Fractional-slot concentrated-winding (FSCW) synchronous permanent magnet (PM) machines have been gaining interest over the last few years. This is mainly due to the several advantages that this type of windings provides. These include high-power density, high efficiency, short end turns, high slot fill factor particularly when coupled with segmented stator structures, low cogging torque, flux-weakening capability, and fault tolerance. This paper is going to provide a thorough analysis of FSCW synchronous PM machines in terms of opportunities and challenges. This paper will cover the theory and design of FSCW synchronous PM machines, achieving high-power density, flux-weakening capability, comparison of single- versus double-layer windings, fault-tolerance rotor losses, parasitic effects, comparison of interior versus surface PM machines, and various types of machines. This paper will also provide a summary of the commercial applications that involve FSCW synchronous PM machines.

Fractional-Slot Concentrated-Windings Synchronous Permanent Magnet Machines: Opportunities and Challenges

Computer Controlled Systems: Theory and Design

This paper reviews the trends in wind turbine generator systems. After discussing some important requirements and basic relations, it describes the currently used systems: the constant speed system with squirrel-cage induction generator, and the three variable speed systems with doubly fed induction generator (DFIG), with gearbox and fully rated converter, and direct drive (DD). Then, possible future generator systems are reviewed. Hydraulic transmissions are significantly lighter than gearboxes and enable continuously variable transmission, but their efficiency is lower. A brushless DFIG is a medium speed generator without brushes and with improved low-voltage ride-through characteristics compared with the DFIG. Magnetic pseudo DDs are smaller and lighter than DD generators, but need a sufficiently low and stable magnet price to be successful. In addition, superconducting generators can be smaller and lighter than normal DD generators, but both cost and reliability need experimental demonstration. In power electronics, there is a trend toward reliable modular multilevel topologies.

Trends in Wind Turbine Generator Systems

This paper presents some design considerations for synchronous machines characterized by a fractional number of slots per pole per phase. The main advantage of this configuration is a smooth torque, which is due to the elimination of periodicity between slots and poles. A second advantage is a higher fault-tolerant capability, making the machine able to work even in faulty conditions. However, the fractional-slot configuration presents a high content of MMF harmonics that may cause an unbalanced saturation and thus an unbearable torque ripple. A method to design fractional-slot machines is illustrated in this paper, including double-layer and single-layer windings. The analytical computation is extended to determine the harmonics of MMF distribution. Their effect is highlighted in isotropic as well as anisotropic machines. Finally, some considerations are reported to avoid unsuitable configurations

Design considerations for fractional-slot winding configurations of synchronous machines

A simple analytical method is developed to compare the combinations of stator and rotor pole numbers in flux-switching permanent magnet (PM) machines in terms of back electromotive force (EMF) and electromagnetic torque The winding connections and winding factors of machines having all poles and alternate poles wound, and different numbers of phases, from two to six, are determined by the coil-EMF vectors Their differences from analyzing the conventional fractional-slot PM machines with concentrated nonoverlapping windings are highlighted The general conditions are established for balanced symmetrical back-EMF waveform It shows that the optimized rotor pole number should be close to the number of stator poles, whereas larger torque can be obtained by the machine with relatively higher rotor pole number The analysis is validated by finite-element analyses and experiment

Winding Configurations and Optimal Stator and Rotor Pole Combination of Flux-Switching PM Brushless AC Machines

In modeling axial field machines, three-dimensional (3-D) finite-element method (FEM) models are required in accurate computations. However, 3-D FEM analysis is generally too time consuming in industrial use. In order to evaluate the performance of the axial flux machine rapidly, an analytical design program that uses quasi-3-D computation is developed. In this paper the main features of the developed program are illustrated. Results given by the program are verified with two-dimensional and 3-D finite element computations and measurements. According to the results, it is possible to evaluate the performance of the surface-mounted axial flux PM machine with reasonable accuracy via an analytical model using quasi-3-D computation.

Modeling of axial flux permanent-magnet machines

A lumped-parameter thermal model is presented for axial flux permanent magnet (AFPM) machines The model provides the steady-state thermal solution to derive the temperatures at different parts of the machine, including the temperatures in the stator windings and the temperature of the magnets Temperature-dependent thermal properties of the materials used in the machine construction as well as the stator winding resistance and consequently copper losses require temperature update in accurate design of the machine Therefore, an iterative coupled electromagnetic and thermal design program is proposed in this study A 5-kW AFPM generator is designed using the proposed program with regarding the thermal behavior of the machine Tests performed on the designed machine verify that the defined thermal resistance network has a high ability to predict the nodal temperatures accurately

Lumped-Parameter Thermal Model for Axial Flux Permanent Magnet Machines

Hybrid and full electric technologies are fast emerging in vehicles and mobile working machines, where electric machines and internal combustion engines are used together to power the systems. Permanent magnet (PM) technology plays an important role here despite the high magnet prices. This paper theoretically and empirically studies the design principles of PM synchronous machines (PMSMs) for hybrid applications, where a high starting torque and a wide field weakening range are needed. Several embedded-magnet PMSM magnetic circuit topologies are considered as possible candidates. A 10-kW PMSM prototype was built and tested. Experimental results verify the theoretical considerations well.

Design Principles of Permanent Magnet Synchronous Machines for Parallel Hybrid or Traction Applications

Control and energy efficiency of PEM water electrolyzers in renewable energy systems

Performance comparison between different machine topologies is not a straightforward task since many variables exist if electromagnetic, thermal and mechanical aspects are taken into account. In this paper some methods to take into account relevant mechanical constraints in the performance comparison are proposed. A comparison study between low-speed axial-flux permanent-magnet machines and radial-flux permanent-magnet machines is provided with introduced mechanical constraints, respectively

Markku Niemela

Papers

Modeling of axial flux permanent-magnet machines

Lumped-Parameter Thermal Model for Axial Flux Permanent Magnet Machines

Design Principles of Permanent Magnet Synchronous Machines for Parallel Hybrid or Traction Applications

Control and energy efficiency of PEM water electrolyzers in renewable energy systems

Performance comparison between low-speed axial-flux and radial-flux permanent-magnet machines including mechanical constraints