GB2127146A - High-frequency heating apparatus - Google Patents

Abstract

High-frequency heating apparatus comprises a heating chamber, a high frequency oscillator for feeding high- frequency waves to said heating chamber, and a door for opening and closing a front opening of the heating chamber. The door is provided with a groove for holding a glass plate (15) which covers a see-through window in the door and a plurality of projections (21) are arranged in the groove at predetermined positions of high electric field to lessen the depth of the groove and thus the temperature of these positions. <IMAGE>

Description

SPECIFICATION High-frequency heating apparatus The present invention relates to high-frequency heating apparatus and has particular reference to a door arrangement for such apparatus.

Conventionally the door of high-frequency heating apparatus, such as a microwave oven, is provided with a see-through window for visual inspection of the progress of cooking of food in the heating chamber during cooking. To prevent microwaves from leaking through the see-through window, a punched plate in the form of a metal plate having a number of small holes formed therein is used forthe window. Further, to prevent particles of food from clogging the small holes of the punched plate, there is provided a cover in the form of a glass plate.

Usually, this glass cover is held in a groove defined by the punched plate, the inner wall of the door and a separate metal member, but since the dielectric constant of glass is higher than that of air, as is known in the art, the effective depth D' of the groove with the glass fitted therein is as follows: D = DW where D = depth of groove with no glass e = dielectric constant of glass Thus, the depth becomes substantially greater than when there is no glass.

Accordingly, the effective dimension from the terminal end of the groove to the inlet approaches XM, with the result that the strength of the electric field at the inlet increases, tending to produce sparks and abnormal temperature rises at the inlet of the groove. Of course, these abnormal phenomena are also influenced to a great extent by the pattern of the electric field in the heating chamber, so that they occur in places where the position of high electric field, due to the pattern of the electric field, coincides with the aforesaid groove depth.

To solve this problem, it is necessary that the groove be as shallow as possible, but in a construction shown in Figure 1, which is a conventional example, the shallowness of the groove is limited, since the inner wall of the door and a separate metal member must be spot welded within the groove in order that the head of a screw may not be exposed at the inner surface of the door.

There is therefore a need for high-frequency heating apparatus which eliminates the aforesaid drawback of the prior art and which has the quality and consistent reliability to prevent occurrence of abnormal heating and sparking in the glass holding groove.

According to the present invention there is provided high-frequency heating apparatus comprising a heating chamber, a high-frequency oscillator for feeding high-frequency waves to the heating chamber, and a door for opening and closing a front opening of the heating chamber, wherein the door is provided with a groove for holding a glass plate which covers a see-through window in the door, and a plurality of projections for controlling the movement of the glass plate are arranged in the groove at predetermined positions of high electric field.

The provision of such projections in the groove decreases the depth of the portions of the groove having a high electric field and hence lowers the strength of the electric field at the inlet of the groove; thus, this arrangement has the effect of preventing occurrence of sparking and abnormal heating.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional view of the region of the door of high-frequency heating apparatus of a conventional type; Figure 2 is a perspective view of high-frequency heating apparatus embodying the present invention; Figure 3 is a back view of the door of the apparatus of Figure 2, with an inner door wall removed; and Figure 4 is a sectional view along the line A-A' in Figure 3.

Referring now to the accompanying drawings, in Figure 2 there is shown high-frequency heating apparatus comprising a main body 1 internally provided with a heating chamber 2 having a front opening openable and closable by a door 3. Numeral 4 denotes an operating panel provided with fluorescent display lamps used to give a display of timer settings and output settings, program selection keys 6, a start button 7, and a cancel button 8.

Numeral 9 denotes the inner wall of the door forming a portion of a choke wall formed on the door 3 and 10 denotes a window for see-through inspection.

In Figure 1, which relates to conventional apparatus but which has features common to said embodiment of the present invention, numeral 11 denotes a choke groove having a depth equal to about 1/4 of a wave-length to be used. The choke groove 11 is defined by an inner wall 9 of the door, an outer wall 12 of the door, and a separate metal member 13 connected to the inner wall as by spot welding. Numeral 14 denotes a punched plate formed with a number of small holes to form a peep window, and 15 denotes a glass plate for preventing particles of food from clogging the small holes. Numeral 16 denotes another glass plate provided outside the door 3, and 17 denotes a handle to be grasped in opening and closing the door 3.

The assembly of inner door 9 and separate metal member 13 is secured to the outer wall 12 of the door by screws 5 to define a groove 20 in which the punched plate 14 and glass plate 15 are clamped and fixed. The numeral 18 denotes a gasket which prevents cracking of the glass and leakage of heat and vapour emanating from the food being cooked.

In addition, since the surface of the glass plate 16 is formed with a coating 19, said screwed portion is hidden and cannot be seen from the surface.

In this conventional apparatus, the shallowness of the groove is limited, since the inner wall 9 of the door and the separate metal member 13 must be spot welded within the groove 20 in order that the head of the screw 5 may not be exposed at the inner surface of the door.

In the case of the embodiment of the present invention, however, as shown in Figures 3 and 4the groove 20 for holding the glass plate 15 is provided with a plurality of projections 21, so that the depth of the portion of the groove provided with the projections is less, by an amount corresponding to the height of the projections, than the portion having no projections. The width e of the projections 21 is about 1/4 of a wavelength to be used, and the distance between adjacent projections (i.e. the pitch) is about 1/4 of the wavelength to be used. In the portions having no projections 21 (in Figure 3, the portions marked with crosses (x)), the spot welding of the separate metal member 13 and the inner door wall 9 is effected.

The positions where the projections 21 are provided are so determined that they are close to the highest electric field due to the mode of distribution of the electric field in the heating chamber 2. As a concrete method therefor, they may be determined by fabricating a door having no projections on an experimental basis, fitting it to a set of trail manufacture, and measuring the temperature throughout the groove 20.

Projections can then be provided at places of high temperature.

The reason why projections must be provided in this way is that when the glass plate is fitted in the groove 20, which must be at least 9 mm deep to provide for spot welding of the inner door wall 9 and separate metal member 13, the effective depth of the groove with respect to microwaves becomes VT times the actual depth, where E is the dielectric constant of glass, as previously mentioned. Thus, if the dielectric constant of glass is, for example, 5 then the effective depth D' of the 9 mm deep groove with respect to the microwaves is: D' =9xW .20mum.

The strength of the electric field at the inlet of the groove is at a maximum when the depth of the groove is about 30 mm, which is 1/4 of the wavelength used, but since the effective depth due to the glass plate being fitted in the groove approaches approximately A/4, as described above, when the maximum point in the distribution of electric field in the heating chamber is in the neighbourhood of the same, sparks and abnormal temperature rises will occur.

The present embodiment prevents this occurrence by providing projections in the groove 20 at places where the electric field is high so as to decrease the depth of the groove 20. At the same time, the presence of the projections 21 decreases the size of the glass plate to be fitted; thus, there is the effect of doubly decreasing the depth of the groove 20, so that aforesaid sparking and abnormal heating can be prevented.

When there is one position of high electric field due to the distribution of electric field in the heating chamber, the next position of high electric field is about 1/2 of the wavelength used away from the first position. Thus, if the width f of one projection 21 is about A/4 and if the distance from the centre of one projection 21 to the centre of another projection 21 is A/2, the intended effect can be obtained with respect to all points of maximum electric field for the pitch P.

With the described embodiment of the invention, the following effects can thus be obtained: (1) Since projection are provided in the glass plate holding groove at positions of maximum electric field due to the pattern of the distribution of the electric field in the heating chamber, abnormal temperature rises and hot spots can be prevented without causing any difficulties in the spot welding of the inner door wall and separate metal member.

(2) Since the provision of projections in the groove makes it possible to decrease the depth of the groove in the corresponding portions and the size of the glass plate at the same time, the depth of the groove can be doubly decreased and hence the strength of the electric field at the inlet of the groove can be made sufficiently low.

(3) Since the width of the projections in the groove is about 1/4 of the wavelength used, even if the frequency of oscillation of the high-frequency oscillator changes more or less depending on load conditions in the heating chamber, a sufficiently consistent effect can be obtained.

(4) If the groove has no projections there is a need for a gasket of sufficiently large cross-sectional area to fill the entire groove in order to hold the glass plate in a stabilized manner. However, the provision of a plurality of projections makes it possible for a gasket of small cross-sectional area to adequately hold the glass plate, which is economical.

Claims (4)

1. High-frequency heating apparatus comprising a heating chamber, a high-frequency oscillator for feeding high-frequency waves to the heating chamber, and a door for opening and closing a front opening of the heating chamber, wherein the door is provided with a groove for holding a glass plate which covers a see-through window in the door, and a plurality of projections for controlling the movement of the glass plate are arranged in the groove at predetermined positions of high electric field.

2. Apparatus as claimed in claim 1, wherein the width of each projection is substantially 1/4 of a wavelength to be used.

3. Apparatus as claimed in either claim 1 or claim 2, wherein the pitch of the projections is substantially of a wavelength to be used.

4. High-frequency heating apparatus substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.