DE1296229B - Use of beryllium as an electrical conductor - Google Patents

Description

The invention is characterized by the use of beryllium, that has cooled to a temperature below 150 ° K, in windings, cables and Connections of electrical machines or devices that are used to generate electrical, magnetic or mechanical energy or intended for converting electrical energy are.

It is known that the weight and space requirements of electrical machines or devices with a simultaneous increase in efficiency by using Ladders kept at a very low temperature are considerably reduced can be. These improvements result from the sharp decrease in the specific Line resistance at low temperatures.

The specific line resistance of a metal is the sum of two Values: the ideal line resistance, which is the interaction of the electrons with corresponds to the thermal oscillations of the crystal lattice, and the remaining resistance, which can be attributed to impurities and defects in the crystal lattice structure is. The ideal line resistance decreases with decreasing temperature, during the remaining resistance is basically independent of temperature.

In the arrangements proposed so far, the following requirements must be met must be taken into account: On the one hand, a sufficiently low ideal line resistance very low temperatures are required to achieve this; you practically have to go with it the temperatures of liquid hydrogen, which boils at 20.4 ° K, or those of liquid neon, which boils at 27.3 ° K, work.

On the other hand, in order to achieve a sufficiently low residual resistance, high purity conductors are required; the proportion of impurities must in general between 100 and 1 parts per million (ppm).

So far, the measure of cooling conductors from very pure has been used Sodium, copper or aluminum applied to around 20 ° K to make the extraordinarily to take advantage of the high conductivity of these metals at very low temperatures.

The purpose of the present invention is to be more economical and technical considerations to select a metal that is an improvement in technology the frozen ladder enables. It is beryllium, the opposite the metals used so far have great advantages for the; Construction electrical Machines or devices with frozen ladders, as it is in the state of industrial Purity at temperatures not quite as low as those of liquid hydrogen or liquid neon can be used. This fact is completely surprising since beryllium, which is intended for industrial processing, has not yet been a special one represents pure metal and since FIG. 1 it can be seen that the value of the specific The line resistance of beryllium is considerable at very low temperatures higher than that of copper or aluminum.

The in F i g. 1 curves shown were based on experiments determined and show the specific line resistance o in microohm centimeters as a function of the absolute temperature for various metals. One can see from this that the specific line resistance of particularly pure aluminum, copper and Sodium each drops considerably when the temperature goes from normal to for example 20 ° K, the boiling point of hydrogen at atmospheric pressure, is lowered.

The concept of the lowest specific line resistance of a However, Leiters only gives an inadequate idea of the gain in performance for electrical devices with frozen conductors. This gain in performance will partly due to the energy required to operate the cooling system used up, and the efficiency of this cooling system is lower, the lower is the operating temperature. The cooling system takes the joule effect in the winding generated heat at a temperature of T ° K and releases it at about the Ambient temperature, i.e. at around 300 ° K.

According to the laws of thermodynamics, an ideal cooling device working under these conditions would have a maximum energy efficiency of In practice, of course, this efficiency is less than the efficiency of the ideal device. The maximum energetic efficiency is here where M is a factor which becomes larger the lower the factor T is. At cooling temperatures between 100 and 4 ° K, the value of M results practically from the following equation: Based on the above formulas, the effective efficiency of a cooling system working with liquid nitrogen at 77 ° K is 0.135, a cooling system working with liquid hydrogen at 20 ° K is 0.02 and a cooling system working with liquid helium at 4 ° K is only 0 , 0015.

Thus, the total power to be expended can be determined as a function of the temperature T of the cooled conductor, the total power to be expended being given on the one hand by the Joule effect and on the other hand by the power consumption of the cooling system which dissipates the released thermal energy to the surrounding atmosphere. The equation applies where P is the total power expended, A is a coefficient dependent on the dimensions of the conductor and t) is the specific line resistance of the metal, which is a function of temperature (see FIG. 1).

Instead of looking at the absolute value P, it is advisable to use this Total loss of power with the power loss Po due to the Joule effect in one Compare copper conductors of the same dimensions to that of a current of the same strength at a temperature of 70 ° C (= 343 ° K), the same as for electrical Machine corresponds to normal operating temperature.

As long as the ratio P: Po is greater than 1 or approximately equal to 1 remains, it is not economical to use frozen conductors with which no real power gain in relation to a machine more common Design with copper winding would be achieved.

F i g. 2 shows the curve for the ratio P: Po as a function the absolute temperature for metals with which P: Po achieves a value below 1 can be.

It can be seen that in the use according to the invention of Industrial grade beryllium for P: Po a value of 0.5 already at temperature of 150 ° K and a value of 0.15 between 60 and 80 ° K, while for example Aluminum, which contains only 40 parts per million impurities, does not form until 25 ° K A value of the same order of magnitude is achieved and is uninteresting at approximately 80 ° K.

That for the experimental determination of the curve shown in F i g.1 and for the determination of the in F i g. The beryllium used in the corresponding curve shown in FIG. 2 is an industrial product with an impurity content of more than 0.1%. It contains approximately 1000 parts per million (ppm) beryllium oxide, 90 ppm iron, 25 ppm aluminum, 20 ppm silicon. 10 ppm nickel, 10 ppm chromium and 5 ppm magnesium.

One of the main advantages of the use of the invention Beryllium as a conductor is due to the fact that this conductor is no longer at such low temperatures must be cooled, as they correspond to liquid hydrogen or liquid neon. You already come to the minimum of at the boiling point of liquid nitrogen Loss of performance.

However, for economic considerations in the invention Procedure does not just matter the slightest loss of performance. It also applies the investments required for the construction and maintenance of the cooling systems consider. The devices for liquefying neon, hydrogen or helium are extremely expensive. In contrast, the plants are used for liquefaction of nitrogen produced industrially, the procurement costs being well below those of the aforementioned devices. It also offers nitrogen over the others Coolants have a variety of advantages: it is cheap, non-toxic and means no risk of corrosion for the ladder. Furthermore, nitrogen is considered excellent Means for electrical insulation and can be liquefied at low cost. In addition, there is the problem of thermal insulation when transferring to the liquid state Much easier to solve for nitrogen than for neon or hydrogen.

The common use of beryllium as industrial grade Head and of liquid nitrogen as a coolant for electrical appliances at low Temperature is a combination that is technical and economic Realities no other pair of metal and cold flow medium is equal. This common use represents an advantageous further development of the present invention Invention. The liquid nitrogen can at a pressure of 0.2 to 10 Atmospheres are used, with the conductor at a temperature between 65 and 80 ° K, preferably between 40 and 150 ° K, is cooled.

With beryllium of greater purity, for example 100 ppm beryllium oxide and has 100 ppm of other impurities, the power loss drops at 77 ° K to around 811 / o of the power loss in a copper winding working at 70 ° C is to be observed. Even better results can be achieved if at 40 ° K and a ratio P: Po of approximately 0.015 is worked.

The following examples describe various machine arrangements with windings, lines or connections made of conductors frozen with liquid nitrogen, for which, however, no independent protection is sought. Example 1 Fig. 3 shows a cross section through a conductor 1 made of beryllium in its thermal insulation. This conductor is intended for the continuous generation of magnetic energy that is intended to excite a large synchronous machine. The conductor 1 through which direct current flows can be solid. It is traversed by a longitudinal channel 2 in which liquid nitrogen flows under a selected pressure between 0.2 and 10 ata. A thermal insulation 3 consisting of reflective shields made of aluminized polyethylene terep'hthalate is arranged between two tight sleeves 5 and 6, between which a vacuum of 10-5 Torr is maintained. The sleeve 5 is inserted into the groove 4 of the machine. Example 2 Fig. 4 shows a cross section through the slot 4 of a stator according to the invention for an alternating current machine. The conductor 1 has alternating current flowing through it and is arranged in an alternating magnetic field. This conductor consists of thin beryllium strips lying on top of one another in order to eliminate the unfavorable influence of Foucault currents and the skin effect. The conductor could also consist of a rope which is formed in a manner known per se from fine wires insulated from one another. The nitrogen, which is liquid for cooling, flows into lines 2 which are provided outside the conductor. This arrangement can be installed in a sheath 6 made of poorly conductive or insulating material, which closes the arrangement so tightly that no liquid nitrogen can escape. Between this shell 6 and an outer shell 5 made of the same material, a thermal insulation 3 consisting of reflective shields is arranged. In addition, a very high vacuum is maintained in the space between the two envelopes. Example 3 Fig. 5 shows a winding 1 which consists of thin beryllium strips or fine wires which are insulated from one another and surrounded on all sides in liquid nitrogen 2. This winding is used, for example, to generate a magnetic field (coils of a circuit breaker) or to convert electrical alternating current (coils of a transformer). The liquid nitrogen 2 simultaneously represents the coolant and also the dielectric insulation medium, since it has the dielectric properties required for such an application. The entire arrangement is located in a double container, between the walls 5 and 6 of which a thermal insulation 3 is arranged under a very high vacuum. Example 4 Fig. 6 shows an overall arrangement. A cooling arrangement 7 feeds the electrical device 8 with liquid nitrogen via heat-insulated lines 9. The liquid nitrogen heated by the loss of electrical power in the device 8, or rather the nitrogen vapor formed by the heat loss, is collected in the line system 10 and fed back to the inlet of the cooling system 7. A pump 11 maintains the desired vacuum in the thermal insulation of the cooling system 7, the electrical device 8 and the lines 9 and 10.

Claims (3)

Claims: 1. Use of to a temperature below 150 ° K cooled beryllium as a current conductor in windings, cables and connections electrical devices used to generate electrical, magnetic or mechanical Energy or for converting electrical energy are intended. 2. Use after Claim 1, characterized in that beryllium industrial as the electrical conductor Purity is used, which is achieved by means of a fluid at a temperature between 65 and 80 ° K is cooled, and that liquid nitrogen is used as the fluid between 0.2 and 10 atmospheres pressure is used. 3. Use according to claim 1, characterized in that industrial grade beryllium as the electrical conductor is used, which with the aid of a fluid to a temperature between 40 and 150 ° K is cooled, and that liquid nitrogen as a fluid between 0.2 and 10 atmospheres pressure is used.