US 3796850 A
Description (OCR text may contain errors)
[ Mar. 12, 1974 PAN DETECTOR FOR INDUCTION 2,497,753 2/1950 Arnot 219/1049 HEATING COOKING UNIT 3,426,166 2/1969 Canceill 335/205  Inventors: William C. Moreland, 11, Export;
Robert A. McCoy, Turtle Creek; Primary ExaminerBruce A. Reynolds Terence D. Malarkey, Monroeville, Attorney, Agent, or FirmE. C. Arenz all of Pa.
 Assignee: Westinghouse Electric Corporation,
Pittsburgh, Pa.  ABSTRACT  Filed: May 1973 A pan detection and control arrangement is provided [21 L N 365,671 for an induction heating cooking unit by providing a reed switch and a pair of permanent magnets disposed relative to the reed switch to hold the reed switch in a  US. Cl 219/1049, 219/10.75, 219/518, closed position in the absence of a sufficient size 335/219 cooking vessel in position overlying the work coil, the lift. C1. reed switch operating to a p p i to permit  Flld of Search 219/10.49, 10.75, 10.77, energization of the work coil when the ferromagnetic 219/518, 519; 335/205, 219, 286; 338/12; mass provided by the cooking vessel is in place to 336/1316" 2; 318/128 shunt a sufficient part of the magnetic field through the vessel and away from the reed switch, Opening of  References and the reed switch permits energization of the work coil.
UNITED STATES PATENTS 3,710,062 1/1973 Peters 219/1049 5 Claims, 5 Drawing Figures 7 i i 22 g 1 24\ 4 t \J 26'- b0 /-28 l\ L L ":T I J T ,q
PATENTEDHARIZ I91 3.796350 SHEET 2 OF 2 PHASE POWER LINE I gggfig' -g OSCILLATOR i 44 48 i I l MANUAL CONTROL FIG. 5.
PAN DETECTOR FOR INDUCTION HEATING COOKING UNIT CROSS REFERENCES TO RELATED APPLICATIONS US. patent application Ser. No. 243,715 and the companion US. patent applications referred to therein disclose circuit arrangements with which the arrangement according to the present invention may be used.
BACKGROUND OF THE INVENTION 1. Field of the Invention:
The invention pertains to the art of pan detection arrangements for use in induction heating cooking appa ratus.
2. Description of the Prior Art:
It has been suggested heretofore, as in US. Pat. No. 2,497,753, that properly locating a mass to be heated by induction heating apparatus can be used to control a switch controlling the energization of the induction heating apparatus. In the noted patent, the presence of a pressing iron, or a cooking vessel, in a proper location results in the attraction and movement of a permanent magnet toward the mass to thereby move means for closing a switch to permit heating of the iron or vessel. In the arrangement of that patent, the permanent magnet and switch is disclosed as being located to the side of the pressing iron to be heated by the induction heating apparatus. Such an arrangement would be inconsistent with the provision of a smooth top cooking platform such as is currently preferred for induction heating cooking apparatus. The general concept of that arrangement could be employed in connection with current induction heating cooking apparatus by locating a similar arrangement in the core of the work coil of the apparatus. However such an arrangement would have several disadvantages with respect to the arrangement according to the invention. Among these disadvantages are that the magnet in moving would likely make an audible sound at the end of its travel, the magnet so located would be more sensitive to concentrated magnetic masses than to a distributed one, the space provided by the core of the work coil is located in a position making it highly useful for other devices such as a thermal sensor, and the mechanism would be exposed to accidental mishandling by service personnel.
An arrangement according to the present invention is considered to obviate all of these objections.
SUMMARY OF THE INVENTION In accordance with an arrangement according to the invention, there is provided safety control means responsive to levels of magnetic flux above and below the given range of values for preventing and permitting, respectively, energization of a work coil, permanent magnet means is disposed relative to the safety control means and to the location of the base wall of a cooking vessel properly located over the work coil to direct a magnetic field with a flux level above the given values through the safety control means in the absence of a ferromagnetic mass at least comparable to the smallest cooking vessel to be used on said coil and in the proper location, with the magnetic field being shunted sufficiently in the presence of the mass in the proper location to reduce the flux level of the field through the safety control means to a value below the given values so that unless an adequate load for coupling is present above the coil, the coil is prevented from being energized.
In the currently preferred form of the invention, the safety control means takes the form of a reed switch located generally in a fringe magnetic field between a pair of permanent magnets which have their poles arranged and are spaced apart such that in the absence of a mass corresponding to a properly located cooking vessel the flux level passing through the reed switch causes it to be closed and in the presence of the mass the field is shunted sufficiently that the reed switch opens.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation in the nature of a side elevation showing flux paths in the absence of a magnetic material vessel in position on a work coil;
FIG. 2 is a schematic representation, also in the nature of a side elevation, showing the shunting effect of the magnetic material vessel on the flux paths;
FIG. 3 is a view in the nature of a vertical cross section showing the relationship between the cooking vessel on the cooking surface, the work coil, and the sensing assembly;
FIG. 4 is a plan view ofa single cooking unit showing the location of the permanent magnets and the reed switch relative to the cooking location; and
FIG. 5 is a block diagram and partly schematic view of the .way in which the arrangement according to the invention may be connected to a circuit for controlling the cooking unit.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the safety control means is shown in its currently preferred form as a sealed, magnetic reed switch 10 having leaf contacts 12 and 14 which extend out through the ends of the glass envelope to provide terminals. The contacts of the reed switch are normally open but are shown as closed in FIG, 1 becuase of the field from the permanent magnets causes sufficient flux to pass through the leaves of the reed switch to force their contact ends to touch each other. The magnetic lines of force, some of which are collectively identified by the numeral 16, are produced by the pair of permanent magnets 18 and 20 which have their poles arranged as shown to provide the magnetic field schematically illustrated. Thus the orientation of the poles of the left magnet 18 is with north at the top and south at the bottom, while the orientation of the right magnet is reversed.
FIG. 2 shows the influence upon the location of the lines of force 16 exerted by placing an extended area mass 22 of magnetic material in proximity to the reed switch 10. As there shown, the extended area mass of magnetice material which corresponds to the bottom wall of a cooking vessel shunts an adequate portion of the field so that the induced magnetism in the leaves 12 and 14 is insufficient to maintain the switch 10 is a closed position.
Incorporation of the arrangement according to the invention in an induction heating cooking unit is best understood in connection with FIGS. 3 and 4. A refractory material such as a glass ceramic sheet 24 provides a cooking surface above the work coil 26. A suitable I work coil may comprise litz wire which is wound in a spiral and then molded in a rubber compound to hold the wire in place with the proper spacing between successive convolutions. The work coil also typically includes an underlying layer 28 of the rubber compound; The work coil is supported from below by an underlying Transite sheet or block 30 which supports the work coil. The Transite block 30 is supported by means not shown herein. A slot 32 is grooved out of a generally central part of the block 30 to receive the reed switch which is held in place by potting it in with a rubber compound such as Dow Corning Sylgard 185. The groove is made sufficiently long to accommodate the projecting terminal ends of the leads, which are connected to two lead wires 34 and 36 (Fig. 3).
In the currently preferred form of the invention, the reed switch should be sufficiently long that the requisite sensitivity is provided with respect only to magnetic objects having a sufficiently extended area as to be comparable to the smallest diameter cooking vessel intended for use for the coil. Reed switches which satisfy this requirement are about two to three inches long (including their terminal ends) such as the Hamlin Company DRS-Z and DRT-S. Examples of permanent magnets 18 and which I have found to function satisfactorily are Allegheny General lndox V magnets having top and bottom face areas of about 2 inches by 1 inch, and about inch thick. Such magnets are adequate for operating reed switches with closing requirements of 70 ampere-turns and opening values of 50 or more ampere turns and are not heated appreciably by the coil of the range. The spacing between the facing ends of such magnets used with the identified reed switch is about 4 inches. However, since variations may occur between one reed switch and another, and between one pair of magnets and another current practice is to locate the magnets relative to the reed switch by first bringing them together sufficiently close that the contacts close, and then bringing a magnetic mass comparable to a cooking vessel bottom into the proper location, then moving the magnets apart until the contacts open. The magnets are then fixed in place and the operation checked. It is noted in this respect that a differential may typically be found to exist between the levels of magnetic flux effecting closing and opening of the reed switch. Thus it is responsive to levels of magnetic flux above and below a given pair of values, rather than a specific given value.
As may be seen from FIG. 4, it is not necessary that the reed switch and magnet be centered exactly relative to the center of the work coil. It is only necessary that it be responsive to the bottom wall of the cooking vessel which is placed generally coincident with the work coil, the location of which is generally indicated by the locating indicia 38 provided on the top surface of the cooking surface.
The manner in which the arrangement according to the invention is incorporated in general circuit arrangements of the noted patent applications for operating an induction heating cooking unit will be described in connection with FIG. 5. The control circuit portion 40 of the arrangement disclosed includes a timing and firing capacitor 42 for the phase controlled rectifier bridge 44. When the pan detector switch 10 is open due to the presence of a pan bottom 22, the capacitor charges up from various sources as disclosed in the noted applications, and is discharged by a semiconductor switch through a pulse transformer which couples into and turns on the phase controlled rectifier bridge.
When the pan detector switch 10 is closed because of the absence of a pan, the capacitor 42 is shorted out through resistor 46 and accordingly prevented from charging up. Hence the phase controlled rectifier bridge 44 cannot be turned on, so no power can reach the power oscillator 48 and work coil 26.
It will be appreciated that the pan detector switch could be incorporated in the circuit in other ways, such as by controlling a relay which in turn controls power in one part or another of the circuit.
The arrangement according to the invention provides the following safety features. It prevents heating of aluminum foil and possible fire hazards of foil covered packages. It prevents inadvertent heating of metallic cooking utensils such as spoons and forks with consequent burning of the users hand. lt also protects electronic equipment and pacemaker wearers from an energized coil without the shielding provided by a cooking vessel.
It is to be noted that in an arrangement according to the invention the switch is located in what may be considered to be a fringe portion of the magnetic field as distinguished from what could be characterized as a main field if the magnets 18 and 20 were of the type in which the poles were at the ends of the bars, and the north pole of 18 were facing a south pole of 20. By locating the reed switch in the fringe area of the field, placing a relatively extended magnetic mass comprising the bottom wall of the cooking vessel is able to shunt sufficient lines of force thereto to permit the reed switch to be moved to an open position.
While the currently preferred arrangement includes the use of the reed switch, it is to be understood that other devices responsive to a magnetic field shunted into and out of position could be used alternatively, one such example being a Hall generator which could be connected to provide an on-off switching arrangement.
What we claim is:
1. In an induction heating cooking unit of the type including a work coil which is adapted to be coupled electromagnetically to a cooking vessel placed in overlying relation to said coil;
safety control means responsive to levels of magnetic flux above and below a given range of values of said flux for preventing and permitting, respectively, energization of said coil;
permanent magnet means disposed relative to said safety control means, and to the location of the base wall of a cooking vessel when properly located over said coil, to direct a magnetic field with a flux level above said given values through said safety control means in the absence of a ferromagnetic mass at least comparable to the smallest cooking vessel to be used on said coil and in said proper location;
said magnetic field being shunted sufficiently in the presence of said mass in said proper location to reduce the flux level of the field through said safety control means to a value below said given values, so that unless an adequate load for coupling is present above said, coil, said coil is prevented from being energized.
2. In a unit according to claim ll wherein:
said safety controls means comprises switching means closely adjacent said coil; and
3,796,850 5 6 said permanent magnetic means comprise a pair of magnetic mass at least comparable to the base wall of permanent magnets on opposite sides of said the smallest cooking vessel to be used with said coil switching means, said magnets being disposed relawhen in overlying relation to said coil comprising: tive to said switching means, and having their poles safety switch means responsive to the presence of arranged, being spaced apart such that in Said 5 levels of magnetic flux above and below a given allsencef of 531d mass the flux f p f l through range of values to assume one and another condisaid switching means causes sa d switching means on, respectively; tolassume one corldmon and h presence 9 permanent magnet means disposed relative to said salfj l the field shunted sufficlemly from f safety switch means and to the location of the base swltchlng means that the flux levfzl through l wall of the cooking vessel properly located over sw tch ng means is reduced sufficiently that said Said coil, to place said Safety switch means in a 3 l meanzassumei i l l magnetic field providing a level of magnetic flux I accfr to C d d above said given range of values in the absence of l is y con r0 means me u as a mdgne 1C ree said base wall of said cooking vessel, said permal5 nent magnet means being so disposed that suffi- 4. In a unit according to claim 2 wherein: cient lines of force are shunted sufficiently in the said permanent magnets have their poles oriented f n f It generally vertically, with the north pole of one g gf l i f fi 9 E g 3 f? uce e eve so magne ic uxmw ic sai sa e y magnet being upwardly facing and the north ole of the other magnet being downwardly facingpand switch means is present to a level below said given with said magnets being spaced apart sufficiently range of f l and that said reed switch is located in a fringe area of elecmcfil f l mefms f controllmg energlzancf" the magnetic lines f f of said am] including said safety switch means in 5. In an induction heating cooking unit of the type inl one n preventmg finerglzatlofl of 531d cluding a work coil underlying a refractory material 11 and In sa d another 00116117410" Permlmng ener' sheet f rmi a ki surface upon hi h a ki gization of said coil, so that unless an adequate load vessel is adapted to be supported and electromagnetf oup ing i Pr in h proper loc ion O erically coupled to the coil when the coil is energized, an lying said coil, said coil is prevented from being enarrangement for controlling the energization of said ergized. work coil in accordance with the presence of a ferro-