US2384982A - Heat treatment of the insulating coverings of electric wires and cables - Google Patents

Description

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o n x G. H. wAL'roN Erm.

Filed March 30, 1945 sept. 1s, 1945.

HEATTREATMENT OF THE INSULATING COVERING OF ELGTRIC WIRES AND SABLES Patented Sept. 18, i945 BEAT TREATMENT OF TBE INSULAI'ING:

COVERINGS F ELECTRIC WIRES AND CA- BLES George Hall Walton, llelsby, near Warrington, Joshua Creer Quayle, Manley, Helsby, and Peter Jones, Kelsall, near Chester, England, asslgnors to British Insulated Cables limited, Prescot, England, a British company Application March 8., 1948, Serial No. 481,083 In Great Britain April 18, 1942 Claims. l('Cl. 21S-4'!) This invention relates to the heat treatment of the insulating coverings of electric wires and cables by the method in which heat is generated within the material by treating it as the dielectric between two electrodes across which a suitable potential difference alternating at a high frequency is maintained. It is known that in such an arrangement the dielectric loss of the material causes it to absorb energy from the electric field resulting in the liberation of heat throughout the part of the material which lies within the electric field. 'Ihe object of the invention is to apply this method in such a way that rapid generation of heat takes place continuously, so that the apparatus can form part of a plant for covering wire or cable and heat treating it continuously. If such an arrangement is to be practically effective it is necessary that the heat treatment should be carried out as rapidly as the covering can be applied, so that there is no necessity for slowing down the covering machine, which may, for instance, be an extruding machine. Accordingly, the apparatus must provide for the application to the covering material of the required amount of heat in a short time and, unless the cost is to be excessive, it must be done in apparatus of a reasonable length. This fixes a high standard for the rate of generation of heat, and it is believed that apparatus previously proposed for high frequency electric heating would not meet this requirement.

It has been found that in order to obtain a satisfactory result it is necessary to satisfy two conditions, which are to some extent conflicting. It is advantageous, in shaping yand distributing the electric field, to make the wire which lies in the centre of the insulating material one of the electrodes operating in conjunction with a second electrode which substantially surrounds the covered wire. It has also been found that it is impracticable to get the required result if the covered wire is used so that a high frequency current has to travel along it through any substantial distance.

For practical results it is obvious that the stress due to the presence of the electric field must not be such as to break down or otherwise injure the insulating material 'within that field, so that a limit is set to the maximum field intensity which can be applied. The rate of generation of heat is dependent upon both the field intensity and the frequency of alternation and the practical limit set to the former necessitates that the latter shall be very high if the rate of heating is to have the desired value. A frequency of from 20 to 200 million cycles per second is the desirable range. Even at the lower end of this range it has been found that current cannot be transmitted along the covered wire through any substantial distance. Owing to the conditions existing with such high frequency currents, the impedance of the wire (in both resistive and reactive components) is high, so that a large voltage has to be applied to it to cause the current to ilow through any substantial distance and the heat which is generated by that current originates within the conductor and, therefore, is not satisfactorily placed for heating the dielectric rapidly. It will give rise to an excessive temperature in the conductor and the directly adjacent insulating material. It has to be appreciated that, with the rate of heating under consideration, a fairly uniform distribution of the generation of the heat throughout the insulating material is essential, since the time available is too short to allow of large heat transfer through material which has not good heat conductivity. ,I

The present invention provides an apparatus oi' the kind indicated which gives the required rate of heating satisfactorily. In this apparatus, while the central wire is used as one of the electrodes for the electric field, it is not connected to the high frequency generating apparatus, but floats at an intermediate potential between two electrodes or sets of electrodes which are connected to the generator. The electrodes of the two sets are connected to the source of high frequency electric energy in such a way that, at any instant, directly adjacent electrodes are of opposite polarity. The members of the first set alternate with the members of the second set along the axis of the apparatus so that any given part of the wire passes alternately through an electrode of the first set and then through an electrode of the second set and then through another electrode of the first set and then through another electrode of the second set and so on.

Each electrode, as above indicated, is of substantially tubular form. It may be a complete tube, or may be split or in parts lying close together. The object in designing this member is that electrically it shall provide a continuous, or nearly continuous, surface surrounding the covered wire to serve as appropriate boundary and uniform distributor of the electric field. The members of the two sets are placed as closely together as is practicable. In general they will only be separated by a distance sufficient to prevent iiashing over between them. With such an arrangement the field (at some instant) passes substantially radially inward from one electrode to the wire and substantially radially Outward from the wire to the next electrode, and it is possible to apply to a considerable length of covered wire an intense high frequency field without any great loss of eiectiveness by the utilisation of the wire as a boundary of the electric field system.

The electrodes themselves, although substantially longer than the gaps between them, are limited in length by the necessity to vobtain a fairly uniform distribution of the field intensity along the material enclosed within each electrode. This can be achieved if each electrode is not larger than one tenth of the wave length.

The invention will be further described by the aid of the accompanying drawing. In this Figure l shows dlagrammatically the arrangement of the electrodes. Figure 2 shows a constructional form of a part of an apparatus for heat treating the insulating covering of an electric cable. Both these views show longitudinal sections.

In Figure l the four electrodes shown are in two sets, the members of which are marked a and b, respectively. It will be seen that these are arranged co-axially end to end and spaced only a short distance apart, the electrodes a alternating with the electrodes b. They thus form a long arrangement through which a wire, such as c, can travel continuously. During this'passage the covering d on the wire, which may for instance be of a rubber Composition, is subjected to an electric eld. For the production of this field the electrodes a are connected to one terminal e of an oscillator g and the electrodes b are connected to the other terminal j of that oscillator. Instead of using the same oscillator for all the electrodes it may be advantageous, for the purpose of getting large power or otherwise, to use a separate oscillator for each directly adjacent pair of electrodes wb.

At some instant the electric field passes radially inward from the surface of an electrode a through the covering d to the wire c and radially outward from the wire c to the surface of a dlrectly adjacent electrode b. The high frequency oscillation or" such a iield subjects the covering d effectively to the rapid generation of heat within it and this process of heating is maintained throughout the movement of the covered wire through the series of electrodes.

A convenient practical construction of the electrode system is one in which each electrode is a complete metal tube and between adjacent electrodes there is a short length of tube of insulating material, for instance ceramic or vitreous material, these conducting and insulating tubes being connected together end to end so as to provide a complete tubular casing, serving to enclose the covered conductor during the period of heat treatment. A part of such a construction is shown in Figure 2. Here a tubular metal electrode h is provided at each end with a tubular insulating piece i by which it is connected at the left-hand side to the end piece of the apparatus and at the right-hand side (not shown) to another tubular electrode similar to the tube h. Any required number of tubes h, with the insulating pieces i may be provided and will be terminated at the right-hand end of the apparatus with another end piece similar to the end piece i.

Each tube h, is mounted in a clamp k carried on a spindle Z, which is supported by an insulator m. The spindle Z serves not only for support, but also to provide the electrical connection from the external supply to the electrode h. By means of the spindles l and insulators m .the connected set of tubular electrodes h. with their insulating pieces i, and their end pieces I, are supported from the casing n which serves as a protective enclosure.

Each insulating piece i consists of a short tube with a flange at each end. Each end of the tubular piece forms a socket to receive the end of the adjacent member h or j as the case may be. The end piece i will also usually be made of metal and can be earthed. Flanges on the insulating tubes i are held between metal flanges o attached to the adjacent tubular parts h or :l and rings p. The latter are divided into two parts so that they may be inserted in the positions shown. They are also provided with studs by. means of which they may be drawn towards the flanges o for the purpose of gripping the end flanges of the members i, gaskets being interposed as shown.

By utilising the form of construction shown, it is possible to build up the requisite length of apparatus in a simple manner from a few similar parts and to support the apparatus without any further connection to the casing n than is provided by the electrical connections l and their insulating supports m. 4

The complete tubular structure provides a convenient means of applying external pressure to `the insulating material during heat treatment.

'Ihis may be done by filling the tube with gas under pressure which may be admitted by a pipe q connected to an end piece 7. To minimise the leakage of pressure fluid from the interior of the tubular structure a gland, such as r, can be arranged in each end piece i. The gland shown consists of a tapering tubular part and a base iiange by which it is held in an enlarged part s of the end piece :i by the clamping arrangement shown. 'I'he gland r is made of such dimensions that it has a slight clearance over the covering of the cable so that this can pass without contact or with only light rubbing contact.

In addition to applying pressure to the surface of the material, the gas will also have the advantage of permitting a higher intensity of electric stress in the region between the covering on the Wire and the tubular electrodes. This will generally be desirable in order to obtain a sufficiently high intensity of field within the covering material on the conductor.

It will often be advantageous to make the tubular structure 7, i, h of such dimensions that there is a clearance round the covered wire so that it does not rub on the surfaces oi the tube as it is drawn through. This is, for instance, necessary in the case where the heat treatment is vulcanisation of a rubber or rubber-like material. If intermediate support is required in long tubular apparatus of this kind, it can conveniently be provided by carriers such as small rollers of glass -or ceramic material placed in the insulating members i so as to be outside the electric field.

For frequencies of from 2O to 30 million cycles per second a suitable length for the insulating spacing pieces between the electrodes is from 1/2 to 3". The larger dimension provides for the case where it is necessary to insert a roller or other support at this place. For many small sizes of rubber covered wires an internal dameter for the tube of 1A" is satisfactory. 4 It will be obvious that for larger sizes of cable this dimension must be increased. It is advantageous t0 keep it as small as is practicable in order that the mos't effective distribution of field within the dielectric material to be heated is obtained. The tube is not necessarily circular in cross-section, but it is advantageous to give it this form when dealing with single core cables or single covered wires. On the other hand, in the case of other forms of cable, such as at twin, the tube may be made of a cross-section similar to that cf the dielectric material to be treated.

What we claim as our invention is:

1. Apparatus for the heat treatment of insulating material. which forms a covering on a wire, by subjecting it to the action of a high frequency electric field, comprising two electrodes of substantially tubular form placed close together end to end, the electrodes being each substantially longer than the gap between them, and means for applying to said electrodes a potential difference alternating at high frequency.

2. Apparatus for the heat treatment of insulating material, which forms a covering on a wire, by subjecting it to the action of a high frequency electric field, comprising two sets of electrodes each of substantially tubular form placed close together end to end, the electrodes being each substantially longer than the gaps between them and the members of one set alternating with those of the other, and means for applying between the v members of one set and those of the other set a potential difference alternating at high frequency.

3. Apparatus for the heat treatment of insulating material, which forms a covering on a wire, by subjecting it to the action of a high frequency electric field, comprising two tubular electrodes placed close together end to end, a tubular insulating piece between said electrodes separating them electrically and joining them mechanically to form a continuous tube surrounding the wire during the treatment of the coveringl and means for applying to said electrodes a potential difference alternating at high frequency.

4. Apparatus for the heat treatment cf insulating material, which forms a covering on a wire, by subjecting it to the action of a high frequency electric field, comprising two sets of tubular electrodes placed close together end to end, the members of one set alternating with these of the other, tubular insulating pieces between the electrodes separating them electrically and joining them mechanically to form a continuous tube surrounding the wire during treatment of the covering, and means for applying between the members of one set and those of the other set a potential difference alternating at high frequency.

5. Apparatus for the heat treatment of insulating material, which forms a covering on a wire, by subjecting it to the action of a high frequency electric field, comprising a tubular structure formed by tubular electrodes separated electrically by tubular insulating pieces, the electrodes and insulating pieces being joined mechanically to form a gas-tight structure, a pressure retaining gland at each end of said structure and means for connecting the interior space of said structure to a source of gas under pressure.

6. Apparatus for the heat treatment of insulating material, which forms a covering on a Wire, by subjecting it to the action of a high frequency electric field, comprising two tubular electrodes placed close together end to end, a tubular insulating piece between said electrodes separating them electrically and joining them mechanically to form a continuous tube surrounding the wire during the treatment of the covering, supports for said tube consisting of electric conductive members, one engaging each electrode, and insulating members carrying said conductive members which pass through them and serve for the connection of an electrical supply to said electrodes.

GEORGE HALL WAL'ION. JOSHUA CREER QUAYLE. vPETER, JONES.

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