A DEVICE FOR INDICATING AN ALTERNATING CURRENT IN A CONDUCTOR

Abstract: An indicator for currents in e.g. a bus bar, which is adapted to be fastened directly to existing installations. The indicator consists of a coil (2) fastened to the bus bar (1) by means of a strap (4). The strap is made of a material having a high magnetic conductivity, so as to constitute the magnetic circuit in the current transformer obtained by fastening the coil to the bus bar. The terminals of the coil (2) are connected to anti-parallel diodes (6, 7) which may both be light emmiting. The iron cross section and coil are dimensioned in order that the core (the strap) is saturated by currents (I) which would otherwise overload the diodes. Alternatively groups of diodes (6', 7'...) may be series connected to different ballast resistors (8'...) in order that currents in excess of certain limits may be indicated.

Description: A device for indicating an alternating current in a conductor The invention relates to a device for the indication of a sum o/f alternating currents in one or several electrical conductors, con¬ sisting of an indicator connected to a coil with many turns which is connected to the current conductors by means of a magnetic cir¬ cuit. Such devices are known are known as current transformers which con¬ sist of a toroid of magnetically conducting material, the primary winding consisting of one or several conductors through the opening in the toroid, and the secondary winding being a toroid winding wound upon the core. From the ends of the secondary winding conductors are connected to an instrument which may be calibrated to indicate • either the actual secondary current from which the primary current may be calculated by means of the transformer ratio or else the primary current directly. Such current transformer circuits are pre¬ cision circuits, and e.g. in bus bar fields one has to build them in or disconnect the bus bars in order to install the current trans¬ former. However, such precision is not always required, and for certain appli¬ cations one may be able to make do with an indication of a current, possibly with a coarse indication of its value. A certain type of ampere meters are known which are built along the lines of a moving- iron instrument, the pole pieces of which are not magnetized by means of a coil current but which directly surround e.g. a bus bar. Equally the so-called "split electromagnet indicators" are of the type mentio¬ ned but they are usually hand-held and not suited for fastening onto a conductor. OMPI_ ^Z-Z°c .It' is the purpose of the invention to provide a current indicator of the type mentioned above, which is adapted to be installed into existing installations without the need for dismantling and which is of low price and great simplicity but which still retains sufHA cient precision for indicating purposes. This is obtained in a device which according to the invention is cha¬ racterized in that the fastening for the secondary coil is constituted by one or several straps which complete the magnetic circuit upon mounting. Through the provisions of claim 2 one obtains a simple indicator for current which reacts at currents above a certain lower limit but which is not liable to overload at large currents. Claim 3 describes the physical placement of the indicating part of the device which makes feasible the mounting of a device on each bus bar in an installation in order to obtain a simple overview of the various currents. Claim 4 provides a way of obtaining a coarse indication of the value of the current by letting the number of Light Emitting Diodes lighting up increase with the current. Claim 5 provides a manner to obtain a well defined reduction of the cross section of the magnetic circuit, this is particularly advanta¬ geous in case the current to be indicated is superimposed on a large direct current which might otherwise saturate the strap. The invention will be described in detail in the following with refe¬ rence to the figure, in which, - Fig. 1 shows an almost completed fastened device according to the invention, and Fig. 2 shows circuit diagrams corresponding to claims 2 and 4, and. Fig. 3 shows possible embodiments of straps according to claim 5. OMPI .In Fig. 1 are seen a bus bar 1 and an encapsulated coil 2, from the terminals 5 of which leads are brought to the indicating part 3. The coil 2 is fastened to the bus bar 1 by means of a strap through the opening 2 of the coil. As is shown in Fig. 1 the bent-back part ofy the strap needs only to be fixed in order to complete the fastening. For reasons of drawing the strap has been shown with a head but the invention is not limited to this embodiment. According to the inven¬ tion the strap is made of a magnetically conducting material, and thereby the current I which is the sum of the -currents in the bus bar, induces a current in the coil which may be detected by the indicating unit 3. In this manner a current transformer is obtained that may be fastened to a bus bar without the need for disassembly. The current transformer will equally react in the case of the current I being a pulsating direct current. The dimensions of the strap are determined by the sensitivity requirements, if needed several straps may be used in order to obtain a sufficient iron cross section. This may be needed when the strap is to surround a number of mutually insulated conductors, the currents of which almost cancel but whose vector sum is still nee¬ ded for indication. The indicating unit may be constructed in any way imaginable but the desirability of cost-effectiveness motivates a con¬ struction as indicated in claims 2 and 4. Fig. 2a shows the circuit diagram for an indicator according to claim 2. The diodes 6 and 7 light up for a large interval of current but the voltage across their terminals does not change appreciably. Hence the number of turns in the coil 2 is determined so that the diodes light up at a sufficiently low current in the bus bar 1. It is desired to obtain indication for low currents and a constant output above those currents irrespective of the value of the current, whereby the indica¬ tor will not be over-loaded in case of short circuit currents. This is in particular obtained by dimensioning the magnetic circuit such that it will be saturated by relatively small currents. Larger primary (bus bar) currents will entail progressively greater .turns ratio er¬ rors so that the secondary current remains almost constant. The coil is dimensioned in a well-known manner in order that the resistance of the winding acts as ballast resistor for the Light Emitting Diodes. Fig. 2b shows a circuit diagram according to claim 4 in which the ac¬ tion is not based upon saturation of the core 4. In this case ballast resistors 8', 8" are dimensioned in a well-known manner so that the diodes 6" or 7" light up for greater currents than those lighting up diodes 6' and 7'. Thus it is possible to indicate that the current is above certain well-defined limits. An embodiment of the invention according to claim 3 is obtained by substituting openings in the casing of coil 2 for the terminals 5. Through these openings the Light Emitting Diodes (e.g. 6, 7 or 6 ' , 6") may be observed. Fig. 3 shows two examples of straps according to claim 5. In instan¬ ces of large direct current components in the current I in the bus bar 1 the strap 4 may be brought into saturation whereby superposi¬ tion of the a.c. components cannot be expected. Reduction of the cross section of the straps in order to avoid this saturation is ob¬ tained according to the invention by punching or cutting material from the strap 4. -g S REX OMPI < χ

Claims: P A T E N T C L A I M S 1. A device for the indication of a sum of alternating currents in one or a plurality of electric conductors 1 , with an indicating unit 3 connected to a multiturn coil 2, which coil iέ connected to the conductors 1 by means of a magnetic circuit, characterized in that the fastening for the coil 2 to the conductors 1 is made by one or a plurality of straps 4 which constitutes the magnetic circuit upon com¬ pletion of the fastening operation. 2. A device for the indication of an electric current according to claim 1 , characterized in that the indicating unit 3 comprises anti- parallel diodes 6, 7 of which at leat one is a Light Emitting Diode. 3. A device for the indication of an electric current according to claim 2, characterized in that the indicating unit 3 is mounted on the coil 2. 4. A device for the indication of an electric current according to claim 1 , characterized in that the indicating unit 3 comprises dif¬ ferent ballast resistors 8 1 , 8", each in series with anti-parallel diodes 6', 7 1 , 6", 7", of which at least one in each group is a Light Emitting Diode, the ballast resistors 8', 8" being dimensioned in or¬ der that the corresponding diodes indicate different currents. 5. A device for the indication of an electric current according to claim 1 , characterized in that the strap or straps 4 have holes 9 or other cross section reducing cuts or punches 10.

Inventors: LYNGSIE O, NORHOLM O
Applicants: KNUDSEN NORDISK ELECT, LYNGSIE O, NORHOLM O, KNUDSEN NORDISK ELEKT AS
Application year: 1980
Main classification: H01F40
IPC codes: H01F3, H01F30, H01F38