WO1997017712A1

WO1997017712A1 - Hybrid switch

Info

Publication number: WO1997017712A1
Application number: PCT/GB1996/002700
Authority: WO
Inventors: John William Mcbride; Jonathon Swingler
Original assignee: University Of Southampton
Priority date: 1995-11-04
Filing date: 1996-11-04
Publication date: 1997-05-15
Also published as: GB9522676D0; AU7324296A

Abstract

An ac hybrid switching system where a power electronic device (figure 2) is used to switch current, and a mechanical device (figure 1) is used to provide physical isolation when the contacts (5, 6) are open and a low resistance current path when the contacts are closed. Arc erosion of the contacts is minimised. The device requires reduced cost contact materials and provides an extended lifetime performance over conventional switching systems.

Description

Hybrid Switch

This invention relates to a hybrid switch.

Conventionally control of power systems has been by means of contactors. The disadvantage of these is that they generate an arc and high temperature plasma which must be dissipated before breaking the circuit. The plasma erodes the surface of the contacts and produces electromagnetic radiation. The erosion limits the life of the devices.

A proposed solution is to use electronic switching which has the benefit of avoiding arcing. However, electronic switching cannot be used with systems which require physical isolation for safety reasons. In power systems a physical gap is required. Thus the solution to this is to combine electronic and mechanical switching to form a hybrid switch. However, generally hybrid switches have highly complex electronics which make them a less attractive option. In accordance with the present invention a hybrid switch comprises a power contact and first switching means, whereby movement of the first switching means away from the power contact causes an electrical connection between the power contact and second switching means to be broken, such that current flow to the power contact ceases.

The present invention provides a switch giving both physical isolation and electronic switching to avoid arcing in a simple construction.

Preferably, a current controlling device is connected across the power contact and the second switching means.

Preferably, the current controlling device comprises a thermistor and a triac.

Preferably the hybrid switch further comprises third switching means mounted outwardly of the first switching means whereby movement of the third switching means away from the first switching means causes an electrical connection between the power contact and the second switching means to be broken before electrical connection between the third and first switching means is broken, such that current flow between the power contact and the switching means ceases. Preferably, the first switching means is coupled to the second switching means by a resilient coupling.

Preferably, the second switching means is coupled to the power contact by a resilient coupling.

Preferably a triac and a resistor are connected across the first and second switching means.

Examples of a hybrid switch in accordance with the present invention will now be described and contrasted with a conventional switch with reference to the accompanying drawings in which:- Figures la to le illustrate an example of a hybrid switch according to the present invention;

Figure 2 is a circuit diagram of the hybrid switch of Figure 1;

Figure 3a shows the VI characteristic on opening of a contact with a conventional switch;

Figure 3b shows the VI characteristic on opening of a contact with the hybrid switch system of Figure 1; and,

Figure 4 illustrates an alternative example of a switch according to the present invention.

Figure 1 shows a hybrid switch 20 according to the present invention. Operation of the switch will be described in detail for opening the switch, but is equally applicable to closing the switch. In Figure la all contacts are closed and current flows directly between a main contact head 11 and a power contact 2. The switch 20 comprises a base 1 at the centre of which is mounted the power contact 2. The power contact 2 is insulated from a connector 4 to a gate contact 5. A first insulating casing 6 insulates the gate contact 5 from a terminal contact 7. A second insulating casing 8 encloses the terminal contact 7. The main contact head 11 is positioned above the terminal contact 7. A triac 9 is connected in series with a resistor 10 across the terminal contact 7. The construction of tne switch 20 is the same in Figures la to le, but the current ceases to flow in some parts of the switch as the contacts 2, 5, 7, 11 are broken sequentially. Figure 2 illustrates a circuit diagram of the hybrid switch of Figure 1. The operation of the switch will be described with reference to both Figures 1 and 2. The first stage of opening the switch is shown in Figure la. The hybrid switch is conducting due to the main contact head 11 being pushed down upon the terminal contact 7 (closing switch S3 12) . The terminal contact 7 in turn makes contact with the gate contact 5 (closing switch S2 13) and the gate contact 5 in turn makes contact with the power contact 2 (closing switch Si 14) .

On opening the switch SI 14 by retreating the main contact head 11 as shown in lb, the gate contact 5 separates from the power contact 2. A potential difference develops across the triac 9 between points TMI and TM2 and points Tl and G. Since the triac 9 is in non-conducting mode no current flows between TMI and TM2. However, a small current flows via resistor Rl 15 and the gate of the triac TMI and G. This current fires the triac 9 into conducting mode which allows full current to flow between TMI and TM2. As long as the voltage between contact pairs 2, 5, 7, 11 remains below the minimum required for an arc discharge no arcing should occur. The voltage required across TMI and G before the triac is triggered is 1.5 volts which is well below what would be expected for an arc discharge to develop.

The next stage of opening consists of the terminal contact 7 separating from the gate contact 5 as shown in Figure lc. This switches off current to the gate by opening switch S2 13. The triac 9 continues to conduct until the potential difference across the switch changes polarity. When the AC current changes polarity, the triac 9 ceases to conduct. The hybrid switch 20 provides an electrical break switching the current off as shown in Figure Id.

The last stage in the opening operation of the hybrid switch is to provide a physical isolating break. The main contact head 11 is further retreated to provide a physical break in the circuit as shown by Figure le. This is the effect of opening switch S3 12.

Figure 3a shows how the VI characteristic on opening the contact with a conventional switch system produces an arc 16 when contact is broken. It can be clearly seen in Figure 3b that in the present invention the triac 9 switches on when contact is broken and therefore no arcing occurs. The voltage across the triac remains constant. Thus the voltage drop across the contacts is kept to a minimum reducing the probability of arcing and thus erosion of the surfaces.

To close the switch the physical break between the main contact head 11 and the top of the terminal contact 7 as shown in Figure le is closed by applying pressure to the main contact head to counter the effect of the springs 15. This results in a contact being made between the terminal contact 7 and the gate contact 5. Further pressure brings the gate contact 5 into contact with the power contact 2 against the effect of the springs 16 so that current flows through the switch.

Figure 4 illustrates an alternative switch design comprising a power contact 21 and first and second switching means 22,23. Opening the switch by reducing the force applied to the first switch means 22 causes contact between the second switch means 23 and the power contact 21 to be broken. As a result current flows around a hybrid circuit 24 comprising a resistor 25, a thermistor 2 and a triac 27. The current flow through the thermistor causes it to heat up until its resistance becomes such that the triac 27 is switched into its non-conducting state and subsequent current flow is only between the first and second switching means.

Claims

1. A hybrid switch, the switch comprising a power contact and first switching means, whereby movement of the first switching means away from the power contact causes an electrical connection between the power contact and second switching means to be broken, such that current flow to the power contact ceases.

2. A switch according to claim 1 further comprising a current controlling device connected across the power contact and the second switching means.

3. A switch according to claim 2 wherein the current controlling device comprises a thermistor and a triac.

4. A switch according to claim 1, further comprising third switching means mounted outwardly of the first switching means whereby movement of the third switching means away from the first switching means causes an electrical connection between the power contact and the second switching means to be broken before contact between the third and first switching means is broken, such that current flow between the power contact and the switching means ceases.

5. A switch according to claim 4 wherein a triac and a resistor are connected in series across the first and second switching means.

6. A switch according to any preceding claim wherein the first switching means is coupled to the second switching means by a resilient coupling.

7. A switch according to any preceding claim wherein the second switching means is coupled to the power contact by a resilient coupling.