GB2030373A - Current Transformers - Google Patents

Abstract

A current transformer for an electrical measuring instrument comprises an iron or ferrite core 1, 2 having two symmetrically disposed air gaps (3) and two secondary windings (4) which are associated with the air gaps (3) and are so connected that voltages induced in them add together. A primary winding 7 is placed symmetrically relative to the air gaps (3). Extraneous field errors thus tend to be cancelled out. <IMAGE>

Description

SPECIFICATION Current Transformer for an Electrical Measuring Instrument The invention relates to a current transformer for an electric measuring instrument.

A current transformer which employs an iron or ferrite core having a large air gap has been disclosed in German Offenlegungsschrift 26 27 031. This known current transformer has the advantage that it is not only suitable for a large measuring range, but is also of simple construction and therefore cheap to produce. The large measuring range is achieved by the linearisation of the magnetisation curve owing to the large air gap. Also, due to the large air gap, the primary and secondary windings can readily be pushed on to the core. However, this known current transformer has the disadvantage that it has a relatively high extraneous field error due to non-symmetrical construction.

The present invention aims to provide an improved current transformer.

According to the present invention, there is provided a current transformer for an electrical measuring instrument, the transformer comprising an iron or ferrite core having two symmetrically disposed air gaps and two secondary windings which are associated respectively with the air gaps and which are so connected that voltages induced therein add together Advantageously, a primary winding is also distributed on the core symmetrically in relation to the air gaps.

An embodiment of the invention will now be more particularly described by way of example, with reference to the accompanying diagrammatic drawing, which illustrates a current transformer for an electrical measuring instrument.

The illustrated transformer comprises two Ushaped core parts 1 and 2, of which legs 1 a and 2a are directed towards one another and are separated from one another by non-magnetic shims for producing two symmetrical working air gaps 3. The two secondary windings 4 are disposed around the legs 1 a and 2a symmetrically in relation to the air gaps 3. They are so connected that the voltages induced in them add together. There are denoted by 5 and 6 corresponding terminal connecting members. A primary winding 7 with two terminal connecting members 8 and 9 is mounted on the yoke of the core part 1, symmetrically in relation to the air gaps 3.

When an extraneous magnetic filed passes through the two core parts 1 and 2, the voltages induced in the secondary windings 4 by this field tend to cancel one another out. In this way, the extraneous field error can be kept optimally low.

It may be appreciated that the illustrated embodiment may be modified to provide a plurality of pairs of symmetrical air gaps such as 3 with respective secondary windings, such that extraneous field errors tend to be cancelled out.

Claims

1. A current transformer for an electrical measuring instrument, the transformer comprising an iron or ferrite core having two symmetrically disposed air gaps and two secondary windings which are associated respectively with the air gaps and which are so connected that voltages induced therein add together.

2. A current transformer according to Claim 1, having a primary winding which is distributed on the core symmetrically in relation to the air gaps.

3. A current transformer substantially as hereinbefore described with reference to the accompanying drawing.

4. An electrical measuring instrument having a current transformer according to any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Current Transformer for an Electrical Measuring Instrument The invention relates to a current transformer for an electric measuring instrument. A current transformer which employs an iron or ferrite core having a large air gap has been disclosed in German Offenlegungsschrift 26 27 031. This known current transformer has the advantage that it is not only suitable for a large measuring range, but is also of simple construction and therefore cheap to produce. The large measuring range is achieved by the linearisation of the magnetisation curve owing to the large air gap. Also, due to the large air gap, the primary and secondary windings can readily be pushed on to the core. However, this known current transformer has the disadvantage that it has a relatively high extraneous field error due to non-symmetrical construction. The present invention aims to provide an improved current transformer. According to the present invention, there is provided a current transformer for an electrical measuring instrument, the transformer comprising an iron or ferrite core having two symmetrically disposed air gaps and two secondary windings which are associated respectively with the air gaps and which are so connected that voltages induced therein add together Advantageously, a primary winding is also distributed on the core symmetrically in relation to the air gaps. An embodiment of the invention will now be more particularly described by way of example, with reference to the accompanying diagrammatic drawing, which illustrates a current transformer for an electrical measuring instrument. The illustrated transformer comprises two Ushaped core parts 1 and 2, of which legs 1 a and 2a are directed towards one another and are separated from one another by non-magnetic shims for producing two symmetrical working air gaps 3. The two secondary windings 4 are disposed around the legs 1 a and 2a symmetrically in relation to the air gaps 3. They are so connected that the voltages induced in them add together. There are denoted by 5 and 6 corresponding terminal connecting members. A primary winding 7 with two terminal connecting members 8 and 9 is mounted on the yoke of the core part 1, symmetrically in relation to the air gaps 3. When an extraneous magnetic filed passes through the two core parts 1 and 2, the voltages induced in the secondary windings 4 by this field tend to cancel one another out. In this way, the extraneous field error can be kept optimally low. It may be appreciated that the illustrated embodiment may be modified to provide a plurality of pairs of symmetrical air gaps such as 3 with respective secondary windings, such that extraneous field errors tend to be cancelled out. Claims

1. A current transformer for an electrical measuring instrument, the transformer comprising an iron or ferrite core having two symmetrically disposed air gaps and two secondary windings which are associated respectively with the air gaps and which are so connected that voltages induced therein add together.

2. A current transformer according to Claim 1, having a primary winding which is distributed on the core symmetrically in relation to the air gaps.

3. A current transformer substantially as hereinbefore described with reference to the accompanying drawing.

4. An electrical measuring instrument having a current transformer according to any preceding claim.