GB2114386A

GB2114386A - Electromagnetic actuator.

Info

Publication number: GB2114386A
Application number: GB08302371A
Authority: GB
Inventors: Flavio Campagnolo
Original assignee: Industrie Zanussi SpA
Current assignee: Industrie Zanussi SpA
Priority date: 1982-02-04
Filing date: 1983-01-28
Publication date: 1983-08-17
Also published as: IT1158616B; ES278778Y; FR2520919A1; IT8245703A0; GB8302371D0; DE3303490A1; IT8245703A1; ES278778U

Abstract

An electromagnetic actuator comprises at least one coil (1) for connection to a power source (2) by way of at least one switch (4) which is controllable for activating or deactivating a movable control member. Connected in series with the coil (1) is at least one PTC thermistor (3) to protect the coil against damage due to prolonged over- current, which may occur if the control member jams. Several coils, with their respective switches may be connected in parallel and protected by one thermistor (Figure 2). <IMAGE>

Description

SPECIFICATION Electromagnetic actuator The invention concerns an electromagnetic actuator, in particular for electrical domestic appliances, for example of the type for controlling hydraulic valves, detergent distributors or the like.

As is known, such electromagnetic actuators essentially comprise a coil which, when the electric current flows therethrough, generates an electromagnetic field which exerts its attractive force to a movable stud which causes activation or deactivation of a movable control member.

At the present time, the coils used in electromagnetic actuators are of dimensions in accordance with the requirements of international safety standards, in particular those standards which require the use of coils which must be capable of operating continuously for a certain number of hours without overheating or completely burning out.

For that purpose, the electromagnetic actuators are provided with over-dimensioned coils in order to meet the requirements of the safety standards, while on the other hand, however, being expensive and undesirably bulky. In addition, it may happen that the electromagnetic actuator remains in operation for a period of time which is longer than that envisaged by the approval tests and the safety standards, such as for example may occur in the case of the electrically operated valve of a washing machine which is started without first being connected to the water main, or as may happen in the case of the electrically operated detergent metering means of a dish washing machine in which the prog rammer jams on a detergent discharge phase.

Such troubles cause the coil of the electromagnetic actuator to overheat and, in the long run, to burn out.

According to the present invention,there is provided an electromagnetic actuator comprising at least one coil capable of being connected to an electrical power source by way of at least one controllable switch for actuating a movable control member, and at least one thermistor, having a positive temperature coefficient, connected in series with said coil.

As will become apparent from the following description, the invention can provide an electromagnetic actuator which makes it possible to use coils which are not over-dimensioned and which are therefore not bulky and expensive, but which have characteristics of great reliability under any operating conditions.

The features and advantages of the invention will be better appreciated by means of the following description, given solely by way of non-limiting example, with reference to the accompanying drawing in which: Figure 1 shows a diagrammatic view of an electromagnetic actuator according to the invention, and Figure 2 is a diagrammatic view of another embodiment of the actuator according to the invention.

The electromagnetic actuator diagrammatically shown in Figure 1 essentially comprises the series connection of a coil 1 and at least one thermistor 3 having a positive temperature coefficient. The actuator is capable of being connected to an electrical power souce 2 by means of a controllable switch 4. Energization of the coil 1 causes an electromagnetic field to be generated, which is capable of actuating a movable control member (not shown), in known manner. As is known, the thermistor 3 with its positive temperature coefficient, also referred to as P.T.C., has the characteristic of experiencing an increase in its ohmic resistance with an increase in temperature whereby, when the current flows through the electromagnetic actuator, the thermistor 3 heats up and the value of its resistance increases proportionately.

Consequently, the progressive increase in the resistance of the thermistor 3 causes a corresponding reduction in the current flowing through the actuator and thus the electromagnetic field generated by the coil 1, down to values which are insufficient to maintain the movable control member in an activated condition.

In the situation where, for the reasons already indicated above, the electromagnetic actuator remains connected for a long period of time to the power source 2, the increase in the resistance of the thermistor 3 is such as to minimize the current flowing in the actuator, thereby avoiding undesirable overheating of the coil 1.

In other words, substantially all the voltage of the power source 2 is dropped across the terminals of the thermistor 3, causing the above-mentioned deenergization of the coil 1. This permits the coil 1 to be under-dimensioned, with a consequent reduction in the bulk and the cost of the entire actuator which however enjoys a higher degree of reliability insofar as the coil 1 is no longer subjected to excessive temperatures.

It will be apparent that the under-dimensioned coils may be used in electromagnetic actuators which have varying operating times and therefore maximum energization periods of greater or lesser length.

For example, in the case of laundry washing machines or dish washing machines, the period of operation of an electrically operated inlet valve is longer than that of an electrical detergent metering means, and therefore the maximum time of energization of the actuator is determined by varying the dimensions of the coil 1, and obviously also depends on the characteristics of the thermistor 3.

In conclusion, if the coil 1 has a low resistance and thus absorbs a relatively high level of current, the increase in temperature of the thermistor 3 will be very rapid and the drop in voltage at the terminals of the coil 1 will also be rapid.

Underthose conditions, the electromagnetic unit ceases to operate within a few seconds and does not have time to overheat, in spite of the high current passing therethrough.

If the coil 1 on the other hand has a high resistance and thus absorbs a low level of current, that results in a reduction in the flow of current through the thermistor 3 so that the thermistor has a longer heating period and the electromagnetic unit oper ates for a longer period of time. In the case of a plurality of electromagnetic actuators (for example three) forming part of a single piece of apparatus, an embodiment of the invention diagrammatically shown in Figure 2 may be used in which a single thermistor 3 is connected in series to three parallel arms, each formed by a respective coil 1, 5 and 6 in series with a corresponding controllable switch 4,7 and 8, only one of these switches 4,7 and 8 being closed at any one time. Each coil 1,5 and 6 is capable of actuating a respective movable member (not shown), with maximum energization times which depend on the dimensions of the coils and actuation of the switches 4,7 and 8.

The electromagnetic actuator according to the invention is used in particular in electrical domestic appliances of the dish washing or clothes washing type, but it will be appreciated that it may also be used with advantage in equipment of different types and for different purposes. It will be appreciated that the electromagnetic actuator according to the invention may be the subject of any modifications according to circumstances, except for the features claimed hereinafter.

Claims

1. An electromagnetic actuator comprising at least one coil capable of being connected to an electrical power source by way of at least one controllable switch for actuating a movable control member, and at least one thermistor, having a positive temperature coefficient, connected in series with said coil.

2. An electromagnetic actuator adcording to claim 1, wherein there are a piurality of such coils which are in parallel with one another and are together in series with the thermistor.

3. An electromagnetic actuator according to claim 4 wherein the coils have respective, individual, controllable switches.

4. An electromagnetic actuator constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.