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     

Analysis of parallel-operated self excited induction generators

The paper presents an iterative solution for the problems related to steady state performance of self-excited induction generators operating in parallel. The analysis is based on voltage and current balance equations derived from an inverse-model for the steady state equivalent circuit of induction machines. The nonlinearity in the magnetization characteristics has been taken into account by piecewise linearisation. The proposed method is general and can be applied for analysis of any number of parallel connected machines. Theoretical predictions and experimental results are presented to study different performance characteristics of the system     

A new synthetic loading for large induction machines with no feedback into the power system

Full-load testing of large induction machines is constrained by the limitations in the power-supply and loading equipment of the manufacturer's facilities, resulting in costly set up time. A new synthetic loading method is proposed based on a bang-bang phase control strategy. The rated power oscillation created is routed to an auxiliary system and the source hydro has to provide only the total losses of the system, without seeing the excessive power swings observed in other synthetic loading techniques. In this technique, only induction machines are used which would enable motor manufacturers to build the test rig in-house. The control stage is very simple to implement and requires only unregulated DC supplies for the excitation windings. The method is suitable for any induction machine and does not requires any set up time. It is possible to strictly maintain constant DEFINE RMS voltage and current at rated values for the duration of the heat runs.     

Determination of air-gap eccentricity in electric motors usingcoherence analysis

This study addresses the detection of air-gap eccentricity based on hearing damage in 5 hp induction machines. The faulty condition of the bearing is caused by accelerated aging processes in seven aging cycles. For the faulty case, the spectra of vibration signals are computed for initial and final aging processes under 100% load condition. Subsequently, the spectra of the current signals for a single phase are also obtained for the initial and aged cases and cross-power spectral coherences between the vibration and current signals are computed     

An efficiency-optimization controller for induction motor drives

When working close to their nominal operating point, induction motors are high-efficiency machines. However, at low load, efficiency is greatly reduced when magnetization flux is maintained at nominal value. Therefore, it is necessary to adjust the flux level according to the load, so as to decrease iron and copper losses. This paper deals with a field-oriented scheme for induction motor drives leading to efficiency optimization. It is based on optimal power factor tracking by adjusting magnetization current component with to respect the torque current component. Simulation results are presented to highlight the feasibility and validity of the proposed method     

Analysis of the induction machine parameter identification

This paper analyzes the numerical identifiability of the electrical parameters of induction machines. Formulations of the single and double-cage induction machine, with and without core-losses in both models, are developed. The impossibility of identifying all the parameters of these models when only external measurements (voltage, current, speed and torque) are used is shown. One proposed solution is the formulation of machine equations by using the minimum number of parameters (which are identifiable parameters). As an application example, the parameters of a double-cage induction machine are identified using steady-state measurements corresponding to different angular speeds     

End effect in linear induction and rotating electrical machines

The finite length of the iron core (stator) has a significant impact on the performance of linear induction machines (LIMs). The performance of a cylindrical symmetric linear induction machine (CSLIM) has been studied considering the impact of the finite stator length. The analytical procedure, originally developed for calculation of plane linear induction machines, has been modified to facilitate the analysis of CSLIM. The procedure is based on Fourier series expansions of the electromagnetic quantities and the air-gap length. This study further provides information on the effect of finite length of tooth in the direction of rotation in rotating electrical machines. As examples, the overheating of the stator core in a 500 MVA synchronous hydrogenerator and the detrimental impact of the end effect on a 506.5 MVA synchronous turbogenerator in a nuclear power station is described.     

电磁感应加热器在吹膜机组上的应用

电磁感应加热器具有加热迅速、温度控制实时准确等特点,应用于塑料吹膜机组上,不但能改变传统电阻丝加热方式工艺落后、高耗能等现状,而且还能使吹膜机组料筒树脂易于塑化,使薄膜产品性能明显提高;同时电磁感应加热器也具有使用寿命长、维护简单、功率大、安全可靠的特点,起到了节约电能和降低环境温度的作用。     

A Synchronous Current Control Scheme for Multiphase Induction Motor Drives

Rotor-flux-oriented control of multiphase induction machines with sinusoidal magnetomotive force (MMF) distribution can be realized theoretically by using only two current controllers in the synchronous reference frame. In practice, however, this is inadequate, as shown in this paper using experimental results collected from five-phase induction motor drive laboratory prototypes. Various asymmetries of the machine and nonideal nature of the inverter necessitate an appropriate modification of the current control scheme in the rotating reference frame. Such a modified current control scheme is developed in this paper, and its ability to provide operation of the machine with a practically perfectly sinusoidal and balanced system of five-phase currents is demonstrated experimentally.      

A novel induction machine design suitable for inverter-drivenvariable speed systems

Induction machines designed for inverter-driven variable speed systems are different from those fed directly from a utility power line. In this paper, a novel design approach for inverter driven induction machines is presented and implemented. This is followed by an investigation on sizing equations and rotor slot shape specifically for this purpose. The proposed approach permits the integration of the design of machines with inverters, comprehensive performance analysis, and system optimization, resulting in 20-30% higher power density for the induction machine than those designed for direct utility power supplies by conventional methods. Simulation analysis and experimental results are presented to substantiate the conclusions     

Qualitative performance assessment of semiconductor switching device,converter and generator candidates for 10 MW offshore wind turbine generators

This paper investigates the possibilities of viable power electronics converters, semiconductor switching devices and electric machines for 10 MW variable‐speed wind turbine generators. The maximum rated power of existing wind turbine configurations is in the range of 6 MW. The proposed alternatives are compared against several technical and economical factors, and their advantages over the present wind turbines are highlighted. A comprehensive performance comparison of modern power semiconductor devices, their electrical characteristics and the key differentiators among them are presented. The power electronics converters considered include all commercially available multilevel voltage source and current source converters as well as the opportunities offered by power electronics building block‐based design. The factors used for the comparison include the converter power range, capacitor voltage balancing, common mode voltage and current, electromagnetic interference emissions, fault ride‐through capability, reliability, footprint, harmonic performance, efficiency and losses, component count, risk of torsional vibration by the harmonics and inter‐harmonics, complexity, ease of back‐to‐back operation and filtering requirements. For the electric machines, this study concentrates on high‐temperature superconducting machines, multi‐phase induction machines and permanent magnet synchronous machines. These machines are compared against existing wind generator technologies in terms of their power range, torque density, efficiency, fault ride‐through capability, reliability, footprint, harmonic performance, ease of fault detection, excitation control, noise and vibration signature, oscillation damping, gearbox requirement, cost and the size of the associated converter. Copyright © 2010 John Wiley & Sons, Ltd.     

An Analytical Approach to Determine the Optimal Resistance for the Three-Series-Resistor Method for Induction Generator Connection

This letter presents a set of analytical formulas to determine the optimal resistor size to be used in the three-series-resistor method of squirrel-cage induction generators connection. The analytical formulation is based on the short-circuit theory of induction machines.      

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     

Transient thermal analysis of induction motors

Induction machines transient thermal analysis has been a subject of interest for electric machine designers in their effort to improve machine reliability and in rotor design optimisation. The study of transient thermal behavior is useful to identify causes of failure in induction machines. This paper presents a 2-D transient analysis of induction machines using the available heat transfer coefficients in literature. A generalised finite element code developed with Galerkin's weighted residual technique is used for analysis. The model is applied to one squirrel-cage totally-enclosed fan cooled machine of 3.7 kW and another surface cooled machine of 5.7 kW. The predicted temperatures compare well with test results. The advantages and limitations of this model are discussed     

Source of induction motor bearing currents caused by PWM inverters

The recent increase of bearing damage in induction machines driven by transistorized inverters has spurred activity on possible causes related to PWM inverters. This paper looks into a typical power conversion system from this perspective. It identifies the existence of the common mode voltages produced in all types of converters. A hypothesis is then proposed to explain the bearing current problem. In particular, it is shown that in an inverter-motor system the common mode voltages generated by a PWM inverter, which are attributed to PWM switching harmonics, couple through parasitic capacitances from stator windings to the rotor body and then returns through the motor bearings to the commonly grounded stator case as a closed loop circuit. The hypothesis is verified by experimental measurement of common mode coupling currents and true bearing currents in a specially modified induction motor. Solutions are then provided to suppress the bearing currents     

Voltage stabilization system for induction generator in stand alonemode

A problem of terminal voltage stabilization for self excited induction generator (SEIG) in stand alone mode is studied. Some means of SEIG terminal voltage stabilization implementing different designs of control schemes are observed. A method for tuning of digital control block by using an optimisation technique is proposed. The method is based on looking for minimum of criterion function that is weighted sum of some system' parameters. Factors of control equation have been obtained and computer modelling was done in order to check out system behaviour in different transient modes. Data of experimental investigations of a prototype are given which confirm the results of simulation. An overall method is based on well established theory for the time domain modelling of induction machines and affords to take into account nonlinearities of induction machines and digital control blocks and seems to be reliable and accurate     

An introduction to electric machine modeling by systems ofnoninteger order. Application to double-cage induction machine

The flowing of induced currents in electric machines, especially in the deep-bars of induction machine cages and the dampers of synchronous machines, implies an increase in the order of the equivalent circuits used to model their frequency response. But, physical meanings of the circuit parameters are often lost. Thus, the authors propose to go back over the modeling of skin effect in order to define noninteger order equivalent circuits which would be linked with the consequences of induced currents on the frequency response. In this paper, the proposed equivalent circuit is based on half-order systems. It is used to represent the frequency response of a 30 kW 4 pole double-cage induction machine and its results are compared with measurements     

Transient Electromechanical Modeling for Short Secondary Linear Induction Machines

Linear induction motors are under development for a variety of demanding applications including aircraft launchers and magnetically levitating trains. These applications require machines that can produce large forces, operate at high speeds, and can be controlled precisely to meet performance requirements. The design and implementation of these systems require fast and accurate techniques for performing system simulation and control system design. We present techniques for modeling and controlling linear induction machines based on a direct and quadrature (DQ) representation of the system dynamics. Simulation results using the DQ representation of the machine dynamics are presented and compared to test data from a Subscale Integration Test Bed developed for the U.S. Navy's Electromagnetic Aircraft Launch System program.      

Minimization of iron losses of permanent magnet synchronous machines

In permanent magnet (PM) synchronous machines, iron losses form a larger portion of the total losses than in induction machines. This is partly due to the elimination of significant rotor loss in PM machines and partly due to the nonsinusoidal flux density waveforms in the stator core of PM machines. Therefore, minimization of iron losses is of particular importance in PM motor design. This paper considers the minimizing of iron losses of PM synchronous machines through the proper design of magnets and slots, and through the choice of the number of poles. Both time-stepped finite element method (FEM) and the iron loss model from a previous study are used in this paper to draw the conclusions.     

Modelling of magnetic saturation in smooth air-gap synchronousmachines

Main flux saturation in d-q axis representation of synchronous machines is at present modelled by selecting either all the winding currents or all the winding flux linkages as state-space variables. However, these two available models are just a tiny portion of the complete set of models that can be obtained by selecting other combinations of state-space variables. This paper presents a general procedure for main flux saturation modelling in smooth air-gap synchronous machines, that is applicable for most selections of state-space variables. The method relies on the concept of `generalised flux' and `generalised inductance', that has been successfully applied in modelling of saturated single-cage and double-cage induction machines. The concept is extended to saturation modelling in smooth air-gap synchronous machines. A number of models, that result from the application of the method for different selections of state-space variables, are presented in detail