DE2740542A1 - Electric cooker plate with magnetic fastening - has permanent magnet operated by exciter coil, to lower heat transfer resistance from hot plate - Google Patents

Abstract

The plate is for domestic cookers, and consists of a conventional cooker plate, with magnetic action as a plate chuck. The cooker plate may be heated in an inner or outer section or in several rings separated by non-magnetisable material. The induction for the magnetic fastening system is supplied by a permanent magnet, which is magnetised or demagnetised by an exciter coil connected to an impulse magnetiser. A soft iron core in a current-carrying coil can instead be used, or a permanent magnet, which is short-circuited by a soft iron section.

Description

Electric hotplate with magnetic attachment

Field of application: For use in household stoves, state-of-the-art and criticism: the hotplates that have been used up to now essentially consist of one Cast plate, the top of which is faced (footprint) and in the bottom the heating resistors are embedded.

The pots, kettles and pans to be heated, as well as cookware called, are with a force, which is determined by their own weight, on the Pressed hotplate. This contact pressure is an essential factor for resisting heat transfer between the hotplate and cookware.

The "Ohm's law of heat engineering" shows the dependency that exists between: heat transfer resistance Rth in ° K / W power to be transferred P in W dem Temperature difference ## in ° K here: between hotplate and cookware P. R @@ A Higher heat transfer resistance therefore results in: 1.) A greater loss of energy by convection and radiation during and after the cooking process, because of the inevitable higher temperature of the hotplate.

2.) It also offers a greater time lag in warming the dead weight only provides insufficient protection against accidents caused by slipping or tipping over or knocking down the cookware.

These disadvantages also mainly apply to ceramic hobs.

In laid-open specification 2 504 516 with the title: Lever arm press and other pressure-exerting devices and objects for electr. Hot plates, was dbs problem of the heat transfer resistance and its dependence on the contact force already recognized, but only incompletely resolved in every respect, because on the one hand is a lever arm press or an object or a device always only at the edge zones of the cookware can exert an increased pressure and thus to a reduced pressure Lead heat transfer resistance and on the other hand is a mechanical device very impractical to use.

In addition, the advantage of a reduced heat transfer resistance be lifted again at the edge zones because the cookware is uneven would be distorted by means of a lever arm press.

It is true that such a device provides better protection against accidents Relation to slipping, tipping over or falling down, but it already poses in itself again a source of danger, as well as the "stove - child safety gate" which has already been made known under the Offenlegungsschrift 2 100 417.

Object of this invention: The invention is intended to provide an additional pressing force apply which then lowers the heat transfer resistance and greater stability creates.

Solution: The object is achieved according to the invention in that under the (2) inner part of the hotplate provided with (1) heating coil a (3) permanent magnet attached which is agnetized or demagnetized with a (4) excitation coil. The excitation coil is connected to a pulse magnetizer. The magnetic one Inference is made via a wedge-shaped cut-out (5) cylinder on which the outer part of the hotplate is attached. The required air gap for a Clamping device is formed by the (7) brass ring between the inner part and outer part is pressed in, wherein an (8) expansion area is to be provided.

After switching on the hotplate, the magnetization process should start not yet go ahead. Instead, the (4) excitation coil goes first supplied by the (10) air gap control device with an alternating voltage which with an open "air gap".

that is, without cookware, it results in a defined current.

If the "air gap" is closed, a sufficiently large one If the cookware is roughly in the center of the hotplate, the current drops below a certain level Value and the (10) air gap control device separates the (4) excitation coil from the AC voltage. Then it triggers the magnetization process on the (11) pulse magnetizer the end.

The circuit also ensures that the cookware is placed on the pan cannot be torn out of the hand by the magnetic force and when in use no insulation process is initiated by aluminum cookware.

Instead of using a permanent magnet in conjunction with a pulse magnetizer You can also get the magnetic attraction with a soft iron core in a current flowing through it Apply coil.

This solution would have the advantage that in the event of a power failure could decrease immediately, however, would have the disadvantage, among other things, that there is constant energy is consumed.

A mechanical "short-circuiting" of permanent magnets would admittedly both Advantages, i.e. independence from the grid and no additional energy consumption, but the disadvantage of the additional mechanical device and its operation with it bring.

Therefore the first possible solution was chosen as an example.

It offers security in connection with energy savings, material savings and no additional controls are required.

The best permanent magnet material would be an Alnico alloy, e.g. Alnico 500, are suitable.

Since in general with electr. No Mannethaftuny hot plates Significant tear-off or displacement forces will occur, one may maximum hotplate temperature of 350 ° C, reversible change in remanence occurs (Loss of adhesive force) of <10 $, valid for the suggested 'Merkstoff, as not consider relevant.

The round rod shape of the permanent magnet and because of the use a pulse magnetizer consisting of relatively few turns of thin wire Excitation coils allow inexpensive manufacture and assembly.

Advantages: 1.) Due to the smaller heat transfer resistance there is a smaller temperature difference between the cookware and the hotplate a. This means, based on a certain cookware with a certain content, a lower temperature of the hotplate, and thus: t.1) Less energy storage in the hotplate 1.2) Less energy loss due to convection 1.3) One lower energy loss due to radiation. A simple experiment shows which The order of magnitude of the losses.

For one liter of water you need 60 kcal to get it from Heat 20 ° C to 800C. With an output of 1 kWh, this would be 260 seconds. last.

In an experiment with an immersion heater of 1 kWh in a pot with 1 liter of water at 209C required a heating time of 310 seconds. measured to reach 600C.

This results in an efficiency of: 0.84 in a test with a Conventional cooking plate of 1 kWh and the same pot, also with 1 Liters of water at 20 ° C required a heating time of 562 seconds. measured to reach 80 ° C.

This results in an efficiency of: 0.46 2.) The power with which the Keeping cookware in place prevents those accidents in particular children Always caused by knocked over or dropped cookware and its hot contents are seriously injured or killed again, as can be seen from accident statistics.

3.) When stirring the food, especially if it is viscous, the housewife no longer needs to hold onto the cookware because it is not can slip more. For disabled people, e.g. amputees, this means an even bigger one Relief and reduction of the risk of accidents.

4.) The magnetic attraction causes the cookware with its entire surface is pressed evenly onto the hotplate, i.e. one over the evenly distributed heating takes place over the entire surface, which causes the burning of the Prevents cooked food, especially in thinner-walled cookware.

5.) This even attraction and good heat conduction make it superfluous to use elaborate and therefore expensive cookware. The additional expenses for the electr. So hot plates with magnetic adhesion can already be purchased at the time of purchase of cooking utensils, at least partially, again.

However, this invention has one imperfection, namely that cookware made of non-magnetizable material or soil continues in the usual Can be used wisely, but the advantages cannot be used which offers the magnet adhesion.

Claims (2)

Patent claims: 1 electric hotplate with magnetic adhesion, characterized in that on customary electr. Hotplates the equally common functional principle of the magnetic clamping plates is used to generate an adhesive force. 1.1 Electric hotplate according to claim 1, characterized in that The heating can take place: 1.1.1 According to Fiour 1, in the inner part 1.1.2 In the outer part 1.1.3 In both parts 1.1.4 In several parts separated by non-magnetizable material Wrestling. 1.1.5 Through-combination in any way of the four above Options. 1.2 Electric hotplate according to claim 1, characterized in that the required induction for the magnetic adhesion system with one of the following three Systems can be applied: 1.2.1 According to Figure 1, with a permanent magnet through an excitation coil that is connected to a pulse magnetizer, for example, is magnetized and demolished. 1.2.2 With a soft iron core in a current-carrying coil. 1.2.3 With a permanent magnet short-circuited by a piece of soft iron can be. 2 control for agnetization and demagnetization processes for adhesive systems which are equipped with magnetization systems as in the Points 1.2.1 and 1.2.2 described, characterized in that the following functional sequences can be achieved according to Figure 2: 2.1 When using system 1.2.1, (9) Contact set that is mechanically coupled to the switch for the heating resistors, at Switch on the control process via connection 3 of the (10) air gap control device the end. The now over the connections 4 and 5 of the (10) air gap control device flowing alternating current which in its size depends on the impedance of the (4) excitation coil depends is interrupted when it falls below a minimum. This is also causes voltage to be applied to connection 6 of the (10) air gap control device, which the magnetization process via a connection with connection 1 of the (11) pulse magnetizer initiates. When the hotplate is switched off, the (9) contact set over Terminal 2 of the (11) pulse magnetizer triggers the demagnetization process. 2.2 If system 1.2.2 is used, there is no demagnetization and instead of the (11) pulse magnetizer, a (12) direct current source is available Supply of the (4) excitation coil, which here must consist of a different winding.