Effects of broken bars/end-ring connectors and airgapeccentricities on ohmic and core losses of induction motors in ASDsusing a coupled finite element-state space method

In this paper, effects of rotor abnormalities such as broken squirrel-cage bars, broken cage connectors and airgap eccentricity on ohmic and core losses of induction motors are presented. In this investigation, a comprehensive time-stepping coupled finite element-state space (TSCFE-SS) model was fully utilized to compute the time-domain elemental flux density waveforms and various time-domain waveforms of motor winding currents useful for core loss and ohmic loss computations. Such investigation is feasible by use of the TSCFE-SS model due to its intrinsic nature and characteristics. The results obtained from the simulations of an example 1.2-hp induction motor clearly indicate that faults due to broken squirrel-cage bars/end-connectors can increase motor core losses in comparison to the healthy case. The results also give the effect of saturation on the core loss distributions within the cross-section of the motor, and indicate the potential for possible excessive loss concentrations and consequent hot spots near zones of bar and connector breakages in the rotor     

On-line current monitoring and application of a finite elementmethod to predict the level of static airgap eccentricity in three-phaseinduction motors

The introduction reviews the real practical problems of airgap eccentricity in large 3-phase induction motors. On-line monitoring methods for diagnosing airgap eccentricity are also discussed and a state of the art review on the application of current monitoring to detect airgap eccentricity is presented. The limitations of the classical MMF and permeance wave approach for predicting the severity of airgap eccentricity are discussed. The time stepping finite element (FE) method and FFT analysis technique are used as `analyses tools' to predict the frequency components in the current (Hz and dB) as a function of static airgap eccentricity. Excellent agreement is obtained between the measured and predicted frequency components (Hz) in the current spectra which are a function of static eccentricity. The FE method is also used to predict the magnitude (dB) of these frequency components in the current spectrum with different levels of static airgap eccentricity. These predictions are much closer to the measured values in comparison to previous attempts using the classical MMF and permeance wave approach. The contents of this paper will be of particular interest to the manufacturers and industrial users of three-phase induction motors     

Induction machine field orientation along airgap and stator flux

Airgap and stator flux field orientation control (FOC) methods are discussed as an alternative to the familiar rotor flux orientation approach. The principal motivation is the relative accessibility of the two alternative fluxes. Motor flux models are developed in a unified manner, and it is shown that inherent coupling between flux and torque exists in the stator and airgap models. Two decoupling methods are presented. Simulation results are given for an airgap oriented control; the performance approaches that of a rotor flux oriented drive     

Analysis of the Double Output Induction Generator Using Direct Three-phase Model Part II - Harmonic Analysis

This paper investigates the harmonic content of the rotor current, rotor voltage and the torque of the variable speed constant frequency double output induction generator (VSCF-DOIG). The waveforms of these variables were generated using a direct three-phase model and then analyzed numerically for their harmonic content using a Fourier series subroutine. In addition, this paper investigates the stator distortion currents that are being induced in the stator windings by the rotor current harmonics at frequencies which are not an integral multiple of the stator fundamental frequency. As a check on the analysis the calculated stator current waveform is compared with the experimental oscillogram.     

Induction motor dynamic and static inductance identification usinga broadband excitation technique

The performance of indirect vector control depends upon accurate prediction of the motor slip angular frequency (ω_s) for the demand torque. The required slip gain depends upon the rotor time constant of the motor (T_r). This value varies significantly over the operating temperature range and saturation level of a typical motor. This variation, if not compensated for, results in a significant degradation in torque production from a vector control system. The saturation effect can be compensated by an adaptive flux model if precise knowledge of the induction motor magnetizing curve is available. The aim of this paper is to present the application of an advanced system identification methodology enabling the off-line estimation of the magnetizing curve (dynamic and static inductance) of induction motors     

Detection of rotor slot and other eccentricity related harmonics ina three phase induction motor with different rotor cages

Detection of rotor slot and other eccentricity related harmonics in the line current of a three phase induction motor is important both from the viewpoint of sensorless speed estimation as well as eccentricity related fault detection. However, it is now clear that not all three phase induction motors are capable of generating such harmonics in the line current. Recent research has shown that the presence of these harmonics is primarily dependent on the number of rotor slots and the number of fundamental pole pairs of the machine. While the number of fundamental pole pairs of a three phase induction motor usually is within one to four (higher pole pairs are generally avoided due to increased magnetizing current), the number of rotor slots can vary widely. The present paper investigates this phenomenon further and obtains a hitherto nebulous theoretical basis for the experimentally verified results. Detailed coupled magnetic circuit simulation results are presented for a four pole, three phase induction motor with 44, 43, and 42 rotor slots under healthy, static, dynamic and mixed eccentricity conditions. The simulation is flexible enough to accommodate other pole numbers also. These simulations are helpful in quantifying the predicted harmonics under different combinations of load, pole pair numbers, rotor slots and eccentricity conditions, thus making the problem easier for drive designers or diagnostic tools' developers. Data from three different induction machines, namely, a 4 pole, 44 bar, 3 hp, a 4 pole, 28 bar, 3 hp and a 2 pole, 39 bar, 100 hp motor have been used to verify the results experimentally. The simulation and the experimental results clearly validate the theoretical findings put forward in this paper     

A hybrid observer for high performance brushless DC motor drives

Brushless DC motor drive systems are used in a wide variety of applications. These drives may be classified as being one of two types: sinusoidal drives in which there are no low-frequency harmonics in the current waveforms and no low-frequency torque ripple; and nonsinusoidal drives in which there is considerable low-frequency harmonic content, both in the current and torque waveforms. Although sinusoidal drives feature superior performance, they are generally more expensive since rotor position must be sensed on a continuous basis, thus requiring an optical encoder or a resolver, whereas relatively inexpensive Hall-effect sensors may be used for nonsinusoidal drives. In this paper, a straightforward hybrid observer is set forth which enables rotor position to be estimated on a continuous basis using information available from the Hall-effect sensors. The proposed observer is experimentally shown to perform just as well as an optical encoder for steady-state conditions and nearly as well as the optical encoder during transient conditions. The proposed scheme provides designers with a new option for rotor position sensing, one which offers an excellent compromise between accuracy and expense     

Online and Offline Rotary Regression Analysis of Torque Estimator for Switched Reluctance Motor Drives

A new torque estimator for switched reluctance motor (SRM) drives based on 2-D rotary regression analysis is presented in this paper. The proposed torque estimator is composed of a bicubic regressive polynomial as a function of rotor position and input current. The regressive coefficients can be computed offline or online from the torque characteristics acquired either experimentally or from numerical computation. Furthermore, a torque estimation method by taking mutual coupling into consideration is proposed. It can be seen that the estimated and experimentally obtained self-coupling and mutual-coupling torque characteristics are in good agreement with each other. In addition, the dynamic torque waveforms with and without the mutual coupling, estimated by the proposed estimator, are found to be virtually the same as those obtained from the bicubic spline interpolation for SRM drives with single-pulse voltage, hysteresis current chopping, as well as with voltage pulse width modulation control. The success of all the case studies being reported is a good validation of the usefulness and accuracy of the proposed real-time torque estimator that, as described in this paper, can be used to quickly estimate the instantaneous output torque of SRM drives.     

Torque ripple analysis of a PM brushless DC motor using finite element method

Three-phase permanent magnet brushless DC motors are widely used. As a function of the rotor position, the torque produced by these machines has a pulsating component in addition to the DC component. This pulsating torque has a fundamental frequency corresponding to six pulses per electrical revolution of the motor. The shape of the torque waveform and, thus, the frequency content of the waveform can be influenced by several factors in the motor design and construction. This paper addresses the various factors that influence the torque waveshape. It is shown that in addition to the basic induced electromotive force (EMF) waveshape, the magnetic saturation in the stator core, and the accuracy in the skewing are also key factors in determining the torque waveshape. Computer simulation using finite element technique has been conducted to study the torque waveform. Simulation results successfully duplicated the torque waveforms measured in experiments under different excitation currents.     

Low torque ripple control of switched reluctance motors usingiterative learning

Torque control of the switched reluctance motor is complicated by its highly nonlinear torque-current-position characteristics. The purpose of this paper is the development of simple and efficient control algorithms for the constant torque control of switched reluctance motors. The approach consists of two distinct steps, i.e., determination of appropriate phase current waveforms for some specified torque and the subsequent generation of suitable phase voltage profiles for faithful tracking of these waveforms by the respective stator windings of the motor. At both the stages of the control design, the principles of iterative learning control have been exploited. Firstly, the desired current waveform is generated by repeated corrections from iteration to iteration starting from the conventional rectangular current profile as the initial waveform. This scheme requires much less a priori knowledge of the magnetic characteristics of the motor. In the second stage, the voltage profiles to be impressed upon the stator phases for the tracking of the desired current waveforms are learnt iteratively. Simulation results show impressive response characteristics for four-phase switched reluctance motor     

Comprehensive salient-pole synchronous machine parametric designanalysis using time-step finite element-state space modeling techniques

This paper details the application of a time-stepping coupled finite element-state space (CFE-SS) model to predict a salient-pole synchronous generator's parameters and performance, including damper bar current profiles and bar losses as well as iron core (including pole face) losses, under various operating conditions. The CFE-SS modeling environment is based on the natural ABC flux linkage frame of reference, which is coupled to a time/rotor stepping FE magnetic field and machine winding inductance profile computation model. This allows one to rigorously include the synergism between the space harmonics generated by magnetic saturation and machine magnetic circuit as well as winding layout topologies, and the time harmonics generated by the nonsinusoidal phase currents, ripple rich field excitation and damper bar currents. The impact of such synergism between these space and time harmonics on damper bar current profiles and losses, iron core losses, various machine winding current, voltage and torque profiles/waveforms is studied here for a 10-pole, 44.9 kVA, 17,143 RPM, 1428.6 Hz, 82 V (L-N), wound-pole aircraft generator     

Commutation torque ripple minimization for permanent magnetsynchronous machines with Hall effect position feedback

A permanent magnet synchronous motor (PMSM) with sinusoidal flux distribution is commonly commutated using discrete rotor position feedback from Hall sensors. A commonly used stator current excitation strategy used in such a system is a six-step current waveform. Application of sinusoidal current waveforms is shown to produce smooth torque in the PMSM. This paper shows how a pseudo-sensorless rotor position estimator may be used with Hall sensors to provide sinusoidal current excitation in place of six-step currents to reduce the torque ripple associated with the six-step strategy. Performance evaluation of the rotor position estimator in a PMSM drive is provided through simulation     

Performance analysis of a three-phase induction motor under mixed eccentricity condition

A substantial proportion of induction motor faults are eccentricity related. In practice, static and dynamic eccentricities happen to exist together. With this point in mind, an analytical approach to evaluate the performance of a three-phase induction motor under mixed eccentric conditions is presented in this paper. A clear and step-by-step theoretical analysis, explaining completely the presence of certain harmonics in the line-current spectrum in the presence of eccentricity, is discussed. More importantly, it is shown for the first time that a link exists between the low- and high-frequency elements of these harmonics. It is also shown that these high-frequency components are not very strong in all types of machine. These results will be useful in generating rules and laws to formulate online tools for machine condition monitoring. Finite-element results to substantiate the inductance values used in the simulation are also included. The analysis is validated by the line-current spectrum of the eccentric machine obtained through simulation using the modified winding-function approach (MWFA) and experimentation.     

Studies of some high solidity symmetrical and unsymmetrical blade H-Darrieus rotors with respect to starting characteristics,dynamic performances and flow physics in low wind streams

Vertical axis wind turbines can be successfully installed in low wind speed conditions but its detailed starting characteristics in terms of starting torque, starting time and dynamic performances have not been investigated thoroughly which is important for increasing the energy yield of such turbines. Amongst their designs, H-Darrieus rotor, in spite of having good power coefficient, possesses poor self-starting features as symmetrical blade profiles are used most of the times. Instead of using symmetrical blades if unsymmetrical or cambered blades are used with high solidity, then starting performance of H-Darrieus rotor along with its power coefficients can be improved. Though this performance improvement measures are correlated with improvement in the starting characteristics, a detailed work in this direction would be useful and for this reason the present work has been carried out. Three types of blade designs have been considered; two unsymmetrical blades namely S815 and EN0005 and one conventional symmetrical NACA 0018 blade, and experiments are conducted using a centrifugal blower test rig for three-bladed H-Darrieus rotors using these three considered blades at low wind streams (4 m/s, 6 m/s and 8 m/s). Considering reality, the effects of flow non-uniformity and turbulence intensity on the rotor performance at optimum condition as well as flow physics have also been studied. It has been found that unsymmetrical S815 blade rotor has higher dynamic torque and higher power coefficient than unsymmetrical EN0005 and symmetrical NACA 0018 blade H-Darrieus rotors.     

Advanced fault diagnosis of a DC motor

This paper presents a model of the DCc motor with an eccentric rotor. The winding function theory shows the effect of eccentricity fault on the motor inductances and the simulation is done using a nonsymmetric air-gap function. A modified equation is presented to show the existence of rotor slot harmonics in the DC motor current. To detect the eccentricity fault, a pattern recognition technique is utilized. The proposed algorithm works at steady state and uses armature current as input. The rotor speed is needed in order to provide the appropriate feature for the classifier. Therefore, rotor speed is estimated from the armature current using the commutation harmonics. The experimental results obtained from a 1/3-hp shunt DC motor verifies the proposed method. In order to cover different motor conditions, data are collected at different shaft speeds for both a healthy dc motor and a dc motor with an unbalanced load which exhibits static eccentricity.     

Performance tests on helical Savonius rotors

Conventional Savonius rotors have high coefficient of static torque at certain rotor angles and a negative coefficient of static torque from 135° to 165° and from 315° to 345° in one cycle of 360°. In order to decrease this variation in static torque from 0° to 360°, a helical Savonius rotor with a twist of 90° is proposed. In this study, tests on helical Savonius rotors are conducted in an open jet wind tunnel. Coefficient of static torque, coefficient of torque and coefficient of power for each helical Savonius rotor are measured. The performance of helical rotor with shaft between the end plates and helical rotor without shaft between the end plates at different overlap ratios namely 0.0, 0.1 and 0.16 is compared. Helical Savonius rotor without shaft is also compared with the performance of the conventional Savonius rotor. The results indicate that all the helical Savonius rotors have positive coefficient of static torque at all the rotor angles. The helical rotors with shaft have lower coefficient of power than the helical rotors without shaft. Helical rotor without shaft at an overlap ratio of 0.0 and an aspect ratio of 0.88 is found to have almost the same coefficient of power when compared with the conventional Savonius rotor. Correlation for coefficient of torque and power is developed for helical Savonius rotor for a range of Reynolds numbers studied.     

Modeling of effects of skewing of rotor mounted permanent magnetson the performance of brushless DC motors

A method for computation of the parameters and performance of permanent-magnet brushless DC motor drives is developed in which the concept of skewing is implemented through the geometries of permanent magnet mounting on the rotor and not through the usual skewing of the armature slots. This technique of permanent-magnet mounting eliminates the 2-D axial symmetry in the resulting magnetic fields. This difficulty is overcome by the use of multiple cross-sectional 2-D finite-element field computations, coupled with a concept of an artificial mutual-coupling inductance between the armature phase windings and the rotor-mounted permanent magnets for induced EMF and torque computations. The computed induced EMF waveforms, motor phase winding current waveforms, and other performance characteristics are found to be in excellent agreement with test data obtained using a 1.2 hp, 120 V brushless DC motor drive system     

水轮发电机转子典型机电故障的不平衡磁拉力研究

针对水轮发电机发生转子偏心和转子绕组匝间短路故障产生不平衡磁拉力,进而引起机组强烈振动的情况,通过有限元仿真软件建立一台550MW水轮发电机二维仿真模型,分别对转子动、静偏心故障及转子绕组匝间短路故障进行模拟,计算各故障情况下的不平衡磁拉力,分析转子偏心程度、方向和转子绕组匝间短路程度对不平衡磁拉力大小和方向的影响。结果表明,不论是发生转子偏心故障还是发生转子绕组匝间短路故障,不平衡磁拉力数值均随故障程度的加重而增大,且不平衡磁拉力方向为诊断转子偏心方向和空载情况下匝间短路故障磁极提供帮助。研究成果为未来研究水轮发电机振动现象和故障检测奠定了基础。     

Instantaneous Shaft Radial Force Control with Sinusoidal Excitations for Switched Reluctance Motors

Unbalanced radial forces acting on a rotor shaft exist in motor applications where the external load is not balanced or when the rotor is not centered causing a nonuniform air gap. These forces are undesirable as they cause motor vibrations. In view of its special structure, the shaft radial force and the torque of a three-phase 12/8 pole switched reluctance motor (SRM) can be separately controlled by proper pole current selection in the energized phase. Therefore, radial forces can be produced intentionally to cancel the existing radial force produced by rotor eccentricity and the unbalanced load inertia. The motor vibrations are thereby reduced. In this paper, a sinusoidal current excitation scheme is proposed for the torque and radial force control of a 12/8 pole SRM. When controlled with the selected sinusoidal currents, the SRM can simultaneously produce the desired shaft radial force in any rotational plane direction and the required rotational torque. As all pole currents are individually controlled, a more sophisticated phase commutation strategy is also proposed that provides smoother torques and radial force ripples.     

The use of a curtain design to increase the performance level of a Savonius wind rotors

In this study, a curtain design has been arranged so as to improve the low performance levels of the Savonius wind rotors. Designed to prevent the negative torque on the convex blade of the rotor, this curtain has been placed in front of the rotor, and performance experiments have been carried out when the rotor is with and without curtain. It has been determined from here that a significant increase can be achieved in the rotor performance by means of the curtain design. Experiments of the curtain design have been conducted in three different dimensions when the Savonius wind rotor is static, and the highest values have been obtained with the curtain 1. Therefore, the curtain designs and curtain angles in which the highest values obtained have been analyzed numerically with Fluent 6.0 program and the results obtained experimentally have been supported with numerical analysis. Moreover, performance experiments have been made for the curtain 1 with which the best performance values have been obtained when the rotor is in its dynamic position, and the results obtained have been given in figures.