Multiple Reference Frames Theory: A New Method for the Diagnosis of Stator Faults in Three-Phase Induction Motors

This paper proposes the use of the multiple reference frames theory for the diagnosis of stator faults in three-phase induction motors. The development of a simplified mathematical motor model allowed the establishment of the equivalent circuits of the motor, in$d!!-!!q!!-!!0$axes, in the presence of stator interturn short circuits. The use of the stationary reference frame, clockwise and counterclockwise synchronous reference frames, allows the extraction and manipulation of the information contained in the motor supply currents in a way that the effects introduced by the fault are easily isolated and measured. A severity factor is defined and the simulation and experimental results presented demonstrate its independence in relation to the working conditions of the motor, such as the load level and unbalances in the voltage supply system. Although the technique is here introduced for the diagnosis of stator faults, it is possible to extend its use for the diagnosis of other asymmetries such as broken rotor bars and air-gap eccentricity.     

Long-shunt and short-shunt connections on dynamic performance of aSEIG feeding an induction motor load

This paper presents a comparative study of long-shunt and short-shunt configurations on dynamic performance of an isolated self-excited induction generator (SEIG) feeding an induction motor (IM) load. The studied IM is suddenly connected to the output terminals of the studied configurations of the SEIG. Both simulated results and experimental results based on laboratory 1.1 kW induction machines are clearly compared to examine the effects of both connections on voltage variation of the studied SEIG. The analyzed results show that the long-shunt configuration may lead to unwanted oscillations while the short-shunt connection provides better voltage variation. The eigenvalue technique is also employed to examine the possible voltage collapse and unstable conditions in the studied SEIG-IM system     

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     

Speed control of a three-phase induction motor based on robust optimal preview control theory

A synthesized method for speed control of a three-phase induction motor (IM) based on optimal preview control system theory is implemented in this article. An IM model comprises three-input variables and three-output variables that coincide with the synchronous reference frame that is implemented using the vector method. The input variables of this model are the stator angular frequency and the two components of the stator space voltage vector, whereas the output variables are the rotor angular speed and the two components of the stator space flux linkage. The objective of the synthesized control system is to achieve motor speed control, field orientation control, and constant flux control. A novel error system is derived and introduced into the control law to increase the robustness of the system. The preview feed-forward controller, which includes the desired and disturbance signals, is used to improve the transient response of the system. A space vector pulse-width modulation (PWM) control technique for voltage source-fed IM is prepared for microprocessor-based control. Spectral analysis of the output voltage is evaluated to predict the effect of the proposed space vector modulation technique on the dynamic performance of the IM. The optimal preview controlled system is implemented, and its applicability and robustness are demonstrated by computer simulation and experimental results.     

An Unsymmetrical Two-Phase Induction Motor Drive With Slip-Frequency Control

This paper proposes a closed-loop control strategy to operate an off-the-shelf single-phase induction motor (IM) as a symmetrical two-phase IM. The proposed control strategy employs the SFC technique to independently control the stator currents of both the main and auxiliary windings, and make them follow a predefined sinusoidal waveform. Simulation and experimental results show that the proposed scheme is successful in operating the conventional single-phase IM as a symmetrical two-phase IM with fast dynamic and transient responses. In addition, the proposed control system achieves cost-effectiveness in both initial and running costs.      

A Complete Time Domain Model of the Induction Motor for Efficiency Evaluation

The saturation of mutual and leakage inductances as well as the eddy currents effects are taken into account to represent an almost complete model of the induction motor. These effects are not trivial, when the stator voltage and/or frequency changes considerably. Therefore, the model is suitable for the motor performance studies under nonsinusoidal voltage waveform supplies. The effects of eddy currents are represented by a simple double cage model equivalent to the deep bar cage. A practical method for finding the double cage parameters is outlined and the results obtained were used in the simulation. The accuracy of the model is emphasized by comparing the simulation results with test results of the stator inrush current during starting under no-load conditions. The model is used for calculating the induction motor losses in the time domain, when the motor is fed from nonsinusoidal voltage waveform supplies. Samples of the obtained results from different voltage waveforms are given together with that of a pure sinusoidal voltage supply for comparison.     

Modeling of Saturated Induction Machines With Injected High-Frequency Signals

This paper analyzes the effects of injecting additional signals in induction machines for the purpose of speed control. A new saturation model able to correctly model the interaction between the added signal and saturation of the motor core due to the main torque-producing flux is presented. The introduction of a variable saturation factor is used to model the variation of the saturation level due to the additional signal. A third harmonic rotor circuit is also introduced to take account of the third harmonic component of the air-gap flux, due to saturation. An additional balanced voltage set is added to the normal supply to analyze the effects of such signals on a saturated induction motor. Simulation results of such a model both at no load and full load are presented together with experimental measurements.     

Laboratory implementation of a neural network trajectory controllerfor a DC motor

The laboratory implementation of a neural network controller for high performance DC drives is described. The objective is to control the rotor speed and/or position to follow an arbitrarily selected trajectory at all times. The control strategy is based on indirect model reference adaptive control (MRAC). The motor characteristics are explicitly identified through a multilayer perceptron type neural network. The output of the trained neural network is used to drive the motor in order to achieve a desired time trajectory of the controlled variable. The neural network controller is assembled in a commercially available PC-based real-time control system shell, using software subroutines. An H-bridge, DC/DC voltage converter is interfaced with the computer to generate the specified terminal voltage sequences for driving the motor. All software and hardware components are off the shelf. The versatility of the motor/controller arrangement is displayed through real-time plots of the controlled states     

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.     

Characteristics and Performance Analysis of a Permanent-Magnet Motor with a Multistacked Imbricated Rotor

The paper studies the parameters and performance of a motor with a multistacked imbricated permanent magnet rotor. An experimental motor with ferrites for excitation is used in the investigation. Methods used in the determination of the parameters, Xd, Xq and the core-loss component, Rc, are described. Their variation with load and terminal voltage are also discussed. These parameters are applied in a model based on the classical two-axis machine theory for performance analysis. Detailed investigation shows that the reactance characteristics under steady-state conditions are similar to a wound rotor machine. Computed results using these values of Xd, Xq and Rc show agreement with experimental results. The significance of Rc is noted.     

Analyses and compensation of rotor position detection error in sensorless PM brushless DC motor drives

In a permanent-magnet (PM) brushless DC motor, the waveform of back electromotive force (EMF) is related to the rotor position; hence, the back EMF can be used for position sensorless control. However, in practical implementation, the terminal voltage or phase voltage is used instead, as the back EMF is difficult to be sensed directly. Thus, detection error of the rotor position can occur. This paper documents the calculations and analyses on the detection error and the motor commutation angle, and presents an error compensation circuit to ensure proper commutation. Finite-element field simulation and experimental results are also given to verify the calculations as well as the compensation circuit.     

汽轮机转子涡动汽流激振力分析与CFD数值模拟

汽轮机转子涡动时轴心偏离静子中心产生轴系失稳的Thomas/Alford汽流激振力,传统的叶顶间隙激振力公式对此不能全面准确评估。该文综合考虑转子涡动以及围带汽封二次流,在动叶通道,根据蒸汽做功分析涡动效应激振力;在叶顶围带汽封,用CFD数值模拟泄漏蒸汽三维粘性流场,确定蒸汽激振力。研究结果表明小的静偏心和动偏心条件下,转子涡动动偏心在动叶通道诱发的激振力要大于静偏心激振力;围带汽封汽流预旋速度对间隙激振力有重要影响;调门不对称进汽也是蒸汽激振力的另一个重要来源。     

偏心度与间隙比对轴流泵流动激励力的影响

叶轮偏心将会引起轴流泵叶轮顶部间隙沿周向不均匀,从而导致附加的流动激励和噪声.利用计算流体力学商业软件Fluent模拟不同偏心度和间隙比的轴流泵非定常流场,研究周向非均匀间隙对泵内压强脉动及叶轮激励力的影响.结果表明:壁面压强脉动强度并不是在最小间隙处达到最大,而是出现在偏离最小间隙处约30°~60°的方向,并随着间隙比增大,脉动强度最大值发生位置逐渐向最小间隙处靠拢;同时,偏心造成的非均匀叶顶间隙引起了轴频处的压强脉动,在间隙比为2.5%下偏心度从0%增加到60%,轴频处的压强脉动相对增量为661.54%;叶轮受到的径向激励力时均值与偏心度呈线性关系,其斜率与间隙比的平方根成正比.     

A solid state direct on line starter for medium voltage inductionmotors with minimized current and torque pulsations

This paper describes a thyristorized static switch which is designed and implemented specifically for frequent direct-on-line starting of medium voltage large induction motors. The circuit configuration chosen utilizes back-to-back connected series thyristor strings on two of the supply lines. Both the starting current and the torque pulsations of the induction motor are minimized by a controlled, nonsimultaneous switching of motor phases to the supply. This reduces the electrical stresses on the power system and the mechanical stresses on the shaft as compared to conventional switching elements. The optimum switching strategy is found by the use of a hybrid mathematical model in ABC/dqO form. The validity of the results obtained for medium voltage, squirrel-cage induction machine are verified by the use of a physical simulator consisting of a low voltage, small induction machine and a shaft torque measuring system. The system has been in operation successfully in an iron and steel plant since 1991     

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     

Comparison between characterization and diagnosis of brokenbars/end-ring connectors and airgap eccentricities of induction motorsin ASDs using a coupled finite element-state space method

This paper describes how a rigorous and comprehensive time-stepping coupled finite element-state space (TSCPE-SS) modeling technique can be utilized in diagnostics and differentiation between induction motor rotor (cage) abnormalities of broken bars/connectors and airgap eccentricities. The model is used for the computation of time-domain performance characteristics, such as the stator phase current waveforms and developed torque profiles including these abnormalities. This is followed by analysis of the current waveforms and torque profiles using fast Fourier transform to obtain their corresponding frequency spectra. Comparison between the TSCFE-SS model's simulation results, which correlate very well with theoretical results, clearly illustrate that rotor bar and/or end-ring connector breakages can be distinguished from static and dynamic airgap eccentricities. This paper also gives an interesting comparison between the effects and implications of these various rotor abnormalities on machine parameters and performance characteristics. Furthermore, the results indicate that frequency components reported earlier to be produced only by the combined effects of static and dynamic airgap eccentricity could be observed in case of either static or dynamic eccentricity. Finally, this paper demonstrates the possible opportunities that can be made use of in noninvasive detection of airgap eccentricities via TSCFE-SS and current signature techniques     

Improved SMS islanding detection method for grid-connected converters

Islanding detection is a mandatory function for grid-connected converters. The popular slip mode frequency shift (SMS) and auto phase shift active islanding detection methods are investigated and an improved (IM)-SMS strategy is proposed in this study. In the proposed method, additional phase shift is introduced to help in stimulating the action of the islanding detection and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection and optimisation is also provided. The islanding detection performance is evaluated through theoretical analysis and verified by digital simulation and experimental results. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability.     

Induction motor stator faults diagnosis by a current Concordia pattern-based fuzzy decision system

This paper deals with the problem of detection and diagnosis of induction motor faults. Using the fuzzy logic strategy, a better understanding of heuristics underlying the motor faults detection and diagnosis process can be achieved. The proposed fuzzy approach is based on the stator current Concordia patterns. Induction motor stator currents are measured, recorded, and used for Concordia patterns computation under different operating conditions, particularly for different load levels. Experimental results are presented in terms of accuracy in the detection of motor faults and knowledge extraction feasibility. The preliminary results show that the proposed fuzzy approach can be used for accurate stator fault diagnosis if the input data are processed in an advantageous way, which is the case of the Concordia patterns.     

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     

Multi-pole permanent magnet synchronous generator wind turbines' grid support capability in uninterrupted operation during grid faults

Emphasis in this paper is on the fault ride-through and grid support capabilities of multi-pole permanent magnet synchronous generator (PMSG) wind turbines with a full-scale frequency converter. These wind turbines are announced to be very attractive, especially for large offshore wind farms. A control strategy is presented, which enhances the fault ride-through and voltage support capability of such wind turbines during grid faults. Its design has special focus on power converters' protection and voltage control aspects. The performance of the presented control strategy is assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a PMSG wind farm equipped with an additional voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride-through control strategy in the active stall wind farm.