A PRESSURE SWITCH PROVIDED WITH A THERMAL CUT-OUT

Abstract: A pressure switch for a compressor or similar apparatus which can supply a fluid under pressure and is driven by an electric motor, comprises a support structure (1), a diaphragm (4) which is fixed to the support structure (1) and is intended to be exposed to the pressurized fluid, electrical contacts (9, 10, 11, 12) mounted on the support structure (1) and arranged for movement towards and away from one another between a position of closure and a position of opening of an electrical-supply circuit of the electric motor, a manually-operated control member (3) movable between two positions in which the electrical contacts are in the positions of opening and of closure of the electrical circuit, and a lever mechanism (14) operatively associated with the diaphragm (4) and with the electrical contacts in a manner such that, when a pressure above a predetermined threshold value is applied to the diaphragm, the lever mechanism brings about movement of the electrical contacts to the opening position, and is characterized in that it comprises a temperature-sensitive thermal cut-out (17) which is mounted on the support structure (1), in series with the electrical supply circuit of the motor, and is arranged to open the electrical-supply circuit automatically when the temperature exceeds a predetermined value, the cut-out being operable manually in order to be reset to a closure position which is stable when the temperature is below the predetermined value, and the cut-out being operatively associated with the control member (3) in a manner such that the cut-out is reset to its closure position as a result of the movement of the control member to its position which brings about opening of the electrical contacts.

Description: A pressure switch provided with a thermal cut-out The present invention relates to a pressure switch or pressure-controlled automatic electric cut-out, to be fitted, -in particular, in machines such as compressors, autoclaves or pumps which can supply fluid under pressure and are driven by electric motors . In particular, the invention relates to a pressure switch provided for breaking the electrical-supply circuit to the motor and discharging the pressurized fluid when the pressure exceeds a predetermined value, of the type comprising a support structure, a diaphragm which is fixed to the support structure and is intended to be exposed to the pressurized fluid, electrical contacts mounted on the support structure and arranged for movement towards and away from one another between a position of closure and a position of opening of an electrical-supply circuit of the motor, a manually-operable control member movable manually between two positions in which it brings about movement of the electrical contacts to their positions of opening and of closure of the electrical circuit, arid a lever mechanism operatively associated with the diaphragm and with the electrical contacts in a manner such that, when a pressure above a predetermined threshold value is applied to the diaphragm, the lever mechanism brings about movement of the electrical contacts to the open position, stopping the moto . The pressure switch also comprises a discharge valve, mounted on the support, for connection to the source of fluid under pressure; when the electrical contacts are in the open position, with consequent stoppage of the electric motor, the valve is brought to its open position in order to discharge the pressurized fluid to the exterior-. The electrical-supply circuit of the motor can be reset to its closure conditio to enable the motor to be started only when the pressure has returned below the predetermined threshold value as a result of the opening of the discharge valve . When the pressure switch is fitted on a compressor, a thermal cut-out is usually associated with the motor and is arranged to,break the electrical-supply circuit of the motor as a result of overheating due, for example, to excessive current absorption. The main disadvantage connected with the above-mentioned solution is that, after a stoppage of the motor due to the opening of the thermal cut-out, the operator may instinctively reset the cut-out to its closure position without previously having isolated the supply circuit; this operation is liable to give rise to sparks which often cause the contacts of the thermal switch to "stick", rendering it inoperative . To overcome the above-mentioned disadvantage, the subject of the invention is a pressure switch of the above-mentioned- type, characterized in that it incorporates a temperature- sensitive thermal cut-out which is mounted on the support structure of the pressure switch, in series with the electrical-supply circuit of the motor, and is arranged to open the electrical-supply circuit automatically when the temperature exceeds a predetermined value, the cut-out being operable manually in order to be reset to a position of closure of the electrical circuit, which position is stable when the temperature is below the predetermined value, and the cut-out being operatively associated with the control member in a manner such that the thermal cut-out is reset to its closure position as a result of the manual movement of the control member to its position which brings about opening of the .electrical contacts. The advantages and further characteristics of the pressure switch according to the invention will become clear from the following detailed description, given "with reference to the appended drawings, in which: Figure 1 is a partially-sectioned, schematic view of a pressure switch according to the invention, Figures 2 to 4 are schematic views which show the functional characteristics of the pressure switch in three distinct configurations, that is, more precisely: Figure 2 shows the pressure switch in the normal operating configuration, Figure 3 shows the pressure switch in the intervention configuration in which the thermal cut J out is opened, and Figure 4 shows the pressure switch in the configuration in which the thermal cut-out is reset or rearmed. The pressure switch to which the invention relates is of a known type and is widely available commercially; for example, the pressure switches concerned are. the single- phase pressure switches produced and marketed by the Applicant under the trademark NE-MA, which are used widely in association with small compressors of low power driven by single-phase electric motors. Naturally, i is intended that the structural principle upon which the invention is based may be used in pressure switches of any type having functions similar to those of the above-mentioned pressure switch. As illustrated in the drawings, the pressure switch comprises a support structure 1 with a cover 2, associated with which is a push-button 3 which is operable manually and can snap between an extended, ON position and a retracted OFF position in which, as will be described in greater detail below, the electrical-supply circuit to a motor M of the compressor is closed and is open, respectively. A diaphragm 4 associated with the support structure 1 is exposed to the fluid pressure when the pressure switch is connected to the pressure source by means of a threaded- connector 5. The pressure switch connects the electrical-supply line to the motor by means of a terminal block.7 which comprises a first pair 9, 9a and a second pair 10, 10a of fixed contacts, and a slider member 13 with which a first pair 11, 11a, and- a second pair 12, 12a of movable contacts are associated. The slider 13 arid the movable contacts associated therewith are movable between a position of opening and a. position of closure of the electrical-supply circuit ,of the motor. The slider 13 can be brought to its position in which the electrical contacts are open by the manual operation of the manual reset button 3. In the " configuration shown, pressure exerted on the push-button 3 brings about axial movement of the slider and opening of the electrical circuit ; when the push-button 3 is returned to its extended position, the slider 13 re-establishes the closure of the electrical circuit under the action of resilient means. Naturally, the invention is _ not intended to. be limited to the specific configuration described above and includes all functionally equivalent solutions . The slider 13 is operatively associated, by resilient means 15, with a snap-lever mechanism 14 ' which in ; turn is operatively associated with the diaphragm 4. When the fluid pressure exceeds the opposing pressure exerted on the diaphragm by a counter-pressure spring 6, the lever mechanism 14 brings about movement of the slider and of the movable contacts to the position in which the circuit is open; at the same time, an air-valve 16 connected to the pressure source is opened, enabling the excess pressure to be discharged. According to the innovative characteristic of the invention, the pressure switch incorporates a thermal cut-out 17 mounted in series with the electrical-supply circuit. This cut-out is typically constituted by a temperature-sensitive, bistable, bimetal-strip cut-out which is arranged to open the electrical-supply circuit when the heating effect produced by the current flowing through the bimetal strip heats the strip to a predetermined threshold value. Bimetal-strip thermal cut-outs are known per se and do <not require detailed description. In the embodiment illustrated, the cut-out comprises an axially-movable rod-like operating member 18 which acts on the strip to reset its closure position, which is stable when the temperature of the strip is below the predetermined threshold value. The rod 18 is operatively associated with the push-button 3 by means of a pressure element 19 in a manner such that, in order to reset the thermal cut-out to its closure position, it is necessary to move the push-button 3 to its OFF position which brings about preliminary opening of the electrical contacts and opening of the air-valve 16. In the configuration shown, the movement of the push-button 3 to its retracted, OFF position, " brings about axial movement of the rod which pushes the strip to its closure position. As stated, Figure 2 shows the pressure switch schematically in its normal-operation configuration in which the movable contacts and the thermal cut-out are in the configuration in which the electrical circuit is closed. Figure 3 shows the configuration in which, as a result of overheating produced, for example, by an excessive absorption of electrical energy, the thermal cut-out has snapped to its open configuration, interrupting the electrical supply to the motor. i i Figure 4 shows the configuration in which, by acting on the push-button 3, the operator has reset the closure position of the thermal cut-out and the movable contacts are correspondingly in their position in which the electrical circuit is open. In order to start the motor, it is necessary to return the push-button- 3 to its extended, ON position which brings about closure of the contacts; since the rod 18 is not connected to the push-button axially, the thermal cut-out maintains its stable position of closure of the electrical circuit after the push-button . has moved to the extended position. It will be appreciated that, by virtue of the above- mentioned characteristics, the pressure switch according to the invention achieves the important advantage of ensuring that the thermal cut-out cannot be reset when the circuit is live; moreover, since the movement of the push-button 3 to its OFF position provides for the discharge of the compressed air in the head ' of the compressor as well as disconnecting the motor, starting of the motor under load (with the head under pressure) is prevented; finally, for the above-mentioned reasons, the motor is prevented from working without protection; this characteristic is particularly important since the operator cannot generally check visually for damage or sticking of the thermal cutout. The incorporation of the thermal cut-out in the pressure switch also eliminates the manual operations connected with the installation of commercial thermal cutouts in the connection line to the motor. Naturally, the principle of the invention remaining the same, the details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example.

Claims: CLAIMS 1. A pressure switch for a compressor or similar apparatus which can supply a fluid under pressure and is driven by an electric motor, comprising: - a support structure (1) , - a diaphragm (4) which is fixed to the support structure (1) and is intended to be exposed to the pressurized fluid, - electrical contacts (9, 10, 11, 12) mounted on the support structure (1) and arranged for movement towards and away from one another between a position of closure and a position of opening of an electrical-supply circuit of the electric motor, - a manually-operable control member (3) movable between two positions in which the electrical contacts are in the positions of opening and of closure of the electrical circuit, respectively, and - a lever mechanism (14) operatively associated with the diaphragm (4) and with the electrical contacts in a manner such that, when a pressure above a predetermined threshold value is applied to the diaphragm, the lever mechanism brings about movement of the electrical contacts to the opening position, characterized in that it comprises a temperature-sensitive thermal cut-out (17) which is mounted on the support structure (1) , in series with the electrical- supply circuit of the motor, and is arranged to open the electrical-supply circuit automatically when the temperature exceeds a predetermined value, the cut-out being operable manually in order to be reset to a closure position which is stable when the temperature is below the predetermined value, and the cut-out being operatively associated with the control member (3) in a manner such that the cut-out is reset to its closure position as a result of the movement of the control member to its position which brings about opening of the electrical contacts. 2. A pressure switch according to Claim 1, characterized in that the thermal cut-out is a bistable, bimetal-strip cutout (17) . 3. A pressure switch according to Claim 1 or Claim 2 , characterized in that the manually-operated control member (3) can snap between a travel-start position and a travel- end position and cooperates with a slider (13) with which the movable contacts (11, 12) are associated, and with an operating member (18) of the thermal cut-out (17) in a manner such that manual movement of the control member (3) from the travel-start position to the travel-end position brings about movement of the slider (13) and of the electrical contacts (11, 11a, 12, 12a) to the 'position of closure of the contacts and movement of the operating member (18) to the position of closure of the thermal cut-out (17) . 4. A pressure switch according to Claim 3 in which the operating member (18) of the thermal cut-out is not connected to the control member (3) so that the thermal cutout is kept in its closure position after the movement of the control member from the position of opening to the position of closure of the electrical contacts .

Inventors: ARRABITO LORENZO, ARRABITO, LORENZO
Applicants: ARRABITO LORENZO, NE MA S P A, ARRABITO, LORENZO, NE-MA S.P.A.
Application year: 2001
Main classification: H01H35
IPC codes: H01H35, H01H37, H01H71