Cooking appliance with a broadband microwave generator and ventilation duct for a cooking appliance

Abstract

Cooking appliance (10) with a broadband microwave generator (16), a cooking chamber (12), a cooking chamber wall (14) which has a wall feedthrough (20), a motor shaft (26) which extends out of the cooking chamber (12) through the wall feedthrough (20), and two microwave traps (22, 24) which are arranged around the motor shaft (26), wherein the microwave traps (22, 24) have different resonant frequencies, characterized in that one of the microwave traps (22, 24) has a λ / 4 trap (22) which surrounds the motor shaft (26) and that the resonant frequency of the λ / 4 trap (22) is between 2.3 GHz and 2.4 GHz.

Description

[0001] The invention relates to a cooking appliance with a broadband microwave generator and a ventilation duct for a cooking appliance.

[0002] Cooking appliances with microwave generators are well known. In these, the microwaves generated by the microwave generator are directed into a cooking chamber containing the food to be heated. The cooking chamber is designed to prevent the uncontrolled escape of microwaves from the cooking chamber, so as not to endanger the user of the cooking appliance or to prevent interference with surrounding electronics. In the event that a motor shaft is guided from outside the cooking chamber into the interior, special attention must be paid to the point where the motor shaft penetrates the cooking chamber wall, because the motor shaft acts similarly to an antenna that can direct microwaves from the cooking chamber to the outside. Typically, either sliding contacts, which electrically connect the motor shaft to the cooking chamber wall, or so-called λ / 4 traps are used, which reflect the microwaves traveling on the motor shaft and thus prevent microwaves from escaping.

[0003] The λ / 4 traps are tailored to the wavelength of the microwaves generated by the microwave generator, so that they only significantly reflect a very narrow frequency band around the frequency of the generated microwave. This is not a problem for cooking appliances with a microwave generator that generates microwaves of a single frequency, because the λ / 4 trap can be adjusted to the frequency of the generated microwaves. However, if the cooking appliance is operated with multiple microwave generators of different frequencies or with a broadband microwave generator, i.e., a microwave generator that generates microwaves over a specific frequency band, a λ / 4 trap around the motor shaft only inadequately prevents microwaves from escaping through the motor shaft.

[0004] DE 20 2005 005 143 U1 discloses a cooking appliance with a drive shaft for a fan impeller. The drive shaft is guided through an opening in the wall of the cooking chamber and surrounded by a tubular, coaxial microwave damping device. A microwave trap is also provided on the inside of the wall, which encloses the drive shaft.

[0005] The object of the invention is to effectively prevent the escape of radiation from the cooking chamber of a cooking appliance with a broadband microwave generator over the entire bandwidth of the microwave generator.

[0006] This object is achieved by a cooking appliance according to claim 1, comprising a broadband microwave generator, a cooking chamber, a cooking chamber wall having a wall duct, a motor shaft extending from the cooking chamber through the wall duct, and two microwave traps arranged around the motor shaft, the microwave traps having different resonant frequencies. The invention is based on the finding that two microwave traps with different resonant frequencies, which are cleverly selected within the frequency range of the microwave generator, enable broadband reflection of the microwaves guided via the motor shaft, thereby dampening the transmission of the microwaves through the motor shaft out of the cooking chamber.The distance between the two resonance frequencies is chosen such that the flanks of the resonance peaks facing each other merge into one another, so that at least in the range between the two resonance frequencies there is a considerable attenuation of the transmitted microwaves.

[0007] One of the microwave traps has a λ / 4 trap surrounding the motor shaft, thus realizing a microwave trap in a simple and proven way.

[0008] The resonance frequency of the λ / 4 trap is between 2.3 GHz and 2.4 GHz, so that the λ / 4 trap covers the low-frequency range of the spectrum of the microwave generator.

[0009] Preferably, the resonance frequencies are selected such that the spectrum of the microwave generator lies largely between the two resonance frequencies or is covered by the interaction of the resonance frequencies, so that an uncontrolled escape of microwaves generated by the microwave generator via the motor shaft is reliably prevented.

[0010] Preferably, the λ / 4 trap is arranged inside the cooking chamber on the cooking chamber wall so that the external dimensions of the cooking chamber are not affected by the λ / 4 trap.

[0011] For example, the λ / 4 trap creates an open circuit for microwaves, whereby microwaves guided via the motor shaft with frequencies very similar to or equal to the resonant frequency of the λ / 4 trap are reflected by the motor shaft.

[0012] In one embodiment, the λ / 4 trap has a dielectric, in particular a ceramic, which enables a compact design of the λ / 4 trap.

[0013] Preferably, the dielectric has a geometric length parallel to the motor shaft which corresponds to the electrical length of a quarter wavelength of a microwave of the desired resonance frequency of the λ / 4 trap, whereby the resonance frequency of the λ / 4 trap can be adjusted as desired.

[0014] In one embodiment of the invention, the cooking appliance has a ventilation duct which is arranged on the cooking chamber wall and which has an opening to the cooking chamber which is aligned with the wall duct, wherein the ventilation duct forms one of the microwave traps, whereby a second microwave trap is realized on the cooking appliance without additional components.

[0015] Preferably, the motor shaft extends through the opening to the cooking chamber of the ventilation duct, so that a simple coupling of the microwaves guided in the motor shaft to the ventilation duct is ensured.

[0016] In a further embodiment, the ventilation duct has a section designed as a conductor, in particular a waveguide, particularly preferably a rectangular waveguide, for microwaves, with the conductor connecting to the opening to the cooking chamber. This provides a blind line for the microwaves.

[0017] For example, the conductor extends perpendicular to the motor shaft, which enables a space-saving design of the cooking appliance.

[0018] Preferably, the geometric length and the cross-section of the conductor are adapted to each other such that the conductor has an electrical length corresponding to a quarter wavelength of a microwave of the desired resonant frequency of the ventilation duct, whereby the resonant frequency of the ventilation duct can be adjusted.

[0019] For example, the ventilation duct has a section, in particular with a grid or a duct section with a round cross-section, which prevents the propagation of microwaves in and through this section and which closes off the conductor on its side facing away from the motor shaft, whereby the ventilation duct is closed off to microwaves but remains permeable to air currents.

[0020] The resonant frequency of the ventilation duct can be between 2.5 GHz and 2.6 GHz, whereby the higher frequency range of the frequency spectrum of the microwave generator is covered by the ventilation duct.

[0021] The invention further relates to a ventilation duct for use in a cooking appliance with a microwave generator, wherein a motor shaft of the cooking appliance is guided through the ventilation duct, and the ventilation duct forms a microwave trap for microwaves of at least one frequency that are guided via the motor shaft and generated by the microwave generator. In this way, a microwave trap is realized without additional components.

[0022] Further features and advantages of the invention will become apparent from the following description and the accompanying drawings, to which reference is made. In the drawings: - Fig. 1 a schematic representation of a cooking appliance according to the invention, - Fig. 2 a perspective view of a λ / 4 trap, which is mounted on a wall duct of a cooking chamber of the cooking appliance of Fig. 1 is arranged, - Fig. 3 a perspective view of a ventilation duct according to the invention, and - Fig. 4 a graph showing the frequency-dependent transmission of microwaves via the motor shaft out of the cooking chamber of a cooking appliance according to the invention.

[0023] In Fig. Figure 1 schematically illustrates a cooking appliance 10 according to the invention. The cooking appliance 10 has a cooking chamber 12 surrounded by a cooking chamber wall 14.

[0024] Furthermore, a broadband microwave generator 16 is provided, which is connected to the cooking chamber 12 via a waveguide 18 or an antenna. The microwave generator 16 is configured, for example, with a semiconductor amplifier.

[0025] The cooking chamber wall 14 has a wall duct 20 in the form of an opening for a motor shaft. On the inside of the cooking chamber wall 14, a λ / 4 trap 22 is arranged around the wall duct 20 and is attached to the cooking chamber wall 14.

[0026] A ventilation duct 24 is provided on the outside of the cooking chamber wall 14, which extends largely along the cooking chamber wall 14.

[0027] The ventilation duct 24 is traversed vertically by a motor shaft 26, with the motor shaft 16 extending from outside the ventilation duct 24 through the duct and the wall duct 20 into the cooking chamber 12. In the cooking chamber 12, a portion of the motor shaft 26 is surrounded by the λ / 4 trap 22.

[0028] A functional part 28 is provided at the end of the motor shaft 26 that extends further into the cooking chamber 12. This functional part 28 can be a fan wheel, as in the embodiment shown, but a turntable or similar device is also conceivable. The motor shaft 26 is driven by a motor 30, which is located at the end of the motor shaft 26 opposite the functional part 28 and outside the cooking chamber 12 and the ventilation duct 24.

[0029] In Fig. Figure 2 shows a perspective view of a section of the cooking chamber wall 14 in the area of ​​the wall duct 20. In this illustration, the cooking chamber 12 is located above the cooking chamber wall 14.

[0030] The motor shaft 26 extends into the cooking chamber 12 and is surrounded by the λ / 4 trap 22. The λ / 4 trap 22 is designed as an annular body, which is constructed as a hollow, rectangular metal housing 32. The metal housing 32 is two-part and has a U-shaped body 34 and a cover 36, which are electrically contacted on the radially outer side of the metal housing 32. On the radially inner side of the metal housing 32, a gap 37 is formed between the body 34 and the cover 36.

[0031] A dielectric 38, also annular in shape, is accommodated in the U-shaped body 34, parallel to the motor shaft 26. The dielectric 38 is, for example, a ceramic and rests against a web 39 of the metal housing 32, which is formed between the legs of the U-shaped metal housing 32. The longitudinal extent L Dof the dielectric 38 parallel to the motor shaft 26 corresponds to the electrical length of a quarter wavelength (λ / 4) of a microwave of the desired resonance frequency ω D .

[0032] Fig. Figure 3 shows a perspective view of a ventilation duct 24. The ventilation duct 24 consists of two sections: a waveguide section 40 and a connecting section 42. The waveguide section 40 and the connecting section 42 are adjacent to each other, allowing airflow through the ventilation duct 24.

[0033] The waveguide section 40 is designed as a rectangular waveguide and has concentric openings in each of two opposite side walls at the end remote from the connecting section 42. The opening in the side of the waveguide section 40 facing the cooking chamber wall 14 forms a coupling opening 44 to the cooking chamber 12 and is aligned with the wall feedthrough 20 or is designed as part of the wall feedthrough 20. The opening opposite the coupling opening 44 forms a bearing opening 46 in which a bearing (not shown) for the motor shaft 26 is provided.

[0034] The motor shaft (in Fig. 3 not shown) extends through the bearing opening 46 and the coupling opening 44 and thus perpendicular to the longitudinal extent of the waveguide section 40.

[0035] The length L Lthe longitudinal extension of the waveguide section 40 from an axis through the centers of the openings 44, 46 to the transition to the connecting section 42 is adapted to the cross section of the waveguide section or vice versa, so that the electrical length of the waveguide section 40 corresponds to a quarter wavelength of a microwave of the desired resonance frequency ω L of the ventilation duct 24.

[0036] In contrast, the connecting section 42 is designed in such a way that propagation of microwaves, at least in the frequency range of the microwave generator, is not possible. In the embodiment shown, the connecting section 42 has a circular cross-section.

[0037] At the end of the connecting section 42 facing away from the waveguide section 40, an opening is provided which opens into the surroundings of the cooking appliance 10, so that a flow connection for air exists between the cooking chamber 12 and the surroundings of the cooking appliance 10 through the ventilation duct 24, consisting of the waveguide section 40 and the connecting section 42.

[0038] To explain the operation of the invention, it is assumed that the microwave generator 16 has a frequency spectrum Δ with a bandwidth of 100 MHz around a center frequency of 2.45 GHz, thus generating microwaves with a frequency of 2.4 GHz to 2.5 GHz. However, this selection should not be considered restrictive in any way.

[0039] The result of a transmission measurement over the length of the motor shaft 16 of the cooking appliance 10 of microwaves in a frequency range from 2.00 GHz to 2.90 GHz is shown in Fig. 4. The amplitudes of the microwaves are compared before and after the two microwave traps 22, 24 in order to determine the attenuation generated by the microwave traps 22, 24.

[0040] It is clearly visible that two frequencies exhibit very strong damping. These frequencies correspond to the resonance frequencies ω D , ω L . Between the two frequencies ω D , ω L a region of strong attenuation is formed which contains the frequency spectrum Δ of the microwave generator 16.

[0041] The microwaves generated by the microwave generator 16 propagate through the waveguide 18 into the cooking chamber 12. There, they are largely absorbed by the food to be heated (not shown). However, a portion of the microwaves couples into the functional part 28 or the motor shaft 26 and propagates along the motor shaft 26 toward the motor 30, which is located outside the cooking chamber 12.

[0042] The dielectric 38 and the gap 37 of the λ / 4 trap 22 act like a blind line for the microwaves propagating in the motor shaft 26, which is short-circuited by the web 39 of the body 34. For microwaves with the frequency ω D the blind line has due to the geometric length L D of the dielectric 38 has an electrical length of λ / 4, so that the end of the dielectric 38 facing away from the cooking chamber wall 14 is for microwaves of frequency ω D acts like an idler and reflects these microwaves. Microwaves with frequencies around the resonance frequency ω D are only partially reflected by the λ / 4 trap 22. The attenuation of the microwaves associated with the reflection is Fig. 4 around the frequency ω D as a drop in transmission through the motor shaft 26.

[0043] The microwaves not or only partially reflected by the λ / 4 trap 22 propagate further towards the ventilation duct 24 and couple with the waveguide section 40. The functional principle of the waveguide section 40 is similar to that of the λ / 4 trap 22. The short circuit is generated here by the transition between the waveguide section 40 and the connecting section 42, and the length L L The blind line is chosen so that it is suitable for microwaves of frequency ω L has an electrical length of λ / 4. Accordingly, an open circuit is generated by the waveguide section 40, and microwaves with the frequency ω L are completely and microwaves with frequencies around the resonance frequency ω L partially reflected. The resulting attenuation of the microwaves is defined as a drop in transmission around the frequency ω L in Fig. 4 can be seen.

[0044] Microwaves with frequencies between the resonance frequency ω Dthe λ / 4 trap 22 and the resonance frequency ω L of the ventilation duct 24 are partially reflected by both the λ / 4 trap 22 and the ventilation duct 24, so that the microwaves of these frequencies also experience considerable attenuation, such as the range between the two resonance frequencies ω D , ω L in Fig. 4 can be seen.

[0045] Thus, through the interaction of the λ / 4 trap 22 and the ventilation duct 24, the exit of microwaves from the cooking chamber 12 is controlled over a frequency range between ω D and ω L effectively prevented.

[0046] Of course, the described embodiment is not to be considered limiting. Thus, the invention can also be used in cooking appliances without a separate heating element. Furthermore, the shape of the waveguide section 40 of the ventilation duct 24 is not limited to a rectangular waveguide. In addition to a rectangular waveguide, other geometries adapted to the desired resonant frequency are also suitable.

[0047] It is also conceivable to realize the transition between the waveguide section 40 and the connecting section 42 of the ventilation duct 24 by means of a grating in order to prevent the propagation of the microwaves into the connecting section 42.

Claims

[1] Cooking appliance (10) with a broadband microwave generator (16), a cooking chamber (12), a cooking chamber wall (14) having a wall duct (20), a motor shaft (26) extending out of the cooking chamber (12) through the wall duct (20), and two microwave traps (22, 24) arranged around the motor shaft (26), the microwave traps (22, 24) having different resonant frequencies, characterized by that one of the microwave traps (22, 24) has a λ / 4 trap (22) surrounding the motor shaft (26) and that the resonance frequency of the λ / 4 trap (22) is between 2.3 GHz and 2.4 GHz. [2] Cooking appliance according to claim 1, characterized by that the resonance frequencies are selected such that the spectrum of the microwave generator (16) lies largely between the two resonance frequencies or is covered by the interaction of the resonance frequencies. [3] Cooking appliance according to claim 1 or 2, characterized bythat the λ / 4 trap (22) is arranged inside the cooking chamber (12) on the cooking chamber wall (14). [4] Cooking appliance according to one of the preceding claims, characterized by that the λ / 4 trap (22) has a dielectric (38), in particular a ceramic. [5] Cooking appliance according to claim 4, characterized by that the dielectric (38) has a geometric length parallel to the motor shaft (26) which corresponds to the electrical length of a quarter wavelength of a microwave of the desired resonance frequency of the λ / 4 trap (22). [6] Cooking appliance according to one of the preceding claims, characterized by that the cooking appliance (10) has a ventilation duct (24) which is arranged on the cooking chamber wall (14) and which has an opening to the cooking chamber (12) which is aligned with the wall duct (20), wherein the ventilation duct (24) forms one of the microwave traps (22, 24). [7] Cooking appliance according to claim 6, characterized bythat the motor shaft (26) extends through the opening to the cooking chamber (12) of the ventilation duct (24). [8] Cooking appliance according to claim 6 or 7, characterized by that the ventilation duct (24) has a waveguide section (40) which is designed as a conductor, in particular as a hollow guide, particularly preferably as a rectangular hollow guide, for microwaves, wherein the waveguide section (40) connects to the wall duct (20) to the cooking chamber (12). [9] Cooking appliance according to claim 8, characterized by that the waveguide section (40) extends perpendicular to the motor shaft (26). [10] Cooking appliance according to claim 8 or 9, characterized by that the geometric length and the cross-section of the waveguide section (40) are adapted to one another such that the waveguide section (40) has an electrical length which corresponds to a quarter wavelength of a microwave of the desired resonance frequency of the ventilation duct (24). [11] Cooking appliance according to one of claims 8 to 10, characterized by that the ventilation duct (24) has a connecting section (42), in particular with a grid or a duct section with a round cross-section, which prevents the propagation of microwaves in and through the connecting section (42) and which closes off the waveguide section (40) on its side facing away from the motor shaft (26). [12] Cooking appliance according to one of claims 6 to 11, characterized by that the resonance frequency of the ventilation duct (24) is between 2.5 GHz and 2.6 GHz. [13] Ventilation duct (24) for use in a cooking appliance (10) with a microwave generator (16), in particular a cooking appliance (10) according to one of claims 1 to 12, characterized bythat a motor shaft (26) of the cooking appliance (10) is guided through the ventilation duct (24) and the ventilation duct (24) forms a microwave trap (24) for microwaves of at least one frequency which propagate along the motor shaft (26) and which were generated by the microwave generator (16).

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