Direct-drive Permanent Magnet Generator Research Articles

A New Zero Waste Design for a Manufacturing Approach for Direct-Drive Wind Turbine Electrical Generator Structural Components

An integrated structural optimization strategy was produced in this study for direct-drive electrical generator structures of offshore wind turbines, implementing a design for an additive manufacturing approach, and using generative design techniques. Direct-drive configurations are widely implemented on offshore wind energy systems due to their high efficiency, reliability, and structural simplicity. However, the greatest challenge associated with these types of machines is the structural optimization of the electrical generator due to the demanding operating conditions. An integrated structural optimization strategy was developed to assess a 100-kW permanent magnet direct-drive generator structure. Generated topologies were evaluated by performing finite element analyses and a metal additive manufacturing process simulation. This novel approach assembles a vast amount of structural information to produce a fit-for-purpose, adaptative, optimization strategy, combining data from static structural analyses, modal analyses, and manufacturing analyses to automatically generate an efficient model through a generative iterative process. The results obtained in this study demonstrate the importance of developing an integrated structural optimization strategy at an early phase of a large-scale project. By considering the typical working condition loads and the machine’s dynamic behavior through the structure’s natural frequencies during the optimization process coupled with a design for an additive manufacturing approach, the operational range of the wind turbine was maximized, the overall costs were reduced, and production times were significantly diminished. Integrating the constraints associated with the additive manufacturing process into the design stage produced high-efficiency results with over 23% in weight reduction when compared with conventional structural optimization techniques.

Research on the design of direct-drive permanent magnet generator for hybrid UAV

As the core of the power system, the performance of the generator is crucial to the flight and operation of the drone. This paper introduces the design and research of a direct-drive permanent magnet generator for gasoline-electric hybrid UAV. A computationally efficient optimization design method was proposed, and the generator used in the 25 kg UAV was designed

Electromagnetic design and performance analysis of large-scale offshore wind generators with modular fractional-slot concentrated winding

Future offshore wind engineering raises demands on higher reliability, better power generating performance, as well as lower manufacturing and transportation cost. This paper presents a direct-driven permanent magnet generator (DDPMG) with modular fractional-slot concentrated winding (MFSCW), to meet the large-scale, low-cost and high-reliability requirements of offshore wind power applications. The corresponding pole–slot combination method and electromagnetic characteristics analysis for this novel generator are proposed and performed in detail, including the winding coefficient analysis, magnetomotive force (MMF) harmonic analysis, cogging torque analysis, finite element simulations and generator parameter optimization. After finite element simulations, a 3.3 MW prototype MFSCW-DDPMG is developed to validate the proposed scheme. The results provide a complete and feasible scheme of pole–slot​ combination and electromagnetic design for future MFSCW-DDPMG engineering applications.

Annual Energy Production Design Optimization for PM Generators Considering Maximum Power Point Trajectory of Wind Turbines

Efficiency optimization is an important goal in the design of permanent magnet generators. However, traditional design optimization methods only focus on improving the rated efficiency without considering the annual cycle for overall efficiency improvement. To overcome this drawback, this paper presents a design optimization method for improving annual energy production (AEP) of wind direct-drive permanent magnet generators. Unlike the conventional efficiency optimization method that only improves the rated point efficiency, the proposed method improves the overall efficiency of the generator during the operating cycle by matching the maximum power point trajectory of the wind turbine. The periodic loss model of the permanent magnet generator is established and further constituted as the objective function to perform the optimization search using a genetic algorithm. Through simulation and experimental verification, the proposed method can obtain a higher AEP compared with the conventional design optimization method, and the proposed method can be extended to other variable speed power generation fields.

Optimal Shape Design of Direct-Drive Permanent Magnet Generator for 1 kW-Class Wind Turbines

Direct-drive permanent magnet generators are becoming an attractive option for highly efficient small-scale wind turbines due to their high-power density and size reduction capabilities. In this study, the optimal shape design of a direct-drive permanent magnet generator for 1 kW-class wind turbines was conducted while considering power generation and weight. Half of the geometry of a single stage in the generator was considered for a electromagnetic analysis under given electrical parameters. In order to construct a response surface model, a sensitivity analysis was conducted on seven design parameters of the proposed generator. The desirability function was used to minimize the weight of the generator while meeting a requirement of the target specification. The results indicated that the optimized design parameters for the generator met the target specification while maintaining the generator’s weight at the same level as the initial design model. From the comparisons with other research, the optimized generator exhibited a higher power generation/weight ratio than the generator with a rated capacity under 3 kW.

Wind power performance assessment at high plateau region: A case study of the wind farm field test on the Qinghai-Tibet plateau

The population of countries with high altitudes accounts for about 20.4% of the world’s population, but the annual electricity consumption is still at a low level, which significantly slows down the development of these regions. Wind energy is suggested as the solution, but the progress is very slow due to challenges in operations from the relatively harsh environment. To overcome this challenge while considering Qinghai-Tibet plateau is one of most representative high altitude regions in the world, we recently developed a 22 MW wind farm at an altitude of over 5000 m in Tibet, China, and conducted a one-year field-test campaign using direct-drive permanent-magnet generators (DDPMGs) and doubly fed induction generators (DFIGs) as a case study. In this paper, we report some main results. During resource assessment, it is found that the wind speeds is between 11 m/s and 22 m/s for most winter time with abundant energy production but relatively significant daily fluctuation due to temperature change. From a technology perspective, we found that both DDPMGs and DFIGs can adapt to the ultra-high-altitude environment. Wind turbines with DDPMGs are reliable because of their gearless design, and turbines with DFIGs have a lower cut-in wind speed, which enables them to generate power in regions with low wind speeds. This wind farm generates nearly 60 million kWh of electricity to support 8410 local households per year. It can also eliminate the need to burn 50.24 million kg of cow dung, thereby effectively reducing the local residents’ dependence on burning organic matter for energy. This study is expected to provide guidance to technology developers and governmental policymakers to plan future wind farms in the high altitude area.

A method for derating power of a MW directdrive wind turbine permanent magnet generator as over allowable working temperature

The use of direct-drive permanent magnet (PM) generators for MW wind application is increasing, because they have higher reliability and less maintenance than geared generators such as geared PM generators and geared induction generators. The price of the direct-drive PM generator is high, taking 10%-15% price of its overall system. Therefore, the generator mush be safely operated to have a long life. For a PM generator, its performance is sensitive to ambient and working temperature so that it needs to be monitored. In this paper, a method is proposed, combining the practical knowledge of a generator and theory to give permissible maximum generated power under certain ambient condition. The stator iron loss of a MW permanent magnet generator in direct-drive wind turbine application is presented as a function of rotor speed or frequency. The generator’s copper loss and electromagnetic torque versus winding temperature and current are investigated. The relationship of voltage, current and active power to the generator temperature is addressed. The given performances can be used to choose the generator’s working parameters such as voltage, current, output power for avoiding generator damaged by over temperature.

A Novel Configuration of a Hybrid Offshore Wind-Wave Energy Conversion System and Its Controls for a Remote Area Power Supply

This article aims to develop a novel hybrid offshore wind-wave energy conversion system (HOW-WECS) configuration which can successfully feed a stable power to the customers of remote communities in near-shore areas or remote islands. The proposed configuration uses a minimum number of converters for the integration of the doubly-fed induction generator (DFIG) based wind energy system with the direct-drive linear permanent magnet generator (DDLPMG) based wave energy system. Advanced control schemes for the DFIG and the DDLPMG are presented to enhance the power extraction from the implemented HOW-WECS. The dynamic and the transient performance of the proposed system and the associated control schemes are tested under various operating scenarios and electrical fault conditions. The dynamic performance was acceptable as the implemented control strategies were able to keep the stator voltage and the stator current of the DFIG sinusoidal and balanced, the frequency excursions are within the acceptable band, and the common ac-bus voltage of the distribution network stable under all operating scenarios. The results prove the effectiveness of the proposed system under transient conditions and the ability of the proposed HOW-WECS to recover quickly to gain its pre-fault conditions once the fault was cleared.

Co-Design Optimization of Direct Drive PMSGs for Offshore Wind Turbines Based on Wind Speed Profile

This paper presents a new method to optimize, from a working cycle defined by torque and speed profiles, both the design and the control strategy of permanent magnet synchronous generators (PMSGs). The case of a 10 MW direct-drive permanent magnet generator for an Offshore wind turbine was chosen to illustrate this method, which is based on the d–q axis equivalent circuit model. It allows to optimize, with a reduced computation time, the design, considering either a flux weakening control strategy (FW) or a maximum torque per Ampere control (MTPA) strategy, while respecting all the constraints—particularly the thermal constraint, which is characterized by a transient regime. The considered objective is to minimize the mass and the average electric losses over all working points. Thermal and magnetic analytical models are validated by a 2D finite element analysis (FEA).

Ferrite‐based axial flux permanent magnet generator for wind turbines

This study presents the development of a framework used to optimise and experimentally validate a novel axial flux direct-drive (DD) permanent magnet generator (PMG) for the offshore wind turbine market. This technology aims to offer significant levelised cost of energy (LCoE) reductions via capital expenditure and operating expense (CAPEX and OPEX) savings – a key objective for the offshore industry. The DD-PMG technology uses ferrite magnets to create the magnetic field, which is a significant source of cost reduction. The use of ferrite could also eliminate an industry wide reliance on neodymium iron boron, the scarce and expensive rare-earth magnet used in existing designs. Another advantage of a ferrite-based design is that it is less sensitive to the cooling problems that currently face existing DD-PMGs. This study describes the development and testing of two prototype machines at nominal 2 and 70 kW power ratings. Moreover, the finite element analysis and analytical steps employed to develop optimised designs together with the experimental verification are presented. The simulated and experimental results show good agreement which provides confidence in the design and modelling work completed.

Design and Analysis of 10 MW Class HTS Exciting Double Stator Direct-Drive Wind Generator With Stationary Seal

The superconducting (SC) generators are expected to have a promising application for offshore wind generation due to the merits of high power density and high efficiency over the conventional generators. In this paper, a 10-MW conceptual high temperature superconducting (HTS) exciting double stator direct-drive wind generator (HTS-DSDDG) with the stationary seal is proposed and investigated. The proposed HTS-DSDDG has two spatially independent stators: to place HTS field windings and copper armature windings, respectively. Thus, it realizes the stationary seal of the cooling system and removes brushes and slip rings and the other additional excitation device; hence offering the advantages of the high reliability and the low operation & maintenance cost. The main design considerations and the initial design specifications of the 10 MW HTS-DSDDG are given in detail. Furthermore, the electromagnetic characteristics of the HTS- DSDDG under no-load, rated load, and three-phase short-circuit fault conditions are analyzed by the finite element method (FEM). Subsequently, its size, weight, and cost are compared with the existing conceptual 10 MW wind generators. The results reveal that the HTS-DSDDG has excellent electromagnetic performances, such as good sinusoidal voltage, low cogging torque, and torque ripple; besides, its output power per volume is twice than that of the permanent magnet direct-drive generator (PMDDG), being comparable to that of the existing conceptual MgB2-HTS direct-drive generators.

Dynamic structural design of offshore direct-drive wind turbine electrical generators

Direct-drive permanent magnet generators for multi-MW wind turbines are low speed high torque electrical machines requiring large, heavy and robust structures to maintain the airgap clearance open and stable. The structural mass of radial-flux generators can be estimated at an early phase of the design process. Using distinct approaches, the integrity of a 3 MW electrical machine structure has been addressed from a dynamic perspective by carrying out modal analyses with a view to minimize its mass. A versatile tool has also been developed to help the engineers with the dynamic design of generator disc structures. Conical structures have been analysed and compared with the baseline disc model obtaining very promising results. Potential improvements have been proposed for more ambitious structural designs.

Real‐time gating signal generation and performance analysis for fully controlled five‐phase, ten‐pulse, line‐commutated rectifier

Fossil fuel prices and air pollution impel many countries to concentrate on renewable energy sources, of which wind energy is considered to be a pillar. Owing to the numerous advantages when compared with its three-phase counterpart, five-phase direct-drive permanent magnet (PM) generators are a key area of focus in power generation with renewable and wind energy systems. The generator output requires a converter, such as an AC–DC rectifier, to match load requirements. The objective of this work is to generate in real time the gating signals of a fully controlled five-phase, line-commutated rectifier, fed from a five-phase PM generator. A mixed-reality environment is used to implement the gating signal generation algorithm for the five-phase rectifier, where the required gating signals are successfully generated, even with some distorted prototype generator voltage waveforms. The performance of the rectifier is investigated practically and validated by using the MATLAB/SIMULINK platform. Moreover, a comparison with a five-phase pulse-width modulated current source rectifier in terms of losses, size, weight, and cost is presented.

The effect of the rotor adjustment on the vibration behaviour of the drive-train system for a 5 MW direct-drive wind turbine

Direct-drive wind turbines, different from the standard geared wind turbines, widely use a direct-drive permanent-magnet generator to avoid the gearbox failures. In the absence of a gearbox in the drive-train system, the direct-drive generator operates at low rotating speeds. Thus direct-drive wind turbines require a larger sized generator (higher weight) to transfer the kinetic energy into electrical energy. The inherent unbalanced magnetic pull force of the generator can have impact on the vibration behaviour of the drive-train system. This paper studies the effect of rotor position and weight adjustment on the vibration behaviour of the drive-train system within a 5 MW direct-drive wind turbine by considering the unbalanced magnetic pull force. The adjustment of rotor position and weight changes the location of the centre of gravity of the drive-train system. The drive-train system which consists of the main shaft, rotor, hub and blades is modelled as a four degree-of-freedom nonlinear system. Both rotor displacement and bearing forces are obtained for a wide range of rotor position and weight under different rotating speeds. The obtained results would provide useful information on the optimized rotor position and mass ratio to improve the performance of the drive-train system.

Commonly Used Wind Generator Systems: A Comparison Note

<p>Amongst all renewable energy generation sources, wind power exhibits fastest growth rate. The increasing number of wind farm installations worldwide demand low maintenance, cost and failure rates with high efficiency. Determining the optimal drive train configuration amongst various configurations available for wind turbines is a challenge. In this paper commonly used, doubly fed induction generator with single stage gear box (GDFIG), doubly fed induction generator with multi stage gear box (DFIG) and the direct-drive permanent-magnet generator (DDPMG) are compared. Modelling of wind turbine with efficiency computations is presented. Considering common wind turbine parameters, performance of GDFIG, DFIG and DDPMG is compared through an experimental study. Considering a reference 5 MW variable speed wind turbine, efficiency of DDPMG is 96% when compared to 93.58%, 93.12% for DFIG and GDFIG. The experimental results presented prove that the DDPMG is a preferable solution considering low cost and high efficiency.</p>

Development of rare earth free permanent magnet generator using Halbach cylinder rotor design

Direct-drive permanent magnet generators (DDPMGs) offer increased reliability and efficiency for wind turbine application over the more commonly used geared doubly-fed induction generators. However, deployment of DDPMGs is limited in the U.S. wind industry due to reliance on NdFeB permanent magnets, which contain critical rare earth elements Nd and Dy. To allow for the use of lower energy density, rare earth free permanent magnets, Halbach cylinders are employed as the rotor in a 3.5 kW PMG to concentrate magnetic flux over the rotor surface and increase magnetic loading. By varying the slot-to-pole ratio in Halbach PMGs (HPMGs), designs are developed, which allow for the use of ceramic, or hard ferrite, strontium iron oxide permanent magnets. At the 3.5 kW scale, the ceramic HPMGs are able to achieve rated performance at reduced average efficiency of between 82 and 87%, which is due to the difference in permanent magnet material properties. For scaling of the ceramic HPMGs to 3 MW, rated performance and high efficiency were achieved on average at rated speed, demonstrating the potential for a rare earth free PMG in commercial scale wind turbines.

Testing the Performance of a Resolution-Level MPPT Controller for PMG-Based Wind Energy Conversion Systems

This paper presents the development and performance evaluation of a maximum power point tracking (MPPT) controller for direct-drive variable-speed permanent magnet generator (PMG)-based wind energy conversion systems (WECSs). The developed MPPT controller is designed to vary the resolution-level $J$ of the wavelet modulation technique used to operate the generator-side $3\phi$ ac–dc power electronic converter (PEC). The variations of $J$ are created by two angles, $\theta _{I}$ and $\theta _{O}$ . The angle $\theta _{I}$ is defined to map the changes on the input side, while the angle $\theta _{O}$ is defined to map the changes on the output side of a $3\phi$ wavelet-modulated ac–dc PEC. The resolution-level MPPT is implemented for performance evaluation on a 3.6 kW PMG-based WECS when operated in grid connection. Test results show good accuracy and dynamic responses, along with minor sensitivity to the variations in system parameters and levels of power delivery to the grid.

Stability analysis and dynamic equilibrium of a Kuroshio generator system

Global resources for conventional energy are currently being exhausted, and several countries worldwide are attempting to develop renewable energy. Current generator systems are a subject of ocean power research. This paper proposes a novel design of a Kuroshio generator system (KGS) that is suitable for the maritime environment of Taiwan (i.e., an average flow velocity of the Kuroshio Current is 1.45 m/s and the flow can be accelerated on Keelung Sill with a depth of 50-250 m). The KGS combined a reliable cable design and simple anchor system at sea and was not affected by motion changes of rotation axes in yaw and roll by way of an appropriate rudder design. An intuitive simulation method applied using MapleSim software was used to create a rigid KGS model. Different modeling frameworks for varied cable design and joint positions were adjusted to meet system requirements. An intuitive simulation method applied using MapleSim software was used to create a rigid KGS model. Different modeling frameworks for varied cable design and joint positions were adjusted to meet system requirements. The stability analysis was performed to determine dynamic equilibrium and motion behavior of the KGS and the combined cable design. The optimal spring stiffness and damper coefficient of polyester fibers were set as 5×10 5 N/m and 3×10 5 N∙s/m in the simulation, respectively. Furthermore, to achieve the torque equilibrium in pitch motion of the KGS, an optimal joint position that was relative to the leading infraedge of the outer duct was set at 2.2 m along the negative surge axis according to their responses in the simulation. Finally, the force and torque generated by the hydrodynamic effect in the KGS and the estimated specifications of a direct-drive permanent magnet generator equipped with an external rotor were imported into the simulation. Consequently, the motion ranges of translation axes in surge and heave were converged within 0.5 m, and the estimated output power in the KGS exceeded 54.8 kW.

Design and Optimization of Electric Continuous Variable Transmission System for Wind Power Generation

A novel brushless electric continuous variable transmission (E-CVT) system is presented and optimized. The proposed system offers an alternative solution for a variable-speed constant-frequency operation of the wind turbine application. The key is to eliminate the gearbox and brushes in the machine and this E-CVT system comprises of two rotors and two stators within one machine. This design inherits and integrates the merits of the direct-drive permanent magnet generator and the doubly fed induction generator with the additional benefit of the improved torque density and the need for a low-cost partial-scale converter. The structure, operation principle, and performance are analyzed. Genetic algorithm is employed to optimize the parameters for maximizing its power density. Time-stepping finite-element method is used to analyze the dynamic performance of the proposed system.

A Novel Multiphase Brushless Power-Split Transmission System for Wind Power Generation

A novel gearless and brushless power-split transmission system is proposed for a variable speed constant frequency wind power generation application. The key is to design a special hybrid axial-radial flux focusing structure, which has an improved utilization of permanent magnet (PM) materials. This design integrates the merits of two popular concepts - the direct-drive PM generator and the doubly fed induction generator. It is direct driven, with high torque density and in the meanwhile, it needs only a partial-scale converter. To improve the stability of the system, five phase currents are used for the control winding. A time stepping finite-element method is used to analyze the dynamic performance of the proposed system.