GB1581415A - Transformer provided with an electrostatic screen between its primary and secondary windings - Google Patents

Description

(54) A TRANSFORMER PROVIDED WITH AN ELECTROSTATIC SCREEN BE TWEEN ITS PRIMARY AND SECONDARY WINDINGS (71) We, OREGA "ELECTRONI QUE ET MECANIQUE", a French Body Corporate, of 74, rue du Surmelin, 75020, PARIS, FRANCE, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to transformers provided with electrostatic screens arranged between their primary and secondary windings, which screens make it possible to prevent interference signals of a higher frequency than that of the supply energy from being transmitted by capacitive coupling.

A transformer for supplying power to a piece of electrical or electronic apparatus contains a magnetic core, which is generally laminated, and various windings, which are isolated from one another and of which those connected to the mains or other AC sources supplying energy are called "primary" windings and those connected to load circuits (rectifiers for example) or loads (motors, relays, filaments, resistors, indicator lamps etc) which consume the energy supplied are called "secondary" windings.

In certain cases, for example where the electrical circuit (a motor) or electronic load circuit produces high currents or voltages of high frequency (a chopping power supply, a horizontal scan generator, an inverter for induction heating) a proportion of which may be transmitted to the input supply, or again where it contains high-gain amplifiers which are sensitive to interference signals originating from the input supply (audiofrequency amplifiers for hi-fi and biological measurement devices such as electrocardiographs or electroencephalographs) it is necessary to insert electrostatic screens, which are generally connected to a reference potential. between the primary and secondary windings and sometimes even between some of the secondary windings.

The reference potential is generally formed by the earth of the magnetic core of the transformer, which is generally connected in turn to the earth of the frame or chassis supporting the circuit.

In the case of superimposed windings, which are formed concentrically (co-axially) around one limb of the magnetic core, a screen of this nature may, for example, be formed from a thin sheet or strip of conductive metal (copper. aluminium, silver) of which the ends are not in electrical contact and which is inserted between the primary and secondary windings, or from a layer of wire of which one end is electrically connected to earth (the magnetic core) and of which the other end remains free.

Furthermore, a French patent application no. 74.31566 which was filed on 18th September 1974 in the name of the company styled Expert Maschinenbau GmbH and which was published on 18th April 1975 as no. 2245061, discloses a transformer comprising primary windings having a large number of turns and secondary windings having only a single turn, which windings are disposed alternately along one limb of the magnetic core with. arranged between each adjoining pair of primary and secondary windings, a screen in the form of an annular loop which is made of a conductive non-magnetisable, so-called earthing, material and which has a transverse division to prevent a current from being induced by the alternating magnetic field in the core.

The present invention has as an object a new kind of electromagnetic screen for a transformer having primary and secondary windings disposed side by side along one of the limbs of a magnetic core.

In accordance with the invention, a transformer has a magnetic core and an electrostatic screen between two windings which are disposed side by side around a limb of the core, wherein said electrostatic screen is in the form of a flat open ended spiral winding of insulated conductive wire which is connected at one end to a point on the transformer representing a reference potential.

The invention will now be further described with reference to the accompany drawings, which are given by way of example and in which: Figure 1 is an elevation view in crosssection of one embodiment of a transformer provided with an electrostatic screen according to the invention.

Figure 2 is a longitudinal section (on line A-A in Figure 1) through such a transformer, and Figure 3 is a partially broken away perspective view showing the complete item including a magnetic core and a former fitted with a screen according to the inven tion.

In Figure 1, which is an elevational cross-section of one embodiment of a transformer 1 according to the invention, reference numeral 2 identifies a laminated magnetic core having three limbs which is made up of stacked laminations 20 of ferromagnetic material and of which the central limb 23 (Figure 3) is surrounded by the tubular part 30 of a former 3 made of a rigid insulating material which is to receive the windings constituting the primary winding 4 and the secondary winding 5.

The transformer 1 according to the invention is of the kind in which the various windings 4 and 5 are arranged side by side along one of the limbs (central limb 23) of the magnetic core 2.

For this purpose. the insulating former 3 has separating partitions in the form of annular flanges or collars 31. 32 situated between the mutual adjacent ends of the windings. which they thereby define. These partitions are formed by thin. flat, insulating walls which are orientated perpendicularly to the geometrical axis of the former 3. to which they are secured. The mutually remote ends of the windings 4. 5 may likewise be defined by similar annular flanges 34, 35.

which may or may not be secured to the former 3 and which are intended to hold the windings 4. 5 in position and to isolate them from the laminated magnetic core 2. which is conductive and connected to the earth of the chassis or frame which carries it. The isolation of the outer ends may however be achieved by some other means.

In the transformer l. where it is a question of inserting between the mutually adjacent ends of the windings 4 and 5 a conductive electrostatic screen to reduce or eliminate the capacitive coupling between the windings, the insulating former 3 includes the parallel separating partitions 31 and 32 which are secured to the former and which are spaced apart by a sufficient distance to allow an electrostatic screen 6 of a novel kind to be inserted between them.

ln accordance with the invention, this electrostatic screen is formed in the present case by a winding 6 in the form of a flat spiral of an insulated (lacquered for example) conductive wire of which the successive turns enclose the previous turns and form only a single layer of substantially concentric contiguous turns which is orientated perpendicularly to the geometrical axis of the tubular part 30 of the former 3, that is to say parallel to the partitions 31, 32 which define the spiral 6 at top and bottom.

The inner end of the flat spiral 6 forming the screen comprises a bare length of wire 60 which is inserted into and held in place in the hollow tubular part 30 of the former 3 through an opening 33 which is made in one of the side-walls of this tubular part 3, thus coming into intimate electrical contact with the central limb of one of the outer laminations 21 of the laminated magnetic core 2 when the latter is assembled, after the windings 4, 5 and 6 have been wound onto the former; in order that the screen so produced may be connected to earth.

The outer end 61 of the flat spiral 6, which must be electrically free (unconnected) if there is to be no induced current, must be fixed in relation to the former, for example by means of a quick-drying adhesive, so that the spiral 6 cannot unwind. To fix the outer end 61 in position it is also possible to use a wedge made of an insulating material to wedge it in place or, when the outer turn of the spiral 6 comes flush with the edge of the partitions 31, 32, to use an insulating clip (of U-shape) which straddles the partitions and enables the end 61 to be held in place.

The wire which is used for the flat spiral is preferably a lacquered copper wire whose outside diameter is of the order of a few tenths of a millimeter up to several millimeters, and the distance between the two parallel partitions 31, 32 must be at least equal to this diameter if the former 3 and the partitions 31, 32 are made of a rigid material, as is generally the case.

The outside dimensions of the screen formed by the flat spiral 6 are preferably at least equal to those of whichever of the windings 4 and 5 has the larger dimensions, so that the electrical fields (lines of force) are substantially only set up between this winding 4 or 5 and the screen 6.

In Figure 3, which is a partially broken away perspective view of the transformer 1 without the windings 4 and 5 of Figure 1, the magnetic-core 2 is formed in the conventional fashion by an E-shaped assembly of laminations 21 which is closed by l-shaped laminations 22. The central limbs of the E-shaped laminations 21 are inserted in an alternate fashion into the tubular part 30 of the former 3 i.e. adjoining, E-shaped laminations 21 are inserted into the part 30 from opposite sides. The laminations 21, 22 are provided with holes 24 which enable them to be fixed together by bolts (not shown) to prevent vibration, and enable attachment lugs to be added which form a mechanical and electrical connection to the supporting chassis (not shown). Laminations of other shapes (F,U,E-E) may equally well be used.

An electrostatic screen of this kind is principally applicable to supply transformers whose primary windings are fed from a 50 or 6()Hz mains supply or from a lowfrequency AC source of 50 to 1,000Hz, for example, such as convertors of the static invertor or motor-alternator kind. Supply transformers of the present inexpensive kind may advantageously be mass produced in large numbers, such as those intended to supply television receivers for example.

Supply transformers employing electrostatic screens 6 in the form of a flat spiral may be single phase, as in the foregoing, or threephase, in which case the three limbs of the magnetic core 2 are all provided with identical formers 3, windings 4, 5 and screens 6. In the case of a single phase transformer, the magnetic core 2 may equally well have two limbs if U and I - shaped laminations are used for example.

It should be noted that it is also possible for example to connect the outer end of the electrostatic screen in the form of a flat spiral 6 to earth rather than its inner end 6().

Moreover, in cases where a transformer has more. than two windings disposed side by side around the same limb of the magnetic core, an electrostatic screen of the type herein disclosed may be arranged between each pair of windings.

WHAT WE CLAIM IS: 1. A transformer having a magnetic core and an electrostatic screen between two windings which are disposed side by side around a limb of the core. wherein said electrostatic screen is in the form of a flat open ended spiral winding of insulated conductive wire which is connected at one end to a point on the transformer representing a reference potential.

2. A transformer as claimed in claim 1, wherein the windings are wound on an insulating former having flanges defining the zone for each winding and wherein the electrostatic screen is wound in a space between two adjacent flanges defining the adjacent ends of said two side by side windings.

3. A transformer as claimed in claim 2, wherein the space between the two adjacent flanges is dimensioned to receive a single layer flat spiral winding forming the electrostatic screen.

4. A transformer as claimed in any preceding claim wherein one end of said spiral winding forming the electrostatic screen is connected to said magnetic core.

5. A transformer as claimed in any preceding claim wherein the magnetic core is a laminated core.

6. A transformer as claimed in any preceding claim, comprising more than two windings disposed side by side and an electrostatic screen between each pair of windings.

7. A transformer intended in particular to supply electrical and/or electronic circuits with alternating voltages and currents, comprising a laminated magnetic core formed by a stack of conductive ferromagnetic laminations, at least one insulating former comprising a tubular part the inside of which is intended to receive one of the limbs of the magnetic core and the outside of which is intended to carry at least two windings which are arranged side by side along it, and means for insulating and separating these windings from one another electrically, which means are secured to the former and are formed by two walls in the form of flanges which are orientated perpcndicularly to the geometrical axis of the tubular part, the said flanges being arranged between the mutually adjacent ends of the windings, and an electrostatic screen in the form of a flat open spiral formed by an insulated conductive wire coiled around the former between the said flanges, one of the ends of this wire being electrically earthed, wherein the tubular part of the former contains an opening situated between the two flanges, and the innermost turn of the flat spiral has a bare part which enters the interior of the tubular part through said opening to come int( contact with the laminations forming the magnetic core, which is connected to earth.

8. A transformer substantially as hereinbefore described with reference to the accompanying drawings.

9. Apparatus including a transformer as claimed in any preceding claim.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. laminations 22. The central limbs of the E-shaped laminations 21 are inserted in an alternate fashion into the tubular part 30 of the former 3 i.e. adjoining, E-shaped laminations 21 are inserted into the part 30 from opposite sides. The laminations 21, 22 are provided with holes 24 which enable them to be fixed together by bolts (not shown) to prevent vibration, and enable attachment lugs to be added which form a mechanical and electrical connection to the supporting chassis (not shown). Laminations of other shapes (F,U,E-E) may equally well be used. An electrostatic screen of this kind is principally applicable to supply transformers whose primary windings are fed from a 50 or 6()Hz mains supply or from a lowfrequency AC source of 50 to 1,000Hz, for example, such as convertors of the static invertor or motor-alternator kind. Supply transformers of the present inexpensive kind may advantageously be mass produced in large numbers, such as those intended to supply television receivers for example. Supply transformers employing electrostatic screens 6 in the form of a flat spiral may be single phase, as in the foregoing, or threephase, in which case the three limbs of the magnetic core 2 are all provided with identical formers 3, windings 4, 5 and screens 6. In the case of a single phase transformer, the magnetic core 2 may equally well have two limbs if U and I - shaped laminations are used for example. It should be noted that it is also possible for example to connect the outer end of the electrostatic screen in the form of a flat spiral 6 to earth rather than its inner end 6(). Moreover, in cases where a transformer has more. than two windings disposed side by side around the same limb of the magnetic core, an electrostatic screen of the type herein disclosed may be arranged between each pair of windings. WHAT WE CLAIM IS:

1. A transformer having a magnetic core and an electrostatic screen between two windings which are disposed side by side around a limb of the core. wherein said electrostatic screen is in the form of a flat open ended spiral winding of insulated conductive wire which is connected at one end to a point on the transformer representing a reference potential.

2. A transformer as claimed in claim 1, wherein the windings are wound on an insulating former having flanges defining the zone for each winding and wherein the electrostatic screen is wound in a space between two adjacent flanges defining the adjacent ends of said two side by side windings.

3. A transformer as claimed in claim 2, wherein the space between the two adjacent flanges is dimensioned to receive a single layer flat spiral winding forming the electrostatic screen.

4. A transformer as claimed in any preceding claim wherein one end of said spiral winding forming the electrostatic screen is connected to said magnetic core.

5. A transformer as claimed in any preceding claim wherein the magnetic core is a laminated core.

6. A transformer as claimed in any preceding claim, comprising more than two windings disposed side by side and an electrostatic screen between each pair of windings.

7. A transformer intended in particular to supply electrical and/or electronic circuits with alternating voltages and currents, comprising a laminated magnetic core formed by a stack of conductive ferromagnetic laminations, at least one insulating former comprising a tubular part the inside of which is intended to receive one of the limbs of the magnetic core and the outside of which is intended to carry at least two windings which are arranged side by side along it, and means for insulating and separating these windings from one another electrically, which means are secured to the former and are formed by two walls in the form of flanges which are orientated perpcndicularly to the geometrical axis of the tubular part, the said flanges being arranged between the mutually adjacent ends of the windings, and an electrostatic screen in the form of a flat open spiral formed by an insulated conductive wire coiled around the former between the said flanges, one of the ends of this wire being electrically earthed, wherein the tubular part of the former contains an opening situated between the two flanges, and the innermost turn of the flat spiral has a bare part which enters the interior of the tubular part through said opening to come int( contact with the laminations forming the magnetic core, which is connected to earth.

8. A transformer substantially as hereinbefore described with reference to the accompanying drawings.

9. Apparatus including a transformer as claimed in any preceding claim.