Safety switching apparatus against dry operation of electrical heating resistances, particularly for washing machines and dishwashers

Abstract

 The invention relates to a safety switching apparatus against dry operation of electrical resistances, particularly for washing machines and dishwashers, with electromechanical or electronic control systems and level switches for monitoring the water level. Security against dry operation of the heating resistances is considerably improved in that two level switches monitor the normal water level and, incorporated in series, control a switch element, in that the switch element triggers the control motor and switches off the water intake valve, and in that the control motor, after expiry of a predetermined time, by means of control contacts, switches the heating resistance into parallel with the switch element in the heating and washing stage following the water intake stage.

Description

[0001] The invention relates to a safety switching apparatus against dry operation of electrical resistances, particularly for washing machines and dishwashers, with a control motor of a program switching mechanism or of a corresponding software/hardware combination with partly and fully electrical program controls and level switches for monitoring and regulating the water level in the appliance.

[0002] Such a safety switch apparatus is known from DE-AS 26 47 616. The two level switches monitor a lower and an upper safety level and are used to increase the number of possible water levels, so that the washing procedure may be carried out with a reduced consumption of energy. However, no additional security against dry operation of the heating resistance is achieved with the two level switches, in comparison to otherwise conventional safety switching arrangements.

[0003] As shown by DE 32 28 744 A1, the electrical heating circuit is normally switched on by a level switch. In this case the control motor for the program switch is preceded by a control contact which closes one or two steps before the control contact which switches on the heating circuit. This circuit ensures that even if the water supply fails and there is phase contact from the heating side when the machine is connected to a grid with "earthing" protection, dry operation is excluded as the program switch cannot pass into the heating stage.

[0004] As shown by DE-OS 19 08 464, a safety switching apparatus is also known which provides a temperature regulator and a temperature restricting means. In this case security against dry operation of the electrical heating resistances is achieved in that at least two contacts of the temperature regulator and of the temperature restricting means, switching the heating directly, are incorporated in series with the heating resistances in such a way that one is incorporated respectively before and after the heating resistances. Also incorporated in the series arrangement is the contact of a level switch, whose fault function however is not monitored.

[0005] The safety circuits mentioned above have basically the following disadvantages:

[0006] The failure of a safety member is not noticed in every case. The redundancy of the respective system is therefore reduced by one stage from the moment of failure.

[0007] The reaction time of the temperature restricting means or of the thermostats limits its effectiveness. Particularly when plastics are used in the vicinity of the heating resistances, severe damage or fire can occur before the safety members respond. Their effectiveness cannot be guaranteed over the entire depth of the appliance in the case of any containers standing at an oblique angle.

[0008] It should not be overlooked also that so-called tubular heating members can burn out when they become overheated, for example when operated in a dry state. This type of fault is not reliably recognised by temperature restricting means or thermostats.

[0009] It is therefore the object of the invention to provide a safety switching apparatus which avoids the named disadvantages of the safety circuits already mentioned, the failure of essential elements of the safety system being easily recognisable to the user at the latest by the end of the ongoing operating cycle, obliging him to take action; the reaction times of temperature restricting means and thermostats play no part, and the risk of dry operation of the heating members is in addition considerably reduced. This object is achieved according to the invention in that two level switches monitor the normal water level and, incorporated in series, control a switch element, in that the switch element triggers the control motor, and in that the control motor, after expiry of a predetermined period, switches the heating resistance, by means of control contacts, into parallel with the switch element in the heating and washing stage following the water intake stage.

[0010] Only when the two level switches indicate and switch the normal level of water is switch-on of the heating prepared, and after a time determined by the control system the heating is switched on by control contacts of the control motor.

[0011] When an electronic control system is used, this procedure may be produced by a corresponding combination of software and hardware.

[0012] By means of the procedure described, the contacts of the level switches are normally switched without carrying current. Thus their reliability, and that of the level switches, is increased.

[0013] By means of parallel incorporation of two level switches with the same switching value, the tolerance relating to the quantity of water is in addition reduced.

[0014] Interrelation of the control contacts of the control motor with the safety switching apparatus is established in that there are incorporated, in series with the heating resistance, itself in parallel with the switch element , two control contacts which switch on the heating resistance during the heating and washing stage, so that the switch element is dependent only on the switching condition of the contacts of the level switches.

[0015] Each of the two level switches, in the outset position, switches a response circuit for the operating motor, said circuit being maintained through control contacts of the control motor only in the heating and washing stage and/or the subsequent rinsing procedures; then after the water is pumped out after the heating and washing stage, the final or intermediate spin can be stopped, of one of the two level switches does not revert to its outset position because of occurrence of a fault. At the end, the user receives washing which has not been spin-dried. Because of this failure he is motivated to call the repair service in order to have the damage rectified before a safety-threatening fault, failure of the second level switch, occurs. Therefore by means of using two level switches, it is not only the redundancy which is increased in a known way. In the configuration described above, the two switches monitor one another.

[0016] According to one embodiment, a relay is used as a switch element. When electronic controls are used, the procedure described above may be realised by a corresponding software/hardware combination.

[0017] The invention will be described in more detail with reference to an embodiment given by way of example and illustrated in the drawings, which show:

Fig. 1:

the circuit diagram of a switching apparatus with hardware components;

Fig. 2:

the associated circuit diagram of the control contacts of the control motor, and

Fig. 3:

the associated circuit diagram for an electronic system.

[0018] The circuit diagram in Fig. 1 shows the safety switching apparatus in the start and water intake stage, in which only the control contact tm5 of the control motor StM (program switch) is closed. A circuit for the water feed valve WV is closed by the closed-contact side 1-2 of the relay R and the control contact tm5. As long as the normal water level is not achieved, the contacts nv1 and nv2 of the level switches NV1 and NV2 remain in their closed positions 1-2. The position of the control contacts tm3 and tm4 is irrelevant. The control motor StM is not initiated in the starting and water intake stage.

[0019] When the level switches NV1 and NV2 reaches their switching points, their contacts nv1 and nv2 adopt the operative position 1-3. Here the switching sequence is not relevant. The contacts nv1 and nv2 switch on the switch element R, which is in the form of a relay, but which may also be in the form of an electronically-operating switching apparatus. By means of the contact r (1-3) of the switch element R, the water intake valve WV is switched off, and the control motor StM triggered and thus switched forward. After expiry of a period of time governed by the control system, the control contacts tm1 and tm2 are closed and the heating resistance H (with any preceding thermostat Th) is switched into parallel with the switch element R. The heating and washing stage is thus initiated. The control contacts tm3 and tm4 are effective in the circuit of the operating motor AM. The wash procedure can begin.

[0020] After the heating and washing stage, the control contacts tm1, tm2 and tm5 are opened, while the control contact tm6 is closed and takes over triggering of the control motor StM, even when the switch element R reverts to the outset position. After a time predetermined by the control system, the water is pumped out, and the level switches NV1 and NV2 reach their switch-back level. The contacts nv1 and nv2 revert to their outset position 1-2. The rinsing and spinning stages proceed in a known way, as the contacts nv1 and nv2 of level switches NV1 and NV2 close the circuit for the operating motor EM, independently of the position of the control contacts tm3 and tm4 which, in effect, as Fig. 2 shows, are not closed during the spin procedure. Therefore the spin procedure only takes place if both level switches NV1 and NV2 have reached their outset position. The control contacts tm3 and tm4 may also be open during each intermediate spin procedure, so that intermediate spinning also is stopped.

[0021] When a fault occurs, the user receives washing which has not been spin-dried, and is obliged to call the repair service to rectify the damage. In this way considerably greater security is achieved against dry operation of the heating resistance H. As the safety switching apparatus is symmetrical in structure, protection is always provided even from an earth-leakage fault by one of the two level switches and a control contact. By means of the control contacts tm3, tm4 and tm6 easily-recognisable yet not safety-threatening breakdowns are deliberately produced for the user, and with as a rule the desired effect - repair service, damage repair - as a consequence.

[0022] When an electronic control system is in use, the procedure described may be realised by a corresponding combination of software and hardware.

Claims

1. Safety switching apparatus against dry operation of electrical heating resistances, particularly for level-regulated washing machines and dishwashers with a control motor of a program switching mechanism or a corresponding software/hardware combination with partly and fully electrical program controls and level switches for monitoring and regulating the water level in the appliance,
characterised in that
two level switches (NV1, NV2) monitor the normal water level and, incorporated in series, trigger a switch element (R), in that the switch element (R) triggers the control motor (StM), and switches off the water intake valve (WV), and in that the control motor (StM), after expiry of a predetermined period, switches the heating resistance (H), by means of control contacts (tm1, tm2), into parallel with the switch element (R) in the heating and washing stage following the water intake stage.

2. Safety switching apparatus according to claim 1,
characterised in that
two control contacts (tm1, tm2) are incorporated in series with the heating resistance (H) itself incorporated in parallel with the switch element (R), said contacts (tm1, tm2) switching on the heating resistance (H) in the heating and washing stage, or establishing a corresponding switching logic system by means of electronic control (hard/software).

3. Safety switching apparatus according to claim 1 or 2,
characterised in that
each of the two level switches (NV1, NV2), in the outset position switches a response circuit for the operating motor (AM), said response circuit being maintained only in the heating and washing stage and/or the subsequent rinsing procedures, via control contacts (tm3, tm4) of the control motor (StM), or establishing a corresponding switching logic system by means of electronic control (hard/software).

4. Safety switching apparatus according to one of claims 1 to 3,
characterised in that
a relay is used as a switch element (R).

Drawing