A method for magnetizing curve identification in rotor fluxoriented induction machines

Operation of an indirect rotor flux oriented induction machine in the field weakening region is usually realized by varying the rotor flux reference in inverse proportion to the speed of rotation. In order to provide the correct stator d-axis current reference at all speeds, it is necessary to incorporate the inverse magnetizing curve of the machine in the controller. The paper proposes an experimental method for identifying the inverse magnetizing curve, specifically developed for the type of vector controlled drives described. The method utilizes the same indirect vector controller and PWM inverter that are used in subsequent normal operation of the drive. It requires that the machine can run under no-load conditions and that the fundamental component of the stator voltage can be measured. The simplicity and accuracy of the method make it well suited for use during commissioning of the drive. The method is verified by extensive experimentation     

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.     

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-/spl omega/ curve correspond to dP/d/spl omega/=0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a DC motor fed from a commercial DC drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.     

Impact of stray load losses on vector control accuracy in current-fed induction motor drives

Vector control accuracy of induction motor drives is affected by variations of motor parameters that are treated in the control algorithm as constant values, and by the phenomena that are not modeled at all and are therefore unaccounted for in the controller. Detuning sources of the first type include variations of rotor and stator resistance, mutual inductance, and leakage inductances, while the second category includes stator iron (core) losses. All these sources of detuned operation have been studied in a considerable depth in the past. It appears that the only potential source of detuned operation, which has never been studied before, is the stray load loss (SLL), which belongs to the category of unmodeled phenomena. This paper develops an analytical model that characterizes detuning due to SLLs in indirect rotor-flux-oriented (RFO) current-fed induction motor drives in steady-state operation by means of the orientation angle error, actual to reference rotor-flux ratio, and actual to reference-torque ratio. A quantitative assessment of the impact of SLL on accuracy of rotor-flux-oriented control is performed, with the necessary motor parameters obtained from IEEE 112-B standard measurements and subsequent equivalent circuit parameter fitting. Detuning is also examined in transient operation. It is shown that, although SLLs are comparable to the iron losses in the studied machine (of approximately the same value in the rated operating point), their impact on accuracy of vector control is much smaller when compared to the iron loss induced detuning.     

A Reconfigurable Motor for Experimental Emulation of Stator Winding Interturn and Broken Bar Faults in Polyphase Induction Machines

The benefits and drawbacks of a 5-hp reconfigurable induction motor, which was designed for experimental emulation of stator winding interturn and broken rotor bar faults, are presented in this paper. It was perceived that this motor had the potential of quick and easy reconfiguration to produce the desired stator and rotor faults in a variety of different fault combinations. Hence, this motor was anticipated to make a useful test bed for evaluation of the efficacy of existing and new motor fault diagnostics techniques and not the study of insulation failure mechanisms. Accordingly, it was anticipated that this reconfigurable motor would eliminate the need to permanently destroy machine components such as stator windings or rotor bars when acquiring data from a faulty machine for fault diagnostic purposes. Experimental results under healthy and various faulty conditions are presented in this paper, including issues associated with rotor bar-end ring contact resistances that showed the drawbacks of this motor in so far as emulation of rotor bar breakages. However, emulation of stator-turn fault scenarios was successfully accomplished.      

Hybrid-secondary uncluttered induction (HSU-I) machine

A new hybrid-secondary uncluttered induction (HSU-I) machine is introduced. The hybrid secondary includes the effective-resistance, inverter, and magnetic switch options. The rotor current is magnetically coupled to the stator through an uncluttered rotating transformer. This machine can be used as a cost-effective slip-energy-controlled adjustable-speed induction motor that operates below synchronism and the wide speed-range motor drives and generators that operate below and above the synchronous speed of the line frequency     

A new energy recovery double winding cage-rotor induction machine

This paper proposes a new double winding induction machine and its speed control methods. The machine consists of two stator windings and one cage rotor. One stator acts as a motor and the other as a generator. By controlling the voltage supplied to the secondary or the generator winding, the rotor speed can be adjusted. The machine has a similar speed control characteristic to that of a slip-ring induction motor equipped with the rotor energy recovery scheme. The construction, principle, equivalent circuit, and speed control schemes of the new machine are presented. The performance characteristics of the machine are analyzed using the equivalent circuit and verified by experimental results.     

Terminal voltage build-up and control of a DFIG based stand-alone wind energy conversion system

The paper presents the voltage build-up process and the terminal voltage control of a doubly-fed induction generator (DFIG) driven by a pitch controlled wind turbine for the supply of autonomous system without any auxiliary source. A control strategy for the complete system including voltage build-up phase is developed with a view to provide as well as possible the required power for load. Indirect stator flux-oriented vector control is proposed to keep the stator voltage constant by means of a back-to-back converter connected to the rotor side, while the management system is supported by the pitch angle and the load shedding controllers. A novel scheme for voltage build-up is presented, which requires no additional hardware support, and physical interpretation of how self-excitation can occur from residual magnetism in the machine core is examined. A reliable start-up process is accomplished by using an appropriate voltage reference ramp which enables minimizing energy loss during the starting. The proposed system is modeled and simulated using Matlab/Simulink software program to examine the dynamic characteristics of the system with proposed control strategy. Dynamic simulation results for different transient conditions demonstrate the effectiveness of the proposed control strategy.     

Lumped-parameter thermal model for induction machines

A novel lumped-parameter thermal model for induction machines that can be included in real-time applications is presented. The model considers just stator, rotor, and environment representative temperatures so that parameter estimation can be carried out. Thermal parameters are experimentally obtained and a sensorless vector control application with a 1-kW induction motor is included showing the effectiveness of the proposed solution.     

Determination of radial-forces in relation to noise and vibrationproblems of squirrel-cage induction motors

The radial electromagnetic forces in induction motors play an important role in the production of audible noise and vibrations. The magnetic flux pulsations at the iron surfaces produce these radial forces, which act on the stator and rotor structures. An analysis for the calculation of the various field harmonics and radial forces in squirrel cage induction motors is presented in this paper. To verify the validity of the analysis, a squirrel cage induction motor is analyzed. Theoretical and experimental results are presented with a view to determine the actual role played by the air-gap harmonic fields on the radial forces. Also, the effects of loading on the radial forces and the ensuing vibrations are closely examined     

A new induction motor drive based on the flux vector acceleration method

A novel control strategy for the induction motor drive, based on the field acceleration method, is presented. The torque is controlled through variations of the stator flux angular velocity. The stator flux is controlled by using a feedforward control scheme, with the stator flux reference vector adjusted so as to obtain the fixed rotor flux amplitude. The applied controller assures a fast torque response, low torque ripple in the steady state, and drive operation with a constant switching frequency. The algorithm includes the improved stator and rotor flux estimation that guarantees the stable drive operation in all operating conditions, even at low speeds. The experimental tests verify the performance of the proposed algorithm, proving that good behavior of the drive is achieved in the transient and steady-state operating conditions.     

Real-Time Speed and Flux Adaptive Control of Induction Motors Using Unknown Time-Varying Rotor Resistance and Load Torque

In this paper, an algorithm for direct speed and flux adaptive control of induction motors using unknown time-varying rotor resistance and load torque is described and validated with experimental results. This method is based on the variable structure theories and is potentially useful for adjusting online the induction motor controller unknown parameters (load torque and rotor resistance). The presented nonlinear compensator provides voltage inputs on the basis of rotor speed and stator current measurements, and generates estimates for both the unknown parameters and the nonmeasurable state variables (rotor flux and derivatives of the stator current and voltage) that converge to the corresponding true values. Experiments show that the proposed method achieved very good tracking performance within a wide range of the operation of the induction motor (with online variation of the rotor resistance: up to (87%). This high tracking performance of the rotor resistance variation demonstrates that the proposed adaptive control is beneficial for motor efficiency. The proposed algorithm also presented high decoupling performance and very interesting robustness properties with respect to the variation of the stator resistance (up to 100%), measurement noise, modeling errors, discretization effects, and parameter uncertainties (e.g., inaccuracies on motor inductance values). The other interesting feature of the proposed method is that it is simple and easily implementable in real time. Comparative results have shown that the proposed adaptive control decouples speed and flux tracking while standard field-oriented control does not.      

A review of RFO induction motor parameter estimation techniques

An induction motor is the most frequently used electric machine in high performance drive applications. Control schemes of such drives require an exact knowledge of at least some of the induction motor parameters. Any mismatch between the parameter values used within the controller and actual parameter values in the motor leads to a deterioration in the drive performance. Numerous methods for induction machine online and offline parameter estimation have been developed exclusively for application in high performance drives. This paper aims at providing a review of the major techniques used for the induction motor parameter estimation. The paper is illustrated throughout with experimental and simulation examples, related to various parameter estimation techniques.     

Power balance considerations for brushless doubly-fed machines

This paper discusses the power balances in a brushless doubly-fed machine (BDFM). Equations of power in the two stator windings and the rotor circuit, showing the distribution of electrical and mechanical powers in the air-gap, are obtained in two different speed ranges of BDFM synchronous mode operation. Comparisons vis-a-vis power distribution are made between the wound-rotor induction machine (WRIM) and the BDFM. A brief analysis of the power balances, along with experimental data points obtained using a laboratory prototype machine, are given     

Application of an Additional Excitation in Inverter-Fed Induction Motors for Air-Gap Eccentricity Diagnosis

In this paper, the application of an additional excitation in induction motor (IM) drives for static, dynamic, and mixed eccentricity diagnosis is proposed. The additional excitation consists in a predefined inverter-switching pattern that is applied on the motor for a short time, while the fundamental excitation is canceled. This excitation was used previously to implement a position estimation strategy. The strategy obtains information about the rotor position from the motor saliencies effects over the zero-sequence voltage. The air-gap eccentricity is a kind of saliency that affects the zero-sequence voltage and allows the use of the additional excitation for eccentricity diagnosis. For the evaluation of the feasibility of this proposal, a multiple-coupled circuit model of the IM is used. The effects of series and series-parallel stator winding connections on the diagnosis signals are shown. Experimental results to validate the proposal are also given. These results show that it is possible to use the diagnosis strategy in a self-commissioning scheme     

A transient induction motor model including saturation and deep bareffect

Transient cage induction motor models for use in inverter-fed drives and controllers are reviewed. A simple transient model is presented that includes rotor deep bar effect and magnetic saturation of the magnetising and rotor leakage flux paths. The improved model requires motor details in the form of simple impedance versus frequency characteristics which can be obtained from a variety of external sources. These can range typically from detailed steady-state finite-element solutions to simple experimental measurements. The model is verified experimentally using a 75 kW, 4 pole vector controlled AC motor drive     

Analysis of a single-phase induction machine with a shiftedauxiliary winding

This paper extents the method of multiple reference frames to the analysis of asymmetrical induction motors with nonorthogonal stator windings, with particular emphasis on the permanent split capacitor (PSC) machine. The predictions of the method are verified by comparison to both experimental results and to results obtained using a machine variable computer simulation. Once verified, the method is used to derive the transfer function relating rotor speed to load torque disturbance, which is of particular interest when analyzing PSC machines driving compressor loads. It is shown that the transfer function exhibits a distinct resonant point, and that the magnitude of the resonant peak increases as the motor approaches synchronous speed. It is also shown that the magnitude of the resonant peak may be lowered by increasing rotational inertia or rotor resistance     

Theory and analysis of three-phase series-connected parametricmotors

This paper presents the steady-state performance of a three phase wound-rotor parametric motor. This type of motor can be practically realized by connection of stator and rotor phases of a conventional wound-rotor induction machine. The analysis is based on the d-q axes model, from which a phasor diagram is presented. The analysis is extended to include the magnetic saturation effect. Comparison between theoretical and experimental results showed a satisfactory agreement proving the validity of the mathematical model as well as magnetic saturation effect representation. Also the motor stability is investigated     

High performance IPMSM drives without rotational position sensorsusing reduced-order EKF

An extended Kalman filter (EKF) based approach for position sensor elimination in interior permanent magnet synchronous motor (IPMSM) drives is presented in this paper. The EKF is capable of estimating system parameters and state variables for the IPMSM by eliminating virtually all influences of structural noises in the vector control scheme. This paper presents a design method of a reduced-order EKF. Position and angular speed of the rotor are obtained through the reduced-order EKF only by measuring stator currents. Also, due to an angle modification scheme with error tracking, the sensorless drive system is robust to parameter variations. Simulation and experimental results are provided to verify the proposed approach based on the reduced-order EKF     

A rule-based acceleration control scheme for an induction motor

This paper presents a rule-based acceleration control scheme that aims to give an inverter-fed induction motor excellent dynamic performance. In every time interval of the control process, the acceleration increments produced by two different voltage vectors are compared, yielding one optimum stator voltage vector which is selected and retained. The online inference control is built using a rule-based system and some heuristic knowledge about the relationship between the motor voltage and acceleration. Because evaluation of integrals is not required and the motor parameters are not involved, the new controller has no accumulation error due to the integrals as in the conventional vector control schemes and the same controller can be used for different motors without modification