Electrical Signature Analysis of Synchronous Motors Under Some Mechanical Anomalies

Electrical Signature Analysis (ESA) has been introduced for some time to investigate the electrical anomalies of electric machines, especially for induction motors. More recently hints of using such an approach to analyze mechanical anomalies have appeared in the literature. Among them, some articles cover synchronous motors usually being employed to improve the power factor, drive green vehicles and reciprocating compressors or pumps with higher efficiency. Similarly with induction motors, the common mechanical anomalies of synchronous motor being analyzed using the ESA are air-gap eccentricity and single point bearing defects. However torsional effects, which are usually induced by torsional vibration of rotors and by generalized roughness bearing defects, have seldom been investigated using the ESA. This work presents an analytical method for ESA of rotor torsional vibration and an experimentally demonstrated approach for ESA of generalized roughness bearing defects. The torsional vibration of a shaft assembly usually induces rotor speed fluctuations resulting from the excitations in the electromagnetic (EM) or load torque. Actually, there is strong coupling within the system which is dynamically dependent on the interactions between the electromagnetic air-gap torque of the synchronous machine and the rotordynamics of the rotor shaft assembly. Typically this problem is solved as a one-way coupling by the unidirectional load transfer method, which is based on predetermined or assumed EM or load profile. It ignores the two-way interactions, especially during a start-up transient. In this work, a coupled equivalent circuit method is applied to reflect this coupling, and the simulation results show the significance of the proposed method by the practical case study of Electric Submersible Pump (ESP) system. The generalized roughness bearing anomaly is linked to load torque ripples which can cause speed oscillations, while being related to current signature by phase modulation. Considering that the induced characteristic signature is usually subtle broadband changes in the current spectrum, this signature is easily affected by input power quality variations, machine manufacturing imperfections and the interaction of both. A signal segmentation technique is introduced to isolate the influence of these disturbances and improve the effectiveness of applying the ESA for this kind of bearing defects. Furthermore, an improved experimental procedure is employed to closely resemble naturally occurring degradation of bearing, while isolating the influence of shaft currents on the signature of bearing defects during the experiments. The results show that the proposed method is effective in analyzing the generalized roughness bearing anomaly in synchronous motors

Electromagnetic forces acting between the stator and eccentric cage rotor

Electromagnetic forces act between the rotor and stator when the rotor is performing eccentric motions with respect to the stator. The aim of this research was to study the charactarestics of the forces and develop the tools to calculate these forces accurately and as quickly as possible. A new method, called the impulse method, is developed into the finite element analysis of the electromagnetic field to calculate the forces for a wide whirling frequency range by one simulation. The idea of the impulse method is to move the rotor from its central position for a short period of time. This displacement excitation disturbs the magnetic field and, by doing this, produces forces between the rotor and stator. Using spectral analysis techniques, the frequency response function of the forces is calculated using the excitation and response signals. The impulse method is based on the assumption of the spatial linearity of the force. The impulse method is utilised in the analysis of the rotor eccentricity. The spatial linearity of the force and the effects of the circulating currents and saturation on the forces are studied herein. The field of investigation is enlarged from the cylindrical whirling motion to the conical motions of the rotor. The modelling of the conical motion requires that the axial variations of the magnetic field be taken into account. This is done by multislice finite element analysis.reviewe

Finite Element Analysis of Motor Eccentric Forces and Effects on Vibration

Machinery trains with motor drives are widely used in industry. Motor related forces may become one of the sources of machinery vibrations. Motor eccentricity is one of the common phenomena that cause forces exerted on motor and the machinery train. The eccentricity between stator and rotor is almost inevitable. Mass unbalance, shaft bending as well as bearing tolerances can introduce the eccentricity. Moreover, modern machines are being designed for higher performance, which often cause significant nonlinear effects. Therefore, a more complete picture of the nonlinear dynamic characteristics brought by motor eccentricity is required to enhance machinery train design, refinement, monitoring and maintenance. The purpose of the work has been to develop accurate modeling methods of electromagnetic forces, especially at static eccentricity fault, and to study the force effects on driven machinery vibrations. This research work focuses on the characterization of the eccentric forces and their effects of the rotor eccentricity fault on vibration. The most accurate numerical method of finite element method is adopted for field analysis in calculation of characteristics including air gap flux density, eccentric force and torques. An improved movement modeling technique is proposed and developed into the finite element analysis of electric machines

Integrating rotordynamic and electromagnetic dynamic models for flexible-rotor electrical machines

The magnetic field within electrical machines causes an interaction between the electrical and mechanical dynamics of the system. In the simplest cases, when the rotor mean position is central in the stator, the interaction manifests itself mainly as a negative stiffness between the rotor and the stator. When the rotor mean position is offset relative to the stator, then components of force arise whose frequency in the stationary frame is twice the electrical frequency of the supply. For induction machines in particular, both the electrical system and the mechanical system may be quite complex dynamically in the sense that over the range of frequencies of interest, it is necessary to consider a number of degrees of freedom in both the electrical part of the model and the mechanical part. This work sets out a structured and formal approach to the preparation of such models. Each different combination of voltage and slip is examined separately. In each case, the first step is to compute the steady-state reference solution for machine currents as a function of time. Then, the electro-magnetic behaviour of the electrical machine is linearised around that reference solution. The result is a linear time-dependent model for the electromagnetic behaviour which is then easily coupled with a linear model for the mechanical dynamics. The mechanical dynamics are usually stationary. Floquet methods can then be applied to determine whether the system is stable and the response of the system to mechanical or electrical perturbations can be computed quickly. The analysis method is applied to a particular three-phase induction machine which has parallel paths integrated into its winding structure in the sense that each of the phases is split into a "Wheatstone-bridge" arrangement following. Currents passing diametrically through a phase in the vertical direction account for the main torque-producing components of stator field. Currents passing diametrically through the phase in the horizontal direction account for transverse forces. The parallel paths can be switched to open-circuit or closed-circuit without affecting the torque-producing function of the machine and all of the stator conductors contribute to torque-production.For a number of combinations of voltage and slip, the machine is stable irrespective of whether the parallel paths are open-circuit or not but the effective damping of the machine for synchronous vibration is shown to be much higher with the parallel paths in closed-circuit

Integrating rotordynamic and electromagnetic dynamic models for flexible-rotor electrical machines

The magnetic field within electrical machines causes an interaction between the electrical and mechanical dynamics of the system. In the simplest cases, when the rotor mean position is central in the stator, the interaction manifests itself mainly as a negative stiffness between the rotor and the stator. When the rotor mean position is offset relative to the stator, then components of force arise whose frequency in the stationary frame is twice the electrical frequency of the supply. For induction machines in particular, both the electrical system and the mechanical system may be quite complex dynamically in the sense that over the range of frequencies of interest, it is necessary to consider a number of degrees of freedom in both the electrical part of the model and the mechanical part. This work sets out a structured and formal approach to the preparation of such models. Each different combination of voltage and slip is examined separately. In each case, the first step is to compute the steady-state reference solution for machine currents as a function of time. Then, the electro-magnetic behaviour of the electrical machine is linearised around that reference solution. The result is a linear time-dependent model for the electromagnetic behaviour which is then easily coupled with a linear model for the mechanical dynamics. The mechanical dynamics are usually stationary. Floquet methods can then be applied to determine whether the system is stable and the response of the system to mechanical or electrical perturbations can be computed quickly. The analysis method is applied to a particular three-phase induction machine which has parallel paths integrated into its winding structure in the sense that each of the phases is split into a "Wheatstone-bridge" arrangement following. Currents passing diametrically through a phase in the vertical direction account for the main torque-producing components of stator field. Currents passing diametrically through the phase in the horizontal direction account for transverse forces. The parallel paths can be switched to open-circuit or closed-circuit without affecting the torque-producing function of the machine and all of the stator conductors contribute to torque-production.For a number of combinations of voltage and slip, the machine is stable irrespective of whether the parallel paths are open-circuit or not but the effective damping of the machine for synchronous vibration is shown to be much higher with the parallel paths in closed-circuit

SIRM 2017

This volume contains selected papers presented at the 12th International Conference on vibrations in rotating machines, SIRM, which took place February 15-17, 2017 at the campus of the Graz University of Technology. By all meaningful measures, SIRM was a great success, attracting about 120 participants (ranging from senior colleagues to graduate students) from 14 countries. Latest trends in theoretical research, development, design and machine maintenance have been discussed between machine manufacturers, machine operators and scientific representatives in the field of rotor dynamics. SIRM 2017 included thematic sessions on the following topics: Rotordynamics, Stability, Friction, Monitoring, Electrical Machines, Torsional Vibrations, Blade Vibrations, Balancing, Parametric Excitation, and Bearings. The papers struck an admirable balance between theory, analysis, computation and experiment, thus contributing a richly diverse set of perspectives and methods to the audience of the conference

Winding Tensor Approach for the Analytical Computation of the Inductance Matrix in Eccentric Induction Machines

[EN] Induction machines (IMs) are critical components of many industrial processes, what justifies the use of condition-based maintenance (CBM) systems for detecting their faults at an early stage, in order to avoid costly breakdowns of production lines. The development of CBM systems for IMs relies on the use of fast models that can accurately simulate the machine in faulty conditions. In particular, IM models must be able to reproduce the characteristic harmonics that the IM faults impress in the spatial waves of the air gap magneto-motive force (MMF), due to the complex interactions between spatial and time harmonics. A common type of fault is the eccentricity of the rotor core, which provokes an unbalanced magnetic pull, and can lead to destructive rotor-stator rub. Models developed using the finite element method (FEM) can achieve the required accuracy, but their high computational costs hinder their use in online CBM systems. Analytical models are much faster, but they need an inductance matrix that takes into account the asymmetries generated by the eccentricity fault. Building the inductance matrix for eccentric IMs using traditional techniques, such as the winding function approach (WFA), is a highly complex task, because these functions depend on the combined effect of the winding layout and of the air gap asymmetry. In this paper, a novel method for the fast and simple computation of the inductance matrix for eccentric IMs is presented, which decouples the influence of the air gap asymmetry and of the winding configuration using two independent tensors. It is based on the construction of a primitive inductance tensor, which formulates the eccentricity fault using single conductors as the simplest reference frame; and a winding tensor that converts it into the inductance matrix of a particular machine, taking into account the configuration of the windings. The proposed approach applies routine procedures from tensor algebra for performing such transformation in a simple way. It is theoretically explained and experimentally validated with a commercial induction motor with a mixed eccentricity fault.This research was funded by the Spanish "Ministerio de Ciencia, Innovacion y Universidades (MCIU)", the "Agencia Estatal de Investigacion (AEI)" and the "Fondo Europeo de Desarrollo Regional (FEDER)" in the framework of the "Proyectos I+D+i -Retos Investigacion 2018", project reference RTI2018-102175-B-I00 (MCIU/AEI/FEDER, UE).Martinez-Roman, J.; Puche-Panadero, R.; Sapena-Bano, A.; Pineda-Sanchez, M.; Pérez-Cruz, J.; Riera-Guasp, M. (2020). Winding Tensor Approach for the Analytical Computation of the Inductance Matrix in Eccentric Induction Machines. Sensors. 20(11):1-25. https://doi.org/10.3390/s20113058S125201

12th International Conference on Vibrations in Rotating Machinery

Since 1976, the Vibrations in Rotating Machinery conferences have successfully brought industry and academia together to advance state-of-the-art research in dynamics of rotating machinery. 12th International Conference on Vibrations in Rotating Machinery contains contributions presented at the 12th edition of the conference, from industrial and academic experts from different countries. The book discusses the challenges in rotor-dynamics, rub, whirl, instability and more. The topics addressed include: - Active, smart vibration control - Rotor balancing, dynamics, and smart rotors - Bearings and seals - Noise vibration and harshness - Active and passive damping - Applications: wind turbines, steam turbines, gas turbines, compressors - Joints and couplings - Challenging performance boundaries of rotating machines - High power density machines - Electrical machines for aerospace - Management of extreme events - Active machines - Electric supercharging - Blades and bladed assemblies (forced response, flutter, mistuning) - Fault detection and condition monitoring - Rub, whirl and instability - Torsional vibration Providing the latest research and useful guidance, 12th International Conference on Vibrations in Rotating Machinery aims at those from industry or academia that are involved in transport, power, process, medical engineering, manufacturing or construction

Electromagnetic Effects On The Torsional Natural Frequencies Of An Induction Motor Driven Reciprocating Compressor With A Soft Coupling

LectureThe electromagnetic field in the air gap of an electric motor is responsible for producing torque between the rotor and stator. The analysis and design of motor-driven equipment trains can be improved by including this electromagnetic (EM) effect. Torsional vibration or unsteady conditions from a reciprocating compressor are superimposed over the steady-state operation of an induction motor. In the past, these phenomena were usually neglected as typical analytic methods were unavailable for the prediction of these special effects. In some cases, not accounting for EM influence leads to substantial errors in the torsional vibration analysis (TVA). Torsional vibration data were obtained on a motor driven reciprocating compressor system with a torsionally soft rubber coupling. This paper shows how the torsional stiffening effect of the electromagnetic field can affect the torsional natural frequencies (TNFs) of a compressor system that utilizes a torsionally soft coupling. The torsional measurements documented herein show that a shift in the first TNF occurred due to the effective torsional spring to ground related to EM. The torsional data confirmed that this effect is significant for a motor-driven reciprocating compressor system with a torsionally soft coupling. Comparisons of the field data are then made with theoretical predications of the TNFs with and without EM effects. A relatively simple methodology for calculating torsional stiffness and damping of EM is shown and yielded good correlation with the measured data. The accurate prediction of all dynamic characteristics of the system becomes more important when the motor is controlled by a variable frequency drive (VFD) over a large speed range so that dangerous torsional resonances can be avoided. As a result of the torsional measurements, the minimum operating speed of this compressor system was increased to provide a sufficient separation margin (SM) from the TNF as recommended by American Petroleum Institute, API 618 for reciprocating compressors [1]. This was accomplished by reprogramming the VFD in the field. To minimize dynamic torque in the rubber coupling, it was also recommended that operating the reciprocating compressor with single-acting cylinders be avoided since such operation produced higher dynamic torque at 1× running speed and excited a torsional natural frequency of the system. In the future, the EM effect should be included in torsional analyses, especially for motor-driven reciprocating compressors with soft couplings