Transient Behavior of Induction Motor Flux and Torque During Run-Up

This paper analyses the various flux components that exist within symmetrical three phase cage induction motors, and how they manifest themselves as torque oscillations during run-up. A small, a medium and a large motor are evaluated to show that recent explanations of this phenomena by way of only the airgap flux may be misleading due to a lack of generally applying to motors of different sizes and parameter combinations.     

Capacitance requirements of a three-phase induction generatorself-excited with a single capacitance and supplying a single-phase load

This paper presents a practical method for computing the minimum capacitance required to initiate voltage build-up in a three-phase induction generator self-excited with a single capacitance and supplying a single-phase load. Attention is focused on the Steinmetz connection which gives superior performance over the plain single-phasing mode of operation. From a consideration of the input impedance of the induction generator and the self-excitation conditions, two nonlinear equations are obtained. Solution of one equation using the Secant method enables the excitation frequency to be determined, and the minimum excitation capacitance can be calculated from the second equation. This solution technique is subsequently employed in an iterative procedure for computing the capacitance required to maintain the terminal voltage at a preset value when the generator is supplying load. Experimental results obtained on a 2-kW induction machine are presented to verify the theoretical analysis where possible     

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     

New approach to determine the critical capacitance for self-excitedinduction generators

This paper presents a new simple approach for computing the minimum value of capacitance necessary to initiate the self-excitation process in three-phase isolated induction generators. The method proposed in this paper is based on the analysis of the complex impedance matrix of the induction generator when loaded with a general inductive load. The advantage of this method is its simplicity since it involves simple algebraic equations and only one equation is solved iteratively to get the value of minimum capacitance. A simple computer algorithm has been developed to predict the minimum value of capacitance necessary for the onset of self-excitation. Using the same approach, the algorithm is modified to predict the minimum value of capacitance necessary to maintain the generator terminal voltage at a preset value under specific load and speed conditions. Computer simulations obtained using the proposed method are compared with those obtained experimentally to confirm the validity and accuracy of the proposed method     

A New Longitudinal End Effect Factor for Linear Induction Motors

The longitudinal end effect in a linear induction motor (LIM) is analysed assuming two travelling magnetic flux density waves in the airgap: the wave travelling with synchronous velocity and the wave representing end effect [1]. A simple equation for the end effect factor which modifies the airgap EMF is obtained, and a simple equivalent circuit incorporating the end effect factor is established. Analysis is compared with measurements from two largescale single-sided LIM's. The equation for the end effect factor is sufficiently accurate to be used for design studies of LIMs. Since end effects are negligible at low speeds, the end effect factor has application for medium and high-speed LIMs.     

Performance optimization of a photovoltaic induction motor pumping system

The performances of a photovoltaic pumping system based on an induction motor are degraded once insolation varies far from the value called nominal, where the system was sized. To surmount this handicap, an improvement of these performances by the optimization of the motor efficiency is described in this paper. The results obtained are compared with those of similar work pieces presented in the literature where the motor effeciency and air gap flux where optimized separatly. The simulation results show that the proposed system allows at the same time to combine the performances of the system with constant efficiency and the simplicity of implementation provided by the system with constant airgap flux.     

The transient and qualitative performance of a self-excitedsingle-phase induction generator

The modeling and transient performance of a single-phase induction generator with series or parallel connected load is the theme of this paper. The system of equations are expressed in terms of flux linkages and includes the effect of magnetizing flux linkage saturation. Generator self-excitation and voltage collapse phenomena are simulated. The balance of the paper deals with the qualitative behavior of the generator using concepts of harmonic balance and system bifurcation     

中压静态转换开关切换冲击电流抑制方法

静态转换开关是实现双电源供电场合下电源之间相互切换的一种有效措施,在中压静态转换开关带变压器负荷系统中,开关切换可能使变压器铁芯产生直流磁链分量,导致铁芯磁饱和,从而导致冲击电流。对此,首先分析了切换电源后直流磁链分量的构成,重点分析了两路电源相位差对直流磁链分量的影响;然后提出了一种基于磁链监测的消除直流磁链分量的切换策略,防止由于直流偏磁导致的变压器铁芯饱和,从而消除切换冲击电流;最终通过多种工况的仿真算例,验证了提出的冲击电流抑制方法的有效性。     

Transient Behavior of the Resultant Airgap Field During Run-Up of an Induction Motor

The present paper deals with the transient behavior of the main airgap field of an induction machine starting from rest. This is often visualized as a revolving field having constant steady-state speed and direction. In this paper the results obtained show the complex movement of the field as a space vector. It is shown that the transient airgap field has a variable speed and amplitude and sometimes moves in the reverse direction. The results are used to explain the occurrence of transient torque reversals and motor operation over synchronous speed.     

An improved control method of buried-type IPM bearingless motors considering magnetic saturation and magnetic pull variation

An improved control method of the buried-type interior permanent magnet (IPM) bearingless motors has been presented. It is shown that the conventional method is not applicable to this type of IPM bearingless motor for loaded conditions. In IPM bearingless motors, the armature reaction flux is present due to high magnetic permeance with thin permanent magnets and small airgap length. An increase in d-axis flux linkage is caused by armature reaction as a torque-component flux is increased. Thus, it is likely that magnetic saturation occurs in the stator teeth. In addition, a magnetic attractive force caused by the displacement force factor is dependent on the armature reaction flux. A new decoupling controller for the IPM bearingless motor considering magnetic saturation is proposed in this paper. It also considers the influence of magnetic attractive force variations. In addition, a new parameter identification method for the decoupling controller is also proposed. The new controller is found quite suitable to realize successful stable operation of the experimental IPM bearingless motor.     

Thermal load levelling in a multilayered wall/roof

An explicit expression for the periodic variation of thermal flux through a multilayered insulated hollow wall/roof, whose one face is exposed to solar radiation and ambient air and the other is in contact with room air at constant temperature, has been derived. to obtain optimum placement of insulation and airgap, numerical calculations for the heat flux through a multilayered insulated hollow wall/roof have been made for a typical hot day (26 May 1978) in Delhi. It is seen that for a given total thickness of concrete, best load levelling is achieved when the thickness of the outer layer is as small as possible. the effect of a water film on heat flux into the buidling has also been discussed.     

Analytical derivation of a coupled-circuit model of a claw-polealternator with concentrated stator windings

A lumped-parameter coupled-circuit model of a claw-pole alternator is derived. To derive the model, analytical techniques are used to define a three-dimensional (3-D) Fourier-series representation of the airgap flux density. Included in the series expansion are the harmonics introduced by rotor saliency, concentrated stator windings, and stator slots. From the airgap flux density waveform, relatively simple closed-form expressions for the stator and rotor self- and mutual-inductances are obtained. The coupled-circuit model is implemented in the simulation of an alternator/rectifier system using a commercial state-model-based circuit analysis program. Comparisons with experimental results demonstrate the accuracy of the model in predicting both the steady-state and transient behavior of the machine     

Axial-Field Electrical Machines - Design and Applications

Axial-field electrical machines offer an alternative to the conventional machines. In the axial-field machine, the airgap flux is axial in direction and the active current carrying conductors are radially positioned. This paper presents the design characteristics, special features, manufacturing aspects and potential applications for axial-field electrical machines. The experimental results from several prototypes, including d.c. machines, synchronous machines and single-phase machines are given. The special features of the axial-field machine, such as its planar and adjustable airgap, flat shape, ease of diversification, etc., enable axial-field machines to have distinct advantages over conventional machines in certain applications, especially in special-purpose applications.     

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     

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     

Effect of Load Models on AC/DC System Stability and Modulation Control Design

This paper investigates important aspects related to the effect of load models on the modulation control design and stability of a modulated ac/dc system. Static load is modeled as a nonlinear function of load bus voltage and dynamic load is modeled by an equivalent induction motor. DC power and reactive power modulations are considered for the modulation controllers. A method for eigenvalue sensitivity calculation is developed to predict the effect of load characteristics on system stability. Eigenvalue sensitivity and simulation results show that static and dynamic load characteristics may have a considerable effect on the system stability. Figure 1 shows an ac/dc power system model used for studying the effect of nonlinear load on system stability. Reactive power modulation gain is obtained via optimal control theory. Figure 2 shows speed response of synchronous generator for a 5% change in reference current (Iref) of the rectifier terminal. Reactive power modulation by static var compensator improves system stability with constant impedance load model. However, reactive power modulation makes the system unstable when the modulation gain is based on constant impedance load model and the actual load is represented by induction motor. Important conclusions resulting from the computations and simulations performed for an integrated ac/dc system are listed below. 1. The dynamic behavior of induction motor load has a significant effect on the system stability. Induction motor in most cases reduces the overall system damping.     

Main flux saturation modelling in double-cage and deep-barinduction machines

The available models of saturated double-cage and deep-bar induction machines are the current state-space model and the flux state-space model, where state-space variables are selected either as stator current and currents of both rotor cages, or stator flux linkage and flux linkages of both rotor cages. This paper presents a number of models of saturated double-cage (deep-bar) induction machines where alternative sets of state-spate variables are selected. The method of main flux saturation modelling relies on recently introduced concept of `generalised flux space vector', which has originally been developed for modelling of saturated single-cage induction machines. The procedure and the novel models are verified by experimental study and simulation of self-excitation process in a double-cage induction generator     

Effect of the absorption coefficient of a transparent outer layer on the thermal flux transmitted through a roof/wall

This paper examines the effect of the absorption coefficient of a transparent slab, which forms the outer layer of a roof/wall, on such factors as average thermal flux passing/through/a roof/wall, maximum and minimum values of this thermal flux and the thermal load levelling. It is seen that a non-completely transparent slab (non-zero absorption coefficient) performs slightly better than a completely transparent slab (zero absorption coefficient) for small thicknesses, as far as the average thermal flux through a roof/wall is concerned. A non-completely transparent slab with an absorption coefficient greater than 3 m^?1 is as effective as a completely transparent slab in introducing thermal load levelling. For both a non-completely transparent slab and a completely transparent slab, maximum, minimum and average values of the thermal flux increase with increasing thickness.     

Incorporating the Effects of Magnetic Saturation in a Coupled-Circuit Model of a Claw–Pole Alternator

A method of representing the effects of magnetic saturation in a coupled-circuit model of a claw-pole alternator is presented. In the approach considered, the airgap flux density produced by each winding is expressed as a function of magnetic operating point. A challenge in the implementation is that the airgap flux densities consist of several significant harmonics, each of which changes at a distinct rate as iron saturates. Despite this complication, it is shown that relatively simple measurements can be used to determine model parameters. The model is implemented in the analysis of several alternator/rectifier systems using a commercial state-model-based circuit analysis program. Comparisons with experimental results over a wide range of speeds and operating conditions demonstrate its accuracy in predicting both the steady state and transient behavior of the systems.     

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