Machine Magnetic Circuit Research Articles

Use of Bulk Superconductors in the Excitation System of Low-Speed Synchronous Generators

This work focuses on implementing HTS bulks in the excitation circuit of low-speed synchronous generators as permanent magnets. In addition to the implementation study, it is also addressed to remagnetize HTS bulks in electric generators. To reduce the maintenance costs and time required for the magnetization process, an in-loco remagnetization is used. The HTS bulks are magnetized in their position inside the machine's magnetic circuit. A pulse-field magnetization (PFM) technique is used, using the stator coils of a linear generator. YBCO and GdBCO samples are tested. The experimental procedure starts with the in-loco PFM to magnetize the HTS bulks and, by applying an oscillatory movement to the linear generator's moving part, measuring the magnetic flux density induced voltage is done. Experimental tests show that, for the FeSi core used in the generator, the YBCO material is most suitable for the excitation circuit.

Modifying the Pulsed-Field-Magnetization Technique for HTS Bulks in Electrical Machines Without Magnetic Field Sensors

This paper studies the high-temperature superconductor (HTS) YBCO and GdBCO bulks pulse-field magnetization (PFM) in loco , that is, when inserted in an electric machine's magnetic circuit. The PFM typically uses cryogenic hall sensors to monitor the magnetization process. However, to install cryogenic hall sensors in the magnetic excitation circuit of an electrical machine may increase the air gap and increase the machine cost. To surpass that, this paper presents an indirect technique capable of monitoring the superconductor magnetization process through the applied pulse voltage and associated electric current transient waveform. Last, different levels and the numbers of pulses were applied to magnetize the HTS bulks inside the magnetic circuit to analyze the resultant trapped magnetic field. Differences between the YBCO and GdBCO are highlighted and discussed.

Design Considerations for the Tooth Shoe Shape for High-Speed Permanent Magnet Generators

This paper presents a study of the effects that the shoe shape of the teeth of electrical machines has on the performance and losses. This is done by considering a concentrated wound, high-speed permanent magnet generator. The paper investigates the influence of the tooth shoe shape on the machine magnetic circuit and the losses distribution based on analytical and finite-element analysis (FEA). A shape coefficient $K_{ {t}}$ is proposed to provide an optimized design reference. A comprehensive analytical tool able to study the variations of the machine performance parameters is proposed. The deduced optimization function is normalized using the non-equilibrium relative weighting method, and then, it is processed via a genetic algorithm to achieve the optimized design. FEA is used to validate the proposed analytical tool and the optimum design.

Electrothermal Combined Optimization on Notch in Air-Cooled High-Speed Permanent-Magnet Generator

A 30kVA, 96000rpm, air cooled high-speed permanent magnetic generator (HSPMG) is investigated in this paper. Considering effects on both the magnetic circuit and heat transfer paths comprehensively, the stator slot notch in this HSPMG is optimized. First, by using the time-stepping finite element method, the transient electromagnetic fields of HSPMG is numerically calculated, and the electromagnetic losses in different components are obtained. Then, after the determination of other mechanical losses in such a machine, a three-dimensional fluid-thermal coupling calculation model is established, and the working temperature distribution in the HSPMG is studied. Thus, the electromagnetic-fluid-thermal coupling analysis method on the HSPMG is proposed, by using which the influences of machine notch height on machine magnetic circuit and cooling air flowing path are investigated. Meanwhile, both the electromagnetic performance and the temperature distribution in HSPMG with different stator notch height are studied, and a series of analytical equations are deduced to describe the variations of machine performances with stator notch. By using the proposed unbalance relative weighting method, the notch height is optimized to enhance the performance of HSPMG. The obtained conclusions could provide reference for HSPMG electromagnetic calculation, cooling system design, and optimization design.

Adaptive control strategy with flux reference optimization for sensorless induction motors

Avoiding mechanical (speed, torque) sensors in electric motor control entails cost reduction and reliability improvement. Furthermore, sensorless controllers (also referred to output-feedback) are useful, even in the presence of mechanical sensors, to implement fault tolerant control strategies. In this paper, we deal with the problem of output-feedback control for induction motors. The solutions proposed so far have been developed based on the assumption that the machine magnetic circuit characteristic is linear. Ignoring magnetic saturation makes it not possible to meet optimal operation conditions in the presence of wide range speed and load torque variations. Presently, an output-feedback control strategy is developed on the basis of a motor model that accounts for magnetic saturation. The control strategy includes an optimal flux reference generator, designed in order to optimize energy consumption, and an output-feedback designed using the backstepping technique to meet tight speed regulation in the presence of wide range changes in speed reference and load torque. The controller sensorless feature is achieved using an adaptive observer providing the controller with online estimates of the mechanical variables. Adaptation is resorted to cope with the system parameter uncertainty. The controller performances are theoretically analyzed and illustrated by simulation.

On determination of inductances for circuit models of AC machines

Abstract The paper discuss a problem of determination of inductances for AC machine windings when saturation of magnetic circuit is not neglected. For such cases, computation of magnetic field distribution in the machine magnetic circuit is a starting point for post processing procedures leading to various values, among others the co-energy in a given area and linkage fluxes of windings. This paper shows how to determine winding inductances in a nonlinear magnetic circuit from these two values and also how to compute directly nonlinear inductances. Problem is not trivial because such inductances are not uniquely determined as for linear case. In the paper a definition of nonlinear inductances is proposed which makes the choice unique.

Towards a global control strategy for induction motor: Speed regulation, flux optimization and power factor correction

A great deal of interest has been paid to induction machine control over the last years. However, most previous works have focused on the speed/flux/torque regulation supposing the machine magnetic circuit to be linear and ignoring the machine power conversion equipments. The point is that speed regulation cannot be ensured in optimal efficiency conditions, for a wide range of speed-set-point and load torque, unless the magnetic circuit nonlinearity is explicitly accounted for in the motor model. On the other hand, the negligence of the power conversion equipments makes it impossible to deal properly with the harmonic pollution issue due to ‘motor – power supply grid’ interaction. This paper presents a theoretical framework for a global control strategy of the induction machine and related power equipments. The proposed strategy involves a multi-loop nonlinear adaptive controller designed to meet the three main control objectives, i.e. tight speed regulation for a wide range speed-reference variation, flux optimization for energy consumption and power factor correction (PFC). Tools from the averaging theory are resorted to formally describe the control performances.

Output Feedback Control for Induction Machine in Presence of Nonlinear Magnetic Characteristic

Output feedback control of induction motors has recently been given a great deal of interest. It consists in resorting to observers to avoid mechanical (speed, torque) sensors and make the control system more reliable. The point is that all existing solutions have been developed based on the assumption that the machine magnetic circuit characteristic is linear which is not true in practice, due to magnetic saturation. Ignoring this saturation effect makes it impossible to meet optimal operation conditions in presence of wide range speed and load torque variations. This paper presents an output feedback control design that accounts this saturation effect. The controller includes an interconnected observer, an optimal flux reference generator and a control law obtained using the backstepping design technique. It is shown that the controller regulates well the motor speed and the rotor flux in presence of wide range variations of the machine speed reference and load torque.

High Gain Observer for Induction Motor in Presence of Magnetic Hysteresis

AbstractThe problem of state estimation in induction motors is considered in this paper. In most previous works, motor observers have been designed using standard models, neglecting the saturation and hysteretic effects in the machine magnetic circuit. As a matter of fact, these effects cannot be ignored especially when considering (speed, torque) control strategies involving wide range flux variations, necessary to meet optimal operation conditions when facing significant load torque changes. In this paper, a high gain observer is designed for induction motors based on a model that accounts for the nonlinear hysteretic nature of the magnetic circuit. The observer provides estimates of the magnetizing current and magnetic variables using available measurements i.e. stator currents and voltages, rotor speed. The observer is formally shown to enjoy nice convergence properties. The theoretical result is confirmed by simulation.

Accounting for magnetic saturation in induction machines modelling

A new model is developed for (uniform air-gap) induction motor that accounts for the saturation feature of the machine magnetic circuit. It is built-up starting from the basic electrical/mechanical laws and turns out to be quite different from the standard model that is widely used in control oriented literature. Specifically, the new model involves state-dependent parameters. An experimental validation using a real 7.5 KW machine proved the good accuracy of the proposed model. A simpler control-oriented version of the model is also developed and shown to be quite representative of the machine.

Effects of converter excitation on the performance of axially laminated anisotropic synchronous reluctance motor drives

An indirectly coupled state space-finite element (ICSS-FE) model of axially laminated anisotropic (ALA) synchronous reluctance motor (SynRM) drive systems is used to evaluate the impact of converter excitation on the machine performance characteristics. The model utilizes an iterative approach which includes the full impact of nonlinearities on the machine magnetic circuit, and which results in the nonsinusoidal flux waveforms in the different parts of the machine. The modeling approach is applied to a prototype ALA SynRM designed for use in hybrid vehicle applications.

Comprehensive salient-pole synchronous machine parametric design analysis using time-step finite element-state space modeling techniques

This paper details the application of a time-stepping coupled finite element-state space (CFE-SS) model to predict a salient-pole synchronous generator's parameters and performance, including damper bar current profiles and bar losses as well as iron core (including pole face) losses, under various operating conditions. The CFE-SS modeling environment is based on the natural ABC flux linkage frame of reference, which is coupled to a time/rotor stepping FE magnetic field and machine winding inductance profile computation model. This allows one to rigorously include the synergism between the space harmonics generated by magnetic saturation and machine magnetic circuit as well as winding layout topologies, and the time harmonics generated by the nonsinusoidal phase currents, ripple rich field excitation and damper bar currents. The impact of such synergism between these space and time harmonics on damper bar current profiles and losses, iron core losses, various machine winding current, voltage and torque profiles/waveforms is studied here for a 10-pole, 44.9 kVA, 17,143 RPM, 1428.6 Hz, 82 V (L-N), wound-pole aircraft generator.

Effects of chopping on core losses and inductance profiles of SRM drives

A coupled finite element-state space (CFE-SS) model of switched reluctance motor (SRM) drive systems is used to evaluate the impact of chopping on the machine performance characteristics. The model is used to predict the impact of chopping on the machine inductance profiles, armature currents, and core losses. The modeling environment utilizes an iterative approach which includes the full impact of nonlinearities on the machine magnetic circuit, that results in the nonsinusoidal flux density waveforms in the different parts of the machine, modeling approach is applied to a four phase SRM. Furthermore, the predicted results are verified by comparison to experimental data.

LOCALIZATION OP THE LOSSES IN AN INDUCTION MACHINE SUPPLIED BY AN INVERTER

ABSTRACT By the measurement of the temperatures in different points of the machine (rotor, stator, coils and magnetic circuit of the stator) and to a simplified thermal model of the motor, the value and the localization of the losses for each type of supply can be deduced. The method consists of determining the distribution of the losses so that the theoretical heating curves obtained from the thermal model coincide with the experimental heating curves. Some parameters used for the elaboration of the then.-al model are poorly known, therefore, we determine them by simple tests of the motor for which the distribution of the losses is well known. Then, we test our method in the case of a sinusoidal supply and we exhibit the extra losses in the rotor, which are difficult to identify with another method. After that, we can study the distribution of the different losses in the motor when it is supplied by an inverter