This invention is concerned with a current transducer for measuring variable currents, particularly A.C. currents.

When measuring currents, it is often necessary to match the value of current being measured with the dynamic range of the measuring instrument; e.g., a high-strength current must be converted to a proportional, weaker current, which is compatible with the measuring ammeter. Several types of current transducers are known to be used for such purpose, among which the most usual are shunts, amperometric transformers and Rogowski coils.

The shunt is the simplest and cheapest device for proportionally reducing a current. It consists of a small resistance in series with a main circuit, intended to derive a voltage signal proportional to the load current flowing through it.

A drawback of the shunt is that its measuring circuit is not insulated from the power circuit: this circumstance makes it unfit for heavy overloads. Other drawbacks are due to the fact that the resistance of the shunt is typically affected by a parasite series inductance and by a parasite parallel capacity. Accordingly, at high frequencies, the parasite inductance will displace the phase of voltage and current, while, at high frequencies, the parasite inductance and capacity together may give rise to resonances. Moreover, high values of the currents will cause large dissipation and consequently heating, which in turn will give rise to variations of the resistance R. This resistance is also affected by the parasite resistances at the connection terminals between cables and shunts, which are unstable and random.

When measuring A.C. currents, certain drawbacks of the shunts may be avoided by using amperometric transformers, which may be used either for purposes of mere protection or with the purpose of converting the quantity to be measured, although some of them have both functions. In the real transformer, the primary and secondary currents are not exactly linked by the nominal transformation ratio: the relationships of the ideal transformer are approximations of the real ones. The differences between the indications that would be delivered by an ideal transformer and the actual indications of the real transfomer are called errors, and may be phase errors as well as value errors.

There are also losses due to the resistance of the copper coils, to the Joule effect, to stray flux caused by failure of a portion of the flux from the primary winding to link with the secondary winding, to the capacitive coupling between adjacent loops and, for transformers with non- ari- wound cores, to the passage of the A.C. magnetic flux through the core and to the hysteresis loop of the core. The Rogowski coil typically consists of an air-cored coil which is wound in a toroidal shape around a conductor in which it is desired to measure a current. The measurement is based on Ampere's law and is made by integrating the voltage across the coil in order to obtain as an output a signal that is substantially proportional to the current to be measured. Even the Rogowski coil is not free from drawbacks, among which is saturation, which may develop when the coil voltage is too high, and slew-rate. Moreover, at high frequencies the self-inductance and the self-capacity of the coil become significant and consequently there may be problems with the phase and the value, and even a resonance.

The main object of this invention is now to obviate the above drawbacks, by providing a current transducer that will reduce the reading errors with respect to the prior art.

Within the above general object, the invention aims at providing a current transducer that is particularly suitable for measuring strong currents, such as the currents flowing in railway overhead wires or train rails.

Another object of the invention is to provide a current transducer that does not interfere with the conductor in which the measurement is to be carriedc out, and that does not require modifications to the conductor.

A further object of the invention is to dispense with ferromagnetic cores and with the attendant problems due to saturation, to stray currents and to hysteresis loops.

The invention has the further object of avoiding the use of active parts, such as integrators and external supplies.

Still another object of the invention is to provide a measuring instrument the is compact and lightweight, highly reliable, and easy to implement at competitive costs.

The above objects, as well as other objects and advantages such as will appear from the disclosure below, are attained by a current transducer for measuring the strength of variable electric currents in conductors, particularly unifilar electric lines, characterized in that it comprises a tube of a conductive material, curved to substantially define a torus to be placed coaxially around a conductor where the electric current is to be measured, the tube being provided, substantially along its entire surface, of a slit lying entirely on a circumferential middle plane of the torus, the ends of said tube facing each other so that a gap is defined therebetween, said tube being provided with first electrical terminals on opposite sides of said slit, for connection to a current measuring instrument. The above objects are also attained by a current transducer for measuring the strength of variable electric currents in conductors, particularly in non-unifilar lines such as rails and the like, characterized in that it comprises a pair of tubes of identical shape, made of a conductive material and arranged side by side whereby a space is defined therebetween for a conductor where the electric current is to be measured, each tube comprising a slit extending substantially from one end to the opposite end of the respective tube, and entirely lying in a middle plane of the tube, each tube being provided with first electric terminals on opposite sides of the respective slit, for connection to a current measuring instrument.

Further features and advantages of the invention will become better apparent from the following disclosure of preferred embodiments of the current tranducer of the invention, and shown, by way of non-limiting examples, in the attached drawings, wherein:

Fig. l shows the mutual arrangement between a loop and a current conductor;

Fig. 2 shows an electric circuit equivalent to the arrangement of Fig. l;

Fig. 3 shows a current transducer according to a first embodiment of the invention; Fig. 4 shows an electric circuit equivalent to the current transducer of Fig. 3, and free from terminals shorted to each other;

Fig. 5 shows a circuit corresponding to the circuit of Fig. 4;

Fig. 6 shows the circuit of Fig. 4, wheretwo terminals of the transducer are connected to each other via an inductor; Fig. 7 shows a current transducer according to a second embodiment of the invention; Fig. 8 shows a circuit corresponding to the circuit of Fig. 7.

With reference to Fig. 1, a conductor 1 carrying an electric current variable in time generates a magnetic field 2 which links with loop 3 and induces an electromotive force therein. If the loop is closed, such electromotive force generates electric currents and an attendant magnetic field whose flux is in opposition to the variation of the magnetic flux linked with loop 3. In particular, the electromotive force V induced in loop 3 is equal to the time derivative of the magnetic flux, with changed sign.

From an electric point of view, the arrangement of Fig. 1 is equivalent to the circuit of Fig. 2, comprising a voltage generator corresponding to the electromotive force and a series impe- dance consisting of the resistance and inductance of conductor 1. According to the invention, a solid of revolution or of translation generated by a conductive loop 3 substantially as shown in Fig. ι is used as a current transducer.

In particular, in a first preferred embodiment of the invention, shown in Fig. 3, the current transducer comprises a tube 10 of a conductive material, shaped to define a torus which is to be placed coaxially around conductor n where the electric current is to be measured. Accordingly, the conductors n with which the current transducer of the first embodiment preferably are unifilar lines, i.e. all wirelike, cylindrical or prismatic conductors liable to be surrounded by a torus as in Fig. 3, without having to be disassembled from their site.

The perimeter of the torus defined by tube 10, and more specifically its external perimeter, is cut with a slit 12, entirely lying in the circumferential middle plane of the torus. Tube 10 can therefore be seen as a solid of revolution around conductor 11, having a C-shaped loop as a generatrix. The revolution, however, is not complete, as the ends of tube 10 face each other, thereby defining a gap 13 inbetween. Thanks to such a gap, tube 10 can be easily placed in toroidal shape around a wirelike conductor, which can be inserted through gap 13. First electric terminals 16 and 17 are attached close to gap 13, on opposite sides of slit 12 and suitable for connection to a conventional ammeter, without any intermediate electrical parts, such as integrators.

Another pair of second electric terminals 14 and 15 are attached to the other end of tube 10, also on opposite sides of slit 12, so that the first and second electric terminals, 14-15 and 16-17, ^ar e placed on opposite sides of gap 13.

Terminals 14 and 15 are preferably connected to each other, so that the circuit is closed. For instance, they may be short-circuit with each other by means of a wire 18, or they may be connected through an inductor 19, for reasons that are explained below. Alternately, slit 12 may be shorted at terminals 16, 17 at the end of tube 10, in this case terminals 14-15 being used for carrying out the measurement.

Tube 10 is electrically equivalent to the circuit shown on Fig. 4: in particular, tube 10 can be thought of as a discrete collection as a plurality of C-shaped loops connected in parallel. Each of the n loops corresponds electrically to a circuit comprising the series connection of a generator of electromotive force Vi, a resistor n, an inductor Li and a mutual inductor li (with i = 1, 2 , n).

Resistor r _; and inductore Li represent the resistance and inductanza of the i-th loop. Inductors li represent the mutual inductances that develop among the loops. Moreover, for each pair of adjacent loops there are two resistances Ry connecting the open terminals of the loops.

Lastly, between the plurality of loops and conductor 11 there must be taken into account, for each terminal 14 and 15 or 16 and 17, a corresponding pair of capacitor/resistor G-Ri, which develop together with conductor 11 where flows the current I under measurement. Ci-Ri e C2-R2, represent, in an equivalent circuit with lumped parameters, the parasite capacities and resistances of the several loops. For symmetry, capacitors Ci and C2 may be regarded as equal to each other and having the same capacity C. Similarly, resistors Ri and R2 may also be regarded as equal to each other and having the same resistance R. In the measurement of current I in conductor 11, it is possible to obtain an equivalent circuit resulting from parallel loops of the circuit of Fig. 4, where terminals 14 and 15 are shorted to each other as in Fig. 3. The equivalent circuit is shown in Fig. 5: it comprises an equivalent generator of electromotive force V-rot, an equivalent impedance Zeq , and a mutual inductor hot which defines, together with inductance I _L of conductor 11, a mutual inductance Mrot. From the following hypothesis, verified in practice,

and if the contribution of the loop-connection resistances R _¾ is neglected, because of the symmetry of the structure, the following relations are obtained:

^V Tot = - j°^ Tot = -ΐ ^ωΜ Το, ¹

2R

V _Tot - i

1 + jcoRC where i is the current in the equivalent loop of Fig. 5, J is the current under measurement in conductor 11 and Φτ _Μ is the equivalent flux linked with the equivalent loop of Fig. 5.

If one considers that the product RC is in practice much larger than is, or can be made so either by inserting dielectric material between conductor 11 and toroidal tube 10, or by increasing the surface of tube 10 facing conductor 11, or by reducing the radius of the torus defined by the tube, or by adding an external capacity between tube 10 and conductor 11, the following relation is obtained i = co ² M _Tot CI Accordingly, it can be seen that the current transducer of the invention linearly links the current I to be measured with the current that may be measured at terminals 14-15 or 16-17.

According to a modification of the first embodiment, of which an electric equivalent is shown in Fig. 6, the quadratic dependence from frequency can be removed in the foregoing relation, by connecting an inductor 19 across terminals 14-15 or 16-17, the inductor being preferably wound in air or on ferrite. In this case, and assuming that the absolute value of the equivalent impedance Z _EQ is much smaller than the product G)L ₁₉ , the foregoing relation becomes

where L ₉ is the inductance of inductor 19. In a second embodiment of the invention, shown in Fig. 7, a current transducer 20 according to the invention comprises a pair of tubes 21 and 22 of identical shape, e.g. prismatic or cylindrical, both made of a conducting material at placed side by side so that they define a space therebetween for a conductor 31 in which it is desired to measure the electric current.

Conductor 31 is preferably not of the air-wound kind, and is not surroundable by a toroidal tranducer according to Fig. 3. According to an example of a typical application of the invention, conductor 31 would be the rail of a railroad.

Tubes 21 and 22 can be regarded as solids of translation, in which the generatrix is a C- shaped loop that is displaced along the axis of the respective tube.

Each of tubes 21 and 22 comprises a slit 21a and 22a, longitudinally extending from end of the tube to the opposite end, and completely lying in the middle plane of the tube.

Moreover, each tube 21, 22 is provided, at its upper end, with first electric terminals 23, 24 and 25, 26 on opposite sides of the respective slit 21a, 22a. A pair 23-24 of the first terminals is used for direct connection to a current measuring instrument, while first terminals 25-26 belonging to the other tube are preferably shorted to each other. Moreover, each tube 21 and 22 comprises second electric terminals 27, 28 and 29, 30, which are arranged on opposite sides of the respective slit 21a and 22a, and substantially at the other end of the tube. Each electric terminal 27 and 28 of first tube 21 is short-circuited with a corresponding electric terminal 30 and 29 of the other tube 22. The connecting cables are preferably braided, in order to improve its behavior at varying frequencies. Tubes 21 and 22 are made fast to each other by means of a support 32 of an insulating material, so that their mutual distance is maintained constant and that the current tranducer 20 may be easily installed.

An equivalent circuit can be defined also for current transducer 20, as shown on Fig. 8, according to what has been taught above with reference to Fig. 4, thereby obtaining a linear relation between the current under measurement and the current read at terminals 23-24.

The current transducer of the invention may be connected directly to a conventional ammeter, without the intervention of active components or external supplies.

Although the transducer of the invention has been developed particularly for overhead lines and rails carrying heavy currents, it can be used for measuring currents in wirelike conductors more generally, whether cylindrical or prismatic.

The preferred embodiments of the invention disclosed above may be modified in many ways within the scope of the attached claims.