GB2026706A

GB2026706A - Current Sensor with Switchable Range-changing Resistor

Info

Publication number: GB2026706A
Application number: GB7925812A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1978-07-27
Filing date: 1979-07-24
Publication date: 1980-02-06
Also published as: DE2833203A1; SE7906328L; FR2432202A1; BE877942A; JPS5519898A; NL7905790A

Abstract

A current sensor comprises an iron-cored transformer whose primary winding (1) is traversed by a current (J) to be sensed, and a secondary winding (3) which is connected across a shunt resistor element (6) provided with selective switching means that is switched to present a resistance value which is such that at currents having differing nominal values, an equal voltage (U) is produced across the resistor element (6). The apparatus can be used for sensing large currents and providing an output to protection or measuring apparatus 12. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Electrical Current Measuring Apparatus The invention relates to electrical current measuring apparatus in which currents of relatively high value in a primary winding of a transformer is fed from a secondary winding inductively linked to said primary winding via a common soft iron core to a measuring instrument or a mains protection arrangement. The ratio of the number of turns on the primary winding relative to the secondary winding is normally determined by reference to the magnitude of the current that is to be measured, and is based on the requirement that, for a given primary current, a predetermined current, for example a current of approximately 1A should flow in the secondary winding.

In order to require as few as possible different types of current transformers, whilst being able to deal with primary currents of different nominal values, i.e. within different limited ranges, and in order to be able to adapt to different circumstances in the operation of a current transformer, it is known to design such current transformers for a plurality of measuring ranges.

In these known designs the switch-over between ranges can be effected either on the primary side or on the secondary side of the current transformer.

Known current transformers with primary switch-over facilities are described, for example, by R. Bauer in a book entitled "Die Messwandler", published in 1953, see pages 78 to 82. This relates to a transformer in which a primary winding consists of a plurality of primary winding sections, which can be selectively connected together in series combinations or in parallel combinations in order to achieve a particular desired transformation ratio for a given nominal primary current.

Known current transformers which can be switched over on the secondary side are described by R. Bauer in a book entitled "Die Messwandler" published in 1952, see pages 82 to 83. This relates to switch-over being effected.

by selected arrangements of the secondary winding in order to modify the transformation ratio in accordance with the relevant nominal primary current.

In known current transformers having switchover facilities, if the outlay is to be economically viable, it is necessary to restrict the arrangement to a relatively small number of different transformation ratios, and thus only provide means to measure currents in a relatively few nominal primary current ranges.

The German Patent Specification No.

2,339,756, describes a current measuring device comprising a transformer with a resistor arranged on the secondary side, the transformer secondary being directly connected to an operational amplifier with a negative feedback path that can be modified by means of a switching device in order to measure currents for different nominal primary current ranges.

One object of the present invention is to provide electrical current measuring apparatus which can be switched over to a relatively large number of nominal value ranges of the current to be measured, but can be produced at lower cost.

The invention consists in an electrical current measuring apparatus comprising an iron-cored transformer having a primary winding which is traversed by the current which is to be measured and a secondary winding, a shunt resistor element connected across the secondary winding and having selective switching means to adjust the value of resistance that is to be presented in accordance with the nominal value of the range of current values to be measured, each selected resistance value being such that an equal voltage of relatively low value appears across the resistor element when the respective nominal current value is flowing, when operating.

The advantage of apparatus constructed in accordance with the invention is that in order to gain a quantity which corresponds to differing nominal values of currents which are to be measured, it is unnecessary to carry out any switch-over directly involving the primary winding or the secondary winding to adjust the turns ratio, and it is merely necessary to switch the resistor configuration accordingly. No significant current errors or phase angle errors occur, as the voltage drop across the resistor is relatively small, i.e. in the order of approximately 100 mV. No elaborate amplifier is required for connection to the resistor, especially where any protection device is to be fed with the resultant output voltage, because it is now the case that to an increasing extent there devices are being designed as electronic circuit arrangements requiring only very small input signal levels.A further advantage of measuring apparatus constructed in accordance with the invention is that it requires no operational amplifier, comprises a relatively small number of components, and is therefore particularly reliable in its functioning.

The invention will now be described with reference to the drawing, which schematically illustrates one exemplary embodiment of a measuring apparatus constructed in accordance with the invention.

The illustrated measuring apparatus comprises a primary winding 1 which is traversed by a current J which is to be measured. The primary winding 1 is conventionally wound upon an iron core 2 which also carries a secondary winding 3 in conventional manner. A shunt resistor element 6 is connected between ends 4 and 5 of the secondary winding 3.

The resistor element 6 is provided with tappings 7, 8 and 9 to which a "mobile" contact element 10 of a switch device schematically illustrated switching unit can be applied. A connecting line 11 links the switch contact element 10 directly to one end 4 of the secondary winding 3. The ends 4 and 5 of the secondary winding 3 are also connected to a load device 12, for example an electronic mains protection device, which is thus supplied with a voltage U which is developed across the resistor element 6 by the current that is to be measured, it being normal to assume that the load device will have an input impedance value that is high relative to the value of the resistor element 6.

The tappings 7 and 9 of the resistor element 6 allow the measuring apparatus to be switched over selectively for different ranges of currents which are to be measured so that in each case, for a current having the relevant nominal current value, the voltage U that is formed is in each case equal in value, and thus corresponds to a standard voltage value. This voltage value is selected in respect of the electronic components contained in the device 12, for example the electronic mains protection device or a measuring instrument, of the display type or recording type, and a typical voltage value is 100 mV.

The apparatus can be produced at favourable cost for measuring currents in widely differing nominal value ranges.

The resistor element 6 of the measuring apparatus can be designed in various ways.

Advantageously, it may consist of a series arrangement of resistances, or of a resistor provided with intermediate tappings, its overall resistance being designed for the smallest nominal values of the current to be measured.

This type of design of the resistor element 6 ensures that even at currents having low nominal values, a voltage equal to a standard value is produced for the currents to be measured in each of the various nominal value ranges, this voltage appearing across the resistor element 6. If a switch-over is to be effected with regard to measuring currents in a higher nominal value range, then the active resistance presented by the resistor element 6 is reduced by selectively connecting the end 4 of the secondary winding 3 to the corresponding tapping, or to the corresponding lunction in the series resistor arrangement. On account of the smaller active resistance now presented, the higher secondary current then produces a voltage equal to the required voltage U. The advantage of this design of the resistor element 6 consists in that it can be switched over by means of a switch device of relatively simple design, which may be an electronic circuit.

However, it is also possible to construct the resistor element 6 from a plurality of resistances that can be selectively connected in parallel, the overall resistance being chosen with respect to the maximum nominal value range of the current to be measured. Then individual ones of the resistors connected in parallel can be separated, in order to measure currents in lower nominal value ranges by presenting a higher effective resistance, so that here again it is ensured that in the varying nominal value ranges the current which is to be measured in each case produces a substantially equal voltage U across the resistor device 6.

Claims

1. An electrical current measuring apparatus comprising an iron-cored transformer having a primary winding which is traversed by the current which is to be measured and a secondary winding, a shunt resistor element connected across the secondary winding and having selective switching means to adjust the value of resistance that is to be presented in accordance with the nominal value of the range of current values to be measured, each selected resistance value being such that an equal voltage of relatively low value appears across the resistor element when the respective nominal current value is flowing, when operating.

2. A measuring apparatus as claimed in Claim 1, in which said resistor element consists of a series arrangement of resistances or a resistor provided with intermediate tappings, and that the overall maximum resistor value is designed for the lowest nominal value of the current ranges to be measured.

3. A measuring apparatus as claimed in Claim 1, in which the resistor element consists of a plurality of resistances which can be selectively connected in parallel, and the minimum overall resistance value is selected for the highest nominal value of the current which is to be measured.

4. An electrical current measuring apparatus substantially as described with reference to the drawing.