WO1999003121A1

WO1999003121A1 - Circuit means for minimising arc generation during switching operations

Info

Publication number: WO1999003121A1
Application number: PCT/GB1998/001886
Authority: WO
Inventors: Paul Roy Gorton
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-07-11
Filing date: 1998-06-26
Publication date: 1999-01-21
Anticipated expiration: 2000-01-11
Also published as: GB9714521D0; AU8224798A

Abstract

Circuit means for minimising the generation of arcing between the two contacts (12, 14) of a switch (11) used to connect or disconnect alternating current electrical power to and from a load (10), includes means (15) responsive to a voltage developed across the two contacts (12, 14) of the switch (11) to connect the load (10) directly to the power supply through the said circuit means until the waveform of the supply voltage passes through zero. The means (15) may comprise a single triggered bidirectional semiconductor device such as a triac.

Description

CIRCUIT MEANS FOR MINIMISING ARC GENERATION DURING SWITCHING OPERATIONS

A common problem that arises when switching heavy or inductive loads, particularly in AC electrical circuits, is that of arc generation between switch contact surfaces. This arcing any lead to burning or even welding of the contact surfaces and, at the very least, a reduction in the life of the contacts. There is also the additional problem of electrical noise being generated when an arc is formed.

Attempts have been made to reduce the problem by the use of a resistor- capacitor network which operates to quench the arc when it is formed. Such a network needs to be matched to the characteristics of the load and therefore networks have to be individually designed for each situation.

It is an object of the invention to provide circuit means for minimising arc generation which is also suitable for a wide range of load characteristics.

According to the present invention there is provided circuit means for minimising the generation of arcing between two contacts of a switch used to connect and disconnect alternating-current electrical power from a power supply to and from a load, which circuit means includes means responsive to a voltage developed across the two contacts of the switch as they separate and operable to connect the load directly to the power supply through the said circuit means until the waveform of the supply voltage passes through zero.

The invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 shows a load connected to an AC supply by way of a switch and one form of the circuit means of the invention, and

Figure 2 shows the use of an alternative circuit means according to the invention.

The circuit means referred to above may take various forms but will preferably comprise one or more semiconductor devices connected in series with the load across the power supply and operable to conduct when a voltage is developed across it on the opening of the switch. One example of a suitable semiconductor device is a triac.

A triac is an semiconductor device which acts as a bidirectional switch between its main terminals 16 and 17 and has a triggering or "gate" electrode 18. The operation of the device is controlled by a low current signal applied to the gate electrode 18. A low current of either polarity passing between main contact 16 and gate electrode 18 is able to control a substantially larger current flowing between the main terminals 16 and 17. To initiate switching of the triac, the control current needs to be of a few microseconds duration only. Once conduction is established between the main terminals of the triac with a current greater than the holding value, the gate current is no longer required until the main current falls below the holding value.

The formation of an arc between two contact surfaces depends upon many factors, in particular the gap between the contacts and the voltage across them. When a resistive load is switched off by a pair of contacts, the worst case occurs if switching occurs when the voltage waveform is at maximum, which in the case of the European 230 Volt standard will be about 325 volts. Hence when the contacts start to open the voltage across them rises rapidly from zero to 325 volts. This will often be sufficient to initiate an arc between the contacts which is extinguished only when the voltage passes through zero. If an inductive load is to be switched, the situation is much worse. When the load current is switched off, the magnetic field collapses suddenly and a back-EMF appears across the load. The voltage thus produced across the opening contacts may therefore be much more than 325 volts.

Referring to Figure 1, a load 10, which may be inductive or resistive, is connected to the live L and neutral N sides of a power supply (not shown) by way of a switch 11. The switch has fixed contacts 12 and 13 and a movable contact 14. The contacts 12 and 14 are closed when power is supplied to the load 10. Connected across the contacts 12 and I is a triac 15, having main terminals % and 17 and a gate electrode 18, and a capacitor 19 and resistor 110 are connected in series between the gate electrode 18 and main terminal 17 of the triac. Power in disconnected from the load by moving the movable contact 14 of the switch from fixed contact 12 to fixed contact 13.

As the switch 11 is opened and contacts 12 and 14 begin to move apart, the voltage developed across these contacts results in a current being developed between the gate electrode 18 of the triac 15 and its main terminal 16 by way of capacitor 19 and resistor 110. The capacitor-resistor combination only passes fast current changes to the triac 15. The control current causes the triac to conduct, bypassing the switch contacts until the load current falls to zero. Hence the circuit prevents the formation of any arc across the opening contacts of switch 11. A small voltage drop occurs across the triac when it is conducting which would lead to unacceptable heating of the device if it was to carry the full load current for any significant period of time. However, in the arrangement described above the maximum time for which the triac will conduct is one half-cycle of _the supply voltage waveform.

It will be noted that, when the switch 11 has changed over completely, the connection between movable contact 14 and fixed contact 13 short circuits any electrical noise which may be developed by the opening of the switch. In addition, the resistor-capacitor network slows the rise of any transient voltages and prevents false triggering of the triac 15. However, if neither of these problems are considered to be serious in any particular situation, the fixed contact 13 of the switch 11 may be discarded.

The situation described above is that existing when the supply to the load is switched off. At switch-on, a similar situation may exist of there is any bounce of the switch contacts, since each small opening of the contacts presents the same situation as exists when the switch is opened. Hence references in this specification to the switch contacts separating should be taken to include the situation which occurs momentarily if the switch contacts bounce when the switch is closed.

Figure 2 shows an alternative to the triac of Figure, which is replaced by two separate unidirectional triggered semiconductor devices 20 and 21 , each with its own resistor-capacitor network connected across it. This arrangement operates in the same manner as the triac of Figure 1, with one or other of the semiconductor devices conducting when triggered, depending on the polarity of the supply voltage at the instant of triggering. In this embodiment it is not possible to provide the electrical noise suppression by way of an additional switch contact, as was possible in the arrangement of Figure 1. However, there are other circuit arrangements using other forms of semiconductor device with which this noise suppression may be provided.

The values of the resistor and capacitor in either arrangement are chosen to suit the particular semiconductor device being used, so that a step in voltage between the trigger electrode of the device and the load of, say, 25 volts will result in the semiconductor device being rendered conducting. The circuit does not require to be changed to suit different load characteristics, so long as the conducting semiconductor device is able to handle the maximum current and voltage requirements of the load for the short period necessary.

The prevention of arcing across the switch contact makes it easier to meet the requirements of any rules concerning the generation of spurious electromagnetic radiation by electrical equipment. In the past it has often been necessary to provide shielding to reduce unwanted radiation.

Claims

1. Circuit means for minimising the generation of arcing between two contacts of a switch used to connect and disconnect alternating-current electrical power from a power supply to and from a load, which circuit means includes means responsive to a voltage developed across the two contacts of the switch as they separate and operable to connect the load directly to the power supply through the said circuit means until the waveform of the supply voltage passes through zero.

2. Circuit means as claimed in Claim 1 which includes at least one semiconductor device connected in parallel with the two contacts of said switch and having a gate electrode across which a triggering voltage is developed when the contacts of the switch separate.

3. Circuit means as claimed in either of Claims 1 or 2 which includes one bidirectional semiconductor device having a gate electrode across which the said triggering voltage is developed.

4. Circuit means as claimed in Claim 3 in which the switch includes a further contact arranged such that when power is disconnected from the load the switch short-circuits any transient voltages developed across the power supply.

5. Circuit means as claimed in either of Claims 1 or 2 which includes a pair of unidirectional semiconductor devices connected in parallel with one another and in parallel with the two contacts of said switch and each having a gate electrode across which the said triggering voltage is developed.

6. Circuit means for minimising the generation of arcing between the two contacts of a switch used to connect electrical power to a load substantially as herein described with reference to the accompanying drawings.