GB2306260A

GB2306260A - Limiting mains voltage fluctuations caused by overcurrents when an appliance is turned on; microwave ovens

Info

Publication number: GB2306260A
Application number: GB9620865A
Authority: GB
Inventors: Longhi Giuseppe De
Original assignee: De Longhi SpA
Current assignee: De Longhi SpA
Priority date: 1995-10-13
Filing date: 1996-10-07
Publication date: 1997-04-30
Also published as: ITMI950700U1; FR2743453A3; GB9620865D0; IT237217Y1; ITMI950700V0

Abstract

In order to limit voltage fluctuations in a household mains supply due to overcurrents when a transformer 6 of a microwave oven is initially connected to the supply, switches 4 and 5 are closed sequentially so that the transformer 6 is initially connected via switch 4 and a current limiting resistance 7 and is then connected directly via switch 5. Switches 4 and 5 are operated by a lever 13 moved by a cam wheel 10 which is rotated by a motor (18, Fig.3) of the timer of the microwave oven. As the timer is already provided, the sequential switching arrangement can be provided at a low additional cost. The delay between closure of switch 4 and closure of switch 5 is of the order of milliseconds. The switch 4 may be a microswitch outside a case 15 of the timer. Alternatively, both switches 4, 5 may be provided inside the timer case 15.

Description

The present invention relates to a safety device for limiting voltage fluctuations in household mains caused by overcurrents when electrical appliances are turned on.

It is known that when an electrical appliance is turned on, therefore when its power transformer is electrically fed, a voltage peak is formed which causes a high voltage fluctuation in the mains.

In particular, when microwave ovens are turned on, a high voltage peak is formed because during the transient state the power transformer behaves as an impedance tending to zero and therefore the voltage tends to infinity.

The voltage fluctuation in the mains, due to overcurrents caused by turning an electrical appliance on, is at present controlled according to statutory regulations. According to the regulations, the voltage peak generated by turning on an electrical appliance, particularly a microwave oven, is suppressed or limited by connecting the electrical circuit of the oven with an electronic circuit controlling a relay which is adapted to provide a delay in closing one of the two feed contacts of the power transformer.

In particular, when the timer motor closes the first feed contact of the power transformer, part of the current feeds the electronic circuit which, by means of the relay, prevents the closing of the second feed contact of the power transformer.

In this phase, all the current passes through a limit resistance adapted to dissipate the current.

When the relay closes the second contact, the current bypasses the resistance and directly feeds the transformer which, having exhausted its transient state, behaves as a natural load.

The use of an electronic circuit and of a relay achieves the intended aims but entails a sensible added cost to that of the microwave oven.

Furthermore, it is necessary to radically modify the electric circuit of the oven with added production complications and a further increase of the production costs and therefore of the retail price of the electrical appliance.

It has to be noted that an electronic circuit and a relay may malfunction especially considering the operative ambience of a microwave oven (grease, humidity, impacts, etc.).

The aim of the present invention is to eliminate the drawbacks of the prior art.

Within this aim, an important object of the invention is to provide a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance, particularly a microwave oven, is turned on, that uses one or more members already present for the operation of the microwave oven.

A further aim of the invention is to provide a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, adapted to be applied on the electrical appliance without substantially add further and costly elements to the original structure of the appliance.

Still a further aim of the invention is to provide a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, which is reliable in the long run regardless of the conditions wherein the electrical appliance, especially a microwave oven, operates.

Still a further aim of the invention is to provide a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, having a low cost which does not modify the retail price of the microwave oven.

The above aim, as well as these and other objects that will be more apparent hereinafter, are achieved by a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, wherein said electrical appliance has an electric circuit comprising peak voltage limit means, said peak voltage being generated when said electrical appliance is turned on said limit means being activated by sequential closure means adapted to close one or more electric contacts of said electric circuit, characterized in that said sequential closure means is defined by at least one of the operating members of said electrical appliance.

Further characteristics and advantages of the invention will be more apparent by the following description of a safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, according to the invention, illustrated, by way of example in the enclosed drawings wherein: FIG. 1 is a schematic view of the safety device according to the invention wherein one of the feed contact of the transformer is defined by a microswitch; FIG. 2 schematically shows the safety device according to the invention wherein the two feed contacts of the transformer are sequentially activated by a cinematic apparatus; FIG. 3 is a simplified diagram sequentially showing the operation of the device according to the invention.

With reference to the above figures, the device according to the invention, generally designated by the reference numeral 1, comprises a peak voltage limit means, generally designated by the reference numeral 2, adapted to limit the peak voltage generated when the electrical appliance is turned on. Peak voltage limit means 2 are activated by sequential closure means 3 adapted to close one or more electrical contacts 4 and 5 of the electric circuit of the electrical appliance, in this particular case a microwave oven provided with a power transformer 6.

Advantageously, sequential closure means 3 is defined by at least one of the operating members of the microwave oven.

In particular, sequential closure means 3 is adapted to close at least a first contact 4 which is adapted to activate peak voltage limit means 2, in particular a limit resistance 7. After a mechanically predetermined time, sequential closure means 3 is adapted to close at least a second feed contact 5 of the power transformer 6 of the microwave oven.

In other words, the sequential closure means comprises a cinematic apparatus associated with the timer of the microwave oven and advantageously adapted to close second contact 5 with a predetermined delay (milliseconds) with respect to first contact 4 by means of a totally mechanical operation of both.

The cinematic apparatus comprises a cam directly operated by the timer microwave oven. The cam is defined by a toothed wheel 10 having one or more lowered zones 11. A lever 13 has a head 12 engaging the lowered zones 11. Lever 13 is hinged to the timer case 15, at hinge 14, and is adapted to sequentially control the closure of electrical contacts 4 and 5.

According to a first embodiment of the invention, shown in FIG. 1, electrical feed contact 4 of limit resistance 7 is, for example, a microswitch arranged outside timer case 15 and activated by the motion of lever 13, through an adapted opening 16 provided in the case.

According to a further embodiment, as shown in FIG. 2, the same lever 13 sequentially operates the closure of the two electrical contacts in order to limit the peak current.

The operation of the device, as schematically shown in FIG.

3, is the following.

Advantageously, the sequential closure of the two electrical contacts 4 and 5 is mechanically controlled by the same motor of the timer provided in the microwave oven for its operation.

In particular, when the microwave oven is turned off, the timer motor 15, through toothed wheel 10 and lever 13, has yet to close the two electrical contacts and therefore to activate power transformer 6.

When through motor 18, the timer closes the first contact 4, the generated current is dissipated by limit resistance 7 for a predetermined time in the order of milliseconds.

Successively, after a time which does not hinder the operation of the limit resistance which would otherwise be of high power, the second contact 5 is closed, mechanically and by means of lever 13 and toothed wheel 10, in order that the current bypasses limit resistance 7 and directly feeds the power transformer. The power transformer has by this time exhausted its transient state and dynamically behaves as a natural load without causing noticeable voltage fluctuations in the mains.

It has been seen in practice that the device according to the invention is particularly advantageous in using preexisting operative members in the microwave oven in order to be extremely reliable and economic in terms of production costs.

The materials employed, as well as the dimensions, may be any according to the specific needs and the state of the art.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims

1. Safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, wherein said electrical appliance has an electric circuit comprising peak voltage limit means, said peak voltage being generated when said electrical appliance is turned on said limit means being activated by sequential closure means adapted to close one or more electric contacts of said electric circuit, characterized in that said sequential closure means is defined by at least one of the operating members of said electrical appliance.

2. Safety device, according to claim 1, characterized in that said sequential closure means defined by at least one operative member of said electrical appliance is adapted to close at least one first contact adapted to activate said peak voltage limit means and, after a mechanically predetermined time, at least one second feed contact of a power transformer of said electrical appliance.

3. Safety device, accordo one or more of the preceding claims, characterized in that said sequential closure means comprises a cinematic apparatus associated with a timer of said electrical appliance.

4. Safety device, according to one or more of the preceding claims, characterized in that said cinematic apparatus comprises a cam acting on a lever adapted to sequentially operate the closing of said electrical contacts.

5. Safety device, according to one or more of the preceding claims, characterized in that said cam is operated directly by said timer of said electrical appliance.

6. Safety device, according to one or more of the preceding claims, characterized in that said cam is a toothed wheel having one or more lowered zones, the head of said lever engaging inside said zones.

7. Safety device, according to one or more of the preceding claims, characterized ifl that said lever is hinged to the case of said timer.

8. Safety device, according to one or more of the preceding claims, characterized in that at least one of said electrical contacts is a microswitch.

9. Safety device, according to one or more of the preceding claims, characterized in that said peak voltage limit means comprises at least one limit resistance arranged in series with said transformer.

10. Safety device, according to one or more of the preceding claims, characterized in that said electrical appliance is a microwave oven.

Amendments to the claims have been filed as follows 1. Safety device for limiting voltage fluctuations in household mains caused by overcurrents when an electrical appliance is turned on, wherein said electrical appliance has an electric circuit comprising peak voltage limit means, said peak voltage being generated when said electrical appliance is turned on said limit means being activated by sequential closure means adapted to close one or more electric contacts of said electric circuit, characterized in that said sequential closure means is defined by at least one of the operating members of said electrical appliance.

o. Safety device, according to one or more of the preceding claims, characterized in that said cam is a toothed wheel having one or more lowered zones, the head of said lever engaging inside said zones.

7. Safety device, according to one or more of the preceding claims1 characterized in that said lever is hinged to the case of said timer.

3. Safety device, according to one or more of the preceding claims, characterized in that at least one of said electrical contacts is a microswitch.

9. Safety device, according to one or more of the preceding claims, characterized in that said microswitch is directly operated by said lever.

10. Safety device, according to one or more of the preceding claims, characterized in that said peak voltage limit means comprises at least one limit resistance arranged in series with said transformer.

11. Safety device, according to one or more of the preceding claims, characterized in that said electrical appliance is a microwave oven.