H. Nomura

A study on stabilities of rotating magnets for superconducting generators

Abstract Stabilities of magnets in a high gravitational field up to 2000G at a rotating speed of 3, 600rpm, are studied to establish the fundamental design criterion of field windings of superconducting generators. It is clarified that magnet stability is greatly improved in a rotational field compared with stationary state, because of the enhancement of natural convection for the cooling of the coils caused by a high gravity. Heat transfer to liquid helium (LHe) are measured using a clean copper and a Formvar coated copper under rotational field. The coefficients are 0.7 × 104Wm−2K−1 and 0.4 × 104Wm−2K−1 at the gravitational field of about 1000G, respectively. They are almost identical to the calculated values based on the normal zone propagation velocities under the same condition.

Discussion of the stable thermal equilibrium current of a superconductor during tests of high-speed rotor windings

Research and development of superconducting generation equipment has been conducted. As a part of the design research for the superconducting generator, the stability of superconducting field winding is being studied. The goal of this study is to formulate a design philosophy for a stable superconducting field winding in the presence of large disturbances. In the present paper, we describe the stable thermal equilibrium current of the superconductor, which is measured during the stability test. Modifying the equal area criterion presented by Maddock et al., a simple graphical scheme has been presented and the minimum stable thermal equilibrium current calculated.

Binding evaluation of the combination between the superconductor and matrix by ultrasonic spectra

Combination of the superconductor and high conductive normal metal is now indispensable in the practical use of superconducting magnet. And the binding property of both materials is one of the key points whether superconductive characteristics are fully attained or the current fails prematurely in the practical magnet. But this binding evaluation is not so easy, because even in the mechanically well-jointed conductor, it often appears poor in electric and magnetic joint in cryogenic temperature. Since we have examined and reported fundamental AE properties of superconductor from UI-91, UI-93, UI-95, and UI-97, we have found the simple but important discovery for getting new information about the binding evaluation. Here we report a new technique for evaluating these bindings by observing ultrasonic spectra when a normal transition is propagated along the superconductor. We have discovered that a good contacting conductor has an emission of this sound with sharp resonating frequency peak or peaks at around the highest spectrum area; from 0.5 to 1.0 MHz. Our interpretation of the evaluations are so simple that we can say that binding degrees are best, good or bad, according to the resonating Q-value at around the highest spectrum of the ultrasonic sound.

Time-frequency distribution of acoustic emission signals induced from a rotating NbTi superconducting winding

Experimental analyses using the time-frequency distribution of acoustic emission (AE) signals were studied in terms of quench propagation along a NbTi superconducting winding in a rotor cooled by liquid helium. Data acquisition of the AE signals was carried out with a sampling frequency of 2 MHz to cover almost the full frequency range of the signals. Short-time spectrum analyses were performed for the AE signals when the voltage between taps at the superconducting winding rose after applying heat energy to the winding. A similar test with a heat pulse of lower energy, which did not cause quench propagation, were also carried out. The spectra in this case are equivalent to the background noise of the AE signals in the quench case, and so the AE components resulting from the quench propagation can be estimated.

AE spectra during super-to-normal transition in a superconducting rotor

Abstract We discovered new quantitative relations regarding AE spectra and super-to-normal transition in a rotating superconducting machine. Hence, we have reported mainly qualitative evaluation of AE from the rotating superconducting machine, such as AE and super-to-normal transition, AE and normal propagation, AE and quench magnitude, AE versus heat and changes in magnetic flux, and a prediction of the area to a certain point [H. Nomura et al. Ultrasonics 34 (1996) 307–309]. By repeating the experiments, interesting quantitative relations were found in the AE spectrums main frequency which shifts, according to changes in the magnetic field, the rotational speed and the transport current. Furthermore, these shifts can be explained using simple arithmetic. These discoveries will be useful for both evaluating and monitoring the superconducting rotor.

How to recognize superconducting rotor sound by AE

New evaluating methods for rotating superconducting magnets have been developed by observing acoustic emission (AE) from the rotating machine. First, by measuring the AE level, we can learn whether the conductor is changing from a super to a normal state or not. Secondly, we can get to the area of the magnet the normal state was propagated in by comparing the AE signals level of each channel. Thirdly, we can estimate how large a quenched magnetic energy to dissipated in the magnet by measuring the recovery of time constant of the AE signals. Fourthly, we can detect the difference between the AE just before and after the jumps of heat flux by analysing the AE spectrum. Finally, we report how we can predict from what areas the incoming natural quench of the magnet will be initiated, by monitoring the AE level of each channel.

A study on stability of rotating magnets

Stabilities of magnets in a high gravitational field up to 2000 G are discussed, based upon the experimental results of forced quench tests in a set of rotating magnets, in order to establish the stability design criterion of field windings of superconducting generators. Relations of propagation velocities, recovery currents, minimum quench energy and heat transfer characteristics are studied, a good agreement between the experimental results and a theory confirmed the improvement of magnet stability in a high gravitational field because of the enhanced heat transfer characteristics.

Experiments on Saddle Shaped Superconducting Magnet for the 1kW MHD Power Generator

We have constructed and tested the saddle shaped superconducting magnet with the bore of 29cm dia. and the magnetic field strength of 24kG for the model MHD Generator and also the refrigerator to cool it down. The construction and the preliminary tests were finished in 1968. We have already reported about them. In the present paper, we describe the details of the experiments of the field energy removals, the S-N transition phenomena and the cooling down of the magnet. We also made the experiments of the MHD power generation using the superconducting magnet and got the power out-put of a few tens of watts. We were able to get much know ledge about MHD power generator with superconducting magnet. In conclusion, we experienced the various essential problems accompanying with the construction of larger scale saddle shaped superconducting magnet and have got the solutions for them.