GB2170052A

GB2170052A - Electromagnetic transformers

Info

Publication number: GB2170052A
Application number: GB08601174A
Authority: GB
Inventors: John Howard Ashmore
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1985-01-21
Filing date: 1986-01-17
Publication date: 1986-07-23
Also published as: GB8501455D0; GB8601174D0

Abstract

A transformer for a line-powered modem has two windings 4,6, on a former 1, these windings being separated by a layer of insulation tape 5 (P.T.FE) which is wider than the axial length of the windings and extends up the end cheeks to increase the creepage path. <IMAGE>

Description

SPECIFICATION Electromagnetic transformers The present invention relates to electromagnetic transformers.

In particular although not exclusively the invention is concerned with such transformers for use as electrical protection barriers where electrical equipment, such as data terminal equipment, is to be connected to telephone lines. The protection barriers are required to protect the telephone lines from dangerous voltages which may unintentionally be applied to the terminal interface lines and vice versa.

While satisfying the protection barrier, or isolation, requirements the transformers have to meet many electrical performance criteria, both d.c. and a.c., which in some ways conflict with the isolation requirements. For example, increased physical separation between windings could provide better isolation, but would result in greater leakage inductances and generally poorer performance as a signal transferring device. Alternatively special bobbins are available with two-piece solid insulating covers for a first winding over which a second winding may be laid. This form of bobbin, however, allows less flexibility in terms of wire gauge and number of turns for the first winding.

According to the present invention in an electromagnetic transformer having at least two concentric windings that are electrically insulated from one another to withstand a predetermined high voltage therebetween, at least part of the electrical insulation between said windings is formed as a winding of a tape of flexible electrically insulating material which is wider than the axial length of either of said concentric windings to provide between the conductors of said concentric windings a predetermined minimum creepage distance over the surface of said electrically insulating material.

Where the windings of the transformer are supported on a former or bobbin of electrically insulating material, comprising a generally tubular centre portion and two generally planar end-cheeks, a first one of said windings being would around the tubular centre portion between said end cheeks, the tape of electrically insulating material may be would directly over said first winding. Due to the width of the tape and the flexibility of the material of which it is made the edges of the tape may deform to extend radially outwards over the inner faces of said end-cheeks, to form a tray in which the second winding may be located. The tape may be a polytetrafluoroethylene.

An electromagnetic transformer in accordance with the present invention, together with electrical equipment incorporating such transformers will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows a sectional view of the transformer, Figure 2 shows a plan view of the transformer of Figure 1, and Figure 3 shows schematically the electric circuit of a line-powered data modem utilising transformers in accordance with the invention.

Referring first to Figures 1 and 2, the transformer comprises a coil former or bobbin 1 having a generally tubular centre portion 2 and generally planar end-cheeks 3. A first winding or set of windings 4 are wound directly on the centre portion 2 substantially over the whole of its length between the end-cheeks 3, and over this winding is laid an insulating layer comprising somewhat-more than one turn of polytetrafluorethylene (PTFE) tape 5. The width of the tape 5 is such that as it is laid over the winding 4 its edges deform so as to extend up the inner faces of the end-cheeks 3 so as to form a trough in which a second winding or set of windings 6 may be laid. Extra turns 7 of a narrower tape of electrically insulating material may be laid over the centre portion of the tape 5 to satisfy any requirement for supplementary insulation.

The edges of the tape 5 increase the creepage distance between the conductors of the windings 4 and 6, this creepage distance being taken as the shortest path, over the surface of the insulating layers, between those conductors. Where the conductors of the winding 6 pass over the edge of the end-cheek 3 a supplementary insulator 8 of PTFE tape is laid to maintain the creepage distance and clearance between the lead-out conductors and the first winding 4.

The electrical insulation between the windings 4 and 6 may be required to withstand relatively high temperatures under fault conditions, and certain types of PTFE tape meet this requirement.

Referring now to Figure 3 a data modem for connection to a-public system telephone line 9 may utilise two transformers in accordance with the present invention, a transformer 10 in the data signal path between the line 9 and a modulator/demodulator circuit 11, and a transformer 12 interconnecting a line-powered oscillator 13 and a rectifying and smoothing circuit 14 by way of which the modulator/demodulator circuit 11 may be energised.

In the case of the transformer 10 which carries both data signals and d.c. line current, the design of the transformer and careful selection of core material with tight tolerance and defined saturation characteristic avoids the use of an additional active line-hold circuit, or additional line-hold coil and capacitor, so that more voltage is available to power the modulator/demodulator circuit 11.

In one implementation the core comprises a laminated, gapped, block core which maintains its characteristics in the line current range 0-125 mA but saturates at the higher currents which may occur under fault conditions. The transformer 10 may be required to limit voltages transferred before a winding fuses to no higher than 50 volts r.m.s.

1. An electromagnetic transformer having at least two concentric windings that are electrically insulated from one another to withstand a predetermined high voltage therebetween, wherein at

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Electromagnetic transformers The present invention relates to electromagnetic transformers. In particular although not exclusively the invention is concerned with such transformers for use as electrical protection barriers where electrical equipment, such as data terminal equipment, is to be connected to telephone lines. The protection barriers are required to protect the telephone lines from dangerous voltages which may unintentionally be applied to the terminal interface lines and vice versa. While satisfying the protection barrier, or isolation, requirements the transformers have to meet many electrical performance criteria, both d.c. and a.c., which in some ways conflict with the isolation requirements. For example, increased physical separation between windings could provide better isolation, but would result in greater leakage inductances and generally poorer performance as a signal transferring device. Alternatively special bobbins are available with two-piece solid insulating covers for a first winding over which a second winding may be laid. This form of bobbin, however, allows less flexibility in terms of wire gauge and number of turns for the first winding. According to the present invention in an electromagnetic transformer having at least two concentric windings that are electrically insulated from one another to withstand a predetermined high voltage therebetween, at least part of the electrical insulation between said windings is formed as a winding of a tape of flexible electrically insulating material which is wider than the axial length of either of said concentric windings to provide between the conductors of said concentric windings a predetermined minimum creepage distance over the surface of said electrically insulating material. Where the windings of the transformer are supported on a former or bobbin of electrically insulating material, comprising a generally tubular centre portion and two generally planar end-cheeks, a first one of said windings being would around the tubular centre portion between said end cheeks, the tape of electrically insulating material may be would directly over said first winding. Due to the width of the tape and the flexibility of the material of which it is made the edges of the tape may deform to extend radially outwards over the inner faces of said end-cheeks, to form a tray in which the second winding may be located. The tape may be a polytetrafluoroethylene. An electromagnetic transformer in accordance with the present invention, together with electrical equipment incorporating such transformers will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows a sectional view of the transformer, Figure 2 shows a plan view of the transformer of Figure 1, and Figure 3 shows schematically the electric circuit of a line-powered data modem utilising transformers in accordance with the invention. Referring first to Figures 1 and 2, the transformer comprises a coil former or bobbin 1 having a generally tubular centre portion 2 and generally planar end-cheeks 3. A first winding or set of windings 4 are wound directly on the centre portion 2 substantially over the whole of its length between the end-cheeks 3, and over this winding is laid an insulating layer comprising somewhat-more than one turn of polytetrafluorethylene (PTFE) tape 5. The width of the tape 5 is such that as it is laid over the winding 4 its edges deform so as to extend up the inner faces of the end-cheeks 3 so as to form a trough in which a second winding or set of windings 6 may be laid. Extra turns 7 of a narrower tape of electrically insulating material may be laid over the centre portion of the tape 5 to satisfy any requirement for supplementary insulation. The edges of the tape 5 increase the creepage distance between the conductors of the windings 4 and 6, this creepage distance being taken as the shortest path, over the surface of the insulating layers, between those conductors. Where the conductors of the winding 6 pass over the edge of the end-cheek 3 a supplementary insulator 8 of PTFE tape is laid to maintain the creepage distance and clearance between the lead-out conductors and the first winding 4. The electrical insulation between the windings 4 and 6 may be required to withstand relatively high temperatures under fault conditions, and certain types of PTFE tape meet this requirement. Referring now to Figure 3 a data modem for connection to a-public system telephone line 9 may utilise two transformers in accordance with the present invention, a transformer 10 in the data signal path between the line 9 and a modulator/demodulator circuit 11, and a transformer 12 interconnecting a line-powered oscillator 13 and a rectifying and smoothing circuit 14 by way of which the modulator/demodulator circuit 11 may be energised. In the case of the transformer 10 which carries both data signals and d.c. line current, the design of the transformer and careful selection of core material with tight tolerance and defined saturation characteristic avoids the use of an additional active line-hold circuit, or additional line-hold coil and capacitor, so that more voltage is available to power the modulator/demodulator circuit 11. In one implementation the core comprises a laminated, gapped, block core which maintains its characteristics in the line current range 0-125 mA but saturates at the higher currents which may occur under fault conditions. The transformer 10 may be required to limit voltages transferred before a winding fuses to no higher than 50 volts r.m.s. CLAIMS

1. An electromagnetic transformer having at least two concentric windings that are electrically insulated from one another to withstand a predetermined high voltage therebetween, wherein at least part of the electrical insulation between said windings is formed as a winding of a tape of flexible electrically insulating material which is wider than the axial length of either of said concentric windings to provide between the conductors of said concentric windings a predetermined minimum creepage distance over the surface of said electrically insulating material.

2. An electromagnetic transformer according to Claim 1 in which the windings of the transformer are supported on a former or bobbin of electrically insulating material, comprising a generally tubular centre portion and two generally planar cheeks, and the tape of electrically insulating material is wound directly over said winding such that the edges of the tape deform to extend radially outwards over the inner faces of said end cheeks, to form a tray in which the second winding is located.

3. An electromagnetic transformer according to Claim 1 or Claim 2 in which the tape of flexible electrically insulating material is of polytetrafluroethylene.

4. An electromagnetic transformer substantially as herein before described with reference to the accompanying drawings.