A comparative study of permanent magnet and switched reluctancemotors for high-performance fault-tolerant applications

Switched reluctance machines (SRMs) have been extensively researched for high performance aerospace applications because of their inherent fault tolerance. Recent work by the authors suggests that with careful design a similar degree of fault tolerance may be achieved with a permanent magnet machine (PM) at a significantly higher torque density than its SRM counterpart. This latter suggestion is of crucial importance in high performance applications, such as aerospace, where a low mass is essential. This paper is concerned with reporting measured results from a fault tolerant PM machine, and using these results to examine the relative merits of SR and PM technology for this application     

Three-dimensional finite-element investigation of current crowdingand peak temperatures in VLSI multilevel interconnections

Multilevel interconnection vias are investigated using a coupled thermal and electrical conduction model in three dimensions and the finite-element method. The concept of using a volume to quantify current crowding is introduced. Close to a contact via it is found that the peak temperature moves between the upper track and the via as a function of geometry     

Conventional and TFPM linear generators for direct-drive wave energy conversion

The archimedes wave swing (AWS) is a system that converts ocean wave energy into electric energy. The goal of the research described in this paper is to identify the most suitable generator type for this application. Of the conventional generator types, the three-phase permanent-magnet synchronous generator with iron in both stator and translator is most suitable, because it is cheaper and more efficient than the induction generator, the switched reluctance generator, and the permanent-magnet (PM) generator with an air-gap winding. The paper also proposes a new transverse-flux PM (TFPM) generator topology that could be suitable for this application. This new double-sided moving-iron TFPM generator has flux concentrators, magnets, and conductors on the stator, while the translator only consists of iron.     

Efficiency trends in electric machines and drives

Almost all electricity in the UK is generated by rotating electrical generators, and approximately half of it is used to drive electrical motors. This means that efficiency improvements to electrical machines can have a very large impact on energy consumption. The key challenges to increased efficiency in systems driven by electrical machines lie in three areas: to extend the application of variable-speed electric drives into new areas through reduction of power electronic and control costs; to integrate the drive and the driven load to maximise system efficiency; and to increase the efficiency of the electrical drive itself. In the short to medium term, efficiency gains within electrical machines will result from the development of new materials and construction techniques. Approximately a quarter of new electrical machines are driven by variable-speed drives. These are a less mature product than electrical machines and should see larger efficiency gains over the next 50 years. Advances will occur, with new types of power electronic devices that reduce switching and conduction loss. With variable-speed drives, there is complete freedom to vary the speed of the driven load. Replacing fixed-speed machines with variable-speed drives for a high proportion of industrial loads could mean a 15–30% energy saving. This could save the UK 15 billion kWh of electricity per year which, when combined with motor and drive efficiency gains, would amount to a total annual saving of 24 billion kWh.     

The modeling of switched reluctance machines with magneticallycoupled windings

This paper presents a method of modeling switched reluctance motors with strongly coupled windings, such as switched reluctance machines (SRMs) with fully pitched windings or with commutating windings. These machines are particularly difficult to model because, unlike a conventional SRM, the flux linking any one winding of the machine is a nonlinear function of rotor position and all phase currents. The problem is overcome by breaking down the instantaneous flux linkages and currents in each phase to give the flux and magnetomotive force (MMF) in each tooth of the machine. The flux and MMF per tooth can then be linked in the model by a simple lookup table, determined from either simulation or measurement. The paper describes the simulation method in detail and then gives extensive comparisons between measurement and simulation results for an SRM with fully pitched windings, operating over a range of operating conditions. The results presented also serve to give an insight into the method of operation of these rather complex machines     

Preliminary performance evaluation of switched reluctance motors with segmental rotors

This paper examines the performance of switched reluctance machines which employ a segmental rotor construction in preference to the usual toothed structure. Two three-phase designs are considered, one in which the windings span a number of teeth and one in which they span a single tooth. Two demonstrators have been built-one for each design type, and their performances are compared with both conventional switched reluctance motors (SRMs) and a rare earth permanent-magnet machine. It is shown how these machines can operate from a standard SRM converter: running test results are presented and there is a discussion of general operating experience, ranging from the measurement of mean torque, voltage, and current-controlled operation to general thermal performance.     

Noise characteristics of cascaded 2nd-order active filters

The absolute noise level at the output of single-amplifier, 2nd-order active filters, suitable for cascade connection, is derived in terms of the sensitivity of the filter response to the amplifier gain. A very simple expression is obtained, which is shown to be valid under practical conditions. A similar expression is obtained for cascaded 2nd-order sections, and a 4th-order example shows that the position of the sections in the cascade has a significant effect on the output noise level. The cascade positions giving lowest noise level are the opposite of those normally used to give maximum dynamic range.     

New winding configurations for doubly salient reluctance machines

This paper develops the concept of employing a fully pitched winding in a doubly-salient reluctance motor. In a conventional switched reluctance machine, there is complete magnetic decoupling between phases, so that torque is produced entirely due to the rate of change of winding self-inductance. This effectively limits each phase to contributing to positive torque production to a maximum of one-half of the cycle of rotation. If the same machine is wound with fully pitched windings, then it can be shown that the new configuration produces torque entirely as a result of changing mutual inductance between phases, while maintaining a unipolar conduction sequence. Compared with the conventional short-pitched winding machine, far better utilization of the electric circuit results, with each phase contributing to positive torque production for at least two-thirds of the cycle of rotation. A subsequent increase in torque per unit volume inevitably results. The application of such an idea to three-phase switched reluctance motors (SRMs) is developed theoretically and through a series of experimental tests upon a 7.5 kW machine, showing static torque curves for the new machine and comparing them to a conventionally wound SRM     

Design principles for a flywheel energy store for road vehicles

This paper introduces a flywheel energy storage device capable of enhancing the fuel economy of a hybrid-type road vehicle. A number of possible drive types are considered and the permanent magnet machine drive is shown to provide the best solution. Reasons for selecting a device using an axial-field configuration with single-rotor and double-stator sections are described. Electrical, magnetic, and mechanical design data are presented for a full-scale prototype device with 240 kJ of usable energy storage and 25 kW of power transfer, operating at speeds up to 50 000 r/min, with a 10% duty cycle     

Permanent-magnet machines with powdered iron cores and prepressedwindings

This paper presents a permanent-magnet servomotor, built with a powdered iron stator, which has been designed to take full advantage of the properties of the material. There are a number of important design features which are not possible with laminated cores. For example, the core back is axially extended over the end windings, the armature core components are subdivided and preformed stator coils are used. The coils are prepressed to form a solid component with an exceptionally high fill factor (78%). The prototype offers far superior performance to a conventionally laminated commercial machine, manufactured in the same frame