WO2004032168A1

WO2004032168A1 - Electrical switching method and apparatus

Info

Publication number: WO2004032168A1
Application number: PCT/GB2003/004282
Authority: WO
Inventors: Robin Alan Radcliffe Wood
Original assignee: Electroheat Plc
Priority date: 2002-10-03
Filing date: 2003-10-03
Publication date: 2004-04-15
Also published as: GB2409771B8; GB2409771B; GB0222881D0; GB0506775D0; GB2409771A; GB2409771A8; AU2003271893A1

Abstract

A switching apparatus (10) for an electrical load (12) on an A.C. Power supply (14) comprises a controller (16) controlling a solid state switch (18) for switching on and off the load, and a relay (20) with contacts (22) in parallel with the solid sate switch (18). When switching on the load (12), the controller (16) first switches on the solid state switch (18), waits for a first predetermined period, and then closes the relay (20) contacts (22). When switching off the load (12), the controller (16) first opens the relay (20) contacts (22), waits for a second predetermined period, and then switches off the solid state switch (18). The relay (20) contacts (22) are thus protected by the solid state switch (18) against arcing and the solid state switch (18) is protected against the problems of overheating and interference generation by the shorting action of the relay (20). The solid state switch (18) can, as an option, be opened after closure of the relay (20) contacts (22) and closed prior to opening of the relay (20) contacts (22). The controller is powered by the A.C. supply.

Description

Electrical Switching Method And Apparatus

The present invention relates to switching on and off of electrical equipment, and particularly relates to such switching of an alternating current (AC) supply through high power equipment such as electrical heaters.

The switching of equipment is advantageously achieved using solid state switchgear, such as triacs, which can be switched on at a zero voltage point of the AC waveform so as to avoid the sudden inrush of current which would be the result of switching the equipment on at, for example, the peak voltage of the AC waveform. Such switching is of particular utility for incandescent light bulbs and electrical heaters, where the high ratio between the switched off resistance and the switched on resistance of the load makes for tremendously high inrush currents.

The heavy standing load current can cause solid state switches to overheat. The characteristics of a solid state switch can generate electrical and radio frequency noise. The present invention seeks to prevent overheating and to eliminate noise.

Another good way to switch loads on and off is by using a relay. Unfortunately it is not possible to arrange for a relay to close at the zero voltage point of a 50Hz or 60Hz supply because of the slowness of operation of any relay compared to the waveform frequencies. It is also an unfortunate feature of relays that their contacts are subject to spark damage. A spark can jump between the relay contacts as they come together to switch a load on. More significant is the spark which can leap between the contacts as they are opened. So, while a relay is a good piece of equipment compared to a solid state switch in that it offers low on resistance and does not experience overheating, its life can be severely compromised because of the spark problem. The present invention seeks to eliminate spark damage to relay contacts. The present invention further seeks to provide the advantages of zero voltage point switching with the advantages of relay switching, without encountering the disadvantages experienced in either.

Finally, the present invention seeks to provide switching equipment which is safe and self disconnecting in the event of any kind of systemic failure.

The present invention consists in a switch apparatus for use in switching a load on an AC supply, said apparatus comprising: controller means, powerable by the supply; solid state switch means, in series with the load, and controllable by said controller means; and relay means, having selectably open or closed contacts in parallel with said solid state switch and also controllable by said controller means; where, when switching on of a load on the supply, said controller means is operable, firstly, to switch on said solid state switch, secondly to wait for a first predetermined period, and thirdly, to close the contacts of said relay means; and, when switching off of a load on the supply, said controller means is operable, firstly, to open the contacts of said relay means, secondly to wait for a second predetermined period, and thirdly to switch off said solid state switch.

The invention further provides an apparatus where the controller is operable to cause the solid state switch to switch on the load at a zero voltage point of the supply waveform.

The invention further provides an apparatus where the controller is operable to cause the solid state switch to switch off the load at a zero voltage point of the supply waveform.

The invention further provides an apparatus where, in the event that the contacts of the relay fail to close, thermal shutdown of the load connection occurs in the event of the solid state switch exceeding a predetermined temperature. The invention further provides that the relay contacts will not close on switching on or off of a load in the event that the solid state switch fails to connect the load.

The invention is further explained, by way of an example, by the following description, to be read in conjunction with the appended drawings, in which:

Figure 1 is a schematic diagram of an example of a switch apparatus according to the present invention.

Figures 2A, 2B and 2C show respective timing charts, to the same horizontal time axis, for the switch on demand, the solid state switch and the relay when all are operating according to the present invention.

And

Figure 3 shows one exemplary flow chart showing one implementation of the activities of the controller when the controller operates according to the present invention.

Attention is first drawn to Figure 1 showing a schematic diagram of one example of an embodiment of the present invention.

A switching apparatus 10 for connecting an electrical load 12 such as a multi-kilowatt electrical heater to an AC power supply 14 comprises a controller 16 which controls a solid state switch 18 in series with the electrical load 12, and further comprises a relay 20 whose relay contacts 22 are in parallel with the solid state switch 18 and close when the relay coil 24 is energised. The controller 16 receives input from a demand switch 26 which indicates, when the demand switch 26 is closed, that it is desired to provide connection from the AC power supply 14 through the electrical load 12. The demand switch could be replaced, as will become clear, by a push button, a pair of push buttons, remote electronic controls, data coupling or any other means capable of giving an indication when the load 12 is to be switched on or switched off.

A current transformer 28 provides energising input, in response to the presence of current through the electrical load 12, to a power supply 30 which is connected through the controller 16 to be switched by the controller 16 selectably provide electrical current through the relay coil 24. The controller 16 also drives an optical isolator 32 which is operative selectaby to control the solid state switch 18, here shown in the form of a triac, to switch on at the zero point of the waveform from the AC power supply 14 and also to switch off, when required, at the zero point of the AC power supply 14 waveform.

A thermal detector 34 detects when the solid state switch 18 exceeds a predetermined temperature, and either directly switches off a series connection 36 in the AC power supply 14, and/or provides a thermal signal 38 to the controller 16 which responds by switching off the solid state switch 18.

Attention is next drawn to Figures 2A, 2B and 2C which show respectively timing charts to the same time horizontal axis for the switch on demand from the demand switch 26, the operation of the solid state switch 18, and the energisation of the relay coil 24.

At the point of closure of the demand switch 40 the controller 16 immediately enables the solid state switch 18 which connects the load 12 to the AC power supply 14 at the soonest moment thereafter 42 when the waveform of the AC power supply 14 becomes zero. The controller 16 waits for a first predetermined period A as indicated by arrows 44 before switching the output of the power supply 30 at a first switching point 46 to energise the relay coil 24 and cause the relay contacts 22 to close in parallel with the connection made by the solid state switch 18. Thus, in a switch on sequence, the advantages of zero voltage point switching are achieved using the solid state switch 18 which are later reinforced by the parallel closure of relay contacts 22. The main current through the load 12 proceeds, thereafter, through the relay contacts 22 short circuiting the solid state switch 18 and silencing any noise generated by the solid state switch 18, the closure being effected without risk of spark damage to the relay contacts 22 which are closing, effectively, in parallel with an already closed switch.

Since the relay coil 24 is powered by a power supply 30 energised by a current transformer 28 which senses the current through the load 12, should the solid state relay 18 fail to connect the load 12 across the AC power supply 14, the relay 20 cannot be energised so that the relay contacts 22 cannot be closed. This automatically avoids the situation where the relay contacts 22 could be closed without the solid state switch 18 already being operational. This is a fail safe condition.

Should the relay contacts 22 fail to close, the solid state switch 18 would begin to heat causing the thermal detector 34 to either disconnect the AC power supply 14 from its connection to the load 12, and/or send the thermal signal 38 to the controller 16 to cause the controller 16 to shut down the solid state switch 18.

When the time comes to disconnect the AC power supply 14 from the load 12, the demand switch 26 is opened at a point of opening 48. The controller 16 instantly switches off the relay at a relay switch off point 50. The controller 16 then waits for a second predetermined period B indicated by arrows 52 before removing the drive from the solid state switch 18. The solid state switch 18 switches off at a zero voltage point in the waveform of the power supply 14. In this way, the relay contacts 22 are already open when the solid state switch 18 switches off. This prevents spark damage to the relay contacts 22.

Attention is next drawn to Figure 3 showing one exemplary flow chart indicating one possible implementation of the activities of the controller 16 when the controller 16 operates according to the present invention. From entry 54 a first test 56 has the controller 16 look to see if it is required to switch on the load 12. If it is, a first operation 58 switches on the solid state switch at a zero voltage point on the power supply 14 waveform, a second operation 60 waits for the first predetermined period and a third operation 62 has the controller 16 energise the relay coil 24 to close the relay contacts 22. Control then passes to a second test 64 which checks to see if there is a need to disconnect the AC power supply 14 from the electrical load 12. If there is, a fourth operation 66 has the controller 16 remove energisation from the relay coil 24 to open the relay contacts 22. A fifth operation 68 then has the controller 16 wait for the second predetermined period after which a sixth operation 70 has the controller 16 switch off the solid state switch 18 at a zero voltage point of the waveform of the AC power supply 14.

Unless provided with a positive result to their tests, the first test 56 and the second test 64 pass control to one another to check what is to be done.

This is just one implementation of how the controller 16 can behave. Other sequences can achieve the same purpose. The controller 16 can be implemented using a computer program, micro-controller, sequencer, hard wired logic or even using inherent delays in components to achieve the desired result.

The controller 16 can be powered from the AC power supply 14 or any other source. While a power supply 30 has been shown as deriving the energy to drive the relay coil 24, another alternative exists where the controller 16 is coupled to detect current flow in the electrical load 12 and to provide energisation to the relay coil 24 if and only if current flow is detected, the power supply to energise the relay coil 24 being derived from the AC power supply by the controller 16.

The solid state switch 18 is shown as being energised via an optical isolator 32. It is to be appreciated that the solid state switch 18 can be any type of solid state switching device and can be energised by any means known in the art whereby switching at the zero voltage point of the AC power supply 14 waveform is achieved.

The invention further encompasses an alternative embodiment where the solid state switch does not switch at a zero voltage point of the power supply, though this form of operation is preferred. This alternative embodiment also permits the solid state switch 18 to protect the integrity of the relay contacts 22 and the relay 20 to shut down the noise from the solid state switch 18 and prevent overheating of the solid state switch.

The activity of the controller 16 also encompasses an alternative manner of operation where, during the switch on sequence, the solid state switch 18 can be switched off after the relay contacts 22 are closed and, during the switch off sequence, the solid state switch can be switched on again before the relay contacts 22 are opened . This alternative mode of operation has the advantage of the solid state switch 18, being switched off nearly all of the time, producing no heat or electrical noise and the disadvantage that, should the switching apparatus 10 become accidentally de-energised, the relay contacts 22 are not protected from spark damage. For this reason, the sequence shown in Figures 2A, 2B and 2C is the preferred manner of behaviour for the controller 16. However, should noise be a problem, this alternative manner of operation can be adopted.

Claims

Claims :

1. A switch apparatus for use in switching a load on an alternating current supply, said apparatus comprising: controller means; solid state switch means, in series with the load, and controllable by said controller means; and relay means, having selectably open or closed contacts in parallel with said solid state switch and also controllable by said controller means; where, when switching on a load on the alternating current supply, said controller means is operable, firstly, to switch on said solid state switch, secondly to wait for a first predetermined period, and thirdly, to close the contacts of said relay means; and, when switching off a load on the alternating current supply, said controller means is operable, firstly, to open the contacts of said relay means, secondly to wait for a second predetermined period, and thirdly to switch off said solid state switch.

2. An apparatus, according to claim 1, wherein said controller means is powered by the alternating current supply.

3. An apparatus, according to claim 1 or claim 2, wherein controller is operative to cause said solid state switch to switch on the load at a zero voltage point of the alternating current supply waveform.

4. An apparatus, according to claim 1, claim 2 or claim 3, wherein said controller is operative to cause said solid state switch to switch off the load at a zero voltage point of the alternating current supply waveform.

5. An apparatus, according to any one of the preceding claims, comprising thermal shutdown means, operative, in the event of the contacts of said relay failing to close, thermally to shut down the load connection in the event of said solid state switch exceeding a predetermined temperature.

6. An apparatus, according to any one of the preceding claims, further comprising closure inhibition means, operative, in the event of said solid state switch failing to connect the load, to prevent closure of said relay contacts on switching on the load.

7. An apparatus, according to any one of the receding claims wherein said first predetermined period is equal to said second predetermined period.

8. An apparatus, according to any one of the preceding claims, where, when switching on a load on the alternating current supply, said controller means, having closed the contacts of said relay means, is operative to switch off said solid state switch, and, when switching off a load on the alternating current supply, said controller means is operative to switch on said solid state switch prior to opening the contacts of said relay means.

9. A method of switching a load on an alternating current supply, said method comprising the steps of: providing solid state switch means in series with the load; providing relay means having selectably open or closed contacts in parallel with said solid state switch; when switching on a load on the alternating current supply, firstly, switching on said solid state switch, secondly waiting for a first predetermined period, and thirdly closing the contacts of said relay means; and, when switching off a load on the alternating current supply, firstly opening the contacts of said relay means, secondly waiting for a second predetermined period, and thirdly switch off said solid state switch.

10. A method, according to claim 9, including the step of employing said solid state switch to switch on the load at a zero voltage point of the alternating current supply waveform.

11. A method, according to claim 9 or claim 10, including the step of employing said solid state switch to switch off the load at a zero voltage point of the alternating current supply waveform.

12. A method, according to any of claims 9 to 11, including the step of, in the event that the contacts of said relay fail to close, and in the event of said solid state switch exceeding a predetermined temperature, thermally shutting down the load connection.

13. A method, according to any one of claims 9 to 12, further comprising the step of, in the event of said solid state switch failing to connect the load, preventing closure of said relay contacts on switching on the load.

14. A method, according to any one of claims 9 to 13, wherein said first predetermined period is equal to said second predetermined period.

15. A method, according to any one of claims 9 to 14, including the steps of: when switching on a load on the alternating current supply, having closed the contacts of said relay means, switching off said solid state switch, and, when switching off a load on the alternating current supply, switching on said solid state switch prior to opening the contacts of said relay means.

16. An apparatus, substantially as described with reference to the appended drawings .

17. A method, substantially as described with reference to the appended drawings .