GB1161350A - Improvements in or relating to Dielectric Heating Apparatus - Google Patents

Abstract

1,161,350. R.F. couplings; microwave ovens. HUSQVARNA VAPENFABRIKS A.B. 23 Dec., 1966 [28 Dec., 1965], No. 57704/66. Headings H4A and H5H. Apparatus for dielectric heating by means of microwaves comprises a substantially closed metal casing into which power is fed from a magnetron by a waveguide leading to the lower end of an upwardly directed funnel-shaped bottom member of said casing. A substantially horizontal non-metallic carrier means is provided at the top of the funnel-shaped member, for an article to be treated. Close below said carrier means there is provided a horizontally disposed secondary aerial or field stirrer, which is arranged to rotate on a vertical, central axis. As described, a dielectric heating apparatus comprises a die-cast or otherwise formed light metal structure providing a hollow magnetron anode body 5, Fig. 1, an outer coaxial conductor 51, a rectangular waveguide 5a, and a lower member 5b of a treatment cavity 10. The lower member 5b is funnel-shaped for impedance matching and terminates in a flange 27 upon which rests a domed, perforated metal upper member 5c provided with a hinge (25), Fig. 2 (not shown), and a handle 29. The members 5b, 5c define a treatment cavity 10 in which food or other material to be heated is supported on a steatite plate 12 which has apertures 48 around its circumference and is located in the upper part of the member 5b. The magnetron body 5 contains a central cathode 1 which is surrounded by resonant anode cavities 3 formed by inwardly projecting walls 4, which are coated with copper or silver. An aerial rod 7 is connected to a wall 4 and is taken coaxially with the bore 51 through a glass or ceramic seal to project into the waveguide 5a. The cathode 1 is supplied with filament heating current and high voltage from a unit 32 which is encapsulated, together with connecting means 8, 9, in a metal casing 37. The magnetron is evacuated through a tube 6 and a horseshoe magnet 30 provides an axially extending magnetic field. Energy from the aerial is coupled via the waveguide 5a and the funnel-shaped lower member 5b into the upper member 5c, in the top of which there is also provided an aerial 13, disposed at right angles to the aerial 7. The aerial 13 is fed via a coaxial cable 14 from a probe 15 projecting into the waveguide 5a. Under the carrier plate 12 an insulating disc 20 is rotatably suspended on a shaft 16 and carries an array 11 of parallel copper strips 19. The disc 20 has blades 17 around its circumference, which cause it to rotate when an air current flows. A plate 50 under the disc 20 prevents food particles &c. falling into the waveguide 5a. Cooling fins 23 are provided on the outsides of the members 5a, 5b and air from a blower 22 is guided by a casing 24 to pass through apertures 21 in the member 5b, the blades 17, the apertures 48, and apertures 49 in the member 5c. A resistor 46, which is in series with the ground end of the high-voltage supply to the magnetron anode, is embedded in insulating material in a groove 47 under the flange 27 of the member 5b and the heat produced keeps that flange and the co-operating flange 26 of the member 5c dry. Mains power supply to terminals (L1), (L2), Fig. 3 (not shown), is connected to a transformer (41), for providing filament heating current for the magnetron, and to a high-voltage transformer (42) through series switches (43), (44), (45). Switch (43) closes when the cathode reaches its operating temperature, switch (44) closes when the upper member of the treatment cavity is closed, and switch (45), which may then be closed, may re-open automatically after a selectable interval. The high-voltage output of the transformer is rectified by rectifiers (33) and a control circuit comprising transistors (34), (35) and a transistor (36) provides shunting for the resistor (46) in order to maintain constancy of the anode current of the magnetron. In an alternative construction, the resonant cavities 3, Fig. 1, and the walls 4 may be formed in a separate copper cylinder, which may be secured in a co-operating recess in the body 5. In another embodiment (Fig. 4, not shown), a magnetron of conventional form, which is detachable from the apparatus, is used and the auxiliary aerial in the top of the upper member of the treatment cavity is omitted. The aerial (7) is contained in an evacuated glass bulb (62) extending from the magnetron, and the part not projecting into the waveguide (5a) is surrounded by a finned diecast outer coaxial conductor (64). Cooling air is blown through apertures (72) in the bottom of the waveguide and, the plate 50, Fig. 1, being omitted, is guided by a calotte-shaped extension (77), extending below the disc (20), into the upper member (5c). The lower part of the apparatus, which may include the transformers (41), (42), Fig. 3 (not shown), is contained in a sheet-metal housing (83). The upper member (5c) is hemispherically shaped and is swung inside the housing (83) when it is opened. Sprung pivot pins (78), Fig. 5 (not shown), are fitted and by pressing these inwards the upper member (5c) may be removed.