WO1999000811A1

WO1999000811A1 - A method of connecting and disconnecting an ac voltage to/from a load, as well as a switch comprising a relay

Info

Publication number: WO1999000811A1
Application number: PCT/DK1998/000277
Authority: WO
Inventors: Morten Bruun-Larsen; Erling Nielsen; Niels Anderskouv
Original assignee: Nkt Research Center A/S
Priority date: 1997-06-25
Filing date: 1998-06-24
Publication date: 1999-01-07

Abstract

For connecting and disconnecting an AC voltage to/from a load (23) via a relay (19) to which an actuator voltage is applied from an actuation circuit, the actuating circuit is connected to a control and measuring circuit (16, 17, 18) which is adapted to control the actuation circuit so that it actuates the relay at an optimum time, said control and measuring circuit (16, 17, 18) being adapted to detect the type of load which is connected to the relay (19). The optimum times of the connection and disconnection sequences of the various types of loads may be determined on the basis of the measurement values of the control and measuring circuit, so that the relay (19) with its contact faces may be controlled very accurately and with least possible wear of the relay itself. The control and measuring circuit (16, 17, 18) can additionally control the relay so that two contact faces of the relay may be kept at a distance which is smaller than the distance between the extreme positions of the contact faces (standby position). The invention thus provides a switch having a relay (19) and a control and measuring circuit (16, 17, 18) which may be produced with very small dimensions, and which is of universal use.

Description

A method of connecting and disconnecting an AC voltage to/from a load, as well as a switch comprising a relay

The invention relates to a method of connecting and disconnecting an AC voltage to/from a load, wherein an actuator voltage from an actuation circuit is applied to a relay connected to the load, and wherein the actuation voltage may be delayed by means of a control and measur- ing circuit.

The invention moreover relates to a switch comprising a relay having two terminals, wherein an actuator voltage may be applied across the terminals to a circuit having a load, and the switch additionally has a control and measuring circuit which is adapted to delay the actuation voltage to the circuit.

With the ever greater requirements of reducing the volume of mechanical switches and the ever greater requirements of accurate switching when connecting and disconnecting loads, the development has been toward controlling mechanical switches by means of electronic circuits. Particularly with a view to prolonging the service life of the switches, measures have been taken to control the switches so that their contact faces are worn as little as possible.

EP 0 571 122 Al, e.g., discloses a switch in which, in order to improve the service life of the switch, a time delay has been introduced from the time when an actuation voltage is applied to the switch and to the time when it actually performs the switching. The principle is that the contact springs of the switch are closed when the voltage difference between the contact springs is ap- proximately zero, while the contact springs are opened preferably when the current through these is 0.

EP 0 353 986 Bl discloses another way of controlling a switch, using an optical sensor for determining the size of the arc which is generated when the switch is actuated, and determining, on the basis of this, a delay in the switch which allows it to switch at zero crossings of voltage or current.

The previously known control circuits for switches have had a quite simple structure per se and have primarily been application-orientated.

Accordingly, an object of the invention is to provide a method of the type stated in the opening paragraph for connecting and disconnecting an AC voltage to/ from a relay which is extremely flexible and gives a multitude of possible applications.

The object of the invention is achieved by the method stated in the introductory portion of claim 1 which is characterized in that the control and measuring circuit, which controls the actuation circuit, is adapted to actu- ate the relay at an optimum time on the basis of the type of load connected to the relay.

Connection and disconnection of an AC voltage by means of a relay may hereby be performed extremely rapidly and precisely with a minimum of wear on the relay which is incorporated in the switching arrangement.

When, as stated in claim 2, the optimum time for the connection of the relay to the load is determined on the ba- sis of the phase of the voltage applied to the load, the time may be determined such that the connection of the relay takes place in that the optimum time is immediately before the zero crossing of a voltage, as stated in claim 3.

When the relay is to be disconnected, it is an advantage, as stated in claim 4, that the disconnection of the relay to the load is determined on the basis of the phase of the current fed to the load, and that this optimum time is immediately before current zero crossing, as stated in claim 5.

As stated in claim 6, the measuring circuit is adapted to determine the type of load connected to the contact on the basis of the phase difference between voltage and current, and to save information on this specific type of load in a storage, which provides the advantage that the contact faces of the switch may be switched to specific phase angles of load current and load voltage, respectively, and that the type of load is determined by apply- ing a short voltage pulse to the load immediately before voltage crossing on the load and then measuring the pulse response. As stated in claim 7, the measuring circuit contains calibration facilities for calibrating variations in mechanical as well as electronic time constants of the switch, a signal from the relay being fed back. This makes it possible to detect the time at which the contact faces of the relay are opened and closed. Changes, if any, in the mechanical time constants cause the measuring circuit to calculate a new delay which is used in future switchings.

For determining the type of load, this may be determined, as stated in claim 7, by applying a short voltage pulse to the load immediately before voltage zero crossing on the load, and then measuring the pulse response when disconnecting immediately before current zero crossing. This provides very safe information on which load is incorporated.

As stated in claim 8, the phase difference may be deter- mined directly on the basis of the zero crossings of the AC voltage and current applied to the load.

When, as stated in claim 9, the clock frequency of the control and measuring circuit is derived from the zero crossing of the AC voltage applied to the load, it is ensured that the time conditions in the switch always follow the time conditions in the connected AC voltage, which means that the switch is automatically calibrated to 50 or 60 Hz power frequency, and also provides a very high precision of control signals to the relay.

A particularly interesting advantage of the method of the invention is that the control and measuring circuit is adapted to determine an arbitrary distance between two contact faces of the relay on the basis of a signal fed back from the actuation coil of the relay, and to maintain the two contact faces at a specified distance. This gives the very great advantage that the contact faces of the relay may assume a standby position, as the two con- tact faces may be caused to be very close to each other before being moved, which results in a very rapid switching.

Further, this method, which is stated in claim 10, may be improved when, as stated in claim 11, the control and measuring circuit is adapted to detect the times at which the contact faces of the relay are opened or closed on the basis of a feedback signal from the relay, as this enables changes in the mechanical time constants to be calculated and a new coupling time to be inserted for fu- ture switching on the basis of the switch characteristic existing at any time.

To prolong the service life of a switch additionally, it is an advantage, as stated in claim 12, that the control and measuring circuit is adapted such that the relay can both connect and disconnect voltage or current to/from the load when this is positive and negative, respec^¬ tively.

As stated in claim 13, it is an advantage that the control and measuring circuit is adapted such that the tem^¬ perature of the contact faces of the relay may be deter^¬ mined.

This particularly provides the advantage that beginning error states in the relay may be detected, which may e.g. be wear on the contact faces of the relay resulting in an increased contact resistance. Information on the state of the relay may be utilized for disconnecting the relay in case of fire risks, or for signalling that the relay should be replaced in the near future.

A particularly elegant way of determining the temperature in the relay is, as stated in claim 13, when the control and measuring circuit is adapted to detect the voltage drop across the contact faces of the relay, when these are closed, as this provides an indication of the dissipated power and thereby the temperature of the relay.

For use in e.g. motor controls or other inductive loads where a so-called soft start is desired, very rapid switching and great precision of such switchings are required of the switch. This may be obtained, as stated in claim 15, in that when the control and measuring circuit has detected whether the load is inductive or resistive with a positive coefficient of temperature, voltage pulses, whose width varies over time, are applied to the relay, said relay being connected at the zero crossing voltage, said relay being disconnected at or immediately before the zero crossing of the current. Further, the combination of the pulse width modulated voltage and the standby position ensures that the switch can switch more rapidly than is the case with traditional on/off control. The actuation voltage thus regulated also provides the possibility of reducing the coil voltage of the relay to a level, which precisely causes the contact faces to be pressed together with a sufficient force, which reduces the coil losses.

As mentioned, the invention also relates to a switch which is of the type stated in the introductory portion of claim 16.

This switch is characterized in that the control and measuring circuit has detection means for detecting the type of the load incorporated in the circuit.

Such a switch is versatile in use of course and provides advantages in terms of production, as stocks of several different switches may be reduced quite considerably to ^'a few standard types .

Expedient embodiments of the switch are defined in claims 17-19.

The invention will now be explained more fully with reference to the figures of the drawing, in which

fig. 1 shows the applied principles of the invention,

fig. 2 shows how the switching times may be calibrated, fig. 3a shows how closing and opening of the switch take place at a capacitive and a resistive load,

fig. 3b shows how closing and opening of the switch take place at an inductive load,

fig. 3c shows how closing and opening of the switch take place when this connects or disconnects an electric mo- tor, or a resistance with a positive coefficient of temperature,

fig. 4 shows a basic structure of a switch according to the invention,

figs. 5a and 5b show time diagrams for the mode of operation of the switch,

fig. 6 shows a set-up in which the switch is controlled by means of a pulse width modulated signal, and

figs. 7A-C show a temporal relation between the action of a pulse width modulated signal on the current of the actuation coils of a relay and the contact face distance of the contact faces of the relay.

In fig. 1, 6 designates a current curve, while 5 designates a voltage curve. As will be seen, voltage and current are phase-shifted with respect to each other, as the voltage 5 lies ahead of the current 6. 1 designates an external signal, which may be applied to a control circuit, as will be explained later, said control circuit being able to provide an internal signal 2 having a time delay 3 so that a relay in a switch may be connected at the time shown by the reference numeral 4. It is thus possible to vary the time when an external signal is ap- plied to an actuation coil, as a control circuit delays the current connection of a relay in a switch.

Since relays in the switch change characteristic as a function of operational time and general ageing, an opti^¬ mum connection or disconnection time for a relay will change. According to the invention, this time may be calibrated, it being possible, as will be explained later, to measure this changed time constant, as a signal is fed back from the relay to a measuring circuit, which allows optimization of the time of the connection or disconnection of the relay.

As will be seen in fig. 2, this figure also shows a cur- rent curve 6 and a voltage curve 5 as well as an inserted time delay 3 for coupling a relay. The time t3 at which it has been possible to connect the relay optimally, is shown schematically by the reference numeral 8. Owing to variations in the characteristic of the relay the optimum time has been changed to the time t2, as shown by the reference numeral 7. This time t2 may be determined on the basis of measurements on the relay so that the time delay changes from t3 to t2, which in turn causes the re^¬ lay to be subjected to less wear as a function of time.

It is shown in fig. 3A how connection and disconnection of a relay take place at a capacitive and a resistive load. As will be seen, the voltage 5 and the current 12 are in phase at the resistive load, while the voltage 5 and the current 10 are phase-shifted with respect to each other at the capacitive load, the voltage being here rearward of the current. As will be seen, the relay is connected immediately at the zero crossing of the voltage shown by the reference numeral 7. If an inductive load is involved, then the current 12 will be rearward of the voltage 5, as shown in fig. 3B . In this case, coupling of the relay will take place optimally at the time shown by the reference numeral 7, i.e. where the voltage difference between the contact faces of a relay is maximum. The reason why no coupling takes place at the zero crossing of the voltage in this case is that an inductive load may cause superimposition of an exponentially decreasing DC current on the AC current, which means in some cases that the core material of the inductive load saturates, resulting in a very strongly increased coupling current.

Finally, fig. 3C shows how connection and disconnection of a relay take place when this is used for connecting or disconnecting an electric motor or a transformer with a coupled resistive load having a positive temperature characteristic .

It will be seen from the figure that this is done by pulsing the voltage to the motor or the transformer, the relay being coupled before zero crossing, shown by the reference numeral 13, which generates a current, shown by the curve 15, which lasts until the relay is discon- nected, shown by the reference numeral 14. As will be seen from the figure, the relay is controlled so that the times of the zero crossings for connecting and disconnecting the relay change, until the relay is closed permanently, which means that the coupling current to the transformer or the motor is minimized. The reference numeral 15 shows the time when the full current has been coupled to the transformer or the motor. It will thus be possible to couple these types of loads softly, which is also called soft start. It is schematically shown in fig. 4 how a switch with a relay 19, which is to connect or disconnect a load 23, is constructed. It should be noted that the relay 19 may be of the so-called three-terminal type which forms the sub- ject-matter of Danish Patent Application No. 169/97. As will be seen in fig. 4, the relay 19 is intended to connect or disconnect an AC voltage shown by the reference numeral 20. Also a control and measuring circuit shown in dashed line by the reference numeral 16 is coupled to the relay 19. This circuit 16 consists of a control circuit 17 which is connected to a measuring circuit 18. The measuring circuit 18 is adapted to measure the current through the load 23 and the voltage shown by the refer^¬ ence numerals 21 and 22. The measuring circuit can hereby determine which type of load 23 is to be connected or disconnected to/from the relay 19. It should be noted that additional sensors may be connected to the measuring circuit 16, such as temperature sensors (not shown) which may be arranged on the relay 19. The measuring circuit 18 may thus send signals to the control circuit 17, which can then control the switching times of the relay, as is explained in connection with the preceding figures.

Fig. 5A shows a timing diagram for the relay when it is to operate at optimum times. On the top line, pulses show symbolic zero crossings for an AC voltage. It is shown below this line that an external signal is applied, which generates an internal signal V_ιnternai after a short period of time to the previously mentioned control circuit, which delays the coupling of the relay shown on the fourth line, where it will be seen that the relay is coupled just before a zero crossing. When the relay is to be disconnected again, this also takes place at a zero crossing, as the external signal, which is fed to the control circuit, causes the internal signal V_ιnternal to be delayed so that disconnection of the relay takes place at a zero crossing.

Fig. 5B shows a corresponding timing diagram, but in this case the relay is set in a standby position, there being applied together with an external signal V_ev_ternai an addi^¬ tional signal V_ιnternai from the control circuit, which, however, is not able to couple the relay, but merely moves the distance between the contact springs on the re- lay. When the time of zero crossing is then reached, the internal signal V_ιnternai is increased, but simultaneously delayed before the relay is coupled at zero crossing. Similar considerations apply to the disconnection of the relay, the internal signal V_ιnternai being lowered slightly, but not more than the relay remains coupled, and only when the internal signal V_ιnternai drops to zero, will it again disconnect the relay after a time delay, shown at t-off, at a zero crossing of the voltage V_=ero crowin .

In order to be able to perform soft start with a mechanical relay, very rapid switchings and a great precision in the switching of the relay are required. This has not been possible previously with traditional relays, but the control principles according to the invention allow soft start to be performed on the mains with mechanical relays .

To satisfy safety requirements for connection and disconnection on the mains, it is necessary to maintain a rela- tively great break distance between the two contact faces when the switch has been broken. This break distance is frequently so great that the make rates of relays are of an order which makes it impossible to use them in circuits where rapid connection is desired. As the control and measuring circuit can position the contact faces at an arbitrary distance between the two extreme positions "broken" and "made", this may be utilized for giving the relay three states, "broken", "made" and "standby", where the "standby" state allows the contact faces of the relay to be positioned at a relatively short distance, thereby increasing the make rate of the relay considerably, cf. the characteristic shown in fig. 7C.

In practice, the standby state is obtained by applying to the actuator coil 24 of the relay, cf. fig. 7A, a con- trollable supply voltage, which, in this case, is a pulse width modulated voltage source, e.g. as shown in fig. 7A. Thus, a voltage level is applied to the actuator coil, causing the contact faces of the relay to have precisely the desired distance. The control and measuring circuit regulates the actuator voltage, so that the actuator voltage exhibits precisely the inductance which is char^¬ acteristic of the desired break distance. It is noted in this connection that the inductance of the actuator coil varies with the air gap in the yoke of the relay coil, and the size of the air gap determines the distance of the contact faces. The inductance of the actuator coil may thus be used as an indication of the distance of the contact faces. When the actuator voltage is generated as a pulse width modulated voltage, the inductance of the actuator coil may be determined by measuring the coil current and determining the first derivative.

As will be appreciated from the foregoing, the invention provides a relay which is extremely universal and may be used within all types of loads where accurate switching times, rapid switching times and least possible wear are desired. The symbolic circuits in figs. 4 and 6 may also be produced with very small dimensions so that the switch with relay and circuit may be constructed as a self-con- tained component.

Claims

P a t e n t C l a i m s :

1. A method of connecting and disconnecting an AC volt- age to/from a load, wherein an actuator voltage from an actuation circuit (24) is applied to a relay (19) connected to the load (23) , and wherein the actuation voltage may be delayed by means of a control and measuring circuit (16, 17, 18), c h a r a c t e r i z e d in that the control and measuring circuit (16, 17, 18), which controls the actuation circuit, is adapted to actuate the relay at an optimum time on the basis of the type of load connected to the relay.

2. A method according to claim 1, c h a r a c t e r ^┬¼ i z e d in that the optimum time for connecting the relay (19) to the load is determined on the basis of the phase of the voltage applied to the load.

3. A method according to claim 2, c h a r a c t e r ^┬¼ i z e d in that the optimum time is immediately before a voltage zero crossing.

4. A method according to claim 1, c h a r a c t e r - i z e d in that the optimum time for disconnecting the relay (19) to the load is determined on the basis of the phase of the current fed to the load.

5. A method according to claim 4, c h a r a c t e r - i z e d in that the optimum time is immediately before current zero crossing.

6. A method according to claims 1-5, c h a r a c t e r i z e d in that the measuring circuit (18) is adapted to determine the type of load connected to the relay (19) on the basis of the phase difference between voltage and current, and to save information on the specific type of load in a storage, and that the type of load is determined by applying a short voltage pulse to the load immediately before voltage zero crossing on the load (23) and then measuring the pulse response and disconnecting immediately before current zero crossing.

7. A method according to claims 1-6, c h a r a c t e r ^┬¼ i z e d in that the measuring circuit contains calibra- tion facilities for calibrating variations in mechanical as well as electronic time constants of the switch, a signal from the relay being fed back.

8. A method according to claims 1-7, c h a r a c t e r - i z e d in that the control and measuring circuit is adapted to determine the phase difference on the basis of the zero crossings of the AC voltage and current applied to the load (23) .

9. A method according to claims 1-8, c h a r a c t e r i z e d in that the clock frequency of the control and measuring circuit (16, 17,18) is derived from the zero crossings of the AC voltage applied to the load.

10. A method according to claims 1-9, c h a r a c t e r i z e d in that the control and measuring circuit is adapted to determine an arbitrary distance between two contact faces of the relay on the basis of a signal fed back from the actuation coil of the relay (19) and to maintain the two contact faces at a specified distance.

11. A method according to any one of claims 1-10, c h a r a c t e r i z e d in that the control and measuring circuit (16, 17, 18) is adapted to detect the times at which the contact faces of the relay are opened or closed on the basis of a signal fed back from the relay (19) ΓÇó

12. A method according to claims 1-11, c h a r a c - t e r i z e d in that the control and measuring circuit (16, 17, 18) is adapted such that the relay (19) can both connect and disconnect voltage or current to/from the load when this is positive and negative, respectively.

13. A method according to claims 1-12, c h a r a c t e r i z e d in that the control and measuring circuit (16, 17,18) is adapted such that the temperature of the contact faces of the relay may be determined.

14. A method according to claims 1-13, c h a r a c t e r i z e d in that the control and measuring circuit (16, 17, 18) is adapted to detect the voltage drop across the contact faces of the relay (19) when these are closed.

15. A method according to claims 1-14, c h a r a c t e r i z e d in that when the control and measuring circuit (16, 17, 18) has detected that the load is inductive or resistive, voltage pulses, whose width varies over time, are applied to the relay, said relay (19) being connected at zero crossings of the voltage, said relay being disconnected at or immediately before zero crossings of the current.

16. A switch comprising a relay (19) having two terminals, wherein an actuation voltage may be applied across the terminals to a circuit having a load (23) , and wherein the switch additionally has a control and measuring circuit (16, 17, 18) which is adapted to delay the actuation voltage to the circuit, c h a r a c t e r - i z e d in that the control and measuring circuit (16,

17. 18) has detection means for detecting the type of the load incorporated in the circuit.

17. A switch according to claim 16, c h a r a c t e r i z e d in that the relay is bistable and integrated with the control and measuring circuit (16, 17, 18) .

18. A switch according to claim 17, c h a r a c t e r - i z e d in that the relay is a capacitive mains contact which is integrated with the control and measuring circuit (16, 17, 18) .

19. A switch according to any one of claims 17-19, c h a r a c t e r i z e d in that the control and measuring circuit (16, 17, 18) is a microelectronic circuit.