• International Journal of Air-Conditioning and Refrigeration
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• v.8 no.2
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• pp.108-118
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• 2000

This paper describes a design of the helium-recondensing system utilizing cascade Roebuck refrigerators. Superconducting generator or motor has the superconducting field wind-ing in its rotor that should be continuously cooled by cryogen. Since liquid helium transfer from the stationary system to the rotor is problematic, cumbersome, and inefficient, the novel concept of a rotating helium-recondensing system is contrived. The vaporized cold helium inside the rotor is isothermally compressed by centrifugal force and expanded sequentially in cascade refrigerators until the helium is recondensed at 4.2 K. There is no helium coupling between the rotor and the stationary liquid helium storage. Thermodynamic analysis of the cascade refrigeration system is performed to determine the key design parameters. The loss mechanisms are explained to identify entropy generation that degrades the performance of the system.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.6
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• pp.896-906
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• 1997

The effect of duplicate flexible disks on the vibrational modes of a flexible rotor system is investigated by using an anlytical method based on the assumed modes method. The rotor model to be analyzed consists of duplicate disks on a flexible shaft. In modeling the system, centrifugal stiffening and disk flexibility effects are taken into account. To demonstrate the effectiveness of the method, a hard disk drive spindle system commonly used in personal computers and a simple flexible rotor system with two disks are selected as examples. In particular, the dynamic coupling between the vibrational modes of the shaft and the duplicate disks is investigated with the shaft rotational speed varied.

• Journal of KSNVE
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• v.9 no.2
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• pp.401-408
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• 1999

A method is presented for the vibration analysis of steam turbine-generator rotor system based on the component mode synthesis method. The motion of each component of the system is described by superposing constraint mode associated with boundary coordinates and constrained normal modes associated with internal coordinates. This method using real fixed-interface modes allows for significant reduction in system model size while retaining the essential dynamic characteristics of the lower modes. The capability of this method is demonstrated in the natural frequency and unbalance response analysis of the steam turbine-generator rotor system in which the dynamics of the pedestal is considered. The results by the present method are compared with finite element method and trnasfer matrix method in terms of the accuracy and computing time.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.4 s.247
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• pp.442-450
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• 2006

This paper describes the new modal analysis method to detect the presence of the breathing crack in a general rotor system with disk asymmetry and stator anisotropy. It is proposed that the modal analysis using directional frequency response functions (dFRFs), which, accounting for the directivity in modes, clears the heavily over-lapping of other harmonics occurring from non-isotropic properties in addition to those due to crack, can provide an effective method to detect the modes by a crack. The simulations from the simple general rotor model show that the r-dFRFs (reverse dFRFs) for asymmetry confirms a good indicator of the presence of the breathing crack and the instability is primarily influenced by the shaft asymmetry than the breathing crack.

• Proceedings of the KIEE Conference
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• 1996.07a
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• pp.412-414
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• 1996

The rotor resistance variation has a large effect on the field oriented control system of induction machine. In this paper, the adaptation technique based on MRAC is used to identify the rotor resistance variation. The criterion function used in the adaptation algorithm is the error function of the two reactive powers of the induction motor. The one is obtained from the voltages and the currents of the stator of the induction motor. And the other is estimated from the rotor flux and stator current. We simulated this control system operated by field oriented control and assured the robustness of the induction motor control system against the rotor resistence variation.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.389-392
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• 1997

This paper presents the dynamic analysis and experiment for rotor systems supported by angular contact ball bearings subjected to axial displacement preload. A dynamic software, which has been developed by the authors for the analysis of rotor systems supported by angular contact ball bearings, is used to simulate a uniform shaft system supported by two angular contact ball bearings with the axial displacement preload varied. Experiments are also performed to validate the simulation. An experimental system is constructed which consists of a uniform shaft, two bearings and a device for adjusting the axial displacement preload. Through a series of simulation and experiment, the dynamic characteristics of rotor systems with angular contact ball bearings subject to axial displacement preload are investigated.

• Journal of the Korean Society for Precision Engineering
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• v.26 no.1
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• pp.130-137
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• 2009

This paper deals with the comparisons of methodologies to express finite rotations accounting for analysis of the rotor system. Researches have been made to predict a behavior of its rotational motion by introducing Euler angles which turned out to be lack in consistency and exactness of the analysis. To overcome this deficiency a new methodology is applied by using both spherical coordinate and quaternion in the rotor rotation and shows its superiority over choices of the Euler angle in terms of kinetic energy and rotation velocity. It is found through numerical examples that quaternion is a more useful and valid tool to derive the ideal numerical model of the rotor system.

• Journal of Aerospace System Engineering
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• v.2 no.3
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• pp.1-6
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• 2008

The tilt rotor aircraft has the flight characteristics which takes off vertically like a helicopter and flies forward like an airplane. Especially, the transition process from a helicopter to an airplane mode requires not only the mixing of control inputs but also the stability and controllability augmentation system(SCAS) in order to keep the safe flight because there are compound flight dynamic characteristics of a helicopter and an airplane including non-linearity, uncertainty. This paper describes the design of SCAS in a lateral motion for the tilt rotor aircraft based on the $H_{\infty}$ control method, which was performed from mathematical model with weighting matrix based on the relationship between the $H_{\infty}$ norm and the sensitivity function. Through simulation analysis for the controller designed on the $H_{\infty}$ control theory, it was shown that this method may be applied to the control design of the tilt rotor aircraft.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.10a
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• pp.52-56
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• 2011

Recently, KARI(Korea Aerospace Research Institute) has been developing a modern 11.5m diameter four bladed bearingless main rotor system, and this rotor system can be used for 7,000lb class helicopter. Flexbeam and torque tube can be considered as the key structural components, and large elastic twist of flexbeam induced by pitch control motion of torque tube can influence the nonlinear aeroelastic behavior. In this paper, the dynamic characteristic analysis results of bearingless rotor system were presented. In order to construct a input model and validate the analysis procedures, calculated results using the comprehensive helicopter analysis program CAMRAD II were compared with the measured natural frequencies and lag damping data from small-scale wind tunnel test. Next, the analysis model was extended to a full-scale model, and the dynamic analysis results were presented.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.15 no.3
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• pp.6-14
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• 2007

In this research, vibration reduction and aeroelastic stability of a composite hingeless rotor hub flexure with viscoelastic constrained layer damping treatment(CLDT) were investigated. The composite flexures with viscoelastic CLDT were applied to hingeless rotor system to improve the in-plane stability of the lead-lag motion causing resonance. The modal test was performed and dynamic properties(natural frequency and loss factor) were acquired. Also, complex eigenvalue analysis(SOLlO7) in the NASTRAN structural analysis module was performed and compared with results of the modal test. To insure aeroelastic stability, damping ratio analyses of the hingeless rotor system with CLDT were accomplished at hovering condition due to collective pitch angle changes. Satisfactory results of increasing structural damping and stability were obtained.