Energy storage systems (ESSs) play a very important role in recent years. Flywheel is one of the oldest storage energy devices and it has several benefits. Flywheel Energy Storage System (FESS) can be applied from very small micro-satellites to huge power networks. A comprehensive review of FESS for hybrid vehicle, railway, wind power system, hybrid power generation system, power network, marine, space and other applications are presented in this paper. There are three main devices in FESS, including machine, bearing, and Power Electronic Interface (PEI). Furthermore, advantages and disadvantages all of them have been presented. In addition a brief review of new and conventional power electronic converters used in FESS, have been discussed. Finally, practical ways to develop this technology in the future are presented.

A comprehensive review of Flywheel Energy Storage System technology

The torque ripple of a permanent magnet (PM) machine is mainly due to the cogging torque and distortions of the back-emf waveforms, winding inductances and current waveforms. Both the cogging torque and back-emf are normally calculated at open-circuit conditions. However, they are affected by the electric loading and magnetic saturation. This paper investigates the influence of the load conditions on the cogging torque and back-emf waveform by employing a frozen permeability finite element technique. Furthermore, the effectiveness of the rotor skew on the minimization of the cogging torque, thus torque ripple, is also highlighted. It is found that the cogging torque magnitude is significantly increased under load conditions due to more flux leakage through tooth tips. However, the more important issue is that the cogging torque periodicity also changes, thus the rotor skew technique becomes less effective. In addition, the back-emf waveform under load conditions contains more harmonics, which leads to more electromagnetic torque ripple. It is also proven that the cogging torque, back-emf harmonics and consequently the output torque ripple are effectively diminished if the machine is skewed by one actual cogging torque period, i.e., when the electrical loading influence is considered. The analysis results are supported by experimental measurements.

Influence of Electric Loading and Magnetic Saturation on Cogging Torque, Back-EMF and Torque Ripple of PM Machines

This paper presents an axial-flux permanent magnet motor optimized by the genetic algorithm (GA)-based sizing equation and the finite-element analysis (FEA). The designed slotted TORUS motor produces sinusoidal back-EMF waveform, maximum power density, and reduced cogging torque. The GA obtained the dimensions of motors with different numbers of slots and the highest possible power density. Electromagnetic field analysis of the candidate motors obtained from GA with various dimensions is subjected through FEA to obtain the motors' characteristics. Based on the GA and FEA results, a candidate motor design is introduced and subjected to FEA for reoptimization and finalization of the motor design. Techniques like modifying winding configuration and permanent magnet skewing are investigated to attain the most sinusoidal back-EMF waveform and reduced cogging torque. FEA and GA simulation results are compared and agreed well to the flux density in various areas of the designed motor at no-load condition. The final designed motor (1 kW, 50 Hz, four poles) is fabricated and tested. Experimental results agree with those of the simulation, both satisfying the desired technical specifications.

Design, Analysis, and Prototyping of an Axial-Flux Permanent Magnet Motor Based on Genetic Algorithm and Finite-Element Analysis

This paper investigates the influence of skewing on torque ripples, including the electromagnetic (EM) torque ripple, and the cogging torque, in permanent magnet (PM) machines. It is found that the effectiveness of skewing largely depends on the axial variation of a torque ripple phase but less on its magnitude under skewing. It is further found that, in both linear and nonlinear cases, the EM and the on-load torque ripple cannot be fully eliminated by skewing one on-load torque ripple period or any other angles, except 360° electrical, which is impractical. This is due to the fact that the EM torque ripple, especially its phase, of each slice is different due to the axial variation of the equivalent current phase advance angle introduced by skewing. In nonlinear cases, the EM torque ripple variation is further aggravated by the axial variation of magnetic saturation. The EM torque ripple as well as the on-load torque ripple may be even increased by skewing, especially when the cogging torque is low and electric loading is high.

Investigation of Torque Ripples in Permanent Magnet Synchronous Machines With Skewing

This paper is about the mechanical design and analysis of a yokeless and segmented armature axial flux permanent-magnet (PM) synchronous machine, which consists of two external rotors and an inner stator. Although this new electrical machine has many advantages such as high torque density, shorter axial length, and high efficiency, its mechanical construction is yet challenging. This is mostly due to the high axial force between the stator and the two rotors, which can be further increased by the inevitable manufacturing and assembly tolerances. Based on the determined geometric dimensions of the electromagnetically relevant components, a reliable mechanical construction is developed in this paper. Subsequently, stress, deformation, thermal, and modal analyses are performed based on finite-element method (FEM). Finally, the first prototype is successfully manufactured and measured. It can be concluded that the proposed construction is reliable and the measured data match well with the results of the electromagnetic analysis.

Development of a Yokeless and Segmented Armature Axial Flux Machine

This paper deals with the experimental verification and electromagnetic analysis of an axial flux machine with a double-sided permanent magnet (PM) rotor and coreless stator windings, based on analytical field computation. Employing a magnetic vector potential and a 2-D analytical model, magnetic fields produced by PMs are obtained. The 3-D problem is solved by introducing a special function for the radial position. Then, the analytical solutions for back emf, electromagnetic torque, and efficiency of the PM-type axial flux machines are derived by using the magnetic field solutions and considering simple equivalent circuits. The analytical results are validated extensively by 3-D FE results. Test results such as back-emf, torque, and efficiency measurements are also presented to show the effectiveness of the analyses.

Improved Analytical Model for Electromagnetic Analysis of Axial Flux Machines With Double-Sided Permanent Magnet Rotor and Coreless Stator Windings

This paper deals with the magnet pole shape design of permanent magnet machines for the minimization of torque ripple based on electromagnetic field theory. On the basis of a magnetic vector potential and a two-dimensional (2-D) polar system, analytical solutions for flux density due to permanent magnet (PM) and current are obtained. In particular, the analytical solutions for mathematical expressions of magnets with different circumferential thicknesses can be solved by introducing improved magnetization modeling techniques, resulting in accurate calculations of electromagnetic torque. The analytical results are validated extensively by nonlinear finite element method (FEM). Test results such as back-emf measurements are also given to confirm the analyses. Finally, on the basis of derived analytical solutions, a reduction of torque ripple can be achieved.

Magnet Pole Shape Design of Permanent Magnet Machine for Minimization of Torque Ripple Based on Electromagnetic Field Theory

For permanent-magnet synchronous motors driven by high-frequency drives, the rotor speed is normally above 30 000 rpm, and it may exceed 100 000 rpm. Designing high-speed motors from both in an electromagnetic and a mechanical aspect requires a different approach from designing conventional motors with low-medium operation speeds. This paper deals with rotor natural frequencies that are particular to high-speed permanent-magnet synchronous motors for turbo-compressor applications. It is very important to accurately predict rotor natural frequencies at the design stage in order to minimize operational failure. On the basis of the finite element method (FEM), this paper predicts the natural frequencies of a designed and manufactured permanent magnet rotor. The experimental results confirm the validity of the FEM calculations of the proposed design process.

Rotor Natural Frequency in High-Speed Permanent-Magnet Synchronous Motor for Turbo-Compressor Application

The rotational loss is one of the most important problems in high-power flywheel energy storage system (FESS) which supplies the electrical energy from the mechanical rotation energy. In our FESS with PM synchronous motor/generator (PMSM/G) and non-contacting system, the rotational loss is mainly generated in the laminated iron stator when PM rotor mounted on the flywheel is rotated. Therefore, this paper presents design and evaluation of a PMSM/G model composed of double-sided Halbach magnetized PM rotor and ironless 3-phase winding stator to minimize speed reduction of the flywheel. For one such motor structure we give analytical formulae for its magnetic field and back-electromotive force in a uniform way via the magnetic vector potential. As a result of experiment, dynamic performance of a manufactured double-sided PMSM/G is evaluated by comparison with those of the slotless PMSM/G in the idling mode. The result shows that the presented PMSM/G can be applicable to the high-power FESS.

Design and Experimental Evaluation of Synchronous Machine Without Iron Loss Using Double-Sided Halbach Magnetized PM Rotor in High Power FESS

In this paper, we investigate the electromagnetic performance of wind power generators with outer permanent magnet rotors on the bases of variations in the turbine characteristics over nominal wind speeds by employing various proposed analytical techniques. We have employed an electromagnetic analytical method for calculating electrical parameters such as back EMF constant, synchronous inductance, and phase resistance on the basis of the magnetic field distribution calculated in previous studies. Moreover, curve fitting and finite element methods have been used for core losses analysis, while the d-q model employing the coordinate transformation theorem has been used for analyzing generation performances. To validate our techniques, we compared the performance results obtained using our analytical methods with those obtained by experimental setups. In addition, we propose optimized wind turbine characteristics for our model from the analysis results of electromagnetic performances obtained by considering three types of turbine characteristics.

Electromagnetic Performance Analysis of Wind Power Generator With Outer Permanent Magnet Rotor Based on Turbine Characteristics Variation Over Nominal Wind Speed

Kyoung-Jin Ko

Papers

Improved Analytical Model for Electromagnetic Analysis of Axial Flux Machines With Double-Sided Permanent Magnet Rotor and Coreless Stator Windings

Magnet Pole Shape Design of Permanent Magnet Machine for Minimization of Torque Ripple Based on Electromagnetic Field Theory

Rotor Natural Frequency in High-Speed Permanent-Magnet Synchronous Motor for Turbo-Compressor Application

Design and Experimental Evaluation of Synchronous Machine Without Iron Loss Using Double-Sided Halbach Magnetized PM Rotor in High Power FESS

Electromagnetic Performance Analysis of Wind Power Generator With Outer Permanent Magnet Rotor Based on Turbine Characteristics Variation Over Nominal Wind Speed