GB2200000A - Switchable torque AC motor operating valve - Google Patents

Abstract

A valve actuator is constructed to drive a valve member into either of two end positions, into one of which the valve member is biased by spring means, or a mid-position. The valve actuator includes an AC synchronous motor in which one of the rotor or stator windings is modified so that the output torque of the motor can be switched between a value in which it overcomes the return force of the spring means at any position of the valve member and a value which exactly balances the return force of the spring means at the mid-position. As illustrated the modified rotor or stator winding comprises two windings (18, 20) which are switched by means of a microswitch (12) between a first arrangement in which they are connected in parallel to provide maximum torque and an arrangement in which only the winding (20) is connected to the power supply to provide the reduced torque. <IMAGE>

Description

VALVE ACTUATORS The present invention relates to valve actuators and, more particularly, to actuators for three-position valves of the type used as divertor valves in central heating control systems.

In domestic central heating systems a divertor valve is located in the hot water outlet from a boiler in order to direct the hot water supply selectively to either or both of heating circuit and a hot water circuit. For this purpose, the divertor valve has three positions and is driven by a valve actuator which includes an AC synchronous motor coupled to the valve member via a gear box. A spring is used to bias the valve member into one position, and the valve member can be driven to and maintained in another.

position by the continuous application of power to the motor so that the valve member is driven until it engages a mechanical stop where the motor stalls. It is also necessary to provide some device for stopping the valve member at a mid-position. Various proposals have been made for this purpose. For example one such valve actuator includes a rectification circuit which is selectively switched into the normal AC power supply to the motor when the valve member reaches the mid-position so that a DC voltage is supplied to the motor. This causes the motor to stall but produces a holding force which holds the motor in the stall position. In another arrangement a resistor is switched into the circuit between the power supply and the motor when the valve member reaches its desired midposition.The result of this resistor being in circuit is to limit the torque generated by the motor so that it just balances the return torque applied to the valve member by the spring in the mid-position. Therefore, the motor stalls and remains stationary. Although effective, there is a considerable power dissipation in both these circuits while the valve member is maintained in its mid-position.

The present invention accordingly provides a valve actuator for a valve member which is biased by spring means into one end position and is selectively movable by the actuator into a second end position or a mid-position between said end positions, the actuator comprising an AC motor having a winding modified so that it is switchable between two arrangements whereby the number of ampere turns of the first arrangement is such that the output torque of the motor is sufficient to overcome the force of the spring means at all positions of the valve member, and the number of ampere turns of the second arrangement is such that the output torque of the motor balances the return force of the spring means at said mid-position, and switch means for switching from the first arrangement to the second when the valve member reaches the mid-position when the valve member is to be stopped in said mid-position.

The actuator of the present invention is advantageous over the previously proposed circuits in that the power dissipation is reduced as there is no external resistor or rectification circuit. Moreover, fewer components are required since it is only necessary to modify one winding in the motor The modified winding may take several forms, for example there may be two windings which are connected in parallel in the f-irst arrangement and only one of which is connected to a power supply in the second arrangement, or the two windings may be connected in series in the second arrangement.Alternatively, the modified winding may comprise a single winding with an intermediate tap, said first arrangement connecting a power supply between one end terminal and the tap terminal of said winding, and said second arrangement connecting the power supply across the end terminals of said winding.

A valve actuator embodying the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is an external view of a valve fitted with a valve actuator of the present invention; Figure 2 is a longitudinal section through the valve of Figure 1; Figure 3 is a plan view of the valve of Figure 1; Figure 4 shows the switching circuit connected for placing the valve member in one end position; and Figure 5 show-s the switching circuit when the valve member is being held in the mid-position.

Figure 1 shows an assembly of a housing 2 containing a valve actuator, and a divertor valve 4 which has an inlet 6 and two outlets 8 and 10. In use in a domestic heating system, the inlet 6 is connected to the hot water outlet from a boiler, and the outlets 8 and 10 are respectively connected to a domestic water heating circuit and a central heating circuit. The valve actuator includes an AC synchronous motor connected by a cable 12 carrying three leads 14, 15 and 16 to a power. source. The AC motor has an output shaft 22 which projects into the body of the valve and carries two obturators 24 and 26 which are outwardly biased by spring means towards the periphery of the housing of the valve 4.

The output shaft 22 and the obturators 24 and 26 comprise a valve member. The shaft 22 is biased by spring means (not shown) within the actuator housing 2 into the position shown in Figure 3 where the obturator 26 closes off the outlet 10 and the inlet 6 is therefore connected solely to the outlet 8. This is defined as one end position of the valve.

Operation of the synchronous AC motor rotates the shaft 22 counter-clockwise as seen in Figure 3 to another end position in which the obturator 24 closes off the outlet 8.

The shaft 22 is stopped in this position by engagement with a mechanical stop (not shown) acting on the shaft 22 within the actuator housing 2. The actuator is operable, as will be described more fully below, in order to stop the output shaft 22 at a mid-position between the two end positions described in which neither of the obturators 24, 26 fully blocks the outlets 8 and 10 so that the inlet 6 is in communication with both outlets. A microswitch 30 is also housed in the actuator housing 2 and is arranged to be operated by a contact on the output shaft 22 when the shaft reaches the mid-position. It will be appreciated that it is not material which of the two end positions or midpositions corresponds to each supply possibility. A gear box may be interposed between the AC motor and the output shaft 22 in the actuator housing 2.

The motor is controlled by a switching circuit shown in Figures 4 and 5. The construction of the valve member and its connections with the hot water supply and the central heating and water heating circuits including the spring biasing into one end position is an essentially conventional design and will not be described herein in further detail.

However, it will be appreciated that any construction that incorporates a spring bias into one end position and a mechanical stop at the other end position together with a micro switch or other device for sensing when the valve member has reached its mid-position can be used with the actuator now to be described.

The AC synchronous motor is connected via terminals 14, 16 to a constant voltage AC power supply, usually the mains electricity supply for a domestic central heating system.

The synchronous motor is of conventional design and is preferably a hysterisis type motor with only a stator coil.

The stator coil is modified by being separated into first and second windings 18, 20. One end of each of the first and second windings 18, 20 is connected to the terminal 16 of the power supply. The position of the microswitch 30 controls whether both the windings are connected to the power supply in parallel as shown in Figure 4, or only the winding 20 is connected to the power supply as shown in Figure 5.

The number of ampere turns of the two windings 18 and 20 is selected so that when connected in parallel they produce a torque on the valve shaft 22 that is sufficient to overcome the return force of the spring throughout the travel of the valve member between the two end positions. Therefore, when the circuit is connected as shown in Figure 4, with the power supply applied between terminals 14 and 16, the motor will drive the shaft 22 against the force of the spring until it reaches the position where the microswitch 30 operates. Winding 18 will now be disconnected (as in Fig.

5) and the torque produced by winding 20 will hold the valve in the position i.e. the mid-position.

If the valve is now required to be fully open the power supply is now connected to terminal 15 and 16 as well as terminals 14 and 16. The motor will now drive the valve member against the force of the spring until it reaches the mechanical stop. If the valve is required to return to the mid-position from this full open position, the power supply is removed from terminal 15 in which case the spring will cause the valve member to return it to a position where the torque produced by winding 20 balances with the torque produced by the spring and the motor stalls.

As illustrated in Figures 4 and 5, the first and second windings 18 and 20 are independent windings connected in parallel to the power supply. It will be appreciated that the circuit could be modified so that only a single physical winding is used which is tapped at an appropriate point.

The complete winding is designed so that it draws a current which is sufficient to produce an output torque from the motor which just overcomes the return force of the spring means at the mid-position of the valve member. However the resistance of the tapped portion is selected so that despite the reduced number of turns in the tapped portion the current drawn is increased so that the motor produces a torque which overcomes the return force of the spring at all positions of the valve member.

In a further alternative embodiment the second winding may be connected into the circuit so that the two windings will be connected in parallel when the valve member is to be driven to the end position and in series when the motor is to be stalled at the mid-position.

Claims (7)

1. A valve actuator for a valve member which is biased by spring means into one end position and is selectively movable by the actuator into a second end position or a midposition between said end positions, the actuator comprising an AC motor having a winding modified so that it is switchable between two arrangements whereby the number of ampere turns of the first arrangement is such that the output torque of the motor is sufficient to overcome the force of the spring means at all positions of the valve member, and the number of ampere turns of the second arrangement is such that the output torque of the motor balances the return force of the spring means at said midposition, and switch means for switching from the first arrangement to the second when the valve member reaches the mid-position when the valve member is to be stopped in said mid-position.

2. An actuator as claimed in claim 1, wherein the modified rotor or stator winding comprises two windings which are connected in parallel in said first arrangement.

3. An actuator as claimed in claim 2, wherein only one of the windings is connected to a power supply in said second arrangement.

4. An actuator as claimed in claim 2, wherein the windings are connected in series in said second arrangement.

5. An actuator as claimed in claim 1, wherein the modified winding comprises a single winding with an intermediate tap, said first arrangement connecting a power supply between one end terminal and the tap terminal of said winding, and said second arrangement connecting the power supply across the end terminals of said winding.

6. A valve actuator as claimed in any one of the preceding claims, wherein the switch means comprises a micro switch operable when the valve member reaches the mid-position.

7. A valve actuator substantially as herein described with reference to the accompanying drawings.