Prospects for magnetization of large PM rotors: conclusions from a development case study

A development feasibility study has been conducted for magnetizing the rotor of a 1.7 MVA, 20-r/min, permanent magnets (PMs) excited synchronous wind turbine generator. Various technical problem areas and their likely influence on the overall design concept, are reviewed. Thus, some particularly important problems are identified: KERI's conventional facilities ensure sufficient energy for magnetizing, pair by pair, of all the PMs of the rotor. A rotor and inductor optimal arrangement for a magnetizing system's configuration is required in order to avoid a wrong, strong magnetization of the neighboring poles. It is also necessary to take into account the mechanical strength of the magnetizing inductor winding to withstand electromagnetic forces. This paper presents the result of the analytical calculation of such a specialized inductor and a PC Spice-based software simulation of the magnetizer circuit; finally, a transient finite element method (FEM) simulation of the inductors connected by the external electric circuit has been done. All three methods confirm the possibility for a successful magnetization, pair by pair, of all rotor poles. Design proposals are detailed and conclusions are relevant for magnetizing large rotors with surface-mounted PMs in general.     

Dynamic study of an electronically brushless DC machine viacomputer simulations

The authors analyze a machine with a conventional DC armature winding on the stator which does not have a field winding connected with slip rings. At positions where the winding would have been attached to a commutator segment, a transistor switch is attached instead. The rotor contains a permanent magnet field. The lap wound stator winding is sequentially commutated by a system consisting of a position sensor and logic circuitry. Analytical expressions for a full pitch lap wound machine are given. A computer simulation of the machine was implemented. The results serve to illustrate the considerations which must be made in the design of this type of machine, in particular the effects of the winding design on commutation and cost efficiency     

Analysis and Optimization of Back EMF Waveform of a Flux-Switching Permanent Magnet Motor

Flux-switching permanent magnet (FSPM) motors have a doubly salient structure, the magnets being housed in the stator and the stator winding comprising concentrated coils. They have attracted considerable interest due to their essentially sinusoidal phase back electromotive force (EMF) waveform. However, to date, the inherent nature of this desirable feature has not been investigated in detail. Thus, a typical three-phase FSPM motor with 12 stator teeth and ten rotor poles is considered. It is found that, since there is a significant difference in the magnetic flux paths associated with the coils of each phase, this results in harmonics in the coil back EMF waveforms being cancelled, resulting in essentially sinusoidal phase back EMF waveforms. In addition, the influence of the rotor pole-arc on the phase back EMF waveform is evaluated by finite-element analysis, and an optimal pole-arc for minimum harmonic content in the back EMF is obtained and verified experimentally.      

Analytical prediction of eddy-current loss in modular tubular permanent-magnet machines

The paper describes an analytical technique for predicting the eddy-current loss in the moving armature of a tubular permanent magnet machine. This loss component is usually neglected in conventional tubular permanent magnet machines since high-order time harmonics in the stator current waveform and space harmonics in the winding magnetomotive force (MMF) distribution are generally considered to be insignificant. However, a relatively new topology of tubular permanent magnet machine, sometimes referred to as "modular", has emerged in which the fundamental component of the stator MMF has fewer poles than that of the permanent-magnet armature, the thrust force being developed by the interaction between a higher order MMF harmonic and the permanent magnet field. Thus, the presence of lower and higher order space harmonics in the winding MMF distribution of a modular machine may gives rise to a significant eddy-current loss in the moving-magnet armature. An analytical model is developed to predict the eddy currents which are induced in the magnets, as well as in any electrically conducting supporting tube which may be employed, and to quantify the effectiveness of axially segmenting the magnets in reducing the eddy-current loss. The validity of the developed model, which is also applicable to conventional designs of tubular permanent-magnet machine, is verified by time-stepped transient finite-element analysis (FEA).     

Design and Analysis of an Interior Permanent Magnet (IPM) Machine With Very Wide Constant Power Operation Range

This paper evaluates the design of a new segmented interior permanent magnet (SIPM) machine with very wide constant power operation range. The machine uses segmentation of the rotor magnets with the help of iron bridges between magnet segments. This provides for an inherent flux-weakening capability to the interior permanent magnet (IPM) machine. A very wide flux-weakening range has been achieved by using a standard distributed stator winding only. The flux-weakening capability of the proposed SIPM motor was compared to a conventional IPM machine. The same stator was used in both machines for direct comparison. The paper also presents the design optimization process and experimental verification of the performance of the proposed machine as a case study of a 42 V alternator operation in which the speed range is from 600 to 6000 r/min.     

Investigation and Simulation of Fields in Large Salient-Pole Synchronous Machines With Skewed Stator Slots

This paper presents the results of an investigative study performed on a large salient-pole synchronous machine with skewed stator slots. The study was carried out to aid the understanding of the phenomena related to electromagnetic field distribution, rotor damper currents and pole face iron losses in machines with skewed stator slots and an armature winding design with a fractional number of stator slots per pole. A hydrogenerator was specially instrumented to measure the variation of magnetic fields along the axial length of the machine and the induced currents in the damper bars on the rotor poles. A computational model has been developed to predict the air-gap flux density, damper currents and rotor iron losses. Measured results are compared with those predicted by simulation.     

Analytic modelling and optimization of slip synchronous permanent magnet wind turbine generator topologies

The modular slip‐synchronous permanent magnet generator (SSPMG) is viewed as an induction‐synchronous machine pair that is electromagnetically decoupled by a free‐rotating rotor that in turn houses two different sets of permanent magnets. This machine pair combines the advantages of both conventional induction and permanent magnet synchronous machines. It therefore has the potential to realize a new path in reliable, robust and cost‐effective wind turbine drivetrains. However, which electromagnetic SSPMG topology is best and how does it compare with conventional drivetrain designs for various capacities? To date, the most published SSPMG advances are specific to winding design, torque quality and performance optimization in the small capacity range. This paper presents optimized analytic electrical designs of modular, radially and axially separable, radial flux SSPMG topologies of capacities ranging from 100 kW to 5 MW. Designs are based on lumped analytic models and are optimized for minimum specific active mass (mass/torque). A rated efficiency of 95 % and an inductive power factor of 0.95 are applied to all designs. The analytic models are validated with transient two‐dimensional finite element analysis results. The best SSPMG topologies are determined and compared with conventional drivetrain designs. The axially separable topology seems to be the best SSPMG design. Copyright © 2014 John Wiley & Sons, Ltd.     

A lightweight low‐speed permanent magnet electrical generator for direct‐drive wind turbines

The evolution, design and test results of a novel permanent magnet generator for use in direct‐drive wind turbines are presented. This generator topology is based on steel C‐core modules (which make up the rotor) and an air‐cored stator winding. This topology allows a reduction in structural mass for large diameter generators, which can lead to lightweight generators. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of toothless stator design on dynamic model parameters ofpermanent magnet generators

The effects of toothless stator design on the dynamic model parameters of permanent magnet (PM) generators are presented. These parameters, which include inductances and induced back EMFs, are determined for a 75 kVA, 208 V, 400 Hz, two-pole, permanent magnet generator. Two particular stator designs, a toothless stator, and a conventional type stator (with iron teeth), are considered. The method which is used to determine these parameters is general in nature and is based on the use of a series of magnetic field solutions. A validation of the computed EMF and inductance values based on experimentally obtained data is given. The results of using these parameters in a state space model in the abc frame of reference to study the effects of a toothless stator design on the PM generator synchronous inductances are presented. Based on that, it is demonstrated that the effects of rotor saliency and armature loading on the machine parameters are minimized in the case of the toothless design     

Transient model of a doubly excited reluctance motor

A transient machine model of a doubly fed reluctance motor is derived by means of winding function and d-q transformation theory. The machine consists of a double-wound stator with four and eight pole sets. The rotor is equipped with six poles. The machine, related to the Hunt motor, has the synchronous speed of a twelve-pole machine. Comparison of simulated results to test results indicates that the higher harmonics in the motor inductances are important for predicting the current waveform     

小型风力发电机电磁设计特点

文章分析了小型风电机组用的永磁同步发电机的电磁设计特点,并与其他普通永磁电机区别,详细介绍了永磁同步发电机各个电磁参数的特点以及对发电机性能的影响。重点介绍永磁同步发电机定子、转子设计的特点,对不同的极数与槽数的配合进行对比分析,总结降低齿槽转矩的规律。     

Impact of Winding Layer Number and Magnet Type on Synchronous Surface PM Machines Designed for Wide Constant-Power Speed Range Operation

This paper thoroughly investigates the impact of the winding layer number and the choice of magnet type on the performance characteristics of surface permanent magnet (SPM) machines with fractional slot concentrated windings designed for wide speed ranges of constant-power operation. This is accomplished by carefully examining the performance characteristics of three different SPM machines designed for the same set of performance requirements drawn from an automotive direct-drive starter/alternator application. These results show that double-layer stator windings yield lower torque ripple and magnet eddy current losses than do single-layer windings, but can contribute to a lower overload torque capability. Although the adoption of sintered magnets leads to the highest machine torque density, bonded magnets result in a significant reduction of the magnet losses because of their much higher value of resistivity.     

Minimization of iron losses of permanent magnet synchronous machines

In permanent magnet (PM) synchronous machines, iron losses form a larger portion of the total losses than in induction machines. This is partly due to the elimination of significant rotor loss in PM machines and partly due to the nonsinusoidal flux density waveforms in the stator core of PM machines. Therefore, minimization of iron losses is of particular importance in PM motor design. This paper considers the minimizing of iron losses of PM synchronous machines through the proper design of magnets and slots, and through the choice of the number of poles. Both time-stepped finite element method (FEM) and the iron loss model from a previous study are used in this paper to draw the conclusions.     

Characterization of coil faults in an axial flux variable reluctance PM motor

Variable-reluctance (VR) and switch-reluctance (SR) motors have been proposed for use in applications requiring a degree of fault tolerance. A range of topologies, of brushless SR and VR permanent-magnet (PM) motors are not susceptible to some types of faults, such as phase-to-phase shorts, and can often continue to function in the presence of other faults. In particular, coil-winding faults in a single stator coil may have relatively little effect on motor performance but may affect overall motor reliability, availability, and longevity. It is important to distinguish between and characterize various winding faults for maintenance and diagnostic purposes. These fault characterization and analysis results are a necessary first step in the process of motor fault detection and diagnosis for this motor topology. This paper examines rotor velocity damping due to stator winding turn-to-turn short faults in a fault-tolerant axial flux VR PM motor. In this type of motor, turn-to-turn shorts, due to insulation failures, have similar I-V characteristics as coil faults resulting from other problems, such as faulty maintenance or damage due to impact. In order to investigate the effects of these coil faults, a prototype axial flux VR PM motor was constructed. The motor was equipped with experimental fault simulation stator windings capable of simulating these and other types of stator winding faults. This paper focuses on two common types of winding faults and their effects on rotor velocity in this type of motor.     

不同工况下3.3 MW永磁直驱风力发电机电磁场仿真分析及损耗修正

文章设计了一台3.3 MW外转子表贴式永磁直驱风力发电机,并对其电磁性能及短路故障情况进行了有限元仿真分析。首先,得出了这台电机在额定工况下的转矩和磁密分布等结果,以及在相间绕组短路和三相绕组短路两种情况下电机转矩、电压和电流等曲线的变化情况;然后,通过三维静磁场仿真,探究了电机定子的径向通风道结构对二维有限元仿真的影响程度,并对铁耗进行了修正;最后,通过电磁场-温度场的耦合迭代仿真,考虑了温度场影响下的电机内部材料特性的变化对电机损耗结果的影响。     

Motors With Buried Magnets for Medium-Speed Applications

A novel type of mechanically robust buried magnet rotor structure is proposed for medium speed permanent magnet machines. A machine utilizing the construction is built, tested, and compared to another machine with traditional V-shaped poles. The machine is also simulated using finite element method and the results are compared to tested values. The obtained results demonstrate the feasibility of the construction.     

A brushless self-exciting three-phase synchronous generatorutilizing the 5th-space harmonic component of magneto motive forcethrough armature currents

A novel brushless self-exciting three-phase synchronous generator is proposed. It consists of three-phase armature windings on the stator, one field winding and one exciting winding with five times as many poles as that of the armature winding on the rotor, and a three-phase reactor connected to the terminal of the armature windings. By utilizing the 5th-space harmonic component of armature electromotive force, small voltage regulation for various loads and no oscillatory tension occurring at the rotor shaft were realized. The basic constitution, principle of operations, and exciting characteristics are described. The experimental results obtained from using a trial generator demonstrated its practical usefulness     

Novel Multiflux Level, Three-Phase, Squirrel-Cage Induction Motor for Efficiency and Power Factor Maximization

In the European Union, the average load factor of electric motors in both industrial and tertiary sectors is estimated to be less than 60%. However, in some industrial sectors, the average load factor for some motor power ranges can be as low as 25%. Most oversized three-phase induction motors operate with low efficiency and power factor, which is, by far, the most important cause for poor power factor in industrial installations. In the low-load operating periods, motor performance can be improved both in terms of efficiency and power factor if the magnetizing flux is properly regulated. In this paper, a multiflux level, three-phase, squirrel-cage induction motor is proposed, in which the efficiency and power factor can be both maximized as a function of load. This novel motor can be a surplus value in industry due to its flexibility, particularly, for variable load applications in which significant energy savings can be obtained, and can also be used as new or rewound general purpose spare motor (with several levels of voltage, magnetizing flux and/or power). The proposed motor has a stator winding with two sets of turns, sharing the same positions in the stator slots (which can be connected either in series or in parallel). Among all the possible stator winding connections, six modes were selected and analyzed (two of which are new). The basic principles for proper connection mode change are discussed. An electronic device and a contactor concept for automatic connection mode change are proposed. As far as the authors know, this concept is described and analyzed for the first time.     

Flux guides for permanent magnet machines

A study for the enhancement of effective flux of PM machines using flux guides is presented. An intrinsic demagnetization curve that considers the leakage flux is introduced. On the basis of a higher effective flux the stator winding can be more compact; consequently, a lighter stator and a higher efficiency permanent magnet (PM) machine can be obtained     

Design criteria for high-efficiency SPM synchronous motors

This paper deals with the design criteria for a high-efficiency permanent magnet synchronous motor. The goal is not pursued by a trivial reduction of the electric and magnetic loadings (which decrease motor losses) but optimizing a set of motor design variables, without increasing the overall dimensions, which are typically imposed as design constraints. The effect of the number of slots and of the inner-to-outer diameter ratio on motor losses is investigated. The possibility of designing a stator with a tooth length lower than the total core length, using soft magnetic composites, is studied. Finally, a criterion is proposed to evaluate the convenience of using a nonoverlapping winding. An analytical approach is adopted so as to allows the obtained results to be useful for a wide variety of permanent magnet machines.