EP0013280B1 - Selectively acting thermal circuit breaker, method for its release and its use for electrical protection - Google Patents

Description

The invention relates to a selectively acting thermal switch and a method for its triggering according to the preamble of claims 1 and 4 and the use of this thermal switch.

Known thermal switches, such as Bimetal strips, fuses or control valves with thermocouples trigger a switching process when a predeterminable temperature value is exceeded with a relatively large time delay. Usually thermal switches are used to interrupt an electrical circuit.

The faulty voltage and its duration, amplitude increase and repetition are of particular importance for the operational safety of electrical devices and systems. Via the electrical distribution network, interference voltages in devices and systems can lead to malfunctions or destruction of semiconductors in sensitive electronic circuits, to malfunctions in control systems, to the loss of information or to flashovers due to poor insulation or insufficiently selected range or air gaps. Switches with a long time delay are unsuitable for protecting such devices. Bimetallic strips also show signs of fatigue after longer use, which lead to a change in the response temperature.

Thermoswitches with a relatively short switching delay using an element made of a shape memory alloy that triggers the switching process are known, inter alia, from DE-A-2 026 629, 2 139 852 and 2 701 884. The relatively large change in shape of this switching element is used to actuate a switch when the temperature rises above the initial temperature of the phase transition from the envy rig to the high temperature phase of the shape memory alloy. The switching element can flow through and be heated directly by the current to be switched. The heating can also be carried out by a heater inserted into the circuit. From DE-A-2 701 884 it is known to interrupt the current in the circuit when a certain maximum current value is exceeded.

A thermal switch which is independent of different critical response temperatures, which can result from different alloy compositions of the switching elements, is known from DE-A-2 724 255. There, the switching element through which the current to be switched is made of a shape memory alloy is connected to a pivotally mounted switching arm which is pulled against an electrical contact by means of a tension spring and also through which this current flows. The tensile force of the spring serves to determine the temperature of the actuating process, which leads to the interruption of the current. This thermal switch responds to temperature, but not a rate of temperature rise. Switches that ensure good selectivity with regard to the switching conditions are required in particular for line and overcurrent protection. After reaching a current tolerance value, the switch should only open if the time change of the current to be monitored exceeds a predefinable limit value and the overcurrent persists for a certain time.

The object of the invention is to ensure the triggering of a switching process as a function of the exceeding of a predeterminable temperature rise rate of a temperature sensor and to provide a thermal switch and an electrical fuse which bring about a switching process or interrupt a circuit when an increasing heat exceeds a predefinable limit value, regardless of the amount of heat flow.

The object is achieved according to the characterizing part of claims 1, 4 and 6. Further developments of the invention are defined in the dependent claims.

The invention is based on the new knowledge of the kinetics of the shape memory effect. An advantage of the invention is that the switching process is selective with respect to the initial value of a change in the temperature in the switching element caused by an overcurrent and not with respect to the transmitted energy. The pretension applied to the switching element by a tensioning device is adjustable so that the switching element can be deformed after cooling down after the passage of a subcritical overcurrent.

• Fig. 1 das Prinzip des Formgedächtniseffektes,
  • a) den Einwegeffekt,
  • b) den Zweiwegeffekt,
• Fig. 2 die Dehnung als Funktion der Temperatur,
  • a) für den Einwegeffekt,
  • b) für den Zweiwegeffekt,
• Fig. 3 das Schema eines einfachen Thermoschalters in einer auch für Prüfzwecke geeigneten Ausführungsform,
• Fig. 4 die chemische Energie G der zwei Phasen Martensit und Austenit einer Formgedächtnislegierung als Funktion der Temperatur T (schematisch),
• Fig. 5 den prozentualen Anteil des Martensits M als Funktion der Temperatur T,
• Fig. 6 den elektrischen Widerstand R einer Formgedächtnisliegierung (a) vor und (b) nach einem Aufheizen als Funktion der Zeit t,
• Fig. 7 das Schema eines mechanischen Modells aus einer Feder K, einer Stossdämpfungseinrichtung mit einer Dämpfungskonstante a und einer Masse m,
• Fig. 8a, 8b,-8c eine halbschematische Darstellung eines Thermoschalters in einem thermoelektrischen Schalter mit Zusatzsicherungselement und mechanischer Schaltverstärkung in verscheidenen Stellungen und
• Fig. 9 ein Funktionsschaltbild des Thermoschalters.

The invention is described below using exemplary embodiments. It shows:

• 1 shows the principle of the shape memory effect,
  • a) the one-way effect,
  • b) the two-way effect,
• 2 shows the strain as a function of temperature,
  • a) for the one-way effect,
  • b) for the two-way effect,
• 3 shows the diagram of a simple thermal switch in an embodiment which is also suitable for testing purposes,
• 4 the chemical energy G of the two phases martensite and austenite of a shape memory alloy as a function of the temperature T (schematic),
• 5 shows the percentage of martensite M as a function of temperature T,
• 6 shows the electrical resistance R of a shape memory bed (a) before and (b) after heating up as a function of time t,
• 7 shows the diagram of a mechanical model consisting of a spring K, a shock absorbing device with a damping constant a and a mass m,
• Fig. 8a, 8b, -8c is a semi-schematic representation of a thermal switch in a thermoelectric switch with additional fuse element and mechanical switching amplification in different positions and
• Fig. 9 is a functional circuit diagram of the thermal switch.

Fig. 1 a) shows the one-way and Fig. 1 b) the two-way shape memory effect. These effects occur with shape memory alloys, ie with thermoelastic martensite, which, after a so-called pseudo-plastic deformation at low temperature, return to their original shape when they are heated above a critical temperature. These alloys exist in a martensitic low-temperature phase lower, face-centered orthorhombic or monoclinic symmetry and in an austenitic high-temperature phase high, cubic body-centered symmetry. In the martensite phase there are no platelets or lamellae with different crystallographic orientations. At a high temperature of, for example, 600 ° C., the alloy is brought into a desired, for example elongated, rod shape and annealed. During cooling, when a critical temperature is exceeded, the austenitic phase changes into the martensitic phase. If deformation is now carried out at a low temperature T ₁ , for example bent, stress-induced or reoriented martensite formation results in a quasi-stable shape which bears the original straight structure. When heating above a temperature T ₂ , which is above a critical temperature, a straight structure spontaneously arises again from the curved effect with the one-way effect. Subsequent cooling does not result in a spontaneous change in shape. However, if the alloy is deformed again at the temperature T ₁ , the described effect is repeated when it is heated to T ₂ .

In the case of the two-way effect, a body which is deformed, for example bent, from a shape memory alloy at temperature T, spontaneously only partially returns to the original shape when heated to temperature T ₂ . When it subsequently cools down to T _{1, it} deforms again spontaneously, but not to the degree of deformation applied from the outside. The two temperature-induced, spontaneous changes in shape can be repeated in successive temperature cycles between T, and T ₂ .

Fig. 2 shows another representation of the memory effects. Depending on the alloy composition, strains or pseudoplastic deformations of up to about 8% can be recovered during heating for the one-way effect shown in FIG. 2a), while strains or deformations of up to 1.5% are known for the two-way effect shown in FIG. 2b) are. While the deformation in the one-way effect drops to zero due to heating and retains this regressed shape when it cools, a deformation of about 2% remains in the two-way effect. When cooling below a certain temperature, a spontaneous deformation of about 1.5% occurs, which spontaneously regresses when the alloy is heated.

• a) durch eine irreversible plastische Verformung über den pseudo-plastischen Bereich der Legierung hinaus, d.h. von mehr als etwa 8% oder
• b) durch eine Temperaturerhöhung um etwa 50 K über die Endtemperatur der Austenitumwandlung hergestellt werden.

The two-way effect can be created by creating dislocations and lattice defects

• a) by an irreversible plastic deformation beyond the pseudo-plastic area of the alloy, ie by more than about 8% or
• b) are produced by increasing the temperature by about 50 K above the final temperature of the austenite transformation.

The part of the deformation leading to dislocations is not recovered during heating. When it cools down again, the residual stress field of the lattice defects favors the regression of martensite orientations, which were generated by the originally applied stress, and leads to a smaller change in shape in the direction of the original deformation during cooling. A small two-way effect is generated in subsequent thermal cycles.

If a tension is exerted on a shape memory alloy, the temperature range of the phase transition shifts to higher temperatures, as indicated by dashed lines in FIG. 2a). Depending on the alloy composition, the conversion from the low to the high temperature phase can be between about -180 ° C and + 250 ° C.

4 shows the chemical energy G of the martensite and austenite phase as a function of the temperature T. At low temperatures, martensite has less energy than austenite. Above a temperature T _o , at which both phases are in thermodynamic equilibrium and have the same chemical energy G, austenite has a lower energy than martensite.

If austenite is to be converted into martensite by cooling, the shape memory alloy must be cooled below T _o , as shown in FIG. 5, since elastic or nucleation energy must be supplied because of the volume difference.

The transformation begins at a martensite transformation start temperature T MS, and is completed at a martensite transformation end temperature T _MF , ie no austenite remains. Even at temperatures above T _MS , however, an externally applied voltage can contribute to the nucleation or chemical energy for the martensite, ie it can stress-induced martensite are generated. If a body made of a shape memory alloy is first cooled below T _MS or T _MF and then deformed, a reorientation of the martensite platelets is produced, among other things, by means of a twin shearing process, which leads to a reversion of the deformation when heated.

The transformation of martensite into austenite begins at a so-called critical temperature T _AS above T _o and ends at a temperature T _AF .

In order to be able to produce a large shape memory effect, the one caused by the phase transition must be. Volume change must be small, otherwise a lot of dislocations are created to adjust the volume differences. Due to an increased dislocation density, the martensite is solidified or hardened and a reorientation due to an applied tension is no longer possible, as is the case with steel. The most important property of a shape memory alloy for an application as a switching element is that work can be done or a tension can be generated when the deformation is converted back when heating. An energy density of 2 MJ / m ³ can be achieved for Ni-Ti base alloys, which is significantly higher than that of bimetal. By hindering the change in shape during heating, stresses of more than 650 MPa can be generated. The achievable force and the path depend on the sample geometry. In the case of a round bar, the length of which is 10 times its diameter, the bending force is 20 times less than the tensile force, but the displacement of the free end in the case of bending is 10 times the displacement in the event of tensile stress.

mit der Anfangsbedingung:

wobei x_max die maximale Dehnung bedeutet, die durch Aufheizen eines Federkörpers als Ergebnis eines Formgedächtniseffecktes erreichbar ist. Unter diesen Voraussetzungen hat die Differentialgleichung (1) die Lösung:

stellt die elastische Energie der Feder dar, die beim Aufheizen in eine Gestaltänderung innerhalb einer charakteristischen Bewegungszeit

mit einer wirksam werdenden Kraft F umgesetzt wird. Die Federkonstante K hängt von der Bewegungsart, wie Spannung, Biegung usw., vom Typ der Formgedächtnislegierung und von der Grösse einer mechanischen Vorspannung ab. Ohne Vorspannung ergibt sich K für Spannung bzw. Kompression zu:

6, curve a shows the resistance R of a wire made of a mechanically loaded or prestressed shape memory alloy before heating up as a function of time t. Curve b was generated by sending a short-term current pulse from a capacitor discharge (875 V, 25 µ F) through this wire and thereby heating it up. The resistance characterizes the structure or phase transition of the alloy. The movement of the wire corresponding to the measured resistance vibration is explained on the basis of a Meohan model shown in FIG. 7, in which K is the spring constant of a spring, m is a mass, a is the damping constant, x is the path or a deflection, or x is the first or second derivative of the path according to time and g mean the acceleration due to gravity. The differential equation applies to this model:

with the initial condition:

where x _{max means} the maximum elongation that can be achieved by heating a spring body as a result of a shape memory patch. Under these conditions, the differential equation (1) has the solution:

represents the elastic energy of the spring, which when heated into a shape change within a characteristic movement time

with an effective force F is implemented. The spring constant K depends on the type of movement, such as tension, bending, etc., on the type of Shape memory alloy and the size of a mechanical preload. Without preload, K for tension or compression results in:

wobei v aas voiumen aes Korpers peqeutet. Oberhalb der kritischen Temperatur T_AS für die Rückverformung entsteht in dem Formgedächtniskörper eine mechanische Spannung σ proportional zu T―T_AS:

wobei σ₀ eine Material konstante ist. Diese Spannung a wird in den in den Fig. 3 und 8a, Bezugsziffer 38, dargestellten Thermoschaltern zur Auslösung eines Schaltvorganges benutzt.If a body or wire made of a shape memory alloy is heated by an electrical current I flowing through it, the relationship applies to temporal temperature changes dT / dt in a temperature range in which there is no shape change caused by a shape memory effect.

where v aas voiumen aes Korpers. Above the critical temperature T _AS for the reshaping, there is a mechanical stress σ proportional to T ― T _AS in the shape memory body:

where σ _{0 is} a material constant. This voltage a is used in the thermal switches shown in FIGS. 3 and 8a, reference number 38, to trigger a switching operation.

The principle of the thermal switch is explained with reference to FIG. 9. Any time-dependent physical quantity f (t), which is to trigger a switching process or provide a switching signal y when a predetermined condition, for example when a certain limit value is exceeded, is assigned in a converter 41 to one of these input quantities f (t) time-dependent heat or thermal energy Q (t) is transformed if it is not already specified as heat. This heat O (t) is transformed in a further converter 42 into an acceleration x, which is compared to a comparator 43 with an acceleration value ẍ _G. For ẍ> ẍ _G , the switching signal y ₁ is present at the output y of the comparator 43, otherwise the signal y ₂ . Synonymous with these signals y ₁ and y ₂ , which are dependent on the output of the comparison, is an “on” or “off” or a “switchover” of a switch.

in Wärme, diese gemäss Gleichung (11) in mechanische Spannung und diese gemässden Gleichungen (7) und (1) in Kraft F bzw. Beschleunigung ẍ transformiert. Der Vergleicher 43 wird durch eine einstellbare Masse in des Körpers 16 dargestellt, wobei die Trägheit dieser Masse zur Realisierung des Beschleunigungsgrenzwertes ẍ_G verwendet wird. Erst wenn eine durch den Formgedächtniseffekt beim Aufheizen des Schaltelementes erzeugte Kraftwirkung bzw. Beschleunigung x diesen vorgebbaren Grenzwert überschreitet, erfolgt eine Auslösung des Schaltsignals bzw. ein Schaltvorgang. Langsam verlaufende Strom- bzw. Temperaturänderungen im Schaltelement 18 lösen somit keinen Schaltvorgang aus. Ebenso lösen Stromänderungen, die Temperaturänderungen ausserhalb der Phasenübergänge zwischen T_AS und T_AF bzw. zwischen T_MSund T_MF entsprechen, keine Schaltsignale aus. Es handelt sich somit um einen selektiv wirkenden Thermoschalter, der unter anderem für ein schnelles Abschalten von Ueberströmen geeignet ist. Dafür wird der Phasenübergang von Martensit in Austenit entsprechend dem Temperaturbereich von T_AS bis T_AF verwendet. Dieser Schatbereich ist dadurch Dimensionierung des Schaltelements, Zusammensetzung der Legierung und mechanische Vorspannung einstellbar.3 and 8a, reference number 38, the time-dependent physical quantity f (t) is an electrical current I (t). The converters 41 and 42 are formed by the switching element 18 from a shape memory alloy. The electrical current I (t) is in accordance with this switching element

into heat, this is transformed into mechanical tension according to equation (11) and this into force F or acceleration ẍ according to equations (7) and (1). The comparator 43 is represented by an adjustable mass in the body 16, the inertia of this mass being used to implement the acceleration limit value ẍ _G. Only when a force effect or acceleration x generated by the shape memory effect when the switching element heats up exceeds this predetermined limit value, is the switching signal triggered or a switching process. Slowly running current or temperature changes in the switching element 18 thus do not trigger a switching process. Likewise, current changes that correspond to temperature changes outside the phase transitions between T _AS and T _AF or between T _MS and T _MF do not trigger switching signals. It is therefore a selective thermal switch that is suitable, among other things, for quickly switching off overcurrents. For this, the phase transition from martensite to austenite used according to the temperature range from T _AS to T _AF . This shading range can be adjusted by dimensioning the switching element, composition of the alloy and mechanical preload.

The thermal switch shown schematically in FIG. 3 consists of the switching element 18 in the form of a round rod or wire 6.5 mm in length and 0.33 mm ^{2 in} cross section made of an alloy of 3 M% Ni, 13 M% Al and 84 M% Cu, compare wire number 9 in Tables 1 and 2. Another example of a switching element is designated there with wire number 8. The upper end of the switching element 18 is held in an upper holder 19 which is immovably attached to a fastening 40. The lower end of the switching element is coupled via a holder 15 with an adjustable mass in the body 16 of 14.8 g and a tension spring 14, which can also be omitted and is set with the spring constant K = 0 in the present example. The tension spring is held immovably on one side in a fastening 13. Mass in the body 16 and tension spring 14 act together as a tensioning device and generate a mechanical tension on the switching element 18 which is deformed pseudo-plastically in the low temperature phase by stretching and which lies below its tensile stress. A current I is sent through the switching element at a room temperature of 20 ° C, which is to be monitored for the occurrence of an overcurrent. If the current-time integral t ₂ sz = ∫ l ² dt, Δt = t ₂ ―t ₁ (13) t ₁ within a time interval with Δt = 2 ms exceeds the value of SZ = 1000 Als, the switching element 18 tears and interrupts it Circuit as shown in Fig. 3b). The switching element acts as an electrical fuse with adjustable sensitivity. If the current rise is faster than a predeterminable value, the mechanical tension u exceeds the tensile stress of the switching element before the mass in the body 16 begins to move. The initial shape of the switching element is chosen so that the distance between the fragments after tearing is large enough to ensure separation of the arc. With a slowly increasing overcurrent, the mass in the body 16 is moved and u will never exceed the breaking voltage of the switching element. The values of m and K are expediently chosen using a two-way memory alloy for the switching element in such a way that the switching element is deformed back by cooling, compare FIG. 2b) after an overcurrent with a subcritical increase has ended. This thermal switch can also be used with a switching element with a one-way memory effect, whereby changes in shape can be used up to about 8%. Instead of a direct interruption of the circuit, the change in position when the switching element is torn can be used in a known manner as a means for triggering another switching operation, if necessary with an intermediate amplification of the switching signal. 3 can be used instead of a fuse in a conventional thermoelectric switch 50 according to FIG. 8, which is known for protecting electrical circuits against slowly or suddenly increasing overcurrents.

8a shows such a thermoelectric switch 50 in a semi-schematic representation, but with a thermal switch 38 which has a switching element 18 with a two-way memory effect, which does not tear when heated and thus cannot directly interrupt the circuit. In contrast to the thermal switch according to FIG. 3, in the thermal switch 38 the upper holder 19 of the switching element 18 is not movably mounted, but is connected to a linkage 12 in the catch arm 23 via a mechanical coupling 20 and is movable against a tension spring 11.

The electrical circuit goes from a connecting line 7 via a fixed contact 6, a contact end 8 of a contact arm 4, which can be pivoted about a linkage 1, via a connecting line 28, an additional securing element 9, a connecting line 22, the upper holder 19 through the switching element 18 lower bracket 15 to a connecting line 17th

• 1) die Anlenkung 12 gegen die Federkraft der Zugfeder 11 nach unten versetzt und
• 2) die Mass in des Körpers 16 und die mit ihr gekoppelte Zugfeder 14 aufwärts bewegt.

If an overcurrent flows through the switching element, which leads to a change in shape or here to contraction due to the shape memory effect, then

• 1) the linkage 12 against the spring force of the tension spring 11 down and
• 2) the dimension in the body 16 and the tension spring 14 coupled to it moves upwards.

For the second movement, the movement time Δt _{m is characteristic} according to equation (8). As soon as the movement is complete, the voltage u disappears, see equation (11).

The first movement will only trigger the switch 50 if u rises above a predeterminable critical value which is sufficient to translate the articulation 12 downward by, for example, 1 mm. Otherwise, the switching element 18 is due to the setting of the values of mass in the body 16 and spring force of the tension spring 14 K ormt by cooling zurückver ^f after the passage of a subcritical overcurrent.

However, if the articulation 12 is deflected downward, the catch arm 23 is pivoted clockwise by an articulation 21, as indicated by dashed lines, and releases an angle arm 37 for pivoting counterclockwise. This pivoting is blocked as long as a hook end 25 of the catch arm 23 bears against a catch end 24 of the angle arm 37. Under the action of a compression spring 2, which is supported on a fastening 3 and which tries to move the contact arm 4 by its articulation 1 clockwise, the angle arm 37 is pivoted counterclockwise by means of a thrust arm 29 by an articulation 26, as in FIG. 8b. The contact arm 4 lifts off the fixed contact 6 and interrupts the electrical connection between the connecting line 7 and the connecting line 28. The push arm 29 is connected to the contact arm 4 by a linkage 5. At its lower end, it has an articulation 27 which can slide in an opening 36 in the angle arm 37 when the angle arm is pivoted. When pivoting, the linkage 27 slides to the left in the opening 36, as a result of which the reset lever 32, which can likewise be pivoted about the linkage 27, is moved into an off position shown in FIG. 8c by means of a push arm 30. The reset lever 32 is under the tension of a tension spring 34 which is anchored in a fastening 35. It is connected to one end of the thrust arm 30 via a movable linkage 31. The other end of the push arm 30 is connected to the push arm 29 by the movable link 27. The switch 50 can also be switched on again with the reset lever 32 by pivoting it clockwise.

The thermal switches according to FIGS. 3 and 8a, reference number 38, are expediently used in addition to the additional securing element 9, which can be connected to the catch arm 23 by means of a mechanical coupling 10 and which responds to other parameters relevant to protection technology.

Rated values for switching elements 18 which are suitable for use in the thermal switch 38 are given in Tables 1 and 2 under wire numbers 1 to 7. The values in Table 1 apply to a room temperature of 20 ° C, a deflection by one of the shape memory effects of 1 mm and a spring constant K = 0 of the tension spring 14. The tension spring 14 can be omitted from the two thermal switches described, but it is suitable for stabilizing the position the Th _l .. noswitch and for applying a bias to the switching element 18th

For the shape memory alloy of the switching element 18, alloys based on Ni Ti, Ni Ti Cu according to DE-A 2 644 041, Cu Zn, Cu Al, Ni Zn are also suitable, inter alia with ternary and other components, in terms of the material properties and the price such as Cu AI Ni, Cu Zn AI Ni etc.

It goes without saying that the invention is not limited to the above. For example, the shape of the switching element 18 can be strip-shaped, tubular, spiral-shaped, curved, etc. The switching element can be deformed by bending, twisting, compressing or stretching. A thermoelectric switch 50 according to FIG. 8 can be triggered by pulling or pushing. The change in position caused by the switching element can be detected in a known manner, for example by means of optical, electrical or hydraulic detectors, amplified and transferred to another switching element.

Claims (6)

1. Thermal switch with a selective action, possessing a switching element (18) which is made from an alloy having a shape-memorising capability and which is located, at least at a first point (15), in a manner essentially permitting free movement, and is rigidly attached to a body (16), characterised in that the mass (m) of this body (16) is adjustable and is assigned to a limiting acceleration-value (x_G) for triggering this thermal switch, it being possible to preset this limiting acceleration-value.

2. Thermal switch with a selective action, according to Claim 1, characterised in that, at a second point (19), the switching element (18) is functionally connected to the device (23) for triggering a switch (50), and to a spring element (11).

3. Thermal switch with a selective action, according to Claim 1 or 2, characterised in that the switching element (18) conducts an electric current which is to be monitored.

4. Method for triggering the switching operation of a thermal switch with a selective action according to Claim 1, this method employing, for the triggering action, the change in the shape of a temperature-sensitive switching element, and the triggering action being executed when the acceleration of this switching element exceeds a limiting acceleration-value which can be preset, characterised in that this limiting acceleration-value (x_G) is set as a function of the mass (m) of a body (16) which is rigidly attached to this switching element (18).

5. Method according to Claim 4, characterised in that the switching element (18) is composed of an alloy having a shape-memorising capability which may be either of the one-way type, or of the two- way type.

6. Use of a thermal switch according to Claim 1, characterised in that the thermal switch is employed as an electrical protection device, the switching element (18) being fixedly mounted at a second point (19), the electric current which is to be monitored being conducted through the switching element (18) and the triggering of the protection device being effected as a result of the break-voltage of the switching element being exceeded.

Images