WO1993019565A1

WO1993019565A1 - An induction panel heater

Info

Publication number: WO1993019565A1
Application number: PCT/IE1993/000014
Authority: WO
Inventors: James Mary Power
Original assignee: Eolas - The Irish Science And Technology Agency
Priority date: 1992-03-20
Filing date: 1993-03-19
Publication date: 1993-09-30
Also published as: AU3647693A

Abstract

An induction panel heater (1) comprises a plate member (2) having a top surface (3) and a bottom surface (4). Six spaced apart angle iron members (7) are welded to the bottom surface (4) of the plate member (2) to form receiving bores (10) for carrying conductors (12) which carry an AC current of approximately 100 amps at 50 Hz for inducing eddy currents in portions (6) of the plate member (2) and the angle iron members (7). Heat generated in the angle iron members (7) by the eddy currents is conducted into the plate member (2) for increasing the quantity of heat available to the plate member (2).

Description

"An induction panel heater"

The present invention relates to an induction panel heater, and in particular though not limited to a panel heater for use in floor heating, for example, for heating the floor of a pig rearing and/or farrowing unit or the like.

It is essential in pig farrowing units that a surface of portion of the floor of the farrowing unit be provided on which newborn pigs may sleep and rest, this portion of the floor surface should be maintained at a temperature, preferably, in the order of 30°C to 40°C. In general, such heated floor surface areas are provided by a heated panel placed in or on the floor. Such heated panels may be either heated by water heating or by electrical heating. In the case of water heating, conduits through which hot water is circulated, in general, are embedded in a panel of the floor of the farrowing unit. However, such water heated panels, while providing adequate heat, tend to be relatively costly to install, to run and to maintain. For example, considerable installation expense is incurred when installing water heated panels in that flow and return hot water supply pipes must be provided to the panel and connected thereto. A boiler is required to provide a ready supply of hot water. The energy conversion efficiency of most boilers tends to be relatively low, thus leading to relatively high running costs of water heated panels. Because of the relatively harsh and hazardous environment of farrowing units, considerable maintenance, in general, is required to hot water heated panels. The environment in farrowing units tends to cause corrosion and deterioration of hot water conduits, and to joints in hot water conduits which in due course lead to leaks, which must be repaired.

In the case of electrical heating, electrical resistance heating elements are embedded in or mounted beneath a panel of the floor. Such electrically heated panels also suffer from a number of disadvantages. Electrically heated panels of this type are susceptible to failure due to the hazardous environment of a pig farrowing unit. Acid in the animal's urine may attack the resistance heating elements thus leading to their failure, and in certain cases, stripping of insulation from cables carrying electrical power to the heating elements. Indeed, it is not unknown for pigs to eat through insulation on cables carrying power to the resistance heating elements. In general, such heating elements are supplied at a relatively high voltage, in many cases, 220 volts mains supply, and any contact of a pig with a live conductor can lead to electrocution of the animal.

Induction heaters, whereby an alternating current flowing in a conductor induces an alternating eddy current in an adjacent electrically conductive material, such as, for example, a metal plate are known. Such an induction heater is disclosed in

British Patent Specification No. 1,513,241. The induction heater comprises a metal plate on which a magnetic circuit is mounted. The magnetic circuit defines a plurality of elongated channels through which electrical conductors extend. Current flowing in the conductors induces eddy currents in the plate. The magnetic circuit is formed from a plurality of laminates of thin sheets of iron or other ferromagnetic material which extend transversely of the channels for directing and guiding magnetic flux. Induction heaters tend to be used where relatively high temperatures are required, typically, 300°C plus, and in many cases significantly higher temperatures. Further, such induction heaters tend to operate at relatively high frequency, generally, higher than 300 Hz_r and in general, at frequencies higher than 10 kHz. Induction heaters, in general, tend to be unsuitable where relatively low temperatures of the order of up to 100°C are required.

Furthermore, because of the need to operate such induction heaters at relatively high frequencies, their cost of installation is relatively high due to the need to provide a power supply at a relatively high frequency. There is therefore a need for a panel heater for heating a pig farrowing unit, and for many other purposes which overcomes the problems of known heating devices.

The present invention is directed towards providing such a panel heater.

According to the invention there is provided an induction panel heater comprising a plate member of ferromagnetic material, an elongated magnetic circuit means adjacent the plate member defining an elongated receiving means, and an elongated conductor extending axially through the receiving means for carrying an alternating current for inducing an eddy current to flow in the plate member substantially parallel to the conductor for heating the plate member, wherein the magnetic circuit means is of an electrically conductive material and is electrically conductive in a longitudinal direction parallel to the receiving means for permitting eddy currents induced by the conductor to flow in the magnetic circuit means parallel to the receiving means for heating the magnetic circuit means.

In one embodiment of the invention the plate member forms part of the magnetic circuit means. Preferably, the plate member forms a longitudinal extending part of the receiving means.

In one embodiment of the invention the receiving means is an elongated receiving bore.

Preferably, part of the magnetic circuit means is formed by an elongated trough shaped member of electrically conductive material having a pair of longitudinal side webs terminating in respective spaced apart free side edges, the trough shaped member being secured to the plate member with the free side edges abutting the plate member to form with the plate member the receiving bore. Advantageously, the trough shaped member is of V-shaped cross-section. Preferably, the trough shaped member is of angle iron.

In one embodiment of the invention the trough shaped member is secured to the plate member by welding the free side edges to the plate member. Preferably, the free side edges are welded to the plate member by respective continuous elongated seam welds.

Alternatively, the free side edges are welded to the plate member by a plurality of short welds at spaced apart locations.

In an alternative embodiment of the invention the magnetic circuit means is formed by an elongated hollow member having a longitudinally extending bore forming the receiving bore for the conductor, the hollow member being secured to the plate member. Preferably, the hollow member is welded to the plate member.

In another embodiment of the invention the conductor is electrically insulated from the magnetic circuit means.

In a further embodiment of the invention a plurality of spaced apart elongated magnetic circuit means are provided defining respective parallel receiving bores, a conductor being provided in each receiving bore. Preferably, the magnetic circuit means are parallel to each other. Advantageously, the conductors in pairs of adjacent receiving bores are connected in series.

In one embodiment of the invention a step down transformer for stepping down an AC mains supply voltage is provided, the transformer comprising an input for receiving mains voltage and an output for applying an alternating voltage and current on each conductor.

Preferably, the voltage across each conductor is in the range of 0.1 volts to 24 volts. Advantageously, the voltage across each conductor is in the range of 0.3 volts to 12 volts. Preferably, the voltage across each conductor is in the range of 0.3 volts to 0.6 volts. Advantageously, the output voltage from the transformer does not exceed 24 volts, and preferably, the output voltage from the transformer does not exceed 12 volts. Ideally, the output voltage from the transformer does not exceed 2 volts.

In another embodiment of the invention the frequency of the alternating current delivered by the transformer is in the range of 20 Hz to 300 Hz. Preferably, the frequency of the alternating current delivered by the transformer is in the range of 40 Hz to 150 Hz. Advantageously, the frequency of the alternating current supplied by the transformer is approximately 50 Hz.

In another embodiment of the invention the alternating current delivered by the transformer through each conductor is in the range of 50 amps to 300 amps.

Preferably, the alternating current delivered by the transformer through each conductor is in the range of 75 amps to 200 amps. Advantageously, the alternating current supplied by the transformer through each conductor is approximately 100 amps.

In a further embodiment of the invention the step down transformer receives an input voltage of approximately 220 volts AC at approximately 50 Hz.

In one embodiment of the invention a temperature control means for controlling the temperature of the plate member is provided. Preferably, the temperature control means comprises a timer controlled switch for switching on and off electrical power to each conductor at predetermined time intervals. Advantageously, the timer controlled switch is provided at the input to the transformer.

In another embodiment of the invention a housing defining a hollow interior region is provided for housing the transformer. Preferably, the housing is mounted on an upstanding member extending upwardly from the plate member. Advantageously, the upstanding member comprises an elongated conduit for accommodating cables between the transformer and the conductors for delivering current from the transformer to the conductors.

In another embodiment of the invention the conductors in the respective receiving bores are connected in series.

In a further embodiment of the invention the plate member has a top surface and a bottom surface, each magnetic circuit means being provided on the bottom surface.

In one embodiment of the invention the panel heater is a floor panel.

The advantages of the invention are many. The panel heater is particular suitable for relatively low temperature heating, for example, providing a heated surface at temperatures up to 100°C, and it is particularly suitable for providing a heated surface at temperatures in the range of 30°C to 40°C. It has been found that the panel heater operates at a relatively high efficiency in the temperature range from 30°C to 100°C. It is believed that this high efficiency is largely achieved by virtue of the fact that each magnetic circuit means is electrically conductive in a longitudinal direction parallel to the receiving means carrying the conductor. This thus permits eddy currents induced by the conductors to flow in the respective magnetic circuit means, and the eddy currents of the magnetic circuit means flow parallel to the conductors thereby heating the magnetic circuit means. The heat in the magnetic circuit means is transferred to the plate member by conduction and/or convection, thereby providing a greater quantity of heat for heating the plate member. In this way, the conversion efficiency of the panel heater for converting electrical power to heat is relatively high. Furthermore, it has been found that the panel heater according to the invention operates at a relatively high efficiency at a magnetic field frequency of 50 Hz, in other words, where the frequency of the alternating current flowing in the conductor is 50 Hz. It is believed that this is because each magnetic circuit means is electrically conductive in a longitudinal direction parallel to the receiving means.

Another particularly important advantage of the invention is that it is safe to use. The panel heater can be operated at particularly low voltages, indeed, at voltages down to 0.1 volt if desired. Thus, there is virtually no danger of electrocution of pigs. Even if the pigs come in direct contact with the conductors because the voltage at which the current is supplied to the conductors is so low there is no danger of electrocution to the pigs. Thus, should insulation on the conductors deteriorate as a result of the environment in the farrowing unit, or should a pig eat through the insulation on the conductors, no harm will come to the pigs. Further, the eddy currents induced in the plate member and in the magnetic circuit means do not present any danger to the pigs. Furthermore, the plate member is effectively decoupled twice from the main supply, firstly, at the transformer, and secondly, by virtue of the fact that the conductors are insulated from the plate member and magnetic circuit means. Another advantage of the invention is that there is no danger of electric shock to humans during high pressure water hosing down of a panel heater even should the panel heater be powered up. Additionally, because of the simplicity of the panel heater, the panel heater is particularly reliable, relatively maintenance free, and can be provided at relatively low cost. The operating costs of the panel heater are also relatively low. The construction of the panel heater provides a heater which is relatively robust, and which can be provided as a portable heater. Accordingly, largely because of the above advantages, the panel heater according to the invention is particularly suitable for use in a pig farrowing unit.

By providing the plate member to form part of the magnetic circuit means, a particularly efficient panel heater is provided. The efficiency of the panel heater is further improved where the plate member forms part of the receiving means. The efficiency of the panel heater is optimised by providing the receiving means in the form of a receiving bore.

The provision of an elongated trough shaped member forming part of the magnetic circuit provides a particularly efficient panel heater, which is relatively simple to manufacture and can be manufactured at a relatively low cost. The trough shaped member may be secured to the plate member by any suitable securing means, however, in general, it is envisaged that welding would be most appropriate and convenient. It is not necessary that the free side edges of the webs of the trough shaped member should be continuously seam welded to the plate member, spot welds at spaced apart intervals or short seam welds provided at spaced apart intervals along the length of the free side edges would be sufficient. However, it will be appreciated that by increasing the length of the welds along the free side edges the heat conducting path between the trough shaped member and the plate member will be increased, thereby increasing the heat conduction efficiency between the trough shaped member and the panel member. This also improves the efficiency of the magnetic circuit. By welding the free side edges to the plate member by respective continuous seam welds along the entire length of the trough shaped member, particularly efficient heat transfer to the plate member is achieved and an efficient magnetic circuit is provided. The provision of the trough shaped member as a member of V-shaped cross-section provides a relatively efficient construction of panel heater, and by providing the trough shaped member of angle iron, a relatively low cost and robust panel heater is provided. The conductors may be insulated from their respective magnetic circuit means by any suitable means, for example, by an air gap being provided entirely around the conductor. The air gap may be provided by locating the conductor in the receiving bore of the magnetic means by means of electrically insulating spacers extending partly or completely circumferentially around the conductor. Alternatively, the conductor may be provided with circumferentially extending insulation, or alternatively, the conductor may be embedded in the receiving bore in an insulating material, such as, for example, expanded polyurethane and the like. Providing a plurality of spaced apart magnetic circuit means defining respective parallel receiving bores for accommodating respective conductors provides a particularly efficient construction of panel heater. Connecting conductors of adjacent pairs of receiving bores in series provides a relatively efficient construction of panel heater.

By operating the panel heater with the current through the conductors being in the range of 75 amps to 150 amps, and at a frequency of 50 Hz, a particularly efficient panel heater is provided, and furthermore, the panel heater is relatively safe. The provision of a temperature control means by a timer controlled switch provides a relatively low cost and convenient means for controlling the temperature of the panel.

The invention will be more clearly understood from the following description of some preferred embodiments thereof given by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of a panel heater according to the invention,

Fig. 2 is a cut-away perspective view of the panel heater of Fig. 1,

Fig. 3 is a side elevational view of the panel heater of Fig. 1,

Fig. 4 is an end elevational view of the panel heater of Fig. 1, Fig. 5 is an underneath plan view of the panel heater of Fig. 1,

Fig. 6 is a perspective view of a detail of the panel heater of Fig. 1,

Fig. 7 is a sectional end view of the detail of Fig. 6,

Fig. 8 is a circuit diagram of the panel heater of Fig. 1,

Fig. 9 is a sectional end view of a detail of a panel heater according to another embodiment of the invention,

Fig. 10 is a sectional end view of a detail of a panel heater according to a further embodiment of the invention,

Fig. 11 is an underneath plan view substantially similar to Fig. 5 of a panel heater according to another embodiment of the invention, and

Fig. 12 is a circuit diagram substantially similar to Fig. 8 of the panel heater of Fig. 11.

Referring to the drawings and initially to Figs. 1 to 8 there is illustrated an induction panel heater according to the invention indicated generally by the reference numeral 1 which is particularly suitable for use as a heated panel for mounting in the slatted floor (not shown) of a pig farrowing unit (also not shown). The panel heater 1 comprises a flat planar plate member 2 of ferromagnetic and electrically conductive material, namely, galvanised coated steel having a top surface 3 and a bottom surface 4. Magnetic circuit means, in this case, six spaced apart magnetic circuits 5 are provided by elongated portions 6 of the plate member 2 and respective elongated trough shaped members, namely, galvanised coated angle iron members 7 which extend parallel to each other and are secured at spaced apart locations to the plate member 2. Each angle iron member is of V- shaped cross-section and is formed by a pair of side webs 8 extending at right angles to each other. The side webs 8 terminate in free side edges 9 which abutt and are welded to the bottom surface 4 of the plate member 2. The portion 6 and angle iron member 7 which form each magnetic circuit 5 form an elongated receiving means, namely, an elongated receiving bore 10 of triangular cross-section for accommodating an electrical conductor 12 of copper for carrying an alternating current for in turn inducing eddy currents in the portion 6 of the plate member 2 and the angle iron member 7 for heating the plate member and the angle iron member 7. In this embodiment of the invention the angle iron members 7 are welded to the plate member 2 by continuous seam welds 13 which extend along the free side edges 9 of the angle iron member 7 from one end to the other. The continuous seam welds 13 provide good heat conduction from the angle iron members 7 to the plate member 2.

Each conductor 12 is electrically insulated from the plate member 2 and the angle iron members 7. A sleeve 14 of electrically insulating material is provided around each conductor 12. The conductors 12 are centrally located in the receiving bore 10 by an electrically insulating material, in this case, expanded polyurethane 15 which is poured into the receiving bores 10 in liquid form after the conductors 12 are centrally located in the receiving bores 10, and the polyurethane material 15 is allowed to expand and cure while the conductors 12 are held centrally located in the receiving bore 10. In this embodiment of the invention the conductors 12 extending through each pair of adjacent receiving bores 10 are connected in series and power as will be described below is applied to ends 18 and 19 of the adjacent conductors 12. The angle iron members 7 forming the magnetic circuits 5 are sufficiently spaced apart so that the magnetic flux formed by the alternating current flowing in adjacent conductors 12 do not interfere with each other. The angle iron members 7 being of angle iron are thus of a ferromagnetic material, and form with the portions 6 of the plate member 2 the magnetic circuits 5, which are complete magnetic circuits. Furthermore, the angle iron members 7 are electrically conductive over their entire length from one end to the other and accordingly, the alternating magnetic flux formed by the alternating current in each conductor 12 induces eddy currents which flow longitudinally from end to end in the corresponding angle iron member 7 in a direction parallel to the conductor 12, but in the opposite direction to the direction of current flow in the conductor 12. Needless to say, since the plate member 2 is also of an electrically conductive material, eddy currents are induced in the portions 6 of the plate member 2 forming the magnetic circuits 5.

A housing 20 of polycarbonate material is supported on an upright member 21 of hollow box section steel extending upwardly from the plate member 2. A step down transformer 25, see Fig. 8 is mounted in the housing 20 for supplying an alternating current at a stepped down voltage to the conductors 12 from a 220 volt 50 Hz AC mains supply. The transformer 25 has a pair of input terminals, namely, a neutral terminal 26 and a phase terminal 27 for connecting to the AC mains supply, and a pair of output terminals 28 and 29 from which the stepped down supply is supplied to the conductors 12 through supply cables 30 and 31. A connecting block 35 in the housing 20 connects the ends 18 and 19 of the conductors 12 to the supply cable 30 and 31, respectively. A bore (not shown) extending upwardly through the upright member 21 communicates with the interior of the housing 20 through an opening (not shown) in the housing 20, and also communicates with the bottom surface 4 of the plate member 2 through an opening 33 for accommodating the conductors 12 to the connecting block 35. A power factor capacitor Cl is connected across the input terminals 26 and 27. A fuse Fl is provided between the phase terminal 27 and the transformer 25. The former 34 of the transformer 25, the output terminal 29 and the plate member 2 are earthed for safety.

Temperature control means for controlling the temperature of the plate member 2, in this embodiment of the invention comprises a variable timer 36 which controls an on/off switch 37 between the phased terminal 27 and the transformer 25 for controlling supply of mains electricity to the transformer 25. The timer 36 is connected to the terminals 26 and 27 and is powered by the mains electricity supply. The timer 36 times a cycle time of 55 seconds, and the on-time period in the 55 second cycle during which the timer 36 holds the switch 37 on is variable from zero to 55 seconds. When set with the on-time period at zero, the timer 36 holds the switch continuously off, and when the on-time period is set at 55 seconds the timer 36 holds the switch 37 continuously on. A knob 38 extending through the housing 20 from the timer 36 permits the on-time period to be varied. To vary the temperature of the plate member 2, the on-time period of the timer is varied. Increasing the on-time period increases the temperature of the plate member 2, since current flows in the conductors 12 for a longer period. Reducing the on-time period decreases the temperature of the plate member 2, since current flows in the conductors 12 for a shorter time period.

In this embodiment of the invention the panel member 2 is 1,200 mm long by 600 mm width by 3 mm thickness. The angle iron members 7 are 40 mm by 4 mm thickness and extend for 1100 mm approximately along the plate member 2, which is substantially the length of the panel member 2. The angle iron members 7 are spaced apart at 85 mm centre to centre. The conductors 12 are of circular cross-section of 35 mm² cross-sectional area. When connected to a 220 volt 50 Hz AC mains supply, the transformer 25 supplies an AC output at 0.6 volts at 50 Hz on the output terminals 28 and 29. The voltage across the terminals 18 and 19 of the conductors 12 is approximately 0.5 volts. For an input current of 500 miHiamps, a 300 amp output is delivered on the output terminals 28 and 29. Since pairs of conductors 12 are connected in series and the pairs of conductors 12 are connected in parallel across the output terminals 28 and 29, the current flowing through each conductor 12 is approximately 100 amps. By setting the timer 36 to time an on-time period of fifteen seconds and thus, an off-time period of forty seconds it has been found that the temperature of the top surface 3 of the plate member 2 is maintained at a temperature of approximately 35°C.

Feet 40 of box section steel extending downwardly from the bottom surface 4 of the plate member 2 support the panel heater 1. A plurality of protuberances 41 are provided on the top surface 3 to form a non-slip top surface.

In use, the panel heater 1 is connected to a 220 volt 50 Hz AC mains electricity supply by connecting the input terminals 26 and 27, respectively, to the neutral and phase lines of the mains supply. The timer 36 is set by the knob 38. An alternating AC current of 100 amps at 50 Hz is delivered through the conductors 12 which in turn induce eddy currents flowing parallel to the conductors 12 in the respective portions 6 and the angle iron members 7 forming the magnetic circuit 5. The eddy currents generate heat in the angle iron members 7 and the portions 6 which in turn heat the plate member 2. Where it is desired to use the panel heater 1 in a pig farrowing unit, the panel heater 1 may be mounted on a slatted floor of a pig farrowing unit, or alternatively, may be recessed into the floor so that the plate member 2 forms part of the floor of the farrowing unit. In practise, a number of panel heaters 1 may be placed side by side to form a relatively large floor area. Where the panel heater 1 or heaters 1 are recessed into the floor, it is preferable that the top surface 3 of the plate member 2 should be aligned with the corresponding top surface of the slatted or other floor of the pig farrowing unit.

As mentioned above to maintain the top surface 3 of the plate member 2 at a temperature of approximately 35°C, the timer 36 is set to time an on-time period for the switch 37 of fifteen seconds followed by a corresponding off-time period of forty seconds. Should a higher or lower temperature be required, the timer 36 is set at the appropriate time setting using the knob 38. It has been found that where the timer 1 is set to hold the switch 37 continuously on the temperature of the top surface 3 of the plate member 1 can be raised through a temperature of approximately 35°C.

Referring now to Fig. 9 there is illustrated portion of a panel heater 50 according to another embodiment of the invention. The panel heater 50 is substantially similar to the panel heater 1 described with reference to Figs. 1 to 8, and similar components are identified by the same reference numerals. The main difference between the panel heater 50 and the panel heater 1 is that each magnetic circuit 5 is formed by an elongated hollow member 51 of square box section steel. An elongated bore 52 extends longitudinally through the hollow member 51, and the bore 52 forms the receiving bore 10 of the magnetic circuit 5 for receiving the conductor 12. The conductor 12 is provided with an insulating sleeve 14 and is set in expanded polyurethane 15 in the receiving bore 10. The hollow member 51 is secured to the plate member 2 by a pair of elongated continuous seam welds 53 extending on opposite sides of the hollow member 51. In this embodiment of the invention the portion 6 of the plate member 2 adjacent the hollow member 51 forms part of the magnetic circuit 5 with the hollow member 51. Although only one hollow member 51 is illustrated, a plurality of spaced apart elongated hollow members 51 extending parallel to each other are secured to the bottom surface 4 of the plate member 2.

Referring now to Fig. 10 there is illustrated a panel heater 60 according to a further embodiment of the invention. The panel heater 60 is substantially similar to the panel heater 1 of Figs. 1 to 8 and similar components are identified by the same reference numerals. The main difference between the panel heater 60 and the panel heater 1 is that each magnetic circuit 5 is formed by an elongated conduit 61 of steel of circular cross- section which is welded to the plate member 2 by a continuous elongated seam weld 63. An elongated bore 64 extending axially through the conduit 61 forms the receiving bore 10.

The conductor 12 surrounded by an insulating sleeve 14 is set axially in the receiving bore 10 in expanded polyurethane 15. In this embodiment of the invention virtually the entire magnetic circuit 5 is formed by the conduit 61 and virtually no eddy currents are induced in the portion of the plate member 2 adjacent the conduit 61. Accordingly, heat generated in the conduit 61 by the eddy currents is transferred through the weld 63 by conduction into the plate member 2. Although only one conduit 61 is illustrated, the panel heater 60 is provided with a plurality of parallel spaced apart conduits 61 welded to the bottom surface 4 of the plate member 2.

Referring now to Figs. 11 and 12 there is illustrated a panel heater 70 according to another embodiment of the invention. The panel heater 70 is substantially similar to the panel heater 1 described with reference to Figs. 1 to 8, and similar components are identified by the same reference numerals. The main difference in the panel heater 70 and the panel heater 1 is in the electrical connection arrangement of the conductors 12. In this embodiment of the invention all the conductors 12 are connected in series, and the conductors 12 extending through the bores 10 formed by the two outer angle iron members 7a terminate in respective ends 18 and 19 which are respectively connected to the cables 30 and 31 extending from the terminals 28 and 29 of the transformer 25. The transformer 25 delivers a 100 amp AC current at a 2 volt output voltage across the output terminals 28 and 29 for a 220 volt 50 Hz AC mains supply input. Accordingly, the voltage across each conductor 12 in each bore 10 is 0.3 volts approximately. The cross-sectional area of the conductors 12 in this embodiment of the invention is 35 mm². Otherwise, the construction and operation of the panel heater 70 is identical to the panel heater 1 described with reference to Figs. 1 to 8.

It is envisaged in certain cases, that a number of panel heaters according to the invention may be connected together. For example, a number of the panel heaters 70 may be connected in series. It is envisaged that twelve of the panel heaters 70 may be connected in series across the transformer 25, and in which case, it is envisaged that the transformer 25 would be rated to deliver a 100 amp output current at a voltage of 24 volt AC and at a frequency of 50 Hz across the terminals 28 and 29. This would thus provide a 2 volt supply across each panel heater 70.

While the panel heaters have been described as being powered by a 220 volt 50 Hz AC mains supply, the panel heaters may be powered by any other suitable alternating current supply, and it will of course be appreciated that the frequency of the alternating current being delivered to the conductors 12 may be greater than or less than 50 Hz. While the panel heaters have been described for use in a pig farrowing unit, it will of course be appreciated that the panel heaters may be put to many other uses, and indeed, it will be appreciated that the panel heaters may be used for space heating any other area, for example, a room. In which case the panel heaters may be floor mounted, wall mounted or indeed, ceiling mounted. The plate member of the panel heater may if desired form a panel in a wall, ceiling or the like. Needless to say, a number of panel heaters could be arranged side by side or at spaced apart intervals on a wall, ceiling or floor as desired. It is also envisaged that the panel heater may be used in an incubator for animal or human use and the panel heater may also be used for plant propagation, plant growth and the like.

While the panel heaters have been described as comprising a planar flat plate member, the plate member may be of other desired shape and construction, for example, the plate member may be curved and furthermore, the plate member may be of any other suitable or desired area. Indeed, in certain cases, it is envisaged that the plate member may be cylindrical to form a cylindrical wall of an incubator, farrowing unit, or indeed, a plate member of cylindrical shape may be used for any other purposes. Where the plate member is of cylindrical construction, the conductors and magnetic circuits may extend either axially relative to the cylindrical plate member or circumferentially around the plate member. Needless to say, the conductors may be provided on the inside or outside of the cylindrical wall.

While the construction of the panel heater whereby the magnetic circuit is formed by portion of the plate member and a trough shaped member secured to the plate member is a particularly advantageous construction, it will of course be appreciated that it is not essential that the magnetic circuit be formed by a trough shaped member. In certain cases, it is envisaged that a bore may be provided through the plate member and the magnetic circuit would accordingly be formed by a portion of the plate member adjacent the bore. It is also envisaged that where portion of the magnetic circuit is formed by a trough shaped member, the trough shaped member may be of any other suitable or desired shape or construction, and in certain cases, it is envisaged that the trough shaped member may be of channel shaped material or the like. It is also envisaged that each trough shaped member may be of inverted top hat cross-section, and the wings extending outwardly from the side webs may be welded to the bottom surface of the plate member by spot welds, seam welds or the like.

It is also envisaged that where the magnetic circuit is formed by a hollow member secured to the plate member, the hollow member may be of other cross-section besides circular or square, for example, rectangular, triangular, hexagonal or indeed, any other desired cross-sections. The hollow member may be welded to the plate member by either spot welds, spaced apart short seam welds or one or more continuous elongated seam welds extending the length of the tubular member, or by any other suitable securing means.

While the magnetic circuits of each panel heater have been described as forming a receiving means in the form of a bore for receiving the conductors, in certain cases, it is envisaged that the receiving means may be of form other than a bore, for example, in certain cases, it is envisaged that the receiving means may be in the form of an elongated channel or the like which would be open along one longitudinal face. In such cases, it is envisaged that each magnetic circuit would be formed by a pair of spaced apart elongated side walls extending downwardly from the plate member which would form with the plate member the receiving channel for receiving the conductor. Alternatively, the channel may be provided by a channel member welded to the plate member with the main web of the channel member welded to the plate member and the two side webs extending downwardly therefrom.

Needless to say, where the parts of the magnetic circuit and conductors are provided externally of the plate member, it is preferable that the conductors and parts forming the magnetic circuit should be provided on a non-working or non-exposed surface. For example, in the panel heaters described with reference to the drawings, it is preferable that the angle iron members, hollow members and conductors should be provided on the bottom surface rather than the top surface of the plate member. Although needless to say, in certain cases, it is envisaged that the conductors and parts forming the magnetic circuit may be provided on a working surface.

It is also envisaged that more than one conductor may be provided in each receiving bore of the respective magnetic circuits, and furthermore, it is envisaged that the cross-sectional area of the conductors may be other than 35 mm². Indeed, in certain cases, it is envisaged that the cross-sectional area of each conductor may be as low as 25 mm², and in certain cases, it may be as low as 16 mm², in which case a single conductor of such cross- sectional area would be sufficient in each bore.

Wh le the plate member has been described as being of steel plate material, the plate member may be of any other suitable ferromagnetic material. Additionally, it will be appreciated that while the trough shape members have been described as being of iron, they may be any other suitable ferromagnetic electrically conductive material. Needless to say, the conductors may be of any other suitable electrically conductive material besides copper.

While the transformers have been described for stepping down the voltage to provide the supply to the conductors at approximately 0.3 to 0.6 volts, the supply voltage to the conductors may be of any other desired voltage but for safety purposes it is preferable that the supply voltage to the conductors should not exceed 24 volts. Advantageously, the supply voltage to the conductors should be less than 3 volts. While a current of a particular value has been described as flowing through the conductors, the current may be of any other suitable or desired value.

It will of course be appreciated other suitable temperature control means besides a timer may be used, and indeed, in certain cases, it is envisaged that a temperature sensor may be mounted on the plate member which would operate an on/off switch in response to the temperature of the plate member. Alternatively, where the panel heater is used in a pig farrowing unit, a temperature sensor may be mounted in any suitable location in the pig farrowing unit for monitoring the temperature therein. The temperature sensor would operate an on/off switch for switching the mains supply to the transformer in response to the temperature of the pig farrowing unit exceeding or dropping below certain predetermined limits. While the conductors have been described as being centrally located in the bores by expanded polyurethane material, any other suitable means for locating the conductors centrally or otherwise in the bore may be used.

Claims

1. An induction panel heater comprising a plate member (2) of ferromagnetic material, an elongated magnetic circuit means (5) adjacent the plate member defining an elongated receiving means (10), and an elongated conductor (12) extending axially through the receiving means (10) for carrying an alternating current for inducing an eddy current to flow in the plate member (2) substantially parallel to the conductor (12) for heating the plate member (2), characterised in that the magnetic circuit means (5) is of an electrically conductive material and is electrically conductive in a longitudinal direction parallel to the receiving means (10) for permitting eddy currents induced by the conductor (12) to flow in the magnetic circuit means (5) parallel to the receiving means (10) for heating the magnetic circuit means (5).

2. A panel heater as claimed in Claim 1 characterised in that the plate member (2) forms part (6) of the magnetic circuit means (5).

3. A panel heater as claimed in Claim 1 or 2 characterised in that the plate member (2) forms a longitudinal extending part (6) of the receiving means (10).

4. A panel heater as claimed in any preceding claim characterised in that the receiving means (10) is an elongated receiving bore (10).

5. A panel heater as claimed in Claim 4 characterised in that part of the magnetic circuit means (5) is formed by an elongated trough shaped member (7) of electrically conductive material having a pair of longitudinal side webs (8) terminating in respective spaced apart free side edges (9), the trough shaped member (7) being secured to the plate member (2) with the free side edges (9) abutting the plate member (2) to form with the plate member (2) the receiving bore (10).

6. A panel heater as claimed in Claim 5 characterised in that the trough shaped member (7) is of V-shaped cross-section.

7. A panel heater as claimed in Claim 5 or 6 characterised in that the trough shaped member (7) is of angle iron.

8. A panel heater as claimed in any of Claims 5 to 7 characterised in that the trough shaped member (7) is secured to the plate member (2) by welding (13) the free side edges (9) to the plate member (2).

9. A panel heater as claimed in Claim 8 characterised in that the free side edges (9) are welded to the plate member (2) by respective continuous elongated seam welds (13).

10. A panel heater as claimed in Claim 8 characterised in that the free side edges (9) are welded (13) to the plate member (2) by a plurality of short welds at spaced apart locations.

11. A panel heater as claimed in Claim 4 characterised in that the magnetic circuit means (5) is formed by an elongated hollow member (51,61) having a longitudinally extending bore (52,64) forming the receiving bore (10) for the conductor (12), the hollow member (51,61) being secured to the plate member (2).

12. A panel heater as claimed in Claim 11 characterised in that the hollow member (51,61) is welded to the plate member (2).

13. A panel heater as claimed in any preceding claim characterised in that the conductor (12) is electrically insulated from the magnetic circuit means (5).

14. A panel heater as claimed in any preceding claim characterised in that a plurality of spaced apart elongated magnetic circuit means (5) are provided defining respective parallel receiving bores (10), a conductor (12) being provided in each receiving bore (10).

15. A panel heater as claimed in Claim 14 characterised in that the magnetic circuit means (5) are parallel to each other.

16. A panel heater as claimed in Claim 14 or 15 characterised in that the conductors (12) in pairs of adjacent receiving bores

(10) are connected in series.

17. A panel heater as claimed in any preceding claim characterised in that a step down transformer (25) for stepping down an AC mains supply voltage is provided, the transformer (25) comprising an input (26,27) for receiving mains voltage and an output (28,29) for applying an alternating voltage and current on each conductor (12).

18. A panel heater as claimed in Claim 17 characterised in that the voltage across each conductor (12) is in the range of 0.1 volts to 24 volts.

19. A panel heater as claimed in Claim 18 characterised in that the voltage across each conductor (12) is in the range of 0.3 volts to 12 volts.

20. A panel heater as claimed in Claim 19 characterised in that the voltage across each conductor (12) is in the range of 0.3 to

0.6 volts.

21. A panel heater as claimed in any of Claims 17 to 20 characterised in that the frequency of the alternating current delivered by the transformer (25) is in the range of 20 Hz to 300 Hz.

22. A panel heater as claimed in Claim 21 characterised in that the frequency of the alternating current delivered by the transformer (25) is in the range of 40 Hz to 150 Hz.

23. A panel heater as claimed in Claim 22 characterised in that the frequency of the alternating current supplied by the transformer (25) is approximately 50 Hz.

24. A panel heater as claimed in any of Claims 17 to 23 characterised in that the alternating current delivered by the transformer (25) through each conductor (12) is in the range of 50 amps to 300 amps.

25. A panel heater as claimed in Claim 24 characterised in that the alternating current delivered by the transformer (25) through each conductor (12) is in the range of 75 amps to 200 amps.

26. A panel heater as claimed in Claim 25 characterised in that the alternating current supplied by the transformer (25) through each conductor (12) is approximately 100 amps.

27. A panel heater as claimed in any of Claims 17 to 26 characterised in that the step down transformer (25) receives an input voltage of approximately 220 volts AC at approximately 50 Hz.

28. A panel heater as claimed in any of Claims 17 to 27 characterised in that a temperature control means (36,37) for controlling the temperature of the plate member (2) is provided.

29. A panel heater as claimed in Claim 28 characterised in that the temperature control means (36,37) comprises a timer (36) controlled switch (37) for switching on and off electrical power to each conductor (12) at predetermined time intervals.

30. A panel heater as claimed in Claim 29 characterised in that the timer (36) controlled switch (37) is provided at the input to the transformer (25).

31. A panel heater as claimed in any of Claims 17 to 30 characterised in that a housing (20) defining a hollow interior region is provided for housing the transformer (25).

32. A panel heater as claimed in Claim 31 characterised in that the housing (20) is mounted on an upstanding member (21) extending upwardly from the plate member (2).

33. A panel heater as claimed in Claim 32 characterised in that the upstanding member (21) comprises an elongated conduit for accommodating cables (30,31) between the transformer (25) and the conductors (12) for delivering current from the transformer (25) to the conductors (12).

34. A panel heater as claimed in any preceding claim characterised in that the conductors (12) in the respective receiving bores (10) are connected in series.

35. A panel heater as claimed in any preceding claim characterised in that the plate member (2) has a top surface (3) and a bottom surface (4), each magnetic circuit means (5) being provided on the bottom surface (4).

36. A panel heater as claimed in any preceding claim characterised in that the panel heater (1) is a floor panel.