Discussion of Induction Motor Effect on Rotordynamics

In this dissertation, the influence of using induction motor in machinery train on rotordynamics is discussed. Two areas are considered – the use of variable frequency drives (VFDs), which control and drive the induction motor; and the unstable forces due motor eccentricity, which is resulted from motor rotor lateral motion. VFDs – The dissertation documents fatigue related mechanical failures in VFD motor machinery due to mechanical vibrations excited by drive torque harmonics which are created by PWM switching. Present effort models the coupled system with full electrical system including DC bus, inverter, motor, and an industrial mechanical system including flexible couplings, gearboxes and multiple inertias. The approach extends failure prediction beyond simple occurrence of resonance, to fatigue life evaluation based on Rain-flow algorithm, which is suitable for both steady state and transient startup mechanical response. The use of multilevel inverters is demonstrated having the possibility actually exacerbate resonance and fatigue failure. The model is also compared to an industrial test case, which provides good agreement. Motor eccentricity – In this dissertation, a MEC modeling method is proposed to calculate both the radial and tangential motor eccentric force. The proposed model is also coupled with the motor electric circuit model to provide capability of transient simulation. FEM (Ansys Maxwell) is used to verify the proposed model. Parametric study is performed on the motor radial and tangential eccentric forces. Also a Jeffcott rotor model is used to study the influence of the motor eccentric force on mechanical stability. A stability criteria of the bearing damping is calculated. The motor radial and tangential eccentric forces are all curved fitted to catch their nonlinearity, which are used in time domain simulation. Nonlinear motions are observed, including limit cycle and jumping phenomena. The results of this dissertation show that both the use of VFDs and the motor lateral motion (motor eccentricity) can cause severe mechanical vibration problem in a rotating machinery train. Both of these two problems need to be carefully concerned in design stage

VSDS Motor Inverter Design Concept for Compressor Trains Avoiding Interharmonics in Operating Speed Range and Verification

TutorialDuring operation of compressor trains by a variable speed drive system (VSDS), integer and non-integer harmonic currents are generated in the inverter. Via the electrical system of the inverter and the motor, an excitation torque is transferred across the motor air gap into the main mass of the motor rotor. The frequency of this excitation may cause torsional resonances. Due to the rapid increase in excitation frequency of integer harmonics, intersections with relevant torsional natural frequencies (TNFs) can in general be avoided within the operating speed range. In contrast, the intersections of the noninteger harmonic excitation frequencies, also called interharmonics, with TNFs within the operating speed range may have an essential impact on the vibration behaviour of the rotating equipment. This aspect has to differentiate between two train configurations. The first are direct driven trains and the second, trains including an intermediate gear. For direct driven trains, only fatigue problems have to be considered. In trains with an intermediate gear, on top of that, interaction of torsional and lateral movement may have a negative effect on the lateral vibration behaviour of the gear rotors. The main focus of this publication is on a simple but effective method for turbo compressor applications that allows avoiding main resonances within the operating speed range caused by intersections of interharmonic excitations with relevant TNFs. This method is based on detailed knowledge of the inverter behaviour and possible design options of the motor itself. This in-depth understanding was developed by correlating numerical and experimental results based on dynamic torque measurements of real turbo compressor trains. During this investigation the mechanically relevant torsional excitations were identified. Therefore, the different types of inverters and their corresponding characteristics had to be analyzed and understood in detail. This knowledge, in combination with possible motor designs, with regard to the number of pole pairs and the most common train configurations (direct driven and/or trains including intermediate gears), is incorporated in this report

All-electric LNG a viable alternative to conventional gas turbine driven LNG plant

The world demand for natural gas which is at an increasing trend has rekindled interest in the production and transportation of Liquefied Natural Gas (LNG) from resource rich areas in Africa, Middle East, Far East, Australia and Russia to customers in Europe, Americas, China and India. The challenges for the future are to produce and transport gas in a cost effective manner to be competitive in the market place. Gas is beginning to play an increasingly important role in energy scenario of the world economy. The easiest ways of getting gas to the market is by pipe lines. However to reach markets far and wide across oceans, gas needs to be converted and transported in liquid form. Competitive pressure and search for economies of scale is driving up the size of LNG facilities and hence the capital requirement of each link of the value chain. Interdependent financing of the various links of the value chain, while maintaining their economic viability, is the challenge that sponsors need to address. The industry is potentially a high risk business due to uncertainty associated with the characteristics of the industry, which calls for high level of investment in an environment of volatility of the price and political and economic changes in the world market. LNG production facilities are becoming larger and larger than ever before to take advantage of economies of scale. These massive plants not only have created new challenges in design, procurement and construction and environment but will create new challenges in operation and maintenance. Innovative technologies and first of a kind equipment applications with a rigorous technology development and a stringent testing plans ensure that the facility will achieve a long term reliable operation. Conventional LNG plants use Gas Turbine as main drivers for refrigerant compressors. To this effect All-Electric LNG has a potential to provide an alternative offer a life cycle advantage over the convention. Hence it would be worthwhile to study the pros and cons and prospects offered by this new technology from an overall life cycle perspective for future of LNG projects. This research is an endeavours in this direction

Design and Analysis of Electric Powertrains for Offshore Drilling Applications

Doktorgradsavhandling ved Institutt for ingeniørvitenskap, Universitetet i Agder, 2016The global energy market is challenged with an ever increasing need for resources to meet the growing demands for electric power, transportation fuels, etc. Although we witness the expansion of the renewable energy industry, it is still the fossil fuels, with oil and gas dominating the scene of global energy supply sector, that provide majority of worldwide power generation.However, many of the easily accessible hydrocarbon reserves are depleted which requires from the producers of drilling equipment to focus on cost-effective operations and technology to compete in a challenging market. Particularly high level of activity is observed in both industry and academia in the field of electrical actuation systems of drilling machines, as control methods of alternating current (AC) motor drives have become an industrially mature technology over the past few decades. In addition, state-of-the-art AC motors manufacturing processes allow to conform to the strict requirements for safe operation of electrical equipment in explosive atmospheres. These two main reasons made electric actuation systems a tough competitor to hydraulic powertrains used traditionally by the industry. However, optimal design of induction motor drives and systematic analysis of factors associated with operation in harsh offshore conditions are still considered as a major challenge. In this thesis, effective methods for design and analysis of induction motor drives are proposed, including aspects of optimization and simulation-based engineering. The first part of the thesis is devoted to studying methods for modeling, control, and identification of induction machines operating in offshore drilling equipment with the focus to improve their reliability, extend lifetime, and avoid faults and damage, whereas the second part introduces more general approaches to the optimal selection of components of electric drivetrains and to the improvement of the existing dimensioning guidelines. A multidisciplinary approach to design of actuation systems is explored in this thesis by studying the areas of motion control, condition monitoring, and thermal modeling of electric powertrains with an aspiration to reach the level of design sophistication which goes beyond what is currently considered an industrial standard. We present a technique to reproduce operation of a full-scale offshore drilling machine on a scaled-down experimental setup to estimate the mechanical load that the designed powertrain must overcome to meet the specification requirements. The same laboratory setup is used to verify the accuracy of the estimation and control method of an induction motor drive based on the extended Kalman filter (EKF) to confirm that the sensorless control techniques can reduce the number of data acquisition devices in offshore machines, and thus decrease their failure rate without negatively affecting their functionality. To address the challenge of condition monitoring of induction motor drives, we propose a technique to assess the expected lifetime of electric drivetrain components when subjected to the desired duty cycles by comparing the effects of a few popular motion control signals on the cumulative damage and vibrations. As a result, the information about the influence of a given control strategy on drivetrain lifecycle is made available early in the design stage which can significantly affect the choice of the optimal powertrain components. The results show that some of the techniques that have a well-proven track record in other industries can be successfully applied to solve challenges associated with operation of offshore drilling machines. One of the most essential contributions of this thesis, optimal selection of drivetrain components, is based on formulating the drivetrain dimensioning problem as a mixed integer optimization program. The components of powertrain that satisfy the design constraints and are as cost-effective as possible are found to be the global optimum, contrary to the functionality offered by some commercially available drivetrain sizing software products. Another important drawback of the dimensioning procedures recommended by the motor drives manufacturers is the inability to assess if the permissible temperature limits given in the standards do not become violated when the actuation system experiences overloads different than these tabulated in the catalogs. Hence, the second most significant contribution is to propose a method to monitor thermal performance of induction motor drives that is based exclusively on publicly available catalog data and allows for evaluating whether the standard thermal performance limits are violated or not under arbitrary load conditions and at any ambient temperature. Both these solutions can effectively enrich the industrially accepted dimensioning procedures to satisfy the level of conservatism that is demanded by the offshore drilling business but, at the same time, provide improved efficiency and flexibility of the product design process and guarantee optimality (quantitatively, not qualitatively, measurable) of the final solution. An attractive direction for additional development is to further integrate knowledge from different fields relevant to electric powertrains to enable design of tailored solutions without compromising on their cost and performance

Bearing Fault Diagnosis Using Motor Current Signature Analysis and the Artificial Neural Network

Bearings are critical components in rotating machinery. The need for easy and effective bearings fault diagnosis techniques has led to developing different monitoring approaches. In this research, however, a fault diagnosis system for bearings is developed based on the motor current signature analysis (MCSA) technique. Firstly, a test rig was built, and then different bearing faults were simulated and investigated in the test rig. Three current sensors, type SCT013, were interfaced to Arduino MEGA 2560 microcontroller and utilized together for data acquisition. The time-domain signals analysis technique was utilized to extract some characteristic features that are related to the simulated faults. It was noticed that the simulated bearing faults have led to generating vibrations in the induction motors, which in turn cause a change in its magnetic field. For classification (identification) of the extracted features, the artificial neural network (ANN) was employed. An ANN model was developed using the Matlab ANN toolbox to detect the simulated faults and give an indication about the machine health state. The obtained features from the captured motor current signals were utilized for training the ANN model. The results showed the effectiveness of using MCSA based on the time-domain signal analysis in combination with ANN in diagnosis different bearings faults

Design Fabrication and Testing of Gearbox for Fault Detection.

This project work focused on the design and testing of gearbox for fault detection. Gearboxes are very important things in many industries applications. Thus the interest for their health monitoring is growing and effective diagnostic techniques and methodologies are the objective of previous studies. Here we are going to detect different types of fault which occurs in gearbox. There are mainly three types of fault occurs. These faults are slightly worn, medium worn, broken teeth. With the help of vibration study (due to gears fault) through oscilloscope we can design such kind of system which can detect what kind of fault a gearbox has. A multistage faulty gearbox is used in the present study. The test setup comprises of a multistage gearbox, driven by an induction motor and loaded by a generator or by a brake motor. The gearbox is an automotive transmission gearbox with five gears including back gear, such as 1st, 2nd, 3rd, 4th and back gears, under synchrony-meshed condition. Here each gear will be tested under the constant load and constant speed (RPM) conditions

Electric otors and Drives in Torsional Vibration Analysis and Design

TutorialTwo types of prime movers are continuously applied for rotating machinery: mechanical and electric drives. This tutorial focuses on electric drives, which are actually composed of separate components: electric motor, frequency converter (optional), and electric network including transformers. The main aim of this tutorial is to explain the phenomena of electric drives related to the torsional vibrations. All the components of an electric drive may affect torsional dynamics of the system. Naturally the main concern is torsional excitations. Another aspect is the electromechanical interaction in the air-gap of the motor that produces electromagnetic torsional stiffness and damping. These and other similar phenomena, related to the electric drives, have been observed on test fields and site conditions. The scope of the paper includes the most common induction, synchronous and permanent magnet motors. First, the various rotor structures of electric motors are discussed. Then, the effects of electromagnetic stiffness and damping on the torsional dynamics are introduced. Next, the main electric excitations are presented and their effect on drivetrain dimensioning is discussed. Various modelling and analysis methods are also presented for drive-trains with an electric motor. The previous sections focus on the motor without any frequency converter. The rest of the paper deals with the variable speed drives. First the main voltage source inverter, current source inverter and load commutated inverter drive types, their operational principles and application areas are introduced. Then the harmonic and inter-harmonic torsional excitations are explained that depend on the configuration and control of the frequency converter. The main functionality of a variable speed drive system is the speed-control feedback loop. This loop is used to maintain the preset speed. Although this feedback loop is also a potential source of instability it can with proper tuning be used to damp low frequency torsional resonances. The analysis guidelines and design principles are distributed throughout the paper. The paper is concluded with summary and main recommendations

Induction motor bearing fault detection using a sensorless approach

Continuous condition assessment of induction motors is very important due to its potential to reduce down-time and manpower needed in industry. Rolling element bearing faults result in more than 40% of all induction motor failures. Vibration analysis has been utilized to detect bearing faults for years. However, vibration sensors and expert vibration interpretation are expensive. This limitation prevents widespread monitoring of continuous bearing conditions in induction motors, which provides better performance compared to periodic monitoring, a typical practice for motor bearing maintenance in industry. A strong motivation exists for finding a costeffective approach for the detection of bearing faults. Motor terminal signals have attracted much attention. However, not many papers in the literature address this issue as it relates to bearing faults, because of the difficulties in effective detection. In this research, an incipient bearing fault detection method for induction motors is proposed based on the analysis of motor terminal voltages and currents. The basic idea of this method is to detect changes in amplitude modulation between the spatial harmonics caused by bearing faults and the supply fundamental frequency. This amplitude modulation relationship can be isolated using the phase coupling property. An Amplitude Modulation Detector (AMD), developed from higher order spectrum estimation, correctly captures the phase coupling and isolates these modulation relationships. In this research, in-situ bearing damage experiments are conducted so that the accelerated life span of the bearing can be recorded and investigated. Experimental results shown in this dissertation are based on different power supplies, load levels, VSI control schemes, and motor operating conditions. Taking the mechanical vibration indicator as a reference for fault detection, the proposed method is demonstrated to be effective in detecting incipient bearing faults in induction motors. If motors are operating at near steady state conditions, then experimental results show that the bearing fault detection rate of the proposed approach is 100%, while no false alarms are recorded

LNG TURBOMACHINERY

TutorialThe International Liquefied Natural Gas (LNG) trade is expanding rapidly. Projects are being proposed worldwide to meet the industry forecasted growth rate of 12% by the end of the decade. LNG train designs in the coming years appear to fall within three classes, having nominal capacities of approximately 3.5, 5.0 and 8.0 MTPA (Million Tons Per Annum). These designs may co-exist in the coming years, as individual projects choose designs, which closely match their gas supplies, sales, and other logistical and economic constraints. The most critical components of a LNG liquefaction facility are the refrigeration compressors and their drivers which represent a significant expense and strongly influence overall plant performance and production efficiency. The refrigeration compressors themselves are challenging to design due to high Mach numbers, large volume flows, low inlet temperatures and complex sidestream flows. Drivers for these plants include gas turbines that range in size from 30 MW units to large Frame 9E gas turbines. Aeroderivative engines have also been recently introduced. This paper covers the design, application and implementation considerations pertaining to LNG plant drivers and compressors. The paper does not focus on any particular LNG process but addresses turbomachinery design and application aspects that are common to all processes. Topics cover key technical design issues and complexities involved in the turbomachinery selection, aeromechanical design, testing and implementation. The paper attempts to highlight the practical design compromises that have to be made to obtain a robust solution from a mechanical and aerodynamic standpoint