US3089017A

US3089017A - Electric heating system

Info

Publication number: US3089017A
Application number: US752384A
Authority: US
Inventors: Eisler Paul
Original assignee: Individual
Current assignee: Individual
Priority date: 1957-08-06
Filing date: 1958-07-31
Publication date: 1963-05-07
Anticipated expiration: 1980-05-07

Description

y 7, 1963 P. EISLER 3,089,017

ELECTRIC HEATING SYSTEM Filed July 31, 1958 4 Sheets-Sheet l wws/vrop Pau/Eis/er' BY 4 {M A T TOPNEY y 7, 1963 P. EISLER 3,089,017

ELECTRIC HEATING SYSTEM Filed July 31, 1958 4 Sheets-Sheet 2 Fig.3.

E94 1 H 5, r Qqi I E 2 H c an E E a E 1 I I l I l i i I 1 I /N VE N TOP Paul [is/er BY .4 Zfl/z A TTORNEY May 7, 1963 P, ElsL 3,089,017

ELECTRIC YSTEM May 7, 1963 P. EISLER ELECTRIC HEATING SYSTEM '4 Sheets-Sheet 4 Filed July 51, 1958 /N V E N TOR Paul [ls/er United States Patent Ofifice Fatented May 7, 1963 3,689,017 ELECTRIC HEATHNG SYSTEM Paul Eisler, 57 Exeter Road, London NW. 2, England Filed July 31, 1958, Ser. No. 752,384 Claims priority, application Great Britain Aug. 6, N57 '7 Claims. (Ql. ZEN-20) The present invention relates to electric step down transformers for use in electrical heating whether for space heating or for heating objects or materials such as water or foodstuffs.

In the ordinary way the function of a transformer is to change the voltage of an alternating current with as little loss as possible compatible with reasonable cost and sound construction. For this reason, transformers normally have a cubic form with high space factor for the core and winding, and are usually made of relatively costly material such as silicon steel for the core which has low hysteresis loss.

The losses in a transformer go up with reduction in the quantity and quality of material in the core; they also go up if the cross section of the conductors is reduced. The losses moreover appear as heat. If now the whole purpose of the transformer is to form part of a heating system, provided the heat can be applied where wanted and the temperatures involved are suited to the purpose it is no disadvantage if part of the useful heat is developed as loss in the transformer, and in certain directions at any rate this can reduce the cost of the transformer not so much by reducing the quantity of material used as by making it possible to use material of cheaper quality as is in fact done in the present invention.

The purpose of using a transformer at all is to enable the heating (apart from the heat produced in the transformer itself) to be effected in a low voltage circuit the voltage in which is below a value which is considered dangerous to touch. Accordingly the secondary voltage is made less than, and usually considerably less than say 48 volts between conductors and between conductors and earth. The secondary winding will be completely insulated and separated from the mains. In view of the purpose of the transformer the primary windings is made suitable for connection to conventional mains in which the supply may be at 100600 volts more usually 110 or 220/250 or 440 volts at 50 cycles.

Briefly stated therefore, the present invention provides an electrical heating system comprising an electrical stepdown transformer, said transformer comprising a primary winding for connection across the mains and a low voltage secondary winding, the transformer dissipating about 20-33% of the electrical energy supplied thereto in the form of heat, means in heat-conductive contact with the transformer for radiating the heat so dissipated, and a low voltage electrical heating resistance connected across the said secondary winding.

As well as serving @as a space heating radiator a transformer according to the present invention is very suitable for use as a hot plate or warmplate in connection with the heating up of food and keeping food hot or warm once it has been heated.

Because a transformer used according to the present invention develops considerable heat within itself but has a high surface to volume ratio, it can be used to operate a heating cycle which gives very rapid production of heat at first with a more or less steep or gradual falling ofl. Such a cycle will conveniently directly heat food stuffs and then maintain them hot. Furthermore it can produce a secondary voltage having a similar character, which is useful for the same purpose.

The invention will be further described with reference 2 to the accompanying diagrammatic drawings which illustrate some embodiments of the transformer and possible modifications thereof as well as one form of circuit in which it may be used. It will be understood that the drawings are given by way of example and that the invention is not confined by them.

FIGURES 1 and 2 are circuit diagrams showing how a heating system according to the invention may be set up.

FIGURE 3 is a part sectional View of a stage in the construction of a first embodiment of transformer.

FIGURE 4 shows in side view the completion of the transformer shown in FIGURE 3 and its embodiment in a space heating radiator.

FIGURE 5 is a similar view to FIGURE 4 of an alternative arrangement of the same basic form of transformer.

FIGURE 6 shows another embodiment of space heating radiator incorporating the same form of transformer as FIGURE 4.

FIGURE 7 is a plan view of a second embodiment of transformer according to the invention.

FIGURE 8 is a perspective view illustrating a stage in the assembly of the transformer of FIGURE 6.

FIGURE 9 is a side view of the third embodiment of transformer according to the invention.

FIGURE 10 is a plan view of FIGURE 9, and

FIGURE 11 is a side view of another embodiment of transformer according to the invention.

FIGURE 1 shows a typical heating system according to the invention. The primary coil is designed to be energised from a normal mains supply say at 240 volts 50 cycles. The secondary coil comprises two windings "11, 12 which can be put in series and parallel by a switch 13. The primary coil may have tappings for regulation purposes operated on the mains side in a conventional manner while the secondary side is controlled by a resistor 15 or tape form which can be extended or which rolls up for instance under spring tension like a measuring tape. Only the extended length is in the circuit either in series with the main external load 14 or on the full secondary voltage constituting itself a minor ex ternal load that is a heating tape used for instance as an alternative small heater instead of the large external heater 14.

The switch-over is effected by switch 16 which has three positions: In the centre position {shown in FIG- URE 1) resistor 15 is in series with load 14 permitting a continuous variation of the ohmic resistance of the total load on the transformer. For full working of load 14 the tape resistor is completely rolled up. The lamp 19 between the two contacts 15b is then dark while the lamp over load 14 is bright. If the switch 16 is in top position, load 14 is cut off and the tape resistor 15 is across the secondary transformer terminals which are connected either in series or parallel according to the position of switch 13. The top position of switch 16 is only possible when the tape 14 is extended at least a certain length so that it cannot run too hot. This is ensured by a mechanical interlocking arrangement symbolized in FIGURE 1 by the bar 15a moving with the free tape end and bearing against a quadrant 16a on switch 16, thus preventing an upward movement of switch 16 until it has passed, that is until the minimum tape length is extended. The lower bar end cannot pass the plunger 16a when the tape resistor 15 rolls up and the bar 15a remains in front of the switch but allows the tape 15 to roll up completely. In the bottom position of switch 16 all loads are off.

The tape resistor 15 is only indicated schematically as a resistor go and return line; in practice it is a heating film of the type described in my co-pending applications Serial No. 747,315, filed I uly 8, 8, now Patent 3 No. 3,033,970, and Serial No. 783,633, filed November 10, 1958, now Patent No. 3,020,378. The contacts 1512 are preferably rollers.

The function of bar 15a is not only to prevent switch 16 being moved into an undesirable position, but also to prevent any switching being eifected at all on the secondary side, on either switch 13 or 16, unless the tape 15 is fully or nearly fully extended. This further locking arrangement is indicated by sliding door 17. As long as the bar 15a is not holding it open against the force of a spring 17a, it covers up both

switches

13 and 16 which therefore cannot be operated.

Naturally these interlocking means are only symbolically indicated in FIGURE 1 and any of the known practical locking devices can be use-d for the symbolized arrangement. The purpose is to ensure that the current at the switch contacts when the switches are operated is reduced to nil or to such a small value that very cheap and light duty switch contacts can safely and reliably be used.

There are fuses 18, indicator lamps 19, both primary and secondary, a thermostat 20 or a temperature controlled automatically resetting circuit breaker and proper earthing connections. When at the beginning of a heating cycle switch 13 is in the series position the transformer is heavily overloaded for a very short time. In this case the secondary supply voltage is twice the usual voltage which is obtained when both coils 11 and 12 are in parallel and consequently the wattage is quadrupled. The transformer and the external heating device 14 will consequently heat up very quickly and the device 20 will switch the mains supply off temporarily when the desired temperature is reached. As soon as this happens, the current supply to coil 8 via leads 9 is switched off. The flux change within the coil exerts a forward force on the bar 10 connected to blocking plunger 13c and situated within the coil. The blocking plunger 13c drops and permits the spring 13a to pull the switch 13 into the parallel position shown in FIGURE 1 where it remains when the device 20 closes the primary circuit again. Spring 13a and plunger 13c are shown here only by way of symbols for standard blocking devices with proper resetting provisions. The latter can be manual or automatic when the temperature has dropped to a pre-set level.

FIGURE 2 is a simplified circuit diagram of a heating system with a transformer having two primary coils 11a, 12a. The system operates with the switch 13- at first parallelling both coils; as soon as the thermostat or temperature controlled automatically resetting circuit breaker 20 breaks the primary circuit for the first time the switch 13 is pulled over into the position in which it connects both coils in series, thus reducing the load to A of what it was before. The resetting of switch 13 for parallel connection of the coils is done manually or automatically when the transformer has cooled down sufficiently. The circuit gives a heating cycle suitable for very rapid heating up where the mains supply has sufficient capacity. This cycle can be made still more steep if two primary and two secondary coils are provided in series parallel connection giving load ratios of 16:4:1.

By Way of example for space heating the transformer may be a single phase mains transformer of 3 kva. made as a fiat panel which dissipates about 1 kw. as heat for space heating while the secondary output of 2 kw. is supplied usually at about 12 volts to a wall-, ceiling, or floor-covering, hereafter called heated wallpaper, which consists of an aluminium foil pattern on a thin layer of plastic film or paper or between two such layers. This and other heated wallpapers and other heating devices which can be used in the heating system of the invention are described in my copending applications Serial No. 747,314, filed July 8, 1958, now Patent No. 2,971,073, Serial No. 747,315, filed July 8, 1958, now Patent No. 3,033,970, Serial No. 783,633, filed November 10, 1958,

4- now Patent No. 3,020,378, and Serial No. 783,609 filed December 27, 1958.

The core and coil cross sections are initially and similarly calculated as for a 0 core transformer with the difference that (a) the core consists usually of very thin, flexible and grain oriented, tapes of very cheap grade steel foil giving high loss instead of the usual grain oriented, high grade magnetic strips and the windings consist of aluminium foil instead of copper wires, that (b) the combined coil and core losses at full load can reach say 20 to 33 percent.

Compared with conventional transformers and taking the diiferent materials into consideration the smaller active cross sectional areas of the windings and core tend to reduce the weight, while the arrangements necessary to give them thin flat shapes, to be described below, tend to have the opposite effect.

The core, see FIGURES 3 and 4, consists of several layers of the thin, highly flexible steel foil tapes 21. The number of layers which can be superimposed on each other, that is the thickness of the stack, is not only restricted by the desired loss and heat dissipation requirements of the present invention, but also by its requirement that the core stack prior to its assembly with the coils and its closure must be very flexible, like a belt. It is for convenience called a belt hereafter. To keep the drawing clear the thickness of the layers has been exaggerated and their number reduced.

While the cross section of a conventional 0 core is a square or a slightly elongated rectangle, the core cross section of the present transformer is so fiat that its long side (:width of core) which is the width of the steel foil or the combined width of narrower paralleled steel foil tapes plus installation between the tape edges is a high multiple of thickness of the foil stack or core. All or some of the steel foils may be slit along their length if, in a particular case or for some only of the foils of a core, the eddy current loss is thought to be too great.

The coils are wound over a removable former of the core cross section so that they Will be able to slide like sleeves over the core belt. They are wound from tapes of aluminium foil with a thin impregnated paper or plastic film tapes 22 interleaved between turns. This paper or film extends over the foil edges. The primary Windings consist of a tape with several parallel strands 23 of aluminilun foil and this tape can be produced as a tape cable element by any of several well known mechanical or printed circuit methods. The inner and outer tape ends are taken out sideways, interconnected, in series .or series parallel to provide the necessary electrical paths, provided with terminals and sealed over with insulation. The secondary winding which requires fewer turns but greater cross section consists of tape with a single wide strand 24 of aluminium foil the inner and outer sides again being taken out sideways. By this means the primary and secondary windings can be made of substantially equal depth i.e. dimension nonnal to the foil of the core where it passes through the winding. The two coils may also be made of substantially equal width. In the drawings the thickness of the foils and insulation have been exaggerated and the number of layers reduced for the sake of clearness.

Impregnated paper or plastic film tape 25-wider than the foilsmay also be used to insulate the superimposed layers 21 of the core and to bond these layers into the long belt-like unit except at both ends of the belt. The interleaving insulating tapes 25 are not cured at the front end and there only hold the foils temporarily together by adhesion. Thus the front end can be opened and the first few inches of each foil layer 21 then become accessible. The insulating tapes 25 stop short of the rear end of the belt so that there the foils are free and bare on both sides.

The assembly of the two coils with the core belt is effected by simply slipping the coils over the belt and joining the belt ends layer by layer, the bare rear end of each layer 21 contacting the non-insulated face of its front end. The joint-with all ends superimposedis pressed together and cured by heat generated for instance by supplying the coils with current.

The bonding of the belt ends by means of the interleaving insulating tapes 25 may be supplemented or replaced by addition of other cements. Generally the bond is additionally secured by a mechanical fixture, clamp, bolts or the like. For this purpose the foils 2 1 at the belt ends may be perforated with oblong holes or other cutouts.

It is to be noted that the closed belt with the coils loosely threaded on is spacious and flexible and very convenient and easy to Work on manually and with tools or machines.

The description has so far referred to a single belt for one transformer. Under mass-production conditions the belt can be produced in continuous lengths limited only by the length in which the foil can be made and laminated fully automatically by a fairly simple machine similar to a paper converting machine. In a similar way many coils can be wound on one axis simultaneously or sliced from one very long cylindrical coil as described in British patent specification No. 700,451; and the coils can be slipped over the continuous belt before it is cut into lengths for single transformers.

The joining and securing of both belt ends can also be done by the automatic machinery. Thus labour cost in transformer production can be reduced considerably and high accuracy can be obtained.

The next step in the transformer production is to stretch the closed belt with the coils on it into a flat panel shape and enclose it between two anodized aluminium or steel sheets. The sheets are suitably decorated for space heating radiator purposes and are associated with a stand or legs and frames. In the preferred method as shown in FIGURE 4 one coil is placed close to the joint indicated at 26 and the other at the diametrically opposite region when the belt is imagined as circle, and then both coils are moved relatively until the belt-free flat coil surface of one coil is in line With the outer surface of the belt which is pressed against the flat surface of the other coil, as shown in FIGURE 3. The belt is so dimensioned that in this position it fits very tightly over the coils.

The joint 26 lies in and preferably fills the space between the coils and thus spaces them apart. This arrangement of the coils on opposite halves of the belt also separates the two parts of the belt which run inside the coils and gives more uniform temperature conditions. Thermal expansion of the belt for example when assembling the transformer with the enclosing sheets while the belt is heated may be used to ensure a tight fit of the belt. But if it is slack it may be tensioned by insertion of wedges and distancing strips.

When a clamping device is used for securing the joint 26 of the belt which features an eccentric bolt or pin, this can also be used advantageously for tightening the belt at this juncture. Tightness or slackness beyond the capacity of these or similarly convenient adjustment facilities require lengthening or shortening of the belt by resetting the joint and indicates an inaccurate dimensioning of the belt length which should be corrected for subsequent production. However, in any particular case if the belt is too short instead of re-setting the joint, the coils may be shifted to bring the joint beyond the outer end of one coil thus enabling the facing ends of the cores to be brought closer together. It would also be possible to put both coils on the same half of the belt and arrive at the configuration of FIGURE 5 or by shifting the joint to one end to bring the coils still closer together. This configuration can indeed be deliberately chosen but the temperature conditions in the core are not so favourable as in the configuration of FIGURE 4. When either configuration is sandwiched between two decorated

metal sheets

27, 28 the total thickness is essentially equal to the thickness of the decorated sheets plus the thickness of the core belt plus the thickness of the coil which in turn and excluding play is the belt thickness plus twice the radial thickness of the cross section of the coil windings i.e. the thickness normal to the foil of the core belt. Thus the total thickness is a little more than twice the thickness of core and coil cross section. Usually it is just over four times the core thickness.

The decorated metal sheets 27, 2-8 hold the transformer under the pressure of bolts, screws or other fixing devices which connect both sheets and keep the sandwich together. The sheets are earthed through their large area of contact with the earthed belt and by additional earth contacts.

In order to minimize noise, potting or other encapsulation of coils and core can be carried out with a minimum of resin and without interfering with the heat flow to the metal sheets through their large contact areas with the coils and core. The resin can be filled with aluminium powder or the like to improve its heat conduction.

Instead of sandwiching the transformer between two metal sheets it can be fixed, for example by clamping bolts, to only one decorated sheet 27, FIGURE 5, which forms the front of the radiator while the back sheet is distant from the rear surface of the transformer by a small air gap 31. The top of the rear sheet 2-8 has a curved hinged lid 32, which operates a switch and a resistor fixed on the transformer as a radiator. Opening the lid switches the main secondary load (wallpaper) on by gradually increasing the value of the resistor across the secondary terminals, switches a decorative indicator light on and allows an airstrearn to flow through the gap between rear sheet 28 and rear surface of the transformer to the top of the sheets and into the room. This heat convection does not take place when the hinged lid is fully closed and the normal secondary load is off.

With no secondary load the transformer would however run too cool and be useful only for drying clothes or similar purposes and not as a space heating radiator of l kw. The lid therefore has two open positions. The second open position so adjusts the resistor across the secondary terminals that the combined transformer loss and the load presented by the resistor amounts to 'l kw. which the panel must dissipate. The airflow is not interfered with in the second open position, but the indicator lights are dimmed.

The description of the example of transformer con struction according to the present invention shown in FIGURES 2 to 5 has referred only to the use of ordinary aluminium and steel foil and to impregnated paper and plastic film as insulants. These materials are perfectly suitable as long as the design by adequate dimensioning of cross sectional and surface areas ensures that at no point a temperature will occur above the limits permissible for these insulants. For so called low temperature radiators running below C. this condition is readily obtainable. For higher temperature work however, be it inside the panel or on the surface of the panel, variations of insulants and/or foils are proposed which will be described later after the description of a further example of the basic construction of the transformer.

The second example is also a transformer constituting and used as a thin flat heating panel. Its conventional analogue is a single-phase, shell-cored, double wound transformer with the primary and secondary windings on the same limb.

Whereas in the first examples the coils were wound with very wide and thin (aluminium) foil tapes, the coil of the second example is made from very narrow foil tape. The axial length of the complete coil is about equal to the tape width if the primary and secondary windings are so wound that they are radially superimposed one over the other. If both windings are wound side by side on the same size former and have the same radial depth, the axial length of the complete coil is then the sum of the widths of the tapes of both windings. The complete coil has the shape of a very elongated oval with straight long sides, and is marked 41 in FIGURES 6 to 8. It can readily be mass-produced, for instance by winding a great number of coils on one long mandrel.

The opening between the straight long sides can be slightly widened or made narrower in order to facilitate the method of assembly of the coil and core proposed by the invention. The elasticity of the coil in this direction is provided by crimping the tapes during winding near the ends of the oval or by elastic interleavings inserted there during winding between given numbers of turns. The elasticity of the coil is not required after final assembly, and does not therefore stand in the way of an encapsulating, potting or curing operating at a later stage.

There are two varieties of core for this coil. The one illustrated in FIGURES 7 and 8 uses the three-limb laminations of heavier gauge, but still flexible steel foil, the other illustrated in FIGURES 9 and 10 consists of preformed and bonded stacks of steel foil which can be bent in the middle and have each foil accessible at the ends prior to and during assembly.

Referring now to the construction illustrated in FIG- URES 7 and 8 these core laminations are made from lacquered narrow steel tape by punching two

slots

42, 43 and three holes 44 in each length which is parted off the tape by severing at 45 the middle limb 46 from one of the two long links of each lamination. The three holes 4 4 are in the centres of the limbs, two large holes in the centre limbs and a small hole in the middle limb. A few end plates of stiff heavy gauge steel are also pre pared, not necessarily with a middle limb, if machining them for insertion of a separate middle member during core assembly is more convenient than bending their middle limbs.

The core assembly is effected by sliding the laminations over the coil from the end as illustrated in FIGURE 8. The middle limb 46 of the lamination is flexed up to slide over the end of the coil, the straight long sides of which have a fair clearance in the

slots

42, 43 of the stampings. The centre limb 4-6 of each lamination springs back into the plane of the lamination as soon as it reaches the coil opening. The lamination can be assembled with the cut in the central limb at the top and bottom alternately. Stacks of the flexible lamination are packed between the stitf end plates which are marked 47. Neighbouring end plates 47 are spaced by washers 48 to leave cooling channels 49. In FIGURE 7 three stacks with two such channels are shown, but obviously any number can be produced. Two insulated long bolts pass through the holes in the outer limbs of laminations and hold the core together round its perimeter. Direct clamping pressure on the middle limbs of the laminations can be obtained by flexible members which are tightened and secured while tight. The flexible member shown in FIGURE 7 is an insulated wire or wire rope 50 with a lead on one end. It is threaded through the holes in the middle limbs and washers between the stacks and is tightened by its other end being wound on a pin 51 held on the last end plate. This pin can be turned by a screw driver or spanner and be secured against unwinding by a ratchet device 51a acting on the head of the pin.

If the middle limb is provided with more than one hole or if there other pathways are provided (for instance notches) for flexible tightening means the described wire or wire rope may be replaced for instance by loops of wire which encircle the limbs in each stack, the loops being tightened by twisting their ends together or drawing them tight by a screw device. This tightening can be effected as each stack is assembled, the loopend being accessible in the opening of the coil. Even when the last stack is assembled there is room in the semicircular end portion of the opening. Alternatively to flexible members, eccentric members lodged in the semicircular end parts of the coil opening may be used for tightening the core stacks, or if the cuts 45 are alternately at the top and bottom, two further long bolts of small diameter might be used at the centres of the long links of the core lamination. The coil play in the slots may be eliminated by the wedging action of an eccentric device or long wedge inserted in the outer ends of the slots. In FIGURES 7 and 8 the thicknesses have been exaggerated and numbers of layers reduced for the sake of clearriess.

The other variety of core illustrated in FIGURES 9 and 10 is made from wide thin steel foil tapes having a plastic cement coating on one side but with narrow strips across the foil width left bare at regular intervals. The foils are cut at the centres of these uncoated strips. The coated length of different foil tapes, that is the spacing of the strips, varies according to the place the cut length of foil is to take in a particular core. The further from one side of the core opening, the wider the spacing. The width of the cut steel tapes is almost equal to the length of the parallel sided part of the coil opening.

A thin stack of the cut foils 61 is assembled in a kind of mould which forms and part cures it into a long narrow U shape. The space between the limbs of the U closely fits the said length of the coil. The bend of the U is not fully cured and remains flexible and the ends of the limbs which were free of cement do not adhere to one another. The inner layers of the limbs are shorter than the outer ones, and the cement edge follows the same length.

The assembly of the coil with two such U parts is done by bending one arm of the U about degrees as shown in chain lines on the right hand side of FIGURE 9, inserting this arm through the elastically widened coil opening, bending the arm back again into the U shape and pressing the coil together again. This process is done with both U parts forming the core either simultaneously or in succession. A bonding layer, cement and/or insulating tape indicated at 62 has been placed on both faces of the coil prior to assembly with the U, so that a press-curing in the final assembly stage bonds core and coil together on all sides. Before this final stage is reached, there are however some more operations.

The next step is to join the free ends of the U limbs over the straight sides of the outside turns of the coil. Starting with the inner foil ends of the U each pair of ends is bent over, superposed and if desired joined together, and a cement and extra insulating layer indicated by the numeral 63 is applied between each pair of foil ends. In FIGURE 9 the thicknesses of the layers have been exaggerated and the numbers of layers reduced for the sake of clearness. A steel tape 64 going round the outside of the core and coil holds the joint under pressure while wedges of the eccentric type which were placed along the inside and/or outside of the long straight sides of the coil tighten up the whole assembly. The wedges along the inside turns may actually be tubes filled with an expanding medium acting like a thermal fuse liquid or a thermostat which switches for instance a part of the load on and off. They may be airfilled pneumatic tubes 65 made of a heat-resisting elastomer inside the coil and/or on the outside coil windings solely for tightening the core after assembly. Terminals or fixing screw are preferably arranged at both axial ends of the core.

Very thin transformer structures can be made in accordance with FIGURES 7 to 10. The transformer can be mounted as described with reference to FIGURES 4 and 6'. Its decorative treatment depends on the temperature at which is desired to run this heating radiator. If the temperature still permits plastic insulation, the transformer can be potted with a formed sheet as mould to save resin, and a decorative print can be placed just under the resin surface of the front face. Or the decoration can be done by laminating as in various known decorative laminated products having a plain or patterned coloured, fabric-like or wood grained surface. For higher temperature running the transformer can be placed in a fiat oil tank the surface of which is then the immediate heating panel, or it can be used without cladding on a stand in front of a reflector. Such oil immersion is also possible with the transformer constructed as in FIGURES 3 to 6.

FIGURE 11 shows a transformer according to the invention adapted to serve particularly for heating food. The transformer itself has a core 66 and a winding '67 arranged similarly to a conventional shell type transformer except that as before the whole structure is made of very flat proportions and the core is built up of very thin flexible material; also cheap materials are used for the core and windings are of aluminium foil. Although the core is built up of pieces to permit assembly there are no bolt holes but the outer limbs are folded over as shown at 68 so that the total height at the side approximates that of the windings 67. Thus it can be firmly clamped by means of a stout plate 69 and bolts 71 which clear the core and press the core against the inturned flanges of a potlike member 72. The flange may if desired be replaced by a complete bottom closure of the member 72 but this is not necessary. If the core at 6 8 is not of sufiicient depth packing can be included. At the top the member 72 is provided with a hinged lid 73 and a hook 74 is formed on its front edge so that the package of food can be suspended when the lid is raised. For the convenience of the user of this structure, it is mounted on a tray 74.

I claim:

1. A transformer assembly comprising a plate-shaped electrical step-down transformer having a primary and a secondary winding, means in heat conductive contact with the transformer for radiating heat dissipated within the transformer and control means for causing the transformer to operate under a predetermined initial load for a predetermined period and for subsequently decreasing the load and maintaining it at a substantially constant, reduced value.

2. A transformer assembly comprising a plate-shaped electrical step-down transformer, said transformer comprising a belt-shaped flexible core of a width substantially more than double its depth, said transformer having a primary winding and a secondary winding which are spaced apart by the maximum distance permitted by the core, and radiating means in heat conductive contact with the transformer for radiating the heat dissipated within the transformer when on load.

3. A transformer assembly as claimed in claim 2 in it) which the radiating means comprises two metal plates between which the transformer is rigidly situated, the distance between the plates being substantially equal to the thickness of the core and one of the windings.

4. A transformer assembly comprising a core and primary and secondary windings, said core being made of separate laminations of thin foil material, and said Windings being made of thin foil material wound into substantially O-shaped form, said windings being placed adjacent one another and forming a plate shaped member which passes through and is substantially completely enclosed by the said core.

5. A transformer assembly as claimed in claim 4 in which the plate shaped member formed by the said substantially O-shaped windings is elongated, having an elongated central opening, the plane of the laminations of said core being transverse to the length of said elongated central opening.

6. A transformer assembly as claimed in claim 5 in which each said core lamination is provided with two apertures through which pass parts of said plate shaped member, the parts of said laminations between the two apertures substantially filling the said elongated central opening.

7. A transformer assembly comprising a core and primary and secondary windings, said windings being composed of separate laminations of thin foil material wound into a substantially elongated O-shaped form, one winding being radially superimposed over the other, and said core being in two separate portions each made of separate laminations of thin foil material which are wound around one of the elongated sides of the O-shaped windings for complete enclosure.

References Cited in the file of this patent UNITED STATES PATENTS 442,649 Talmage Dec. 16, 1890 1,546,885 Burnham July 21, 1925 2,027,405 Smede Jan. 14, 1936 2,544,845 Link Mar. 13, 1951 2,635,168 Lerza et a1 Apr. 14, 1953 2,702,936 Hurt Mar. 11, 1955 2,764,802 Fiertag Oct. 2, 1956 2,771,587 Henperson Nov. 20, 1956 2,840,680 Mills June 24, 1958 2,879,367 McLean Mar. 24, 1959

Claims

1. A TRANSFORMER ASSEMBLY COMPRISING A PLATE-SHAPED ELECTRICAL STEP-DOWN TRANSFORMER HAVING A PRIMARY AND A SECONDARY WINDING, MEANS IN HEAT CONDUCTIVE CONTACT WITH THE TRANSFORMER FOR RADIATING HEAT DISSIPATED WITHIN THE TRANSFORMER AND CONTROL MEANS FOR CAUSING THE TRANSFORMER TO OPERATE UNDER A PREDETERMINED INITIAL LOAD FOR A PREDETERMINED PERIOD AND FOR SUBSEQUENTLY DECREASING