Analysis of Self-Excited Synchronous Reluctance Generator

A steady-state model and a transient model of self-excited synchronous reluctance generator are developed for stand-alone operation. Saturation is considered to predict the actual behavior of the machine. The proposed linearized model is applied to obtain eigenvalues for steady-state stability analysis under different loading conditions. The effects of active, reactive, apparent power output and excitation capacitance on the steady-state performance of self-excited synchronous reluctance generator are analyzed. Eigen value sensitivity is calculated by varying circuit parameters from 50% to 0% of their standard values at 100 μF, 125 μF and 150 μF excitation capacitances. The proposed transient model is applied to observe the transient behavior of machine such as load angle, speed and electromagnetic torque when the machine is subjected to a three-phase symmetrical short-circuit fault across the machine terminals. The load angle sensitivity to machine circuit parameters is investigated

New approach to steady-state and dynamic nonlinear modelling of laminated salient-pole alternator systems

A complete mathematical model for a generator system consisting of an isolated laminated salient-pole alternator, exciter and prime mover is presented, with emphasis on the inherent electromagnetic nonlinearities in the alternator and its exciter. An equivalent circuit, representing the rotor circuits accurately, has been adopted to model the al ternator in the dqo reference frame. A computer program has been developed to calculate the unsaturated parameters of the model using the machine design data. A new approach has been developed to account for the electromagnetic saturation effects on the model reactances. Consequently new saturation factors, based on the machine design particulars have been deri ved. The advantages of these saturation factors, compared with conventional factors, are that both mutual saturation effects between the main and leakage fluxes, and between the direct- and quadratureaxis fluxes are considered. A mathematical nonlinear model, utilising the new saturation factors, is presented for a system containing an isolated laminated salientpole alternator and a direct thyristor static exciter. A digital computer program has been developed to simulate the system. The predicted results, for some steady state and dynamic candi tions, showgood agreement with test results and clear improvement over those obtained if saturation is either neglected or considered using the conventional saturation factors. At high saturation levels, the conventional method of calculating the machine transformer voltages, using static saturated reactances, gives unacceptable errors. A method for calculating these voltages correctly, in models utilising the currents as state space variables, is presented using new derived dynamic saturated reactances. This dynamic reactance concept is presented in a generalised form so that it can be applied to any machine with different saturation factors. The previous mathematical model of the alternator system has been modified according to the dynamic reactance concept, and the computer program has been developed accordingly. The predicted results confirm the need to apply this concept especially to dynamic conditions characterised by high saturation levels. To extend the analysis to a wider range of loading conditions, the alternator has been modelled in the abc reference frame. The unsaturated, static and dynamic saturated reactances of the machine in this reference frame have been obtained using conventional dqo-abc transformation techniques. Starting from the fundamental machine relations, a new set of equations, in the phase reference frame, has been derived employing the new dynamic reactance concept. A comprehensive system consisting of an isolated laminated salientpole alternator, brushless exciter, thyristor divert automatic voltage regulator and a diesel prime mover has been studied. Both the alternator and the exciter have been modelled in the abc frame to comply with the nature of rectifier loading associated with the exci ter. A complete steady state and dynamic mathematical model is presented where the t~r technique has been applied to the dynamic variable topology of the system electrical circuits. The model presented covers all the possible modes of operation associated with the exciter rotating bridge rectifier circuit. A digital computer program has been developed to simUlate the system. The predicted results obtained using the new set of saturation factors in conjunction with the dynamic reactance concept show good agreement with the test results. The study presented confirms the validity of the mathematical models developed for the alternator systems. Also, it supports the metlxxi by which the electromagnetic nonlinearity has been accounted for

Asynchronous performance analysis of a single-phase capacitor-start, capacitor-run permanent magnet motor

This work presents a detailed analysis of the asynchronous torque components (average cage, magnet braking torque and pulsating) for a single-phase capacitor-start, capacitor-run permanent magnet motor. The computed envelope of pulsating torque superimposed over the average electromagnetic torque leads to an accurate prediction of starting torque. The developed approach is realized by means of a combination of symmetrical components and d-q axes theory and it can be extended for any m-phase AC motor - induction, synchronous reluctance or synchronous permanent magnet. The resultant average electromagnetic torque is determined by superimposing the asynchronous torques and magnet braking torque effects

Performance estimation of interior permanent-magnet brushless motors using the voltage-driven flux-MMF diagram

The flux-magnetomotive force (flux-MMF) diagram, or "energy conversion loop," is a powerful tool for computing the parameters of saturated interior permanent-magnet brushless motors, especially when the assumptions underlying classical dq theory are not valid, as is often the case in modern practice. Efficient finite-element computation of the flux-MMF diagram is possible when the motor current is known a priori, but in high-speed operation the current regulator can lose control of the current waveform and the computation becomes "voltage-driven" rather than "current-driven." This paper describes an efficient method for estimating the motor performance-average torque, inductances-by solving the voltage-driven problem. It presents experimental validation for a two-pole brushless interior permanent-magnet motor. The paper also discusses the general conditions under which this method is appropriate, and compares the method with alternative approaches

Analytical prediction of the electromagnetic torques in single-phase and two-phase AC motors

The single-phase and two-phase versions of AC motors can be modelled by means of the two-axis (d-q) theory with sufficient accuracy when the equivalent circuit parameters are correctly estimated. This work attempts to present a unified approach to the analytical prediction of the electromagnetic torque of these machines. Classical d-q axes formulation requires that the reference frame should be fixed on the frame where the asymmetries arise, i.e. the stator and rotor. The asynchronous torques that characterize the induction motors are modelled in a stationary reference frame, where the d-q axes coincide with the physical magnetic axes of the stator windings. For the permanent magnet motors, that may exhibit asymmetries on both stator and rotor, the proposed solution includes: a series of frame transformations, followed by symmetrical components decomposition. As in single-phase and two-phase systems the homopolar component is zero; each symmetrical component – negative and positive – is further analysed using d-q axes theory. The superposition principle is employed to consider the magnets and rotor cage effects. The developed models account for the most important asymmetries of the motor configuration. These are, from the stator point of view, different distribution, conductors' dimensions and number of effective turns, non-orthogonal magnetic axes windings and from the rotor point of view, asymmetrical rotor cage, variable reluctance, and permanent magnets effect. The time and space harmonics effect is ignored. Test data are compared with the computed data in order to observe how the simplifying assumptions affect the level of accuracy. The analytical prediction methods make possible torque computation according to the nature of the torque being computed, namely, induction, reluctance and excitation (permanent magnet). The results are available for quasi steady-state, steady-state (rated or synchronous speed) and dynamic analyses. All the developed mathematical models can be used in preliminary design for further optimisation and accurate estimation in complex numerical models. Another important feature of the analytical models for single-phase and two-phase AC motors, is that they can be directly implemented in any suitable electrical drives control strategy.reviewe

New techniques for the determination of synchronous machine parameters

The ever-increasing complexity of power systems is leading to more severe problems of system stability, and considerations of this are requiring a more detailed modelling of the system generators than has hitherto been undertaken. The induced eddy currents in the iron body of the machine, and the considerable active resistance of this iron, have previously been regarded as fully accounted for by a single damper circuit on each axis of the machine. However, recent work has shown that this is not an adequate representation, and that calculations on this basis may exhibit considerable discrepancies with practical measurements of the transient performance of the machine. Measurements of the generator parameters has been based on a single damper representation, and it is now necessary to reexamine carefully the assumptions underlying these tests and to determine if they can be used to provide an improved machine model. The main aim of this thesis is to investigate the possibility of using step-response tests to obtain the parameters of a machine model with two damper windings on each axis. From an investigation of this form of equivalent circuit, expressions in the form of transfer functions are developed for the operational inductance of the machine, and these are correlated with measured results to provide the necessary numerical data. Some of the results required can also be obtained from well-established test procedures, and these are therefore used to provide further confirmation of the accuracy of the new machine representation

Aspects of magnetisation and iron loss characteristics in switched-reluctance and permanent-magnet machines

In the first section, the magnetisation characteristics of the switched-reluctance motor are examined. Measurements have been carried out using both static and dynamic test methods. The test data has been compared with simulation results from analytical design programs and finite element models. The effects of mutual coupling on the magnetisation characteristics are investigated through measurement and simulation. Results show that the degree of mutual coupling is strongly dependent on the winding arrangement of the machine. In the next section, the difficulties in measuring the properties of permanent-magnet machines are discussed in detail, and solutions to common problems proposed. The measurement and analysis methods used for the switched-reluctance motor are further developed for analysis of permanent magnet machines. Techniques for determining the variation in synchronous reactances and permanent magnet flux are presented. Finite element simulations are used to show the variation of magnet flux under loading, a condition ignored in classical analysis methods. The final section discusses the analysis of magnetisation characteristics of electrical sheet steels. Comparison is made between measurements carried out on single sheet tester and Epstein square test rigs. The iron losses of a typical non-grain-orientated steel are measured under both sinusoidal and nonsinusoidal flux density conditions. The iron losses are shown to increase significantly when higher harmonic components are introduced to the flux density waveform. The difficulties in modelling the nonlinear iron loss characteristics of electrical steels are considered

Systematic development of equivalent circuits for synchronous machines.

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