Excitation Of Torsional Vibrations Research Articles

Research on the Torsional Vibration Response Characteristics of Parallel Operation Steam Turbine Generator Unit Shafting

With the continuous increase of unit capacity and the widespread application of series compensation devices in power transmission systems, the risk of coupling oscillation between units and power grids and between units has increased significantly, which seriously endangers the safe operation of units and power grids. In this paper, the torsional vibration response of each unit after grid failure is calculated by establishing a lumped mass model of the shafting torsional vibration of the parallel unit. The effects of fault type, series compensation degree and fault location on the torsional vibration of parallel unit shafting are studied. The results show that the three-phase short-circuit fault in the grid has a greater impact on the unit; the influence of series compensation degree on the torsional vibration of the shaft system decreases first and then increases; when the transmission line fails, it will produce a certain torsional vibration excitation effect on the shafting of all units; when a short-circuit fault occurs at the outlet of a generator, the torsional vibration excitation of the unit is the largest, but it will also produce a certain degree of torsional vibration excitation to other units in the same network.

Concept and electromechanical-coupling modeling of a torsional vibration excitation method

The sandwiched type torsional piezoelectric actuators have widely been employed in the fields of ultrasonic cutting, ultrasonic drilling, ultrasonic welding, ultrasonic wire drawing, ultrasonic spraying, and microdroplet generation, due to their unique vibration form. However, the circular piezoelectric ceramic plate polarized in the circumferential direction holds a complicated manufacturing process, a long processing period, and a high price. It can only be used for the excitation of the torsional vibration of circular rods, and cannot be used for square cross-section beams, severely restricting its applications. In order to solve the above problems, a novel excitation method is proposed for generating torsional vibration in the square cross-section beam in this study, which uses the d15 vibration mode of rectangular piezoelectric ceramic plates to produce the torsional vibration, and a novel transfer matrix model of square torsional piezoelectric element is created. Then a case study is conducted in this paper to confirm the proposed excitation method and the developed model, in which two novel sandwiched type torsional piezoelectric actuators are designed. Moreover, electromechanical coupling models of these two torsional piezoelectric actuators are developed using the transfer matrix method, in order to reveal their dynamics behaviors. Finally, the finite element method and the experimental investigations are adopted to analyze and test the frequency response characteristics and vibration shapes of the proposed piezoelectric actuators. Both simulation analysis and experimental tests verify the effectiveness of the proposed excitation method and the correctness of the developed electromechanical coupling dynamic models. The proposed excitation method provides a new idea for the design of sandwiched type torsional piezoelectric actuators, and the developed theoretical model lays a theoretical foundation for the further research of torsional piezoelectric actuators.

Evaluation of Input-Shaping Control Robustness for the Reduction of Torsional Vibrations

Aircraft drivetrains connect the engine to the electrical power system. In most cases, the drivetrains are relatively flexible and have vibration modes with values below 100 Hz to reduce weight and size. Therefore, electrical loads’ connection and disconnection may excite torsional vibrations in the machine's shaft, reducing the drivetrains’ lifespan. This interaction is known as electromechanical interaction. This issue can be mitigated using an input-shaping strategy, which reduces the excitation of torsional vibrations by connecting the electrical loads following a pattern, dependent on the drivetrain's natural frequencies. However, since this method is based on the knowledge of the vibration modes attributes, it can be susceptible to parameter's uncertainty. In this article, a pulsating input shaping method's robustness is assessed, analyzing simulation and experimental results. The effect of the inductances is analyzed, and a strategy to reduce its effect is proposed. Furthermore, the effect of uncertainty in the mechanical parameters is evaluated, and theoretical analysis is carried out to establish safe operating limits. The theoretical analysis is experimentally validated.

Instantaneous Electromagnetic Torque Components in Synchronous Motors Fed by Load-Commutated Inverters

This paper proposes time-domain analytical expressions of the instantaneous pulsating torque components in a synchronous machine air gap when supplied by a load-commutated-inverter (LCI) system. The LCI technology is one of the most used variable frequency drives when very high power and low speed are required in applications such as pipeline recompression and decompression, as well as liquefied natural gas compression. In such applications, synchronous motors are used because of their high efficiency resulting from a separated supply of the current to their rotor through the excitation circuit. These applications usually have long and flexible shafts, which are very sensitive to torsional vibration excitation when their natural frequencies interact with any external torque applied to the shaft. A torsional analysis is required by international standards to assess the survivability of the shaft through the overall speed range of the motor. Therefore, the magnitude and frequencies of the motor air-gap torque are needed for such evaluation. The proposed developments are supported by numerical simulations of LCI systems in a large range of operation range. From the simulation results, torque harmonic families are derived and expressed in a parametric form, which confirm the accuracy of the proposed relationships.

Excitation method and electromechanical coupling dynamic model of a novel torsional piezoelectric actuator

Torsional piezoelectric actuators have been widely applied in many fields due to their specific vibration mode. However, the conventional torsional vibration piezoelectric actuators require circumferentially polarized piezoelectric ceramics or special structural designs, increasing manufacturing difficulty and processing costs, and seriously limiting their applications. In order to solve these problems, a novel excitation method of torsional vibration is proposed in this study through the arrangement of the regular piezoelectric ceramic plates polarized along their thickness direction, and then two surface-bonded type torsional piezoelectric actuators are constructed to stimulate the odd and even order torsional vibration modes, respectively. A novel transfer matrix model of describing the torsional vibration, which involves in the mechanical and electrical properties, is created first for the piezoelectric composite beam element. Then a general electromechanical coupling dynamic model is developed to reveal dynamic behaviors of the proposed surface-bonded type torsional piezoelectric actuators. In addition, the finite element simulation and experimental investigations are carried out to confirm the validation of the proposed excitation method of torsional vibration and to verify the correctness of the developed electromechanical coupling dynamic model. The comparisons showed that the results calculated by the developed theoretical model, simulated by the finite element method, and tested by the experiments matched well, indicating that the developed electromechanical coupling dynamic model is correctness and the excitation method of torsional vibration is effective. Finally, the output characteristics of the two proposed actuator prototypes are measured, further confirming the feasibility of the proposed torsional vibration excitation method.

Reduction of Torsional Vibrations Excited by Electromechanical Interactions in More Electric Systems

This paper studies the excitation of torsional vibrations by electromechanical interaction after the connection of electrical loads. Electrical generation and power system in aircraft are becoming of key importance, especially considering the current roadmap for aerospace systems' electrification. However, since the drivetrain which links electrical generators to the main engine is relatively flexible in most aircraft structures, torsional vibrations can be excited by the sudden connection or disconnection of electrical loads, thus increasing the drivetrain's fatigue and reducing its reliability and lifetime. Because of the increased amounts of intermittent electrical loads on aircraft systems, which excite torsional vibrations through electromechanical interactions, the electrical load connections are investigated in this paper. Specifically, a method for reducing torsional vibrations in aircraft drivetrains is proposed to extend their lifespan. This method is applied directly to the load being connected and it proposes the connection of the electrical loads following a pulsating pattern, for which the pulse connection time is determined as a function of the drivetrain vibration modes. Simulations results show that the proposed method provides a significant reduction in the drivetrain shaft torsional vibrations. Experimental results validate the simulation data showing the benefits this method can provide in drivetrain reliability, weight reduction and cost.

Torsional vibration reduction of rotating shafts for multiple orders using centrifugal double pendulum vibration absorber

Centrifugal Pendulum Vibration Absorbers (CPVAs) reduce the torsional fluctuations of rotating shafts at fixed vibration order by changing the natural frequency in accordance with the excitation frequency. Attenuation of multiple orders is necessary in the case of the rear wheel drive vehicle and cylinder deactivation applications for achieving refined vehicle performance. Single pendulum vibration absorbers attenuate only one vibration order at a time and the pendulum length must be reduced for higher orders in the order of (1/n2) times the distance between the center of rotation and pivot point to attenuate nth order vibration. Use of single pendulum absorbers for reducing higher orders has limitations due to manufacturing inaccuracies and occurrence of wear of joints over prolonged usage which reduces the effectiveness of absorber in reducing the torsional vibration. In this study, a centrifugal double pendulum vibration absorber (CDPVA) is proposed to make the design easy for higher orders by deploying two pendulums in series. The CDPVA is designed to attenuate the fundamental excitation order (n) and its second harmonic (2n) without significant reduction in the pendulum length hence making the design robust for prolonged usage. The mass and length values of the double pendulum are tuned to match the two natural frequencies correspond to the fundamental and second harmonic order of the torsional vibration excitation. Torsional springs are used at the pivot joints of the double pendulum to counteract the gravitational effects at low speeds and the influence of spring stiffness on the natural frequencies is analyzed. The nonlinear second order equations of motion of pendulum masses and rotor are solved analytically using method of multiple time scales and the stability characteristics of pendulum motion are analyzed at the excitation frequencies with different torque levels. Numerical simulation of complete nonlinear equations is carried out and the analytical results are compared with the simulation results. The designed CDPVA for 2nd and 4th order is tested on a 4-cylinder the rear wheel drive vehicle and considerable reduction is achieved in the torsional vibration of the drive shaft and also in the vibration levels at seating locations. The proposed methodology helps in designing the CDPVAs for attenuating the torsional vibration of multiple higher orders yet avoiding impacts in low speed operation.

Modelling of reduced electromechanical interaction system for aircraft applications

Rotational systems such as aircraft engine drivetrains are subject to vibrations that can damage shafts. Torsional vibrations in drivetrains can be excited by the connection of loads to the generator due to electromechanical interaction. This problem is particularly relevant in new aircraft because the drivetrain is flexible and the electrical power system (EPS) load is high. To extend the lifespan of the aircraft engine, the electromechanical interaction must be considered. Since real-time constants of the electrical and mechanical systems have very different magnitudes, the simulation time can be high. Furthermore, highly detailed models of the electrical system have unnecessary complexity for the study of electromechanical interactions. For these reasons, modelling using reduced order systems is fundamental. Past studies of electromechanical interaction in aircraft engines developed models that allow the analysis of the torsional vibration, but these are difficult to implement. In this study, a reduced order electromechanical interaction system for aircraft applications is proposed and validated using experimental results. The proposed system uses a reduced drivetrain, simplified EPS, and sensorless measurement of the vibrations. The excitation of torsional vibrations obtained is compared with past studies to prove that the reduced order system is valid for studying the electromechanical interactions.

Root cause analysis of locomotive wheel tread polygonisation

Periodic out-of-round or polygonised wheels were detected on a fleet of high adhesion locomotives operating within South Africa. The polygonised wheels increased the vertical load spectrum of the locomotive resulting in accelerated component failures. During the initial investigation it was found that torsional axle vibration could lead to the polygonisation of the wheels. The torsional vibration of the locomotive axle was confirmed theoretically and experimentally through various physics models and on-track measurements. Based on the results, two potential torsional vibration excitation mechanisms were identified. A link was also established between the torsional vibration frequency, vehicle operating speed and the polygonisation order. Based on the similarities of the two excitation mechanisms a vibration suppression system was considered and tested. A reduction in the torsional vibration amplitude was observed when the suppression system was active.

Laser Excitation of Torsional Vibrations in Optical Microfibers

Comparative analysis of mechanisms of the laser excitation of torsional vibration modes in optical microfibers shows that special attention should be devoted to the Sadovsky effect manifested in polarization-anisotropic optical microfibers. Estimations of the efficiency of this mechanism for the excitation of torsional vibrations and polarization modulation in the proposed types of anisotropic quartz microfibers shows the possibility of creating new types of polarization elements—optically controlled fiber polarization devices and sensitive elements for resonance fiber-optic sensors.

Лазерное возбуждение крутильных колебаний волоконных микросветоводов

AbstractComparative analysis of mechanisms of the laser excitation of torsional vibration modes in optical microfibers shows that special attention should be devoted to the Sadovsky effect manifested in polarization-anisotropic optical microfibers. Estimations of the efficiency of this mechanism for the excitation of torsional vibrations and polarization modulation in the proposed types of anisotropic quartz microfibers shows the possibility of creating new types of polarization elements—optically controlled fiber polarization devices and sensitive elements for resonance fiber-optic sensors.

Stability Analysis of Rotating Blade Vibrations Using Lyapunov Exponents

This paper presents an analysis of the vibration stability of a rotating blade due to shaft torsional vibration excitation. The governing equation adopted in the study is a Hill’s type linear second order ordinary differential equation with multiple harmonically variable coefficient terms. The differential equation of the system is rewritten as two coupled first order ordinary differential equations. The stable and unstable regions are determined by the Lyapunov characteristics exponents on parameter space (grid) relating to the rotor speed, the torsional vibration excitation frequency and the blade natural frequency. The results are contrasted to those obtained by the strained parameter method (a perturbation technique), an excellent match is observed for small torsional vibration amplitudes .

Design and analysis of a multi-stage torsional stiffness dual mass flywheel based on vibration control

This paper is aimed to investigate the kinetic parameters matching and designing method for a multi-stage torsional stiffness dual mass flywheel (DMF) based on torsional vibration control. Using the numerical analysis methods, three critical technological problems are resolved subsequently. The first one is that the kinetics simulation model of vehicle drivetrain is established, and the actual torsional vibration excitation of crankshaft and different gear pairs engaged multi-operating conditions for drivetrain are taken into account during modeling. The second one is that the kinetic parameters sensitivity of DMF with respect to torsional vibration control are obtained and the range of each kinetic parameter (i.e. rotation inertia ratio, torsional stiffness and damping) is derived further. The last one is that the parameters matching and designing methods for a multi-stage torsional stiffness DMF, considering the constraint conditions that transmission torque, relative angle and distribution of natural frequencies of DMF-drivetrain, are carried out. The results show that the torsional vibration of drivetrain is controlled effectively after matching a three-stage torsional stiffness DMF and the amplitude of angular velocity at input end of gearbox decreases significantly, which verifies that the designing and matching methods are effective and the result can thus be used in vibration control for vehicle drivetrain.

Stability Analysis of Rotating Blade Vibration due to Torsional Excitation

This paper presents an approximate analysis of the vibration stability of a rotating blade due to shaft torsional vibration excitation. The governing equation adopted in the study is a Hill's type linear second order ordinary differential equation with multiple harmonically variable coefficient terms. The strained parameters method, a perturbation technique, is utilized in developing the stability transition curves in the plane of parameters related to the rotor speed, the torsional vibration excitation frequency and the blade natural frequency. The stable and unstable regions obtained by perturbations are contrasted to those obtained by numerical stability analysis performed using Floquet theory and an excellent match is observed for small torsional vibration amplitudes. Numerical integration of the original equation at selected points in the predicted stable and unstable regions showed that the predicted behavior of the responses is correct, wherein the unstable regions growing blade vibration is exhibited.

Unbalanced Operation of HVDC Converters in Asynchronous Linksin Torsional Stressing of Turbine-Generator Shafts

This article analyzes the impact of unbalanced and balancedoperation of HVDC link rectifier bridges on ripple current superimposed on the direct current in asynchronous links in the excitation of torsional vibrations in steam turbine-generator-exciter shafts. The effect of nonideal conditions such as unbalanced rectifier ac supply voltage, unbalanced phase inductance and transformer tappings, and firing errors on the spectrum of dc harmonic voltages compared with the ideal case is discussed. These voltages give rise to dc harmonic currents at the rectifier that may excite sympathetic shaft torsional vibrations in the machine shaft.First, frequencies at which sympathetic shaft torsional vibrations would be excited by modulation product harmonic currents due tounbalanced and balanced operation of 50 Hz/50 Hz asynchronous links as a function of deviation in the system frequency are illustrated. Amplitude of shaft torsional torque due to resonant torque excitation due to harmonic current injected into the ac system by the inverter is then discussed. The article thenconsiders how the steady torque excitation depends on the noncharacteristic harmonic current that is injected into the ac system by the inverter. Tables and graphs are presented to illustrate the significance of bridge unbalances on rectifier harmonic ripple voltage (and hence on the associated currents thatare superimposed on the HVDC line current at the rectifier and subsequently at the inverter), which result in torsional torque in turbine-generator shafts. The relative amplitude of noncharacteristic currents injected into the ac system by the inverter operating unbalanced and balanced is indicated. Both 6- and 12-pulse bridges are analyzed.

Proposed 4 GW Russia-Germany link-impact of 1 GW inverter station on torsional stressing of generators in Poland

The European power systems are in the process of a new orientation which will facilitate and promote exchange of electric power between Eastern and Western European nations. One such example is the 4 GW HVDC link from Russia to Germany where a 1 GW power converter substation has been proposed for Central Poland. This paper considers the impact of the proposed 4 GW Russia-Germany HVDC link on the Polish grid network and possible excitation of torsional vibrations in shafts of steam turbine-generator exciter units connected to the system by noncharacteristic currents due to the link. Excitation of torsional vibrations in machine shafts due to modulating currents and the level of risk of damage to machine shafts due to subsynchronous currents injected into the network by the inverter is assessed. Dynamic parameters of system loads and the power factor of loads which influence system scaling factors are also discussed. The paper also examines the most probable location of the proposed 1000 MW converter on the Polish grid network. System scaling factors are evaluated for different system configurations for each generator at risk and generators most at risk are identified. The paper then investigates transient behaviour of the Polish power system following loss of the proposed link at full import. Transient stability performance is also evaluated for a range of system faults of different fault durations, distances and locations from power plants.

The excitation of torsional vibrations of a nickel wire

Nickel wires have mostly a helical <111>-texture and therefore an anisotropic part of magnetization when subjected to an axial constant field. A torsional pendulum with such a wire as spring can be set into vibration by a superposed axial alternating field.

Elastic and piezoelectric properties of paratellurite (TeO 2)

The elastic and piezoelectric properties of single crystals of paratellurite (TeO 2) are investigated. In the <110 > direction, shear waves may propagate with the extremely low phase velocity of 0.6 × 10 3 m s −1. The coupling coefficient for shear waves in the <001 > direction is of the order of 10%. Direct excitation of torsional vibrations is possible using a very simple electrode configuration. There are certain directions in which the phase velocity of shear waves is independent of temperature.