JPS59218712A - Protective circuit for superconductive coil - Google Patents

Abstract

PURPOSE:To enable to sharply change the resistance value of a resistor by a method wherein the resistor is composed of the metal such as copper, aluminum and the like having a large resistance changing rate, the resistor is cooled by liquid nitrogen before application of a current, and the liquid nitrogen is evaporated by heating the resistor. CONSTITUTION:The titled protective circuit consists of a superconductive coil 1, a cryostat 2, a resistor 3, a liquid nitrogen chamber 4, a switch 5, with which coil current will be cut off in order to move the coil current to the resistor, and a power source 6 which excites the coil. Also, when an ordinary resistor 7 is used in parallel with a resistor 3 which is cooled by liquid nitrogen, the range of variation in resistance value of synthesized resistance is made smaller, and the thermal input to the resistor by the cooling of liquid nitrogen can be reduced, and the constitution of the resistor can be simplified as a whole. As the material having the large temperature variation, an incandescent lamp can be used, and the same effect can be obtained by the resistor which is formed by having a tungsten wire sealed up in a vacuum atmosphere.

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The present invention relates to a protection circuit for superconducting coils.

[Prior art and its problems]

Since the conventional protection circuit uses a normal resistor, the resistance value hardly changes. Therefore, when trying to quickly release the energy of the superconducting coil, it is necessary to increase the resistance value of the discharge resistor. Therefore, the voltage generated when transferring the coil current to the discharge resistor increases. Therefore, the switch that cuts off the coil current needs to have high voltage DC current cutting ability.
There was a risk of insulation breakdown due to transient voltage when the current was cut off.

[Purpose of the invention]

This invention lowers the discharge resistance value when the current is cut off,
The purpose of the present invention is to provide a protection circuit that suppresses the voltage generated when current is cut off, but when the temperature of the resistor rises due to the heat generated by the resistor itself, the resistance value increases, making it possible to quickly release energy.

[Summary of the invention]

The resistor is made of a metal such as copper or aluminum, which has a large resistance change depending on temperature, and is cooled with liquid nitrogen before applying current.The heat generated by the resistor evaporates the liquid nitrogen. By achieving a temperature change from nitrogen to around room temperature, the resistance value of the resistor can be significantly changed.

〔Effect of the invention〕

By changing the temperature of the resistor from liquid nitrogen temperature to room temperature, the resistance value can be significantly changed relatively easily.

[Embodiments of the invention]

FIG. 1 shows a protection circuit for a superconducting coil according to the present invention. In the figure, 1 is a superconducting coil, 2 is a cryostat, 3 is a resistor, 4 is a liquid nitrogen container, 5 is a switch that cuts off the coil current in order to transfer the coil current to the resistor, and 6 is a power source that excites the coil. It is. FIG. 2 shows the relationship between temperature and electrical resistivity of oxygen-free steel.

As can be seen from the figure, the resistivity changes by approximately one order of magnitude between liquid nitrogen temperature ('77K) and room temperature (300K).

That is, since the initial resistance value is about 10% of the resistance value at room temperature, the voltage generated when the coil current is cut off is also about 10% of the final generated voltage value.

Therefore, by using a no-fuse breaker with a DC breaking voltage of about 250V, the voltage generated by the resistance can be reduced to 10V.
A protection circuit with a voltage of 00 to 2000 V can be constructed.

[Other embodiments of the invention]

FIG. 3 shows another embodiment according to the invention.

7 is an ordinary resistor, which is used in parallel with resistor 3 cooled with liquid nitrogen. When used in this manner, the range of change in the resistance value of the combined resistor becomes smaller, but the heat input to the resistor due to liquid nitrogen cooling can be reduced, and the resistor structure as a whole becomes easier.

A numerical example is shown below. Carrying current 1000A1
When the energy of a superconducting coil with stored energy l MJ is released at a maximum generated voltage of 1000 V, the combined resistance value is 1 Ω. If the resistance value of the normal resistor is 1.5Ω and the resistance value of the liquid nitrogen cooling is changed from 0.3Ω to 3Ω, the initial combined resistance value is 0.25Ω, which is 100Ω.
Since the voltage generated by energizing A is 250V, it is possible to break it off using, for example, a no-fuse breaker. The heat load on the liquid nitrogen cooling resistor at this time is 333 KJ, which is 1/3 of the total. Assuming that the resistor is made of copper, from the liquid nitrogen temperature, loo′c″!, n
The heat capacity when the A degree change can be omitted is 90 J/g 1. Therefore, 3.7 kg of material is required. Assuming that the resistivity at room temperature is 1.8X10'Ωm, to make it 3Ω, the cross-sectional area is 1.55+++ffl, and the length is 250m.
becomes. This means that a line with a diameter of 1.45 mm is made with a diameter of 2
When wound in a coil shape between 00 and 1 round trip with non-inductive winding, the length is 30
It is large enough to have 0 groups.

An incandescent light bulb has a resistance value that changes significantly with temperature, and a similar effect can be obtained by sealing a tungsteer wire in a vacuum and using it as a resistor.

[Brief explanation of the drawing]

Fig. 1 is a protection circuit diagram of a superconducting coil according to the present invention, Fig. 2 is a characteristic diagram showing an example of resistivity change due to temperature, and Fig. 3 is a characteristic diagram showing an example of resistivity change due to temperature.
The figure is a protection circuit diagram of a superconducting coil showing another embodiment of the present invention. 1...Superconducting coil 2...Cryostat 3.7...Resistor

Claims (4)

[Claims] (1) A protection circuit for a superconducting coil characterized by using a material whose resistance value changes greatly with temperature in a discharge resistor that is connected in series with the superconducting coil to reduce the coil current when the superconducting coil transitions to normal conductivity. . (2) The six superconducting coil protection circuits according to claim 1, wherein the discharge resistor is cooled with liquid nitrogen. (3) The superconducting coil protection circuit according to claim 1, wherein the discharge resistor is constructed by connecting a resistor and a resistor cooled with liquid nitrogen in parallel. (4) A protection circuit for a superconducting coil according to claim 1, wherein the discharge resistor is constructed by connecting the resistor and a tungsten wire placed in a vacuum or an inert gas in parallel.