H.B. Ertan

Optimum geometry for torque ripple minimization of switched reluctance motors

For switched reluctance motors, one of the major problems is torque ripple which causes increased undesirable acoustic noise and possibly speed ripple. This paper describes an approach to determine optimum magnetic circuit parameters to minimize low speed torque ripple for such motors. The prediction of torque ripple is based on a set of normalized permeance and force data obtained from numerical field solution for doubly-salient geometries. For that purpose a neural net is trained to extract the data needed to predict the torque produced by a given geometry and excitation at any position of teeth. Hence the static torque curve can be constructed and torque ripple can be found. The accuracy of the approach developed is illustrated by comparing measured and predicted torque for a switched reluctance motor. The optimum parameters for minimum torque ripple conditions are sought using the augmented Lagrangian method. The paper presents the optimization results, and then proceeds to determine the range of geometric parameters which keep the torque ripple within /spl plusmn/10 of the optimum value.

Various induction generator schemes for wind-electricity generation

Abstract In this paper, various generator types and control systems employed in wind energy conversion systems have been reviewed with particular emphasis on systems including induction generators. Advantages and disadvantages of these systems and relative generator sizes are also discussed. Variable shaft speed schemes employing induction generators have been studied more closely and the important aspects of the findings are emphasized. Throughout the paper, the importance of maximizing the conversion efficiency is stressed. The parameters controlling the energy conversion are identified. Simple methods have been developed to determine the optimum parameters for a constant speed synchronous generator wind energy conversion system and a variable speed squirrel-cage induction generator wind energy conversion system. This last configuration is shown to have advantages from the viewpoint of energy production as well as some other aspects including simplicity and robustness.

Calculation of parameters of single-phase PM motor for design optimization

This paper presents methods of calculation of parameters of single-phase permanent-magnet (SPPM) motor, in terms of motor dimensions and material properties, which are utilized in the dynamic model of the motor. The intention of the study is to develop means of SPPM performance calculations, which lend themselves to be employed within a mathematical design optimization approach. The calculated parameters are compared with measured values and are shown to be accurate for the purpose of the study.

Comparison of PWM and PFM induction drives regarding audible noise and vibration for household applications

This paper is aimed at comparing the performance of pulse frequency modulation (PFM) and pulsewidth modulation (PWM) techniques regarding audible noise generated from inverter-driven induction motors. For the purpose of illustrating the performance of the two modulation techniques, a drive developed for washing machine applications is considered. First, the measured and simulated harmonic content of this inverter is compared with the measured harmonic spectrum of a three-phase input-output commercial variable-frequency inverter. It is found that despite the 8-bit processor inexpensive implementation, the total harmonic distortion (THD) of PFM is comparable to the THD of PWM in more sophisticated applications. It is discovered that the harmonic spectrum of PFM exhibits harmonics of smaller magnitude distributed over 0.9f/sub o/--2.1f/sub o/ (base switching frequency) range, as opposed to a smaller number of dominant harmonics of PWM. Next, the prototype drive is programmed to generate both sine-PWM and PFM waveforms. The motor is driven at several frequencies and the audible noise and vibration level of the motor is measured. It is observed that the type of current harmonic content of PFM leads to reduced noise and vibration.

External search coil as a means of measuring rotor speed of an induction motor

The study in this paper investigates whether the air gap harmonics can be identified by using an external search coil placed on the frame of an induction motor. For this purpose the study focuses on the prediction of rotor speed of an induction motor from external search coil emf, using spectral analysis techniques. In the first stage of the study the type of search coil which maximizes the induced emf is sought, also the position of the coil on the induced emf is investigated. Several motors are tested with a sinusoidal supply as well as a PWM inverter power supply and the search coil voltage is sampled. FFT analysis of the recorded data is performed in Matlab environment. An algorithm is developed for identifying the rotor speed using the FFT analysis results. It is shown that the developed algorithm is capable of predicting the motor speed under various load conditions and motor speeds. The speed estimation accuracy of this approach is also discussed. In short, this paper illustrates that the induced emf in an external search coil contains information on air gap harmonics, and these harmonics can be identified even when the motors are supplied with a PWM inverter.

A method for stator resistance measurement suitable for vector control

In this paper a method is introduced which can be used to predict the stator resistance of an asynchronous motor from current and voltage measurements without interrupting the controller or motor operation. This approach takes minimal microcontroller time. It is illustrated through realistic simulations that it can be succesfully used for stator resistance measurement and update in field orientation schemes under both accelerating conditions and constant speed operating conditions of the motor maintaining output torque within 0.5% of the reference value even when a 10% abrupt change occurs in stator resistance of the motor. Work is in progress for implementation of the scheme.<<ETX>>

A sliding mode controller to minimize SRM torque ripple and noise

This paper is an attempt to eliminate the torque pulsation of switched reluctance (SR) motors. It is well known that pulsed radial and tangential forces are the source of acoustic noise problems often mentioned in relation to SR motors. In this study, to control the force pulsations, phase voltage is selected as the control variable contrary to what is often encountered in the literature. The authors believe that cost effective implementation of this choice is now technically possible. Selection of appropriate phase voltages is achieved by sliding mode control in order to produce the pro-defined torque references for each phase. The paper first illustrates via simulations that such an approach effectively eliminates the torque ripple and follows the defined reference, if the derivative of error function is appropriately chosen. The performance of the motor with different choice of sliding mode functions is given. The performance of the final choice is also experimentally illustrated. Various application issues of this type of controller are discussed.

Prediction of torque and inductance displacement characteristics of asymmetrically slotted variable-reluctance motors using a simplified model for numerical field solution

For prediction of static and dynamic performance of doubly-salient motors, it is essential to know their flux linkage-position-excitation characteristics and also the static torque characteristics. At the design stage determination of these characteristics presents difficulties because of highly nonlinear behavior of the magnetic circuit. It is possible to use numerical field solution of the complete motor to obtain this information. This, however, requires expertise on a professional program and may be expensive if used to search for the best design. This paper shows that a reduced model can be used to obtain the desired information accurately. It is also shown that in fact obtaining field solutions just for a pair of teeth is enough for accurately predicting the flux linkage and torque characteristics of a motor. The approach introduced here makes possible searching for an optimum design (even on a PC) for maximizing average torque or reducing noise and vibration problems, since the effort for producing the model and computation time are greatly reduced.

An integral equation approach to the computation of nonlinear fields in electrical machines

A numerical method based on an integral equation formulation, for the computation of nonlinear magnetostatic field, in two dimensions in cylindrical polar coordinates is given. The correctness of the method is illustrated by solving two linear two-dimensional magnetic field problems which have readily available analytical solutions. The dependence of the accuracy of the solution on the number and distribution of the meshes is studied on these examples. The method is then applied to the computation of the nonlinear field of a small salient pole synchronous machine. The technique used to accelerate the solution and other aspects of the solution are discussed. The program is used for the computation of magnetization characteristics, zero-power-factor characteristics, terminal voltages and their harmonics, and the field distribution in the air gap. Computed characteristics are compared with measurements. The results obtained illustrate that accurate and reasonably quick results are obtainable using this method. >

Choice of pole number of two-rotor axial-flux PM motor for control moment gyroscope

Modern low earth orbit satellites use control moment gyroscopes (CMG) for maneuvering. GMCs generally use a brushless DC motor to run an inertial wheel at about 10000 rpm. In this paper axial-flux BLDC motor is considered for this task with a view to design a low volume, low weight CMG. Various axial-flux (AF) motor topologies are possible. However, two-rotor AF topology is found to be more advantageous. The specifications of an existing CMG design, based on a commercial BLDC motor are taken as reference. The approach adopted for the design of the AF motor, to meet this specification is described, while minimizing the motor volume and maximizing the motor inertia. This approach is used for designing two axial flux motors one with 2-poles the other with 6-poles. The resulting CMG designs, based on AF motor are compared with the existing CMG. It is found that the new design promises to reduce the volume of the CMC by nearly 60%.