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This paper presents results from a design study on the feasibility of employing high-efficiency switched reluctance (SR) machines in minimal hybrid-electric vehicles. The application requirements are presented and highlight the constraining influences of the vehicle drive-line topology on the machine design. The benefit of continuous phase current excitation is reported for the first time, demonstrating that constant power at an extended-speed operation can be realized with a higher number of phase winding turns per pole than would otherwise be achieved with conventional discontinuous current control. Thus, the torque/Ampere capability, when operating at or below base speed, is not as significantly compromised, an important consideration for the power inverter rating and, hence, drive system cost. The design procedure and simulated results are validated by measurements from a prototype machine. The results demonstrate the potential of SR technology for high-performance low-cost automotive applications, which often combine arduous environmental and volumetric constraints. In addition, the results highlight the benefits of continuous current control for extended-speed operation. Index Terms--Automotive power-trains, electric vehicles, minimal hybrid-electric vehicles, switched reluctance machines.

A 3-D lumped-parameter magnetic circuit model is developed for a single-phase flux-switching permanent-magnet motor. Particular attention is given to representing the complex air-gap flux paths by equivalent permeances so as to accurately model the asymmetry in the air-gap field distribution and to determine the back-EMF and inductance waveforms, as well as the average static torque. Leakage fluxes external to the stator outer surface and the end surfaces on the back-EMF waveform are also taken into account. The developed model is used to investigate the influence of the motor axial length and magnet dimensions on the end effect. Good agreement between predicted, finite element-calculated, and measured results is achieved. Index Terms--Flux switching, lumped-parameter magnetic circuit (LPMC), permanent magnet (PM), single phase.

A general analytical model, formulated in 2-D polar coordinates, is developed to predict the unbalanced magnetic force, which results in permanent-magnet brushless ac and dc machines having a diametrically asymmetric disposition of slots and phase windings. It is shown that the unbalanced magnetic force can be significant in machines having a fractional ratio of slot number to pole number, particularly when the electric loading is high. The developed model is validated by finite-element calculations on 9-slot/8-pole and 3-slot/2-pole machines. In addition, the unbalanced magnetic force has been measured on a prototype 3-slot/2-pole machine and shown to be in excellent agreement with predicted results. Index Terms--Brushless machines, magnetic field, permanent magnet (PM), unbalanced magnetic force, unbalanced magnetic pull.

This paper presents a computationally efficient technique in determining the influence of the finite axial length of the permanent magnets on the rotor eddy-current loss in modular brushless ac machines. The technique is employed to quantify the effectiveness of axially segmenting the permanent magnets of four-phase, eight-slot, ten-pole and five-phase, ten-slot, 12-pole brushless machines, which have been designed to meet the performance requirements of an electromechanical flight control surface actuator for a 'more-electric' aircraft. Index Terms--Eddy currents, permanent-magnet machines.

This paper describes an improved implementation of direct torque control (DTC) to a permanent-magnet brushless dc (BLDC) drive. The commutation torque ripple, which occurs every 60[degrees] dec. in a conventional three-phase BLDC machine with two-phase 120[degrees] elec. conduction, is reduced by employing a hybrid two- and three-phase switching mode during the commutation periods. It is based on the criterion of minimizing the error between the commanded torque and the estimated torque and does not require knowledge of the conduction duration of the three-phase switching mode. It adaptively adjusts the phase-current waveform to maintain constant electromagnetic torque so that commutation torque ripple, which would have resulted with conventional DTC, particularly at high rotational speeds, is effectively eliminated, as confirmed by both simulations and measurements. Index Terms--Brushless dc (BLDC) drives, current commutation, direct torque control (DTC), permanent-magnet (PM) motor, torque ripple.

While methods of determining the iron loss in rotating permanent-magnet (PM) machines have been investigated extensively, the study of iron loss in linear machines is relatively poorly documented. This paper describes a simple analytical method to predict flux density waveforms in discrete regions of the laminated stator of a tubular PM machine, and employs an established iron loss model to determine the iron loss components, on both no load and on load. Analytical predictions are compared with the iron loss deduced from finite-element analyses for two tubular PM machine designs, and it is shown that if a machine has a relatively high electrical loading, the on-load iron loss can be significantly higher than the no-load value. Index Terms--Analytical model, iron loss, linear machines, permanent-magnet (PM) machines.

The application of direct torque control (DTC) to brushless ac drives has been investigated extensively. This paper describes its application to brushless dc drives, and highlights the essential differences in its implementation, as regards torque estimation and the representation of the inverter voltage space vectors. Simulated and experimental results are presented, and it is shown that, compared with conventional current control, DTC results in reduced torque ripple and a faster dynamic response. Index Terms--Brushless dc (BLDC) drives, direct torque control (DTC), permanent-magnet motor.

This paper presents a comparative study of three-phase permanent-magnet brushless machines in which the slot and pole numbers are similar, with reference to conventional brushless dc machines in which the ratio of the slot number to pole number is usually 3 : 2. Three different motor designs are considered. Two have equal tooth widths, with one having a coil wound on every tooth and the other only having a coil wound on alternate teeth, while the third machine also has coils wound on alternate teeth but these are wider than the unwound teeth while the width of their tooth tips is almost equal to the rotor pole pitch in order to maximize the flux linkage and torque. Analytical and finite-element methods are employed to predict the flux-linkage and back-electromotive-force waveforms, and the self- and mutual-inductances, and these are shown to be in good agreement with measured results. It is also shown that the third machine is eminently appropriate for brushless dc operation. Index Terms--Back electromotive force (EMF), brushless machines, flux linkage, fractional slot, permanent magnet.

The sensorless control of brushless machines by detecting the third harmonic back electromotive force is a relatively simple and potentially low-cost technique. However, its application has been reported only for brushless dc motors operating under normal commutation. In this paper, the utility of the method for the sensorless control of both brushless dc and ac motors, including operation in the flux-weakening mode, is demonstrated. Index Terms--Back electromotive force (EMF), brushless drives, permanent-magnet machines, sensorless control.

This paper discusses the suitability of four-phase, five-phase, and six-phase modular machines, for use in applications where servo characteristics and fault tolerance are key requirements. It is shown that an optimum slot number and pole number combination exists, for which excellent servo characteristics could be achieved, under healthy operating conditions, with minimum effects on the power density of the machine. To eliminate torque ripple due to residual cogging and various fault conditions, the paper describes a novel optimal torque control strategy for the modular permanent-magnet machines operating in both constant torque and constant power modes. The proposed control strategy enables ripple-free torque operation to be achieved, while minimizing the copper loss under voltage and current constraints. The utility of the proposed strategy is demonstrated by computer simulations on a four-phase fault-tolerant drive system. Index Terms--Fault tolerant, modular machines, optimal control, permanent-magnet machines, servo drives.

For high-speed machines, in particular, it is very important to accurately predict natural frequencies of the rotor at the design stage so as to minimize the likelihood of failure. Finite-element analysis and experimental measurements are used to establish the natural frequencies and modes of the rotor of a high-speed permanent-magnet brushless motor, and to assess the influence of leading design parameters, such as the active length, the shaft diameter and extension, the bearings, and the material properties. Index Terms--Brushless motor, high speed, mechanical resonance, permanent-magnet machine, vibration.

The application of flux-observer-based sensorless control to permanent-magnet brushless ac motor drives is described. Current methods of speed estimation are assessed, both theoretically and experimentally, and an improved method, which combines the best features of methods in which speed is derived from the differential of rotor position and from the ratio of the electromotive force to excitation flux linkage, is proposed. Its performance is verified experimentally. Index Terms--Brushless ac drives, flux observer, permanent-magnet machines, sensorless control, speed estimation.

An enhanced online optimal control strategy, which maximizes the flux-weakening performance of a brushless ac motor, is described, and applied to motors having different rotor topologies: interior (radial or circumferential), inset, and surface-mounted magnet. It enables the maximum inherent power capability of a brushless ac motor to be achieved independent of any variation in its parameters, and facilitates maximum efficiency over the entire speed range. It also results in good transient dynamic performance, since it is coupled with feedforward vector control based on optimal current profiles. Index Terms--Brushless ac drive, flux weakening, optimal control, permanent-magnet machine.

This paper considers the robust control of an active radial magnetic bearing system, having a homopolar, external rotor topology, which is used to support an annular fiber composite flywheel rim. A first-order dynamical compensator, which uses only position feedback information, is used for control, its design being based on a linearized one-dimensional second-order model which is treated as an interval system in order to cope with parameter uncertainties. Through robust stability analysis, a parameterization of all first-order robustly stabilizing dynamical compensators for the interval system is initially obtained. Then, by appropriate selection of the free parameters in the robust controller, the [H.sub.2] norm of the disturbance-output transfer function is made arbitrarily small over the system parameter intervals, and the [H.sub.[infinity]] norm of the input-output transfer function is made arbitrarily close to a lower bound. Simulation and experimental results demonstrate both stability and performance robustness of the developed controller. Index Terms--Disturbance attenuation, interval systems, magnetic bearing, robust stabilization.

The eddy-current loss in the permanent magnets of brushless ac machines is usually neglected, since the fundamental air-gap field usually rotates in synchronism with the rotor, and time harmonics in the current waveform and space harmonics in the winding distribution are generally small. However, an important category of brushless ac machine design is emerging in which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy currents. An analytical model is developed to predict rotor-induced eddy currents in such machines, and to quantify the effectiveness of circumferentially segmenting the permanent magnets in reducing the rotor loss. Index Terms--Brushless machines, eddy currents, losses.

Flux-switching permanent-magnet (FSPM) brushless machines have attracted considerable interest as a candidate machine technology for applications requiring high torque density and robust rotors. To date, published findings have focused exclusively on single- and three-phase FSPM machines. This paper investigates FSPM brushless machines of higher phase numbers by means of a detailed comparison of the electromagnetic performances of three-, four-, five-, and six-phase variants within the specific context of aerospace machine. Machines having both all poles and alternate poles wound are investigated, with the latter offering scope to reduce mutual coupling between phases so as to achieve improved fault tolerance. The finite-element (FE)-predicted electromagnetic performances in both machines, such as electromotive force waveform, winding inductance, cogging torque, and static torque, are validated by the experiments made on a small-scale five-phase FSPM machine. The nature of the machine specification requires that consideration must be given to mechanical stress in the rotor and the tradeoff with electromagnetic design considerations, notably the degree of rotor saliency which can be incorporated. Therefore, a mechanical FE study of the rotor mechanical stresses of multiphase FSPM machines is also comparatively assessed. [ABSTRACT FROM AUTHOR]

This paper presents an improved signal-injection-based sensorless-control method for permanent-magnet brushless ac (BLAC) motors, accounting for the influence of cross-coupling magnetic saturation between the d- and q-axes. The d- and q-axis incremental self-inductances, the incremental mutual inductance between the d-axis and q-axis, and the cross-coupling factor are determined by finite-element analysis. An experimental method is proposed for measuring the cross-coupling factor which can be used directly in the sensorless-control scheme. Both measurements and predictions show that a significant improvement in the accuracy of the rotor-position estimation can be achieved under both dynamic and steady-state operation compared with that which is obtained with the conventional signal-injection method. [ABSTRACT FROM AUTHOR]

A key issue in the direct torque control of permanent-magnet brushless dc motors is the estimation of the instantaneous electromagnetic torque, while sensorless control is often advantageous. A sliding-mode observer is employed to estimate the nonsinusoidal back-electromotive-force waveform, and a simplified extended Kalman filter is used to estimate the rotor speed. Both are combined to calculate the instantaneous electromagnetic torque. The effectiveness of this approach is validated by simulations and measurements. [ABSTRACT FROM AUTHOR]

The performance of the conventional predictive current control (PCC) when applied to a permanent-magnet brushless ac (PM BLAC) drive is analyzed. A technique is proposed to reduce the current errors that arise due to the inaccuracies in the system parameters and the nonideal behavior of the inverter during a steady-state operation. In addition, a current-regulated delta modulator is employed to achieve a fast dynamic response during a transient operation. The performance of the proposed PCC, which is simple to implement on a low-cost fixed-point DSP, is demonstrated experimentally. [ABSTRACT FROM AUTHOR]