Steady-state Equivalent Circuit Research Articles

A Simplified Analysis of a Constant Voltage Single Phase Self-Excited Induction Generator

ABSTRACT This article presents a simple method to obtain lagging power requirement of a single-phase cage induction machine as a self-excited induction generator (SEIG) in stand-alone mode. A static capacitor bank is considered to self excite the machine and to maintain its terminal voltage constant. Performance characteristics are then evaluated using the simple method and considering an approximate steady state equivalent circuit of the machine. The effects of speed and load variation on excitation requirement of the machine and on generated frequency have also been studied for different types of load. Experimental results are included.

Modeling and simulation of ac–dc buck-boost converter fed dc motor with uniform PWM technique

PWM controlled rectifiers can efficiently and economically be employed in low and medium power applications of dc drives and in front-end converters of rectifier–inverter systems while maintaining the advantages of design simplicity and operation reliability of naturally commutated schemes. Due to the high dc voltage that is produced which is greater than the peak voltage of the utility supply, the ac–dc buck-boost converter is especially suited as a front-end power source in variable-speed drive systems to convert the utility supply voltage into a variable dc link voltage where a single-phase or a three-phase utilities power supply is available. In this paper, the dynamic model and steady state equivalent circuit of a single-phase ac–dc buck-boost converter fed dc motor with uniform PWM control is presented. The waveforms of voltage and current, the input and output characteristics of the converter are discussed and verified. Measured, computed and simulated results are shown to be very close and the model is proved to be efficient and accurate.

Experimental characterization procedure for use with an advanced induction machine model

An advanced induction motor model that includes stator leakage saturation, rotor leakage saturation, magnetizing saturation, and distributed system effects in the rotor circuits has been set forth. This model is considerably more accurate than traditional models, particularly in terms of predicting switching-frequency dynamics. The model proposed is very general in terms of the range of magnetic properties that can be incorporated. This paper provides suggestions for specific forms for the leakage and magnetizing characteristics and derives the resulting small-signal impedance and large-signal steady-state equivalent circuit. Based on these results, a test procedure for experimentally characterizing the machine is developed. The application of the procedure to a 50-hp test machine is included as an example.

Direct solution method for initialising doubly-fed induction wind turbines in power system dynamic models

The modelling of a doubly-fed induction generator (DFIG) wind turbine in large power system dynamic simulation software is discussed, and the steady state equivalent circuit and reduced order dynamic DFIG models are described. A direct solution method for obtaining injected rotor voltages, which satisfy the dynamic model control objectives, is derived from the approximate steady state model. An initialisation procedure is given for the reduced order dynamic DFIG machine model. Simulation results are presented from initialised dynamic DFIG wind turbine models. PQ and PV bus representations of the DFIG for load flow studies are compared and simulated with typical turbine and network data for wind farm installations.

Simplified steady-state model of an SCR-inverter-based load-commutated commutatorless series motor

This paper presents a simplified analytical model of a silicon-controlled-rectifier based, load-commutated inverter driven, commutatorless series motor to predict its speed-current, torque-current and torque-speed characteristics. The model has been formulated, keeping in view the steady state equivalent circuit of a conventional DC series motor. The proposed analytical expressions relating torque and current with speed are found to follow an inverse law and that relating torque and current is found to be parabolic, as is expected in a "series" machine. The drive has been implemented, loaded with a DC generator and experimentally similar characteristics have been obtained, as predicted by the analytical model. The characteristics obtained experimentally are seen to follow the ones, predicted by the analytical model closely.

Transient model for induction machines with stator winding turn faults

A transient model for an induction machine with stator winding turn faults is derived using reference frame transformation theory. A state-space representation of the dynamic equations, suitable for digital simulation, is provided. Steady state equivalent circuits are derived, from which the sequence components of the line currents can be estimated. Experimental results are provided to validate the derived model. This model would find use in the design of turn fault detection systems for induction machines.

Graphical phasor analysis of three-phase PWM converters

A graphical steady-state analysis technique based on the complex 0-sequence, forward-rotating, and backward-rotating (0fb) phasors is described for balanced three-phase PWM inverters, rectifiers and cycloconverters. The technique avoids the cumbersome algebra of the 0fb transformation, and the coupling between the subcircuits resulting from the real direct-axis/quadrature-axis (dq) transformation. Using symmetric and de-coupled subcircuits that are time-invariant under steady state, the graphical method makes the analysis of three-phase converters straightforward and insightful. In the paper, time-invariant transient models with respect to an arbitrary reference node are first derived for all three-phase converter components. Application of the component models to steady-state analysis is demonstrated for the buck-boost inverter. The steady-state equivalent circuits for the popular buck and boost inverters, rectifiers, and cycloconverters are given along with the steady-state values for their capacitor voltages and inductor currents.

Starting characteristics of motor in inverter-motor drive system

When the induction motor is fed from the PWM VSI inverter, with and without torque enhancing function, the variations of the starting torque and the starting current versus the starting frequency are analyzed, based on the steady state equivalent circuit of the induction motor. The corresponding expressions are derived, and the corresponding curves are also drawn, from which the initial starting frequency range can be determined properly and easily. The initial starting frequency range is the common range, in which the starting torque should be high enough and the starting current low enough. Some other useful formulas are also derived. These results are significant for electrical drive designers to make the starting schemes. The effects of the increase of the stator and the rotor resistances on the starting torque and the initial starting frequency range are also discussed, which is caused by the skin effects of the high order harmonics present in the PWM inverter output voltage.

Autonomous induction generator with single phase rotor

An autonomous induction generator (IG) with an asymmetric configuration, both the stator and the rotor are single-phase, is shown to be capable of generating modulated waveforms. The experimental results are demonstrated to be in good agreement with the simulation waveforms and both strengthen the theoretical part. The developed equations can help as an IG design guideline. The modulated waveform periods are shown to be in a continuous transient state in a chaos manner. The IG is also explained by its steady-state equivalent circuit.

Determination of Induction Motor Efficiency Using Non-Linear Programming

This paper deals with determination of induction motor efficiency, as one of the qualitative indexes of machine operation, by means of acalculation and measurement during its operation. The possibilities of the efficiency calculation are described and it is explained that the simplest method for the efficiency determination is a calculation from a steady state equivalent circuit of the machine. This method is improved by an optimization of the induction machine equivalent circuit parameters based on the measured quantities during steady state operation of the machine. Optimization utilises non-linear programming.

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.

Steady-state analysis of an isolated self-excited induction generator driven by regulated and unregulated turbine

This paper examines the steady-state analysis and performance of an isolated three-phase self-excited induction generator (SEIG) driven by regulated and unregulated turbines. For the case of a regulated turbine, the equivalent circuit is solved with speed as a constant parameter, while for the unregulated turbine, the speed is considered as a variable which depends on the shaft torque according to the turbine characteristics. The no-load speed is considered as a constant independent parameter, which depends on wind speed which is assumed constant in this analysis. The steady-state equivalent circuit is solved using the node-admittance method, and the shaft torque is expressed in terms of the rotor current. The Newton-Raphson method is used to solve the system nonlinear equations. For the present investigation, a linear speed-torque characteristic is considered, but the method of analysis applies equally well to nonlinear characteristics. Experimental investigations on a 1 kW three-phase induction generator driven by a separately excited DC shunt motor have confirmed the accuracy of the proposed method of analysis.

A novel analysis of parallel operated self-excited induction generators

This paper presents a novel approach based on eigenvalue and eigenvalue sensitivity analyses to predict both minimum and maximum values of the capacitance required for the self-excitation of parallel operated three-phase induction generators. Numerous numerical methods based on steady-state equivalent circuit models have been proposed to find the minimum capacitance of a self-excited induction generator by solving simultaneous nonlinear equations. Steady-state and sensitivity analyses of different capacitance values concerning various system limits are performed. Transient analyses of the studied induction generators under different loading conditions are also carried out. The results show those induction generators with different speeds can be operated in parallel properly, but the requirement before they may be connected in parallel is the phase sequence of the running and incoming induction generators must be the same. Experimental results obtained on 1.1 kW induction machines confirm the feasibility and effectiveness of the proposed approach.

Excitation requirements for stand alone three-phase induction generator

Self excitation in induction machines depends on appropriate combination of speed, load and excitation capacitance. This paper presents direct methods derived from loop and nodal analyses to find different criteria for maintaining self-excitation. Unlike available techniques, the inverse-model for the steady state equivalent circuit has been used. This is found to be of much assistance in the analysis. Besides determination of speed-limits for fixed terminal parameters, and capacitance requirement under varying speed and load, a method to test the possibility of self-excitation for known speed, load and terminal capacitance has also been presented.

Nonlinear parameter estimation of steady-state induction machine models

The use of modern identification techniques in determining the values of the steady-state equivalent circuit parameters of a three-phase squirrel-cage induction machine is discussed. The identification procedure is based on the steady-state phase current versus slip and input power versus slip characteristics. The proposed identification algorithm is of a nonlinear kind. The machine parameters are obtained as the solution of a minimization of least-squares cost function of the difference between calculated and experimental steady-state characteristics. Simulation, as well as experimental results concerning the application of the proposed algorithm to the modeling of a 1.5 kW wound-rotor three-phase induction machine, are presented.

A novel analysis on the performance of an isolated self-excited induction generator

This paper presents a novel approach based on eigenvalue and eigenvalue sensitivity analyses to predict both minimum and maximum values of capacitance required for the self-excitation of a three-phase induction generator. Numerous numerical methods based on steady-state equivalent circuit models have been proposed to find the minimum capacitance of self-excited induction generators by solving simultaneous nonlinear equations. Steady-state and sensitivity analyses of different capacitance values with respect to various system parameters are performed. Transient analyses of the studied induction generator under different loading conditions are also carried out. Experimental results obtained on a 1.1 kW induction machine confirm the feasibility and effectiveness of the proposed approach.

Capacitance requirements of self-excited induction generators

A simple method for computing the minimum value of capacitance, C min. required for initiating voltage build-up in a three-phase self-excited induction generator (SEIG) is presented. Based on the steady-state equivalent circuit model, a consideration of the circuit conductances yields a sixth-degree polynomial in the per-unit frequency. The polynomial can be solved for real roots, which enables the value of C/sub min/ to be calculated. Critical values of load impedance and speed, below which the machine fails to self-excite irrespective of the capacitance used, are found to exist. Closed form solutions for C/sub min/ are derived for no-load and inductive loads. Using the same numerical approach, an interative procedure is developed for predicting the capacitance required for maintaining the terminal voltage at a preset value when the generator is supplying load. Experimental results obtained on a 2 kW induction machine confirm the feasibility and accuracy of the proposed methods.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparative study on the performance of a commercially designed induction generator with induction motors operating as self excited induction generators

The paper presents the performance characteristics of different induction machines of various ratings operating as a capacitor self-excited induction generator. Their computed performance is compared with commercially designed line excited induction generators operating as self excited induction generators. An algorithm, using the Newton-Raphson method and steady state equivalent circuit for the machine, is developed to compute the excitation requirement for maintaining a constant terminal voltage at different loads. This technique is also used to compute the number of capacitor steps required to load the machine, from no load to full load, while maintaining terminal voltage within specified limits. The optimum output power of the machine is computed without exceeding the rated stator current for its rated speed. The voltage range within which the machine is expected to operate at or above its rated capacity in stand alone mode, but without being overloaded, is also obtained. The effect of speed on excitation requirements is studied.

A new look at the effects of unbalanced voltages upon synchronous and induction machines

Recently developed polyphase machine demonstration and analysis software is used to explore a phenomenon whose effects have been observed for many years: namely, that under certain conditions, significant imbalances in stator currents of polyphase AC machines (and thus in the phase power distribution and the internal steady-state magnetic field) can result from relatively minor imbalances in machine terminal voltage at certain loading levels. Using standard steady-state equivalent circuit models of synchronous and induction machines, it is shown that the presence of severe levels of current imbalance can be easily and effectively demonstrated. It is also shown how these effects can be readily simulated and analyzed in order to help underscore the need for stringent voltage imbalance limits.

PERFORMANCE ANALYSIS OF SELF-EXCITED INDUCTION AND RELUCTANCE GENERATORS

ABSTRACT The paper presents a suggested method for ac power generation from a constant speed drive using capacitor excited induction and reluctance machines. The proposed arrangement is adapted to use one stator frame and two rotor types, namely, salient poles reluctance and squirrel-cage induction rotors. Operational and steady-state equivalent circuits for both types are developed and used to predict the dynamic and steadystate behaviours under load conditions. This analysis depends mainly upon the computation of the power angle for a given machine parameters, capacitor ratings, and load conditions at a constant speed. Further, stability limits of both generators are investigated by developing the active-reactive power (P-Q) diagram for each generator. Conditions required to achieve self-excitation are also given. Performances of the two generator types are evaluated theoretically and experimentally.