In accordance with the invention, the compensating current is achieved in the following manner: OMPΪ A phase comparator 6 has both its inputs connected to the connection point for the modulating voltage source 5 and the resistor 4, and also to the connection point for the resistor 4 and the transformer secon¬ dary winding 3, the. comparator being such as to measure at two conse- cutive zero passages the separation in phase between the voltage of the modulating voltage source 5 and the voltage across the transformer secondary winding 3.The comparator 6 is further adapted to generate a control signal in response to the difference between both these mea¬ sured separations in phase. This control signal, which is thus propor- tional to the difference between the separation phase in two consecu¬ tive zero passages, is applied to a controlled compensating current generator (7), which has its output connected to the connection point for the transformer secondary winding 3 and the resistor 4. The compensating generator 7 is adapted for feeding a current, in res- ponse to the mentioned control signal, for compensating the flux in the core 1 caused by the current in the primary winding 2, until the difference between two consecutive, measured separations in phase is zero. When such is the case, the current from the compensating current generator 7 constitutes a copy, transformed over with the transforming ratio, of the current in the primary winding 2. This current may then be measured in a manner known per se for obtaining measure of the cur¬ rent in the primary winding 2. A DC or a low-frequency AC current may thus be transformed in a simple manner, with the aid of the apparatus in accordance with the invention.

Claims: CLAIM Method of measuring a DC current or a low-frequency AC current with the aid of a transformer having a first and a second winding (2 and 3), where the current is supplied to the first winding (2 ) , and where for compensating the flux in the transformer core (1) caused by the current in the first winding, there is supplied a compensating current to the second winding (3) having a value corresponding to the trans¬ formed value of the current in the first winding, characterized in that a modulating voltage is applied to the second winding (3) via phase- shifting"network (4), the separation in phase between the modulating voltage and the vol¬ tage across the second winding (3) of the transformer is measured at two consecutive zero passages, the deviation between both these separations in phase is measured, a signal corresponding to the deviation is generated, and a compen¬ sating current generator (7) is controlled with the aid of this sig-. nal, the current from the generator being fed to the second winding(3) of the transformer, and in that the deviation between the separations in phase is once again measured, and the process is repeated with continuous addition to the previous deviation values until the deviation becomes zero, the cur¬ rent value from the compensating current generator (7) then giving the measured current. OMH

Inventors: HAGELIN FRANK HELGE, HAGELIN, FRANK, HELGE
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1983
Main classification: G01R19
IPC codes: G01R17