The Brushless Doubly-Fed Generator (BDFG) shows the great potential for use in large variable speed wind turbines due to its high reliability and cost benefits of a partially-rated power electronics converter. However, it suffers from the compromised efficiency and power factor in comparison with conventional doubly fed induction or synchronous generators. Therefore, optimizing the BDFG, especially the rotor, is necessary for enhancing its torque density and market competitiveness. In this paper, a novel cage-assisted magnetic barrier rotor, called the hybrid rotor, is proposed and analyzed. The detailed analytical design approaches based on the magnetic field modulation theory are investigated. In addition, the machine losses and mutual inductance values using the proposed rotor designs are calculated and their performance implications evaluated. Finally, the comparative experimental results for two BDFG prototypes are presented to verify the accuracy and effectiveness of the theoretical studies.

Aiming at the shortcomings of complex mechanical transmission structure and low efficiency of the existing belt conveyor driven by asynchronous motor, this paper focuses on the investigation of a new type of low-speed high-torque-density direct-drive permanent magnet synchronous motor (LHDPMSM) for driving the belt conveyor. The proposed LHDPMSM utilizes external rotor structure to achieve the goal of fully direct drive of the belt conveyor. First, the topology and advantages of the straight-shape external-rotor LHDPMSM is introduced. Then the selection rules of the pole-slot combination are studied. Furthermore, an improved subdomain model is established based on the idea of segmentation for fast calculation of the no-load magnetic field of the proposed straight-shape external-rotor LHDPMSM. Moreover, the operating performance of the proposed LHDPMSM with load fluctuations was verified by FEA. In addition, an efficient cooling system based on Z-shape water channels and hollow shaft was designed, and a LPTN model was established for temperature fast calculation. Since the cooling system has a negative impact on the shaft mechanical strength, the mechanical strength of the shaft and other fragile components of the LHDPMSM was analyzed to ensure the safe operation of the motor. Finally, a prototype is manufactured and tested to validate the design concept introduced in this paper.

The brushless doubly-fed generator (BDFG) has been considered as a promising alternative for variable speed system. The rotor structure is one of the key factors affecting the performance of BDFG and it has become the research hotspot. This paper presents a novel hybrid rotor for BDFG. The hybrid rotor structure is the combination of the reluctance structure and cage structure. In order to explain the operation principle of BDFG with hybrid rotor, the magnetic field modulation is analyzed using the analytic method. The prototype of the proposed BDFG is manufactured to verify the rationality and feasibility. The inductance is measured and no-load and load performance are tested based on the prototype. The experimental results show that the BDFG with hybrid rotor is well applicable in variable speed system and has a good application prospect.

The brushless doubly fed generator (BDFG) has been a research focus of wind power generation due to its inherent promising features. Although it is important to consider the effect of losses on the design and analysis of the BDFG, there are a few related comprehensive studies. Moreover, a hybrid rotor is studied to improve the performance of BDFG, which brings greater challenges to the loss calculation. The aim of this paper is to propose the accuracy calculation methods of core loss and copper loss for the studied BDFG with hybrid rotor. The core loss calculation model which taken the influence of harmonics, alternating magnetization, rotating magnetization and two-dimensional magnetic property of material into account is proposed based on the analysis of the magnetic field characteristics. In order to calculate the copper loss considering the skin effect, the inductive current of the assisted cage bars which is difficult to obtain directly is calculated. The accuracy and correctness of the proposed approaches and models are validated by experimental results from a 25kW prototype BDFG with hybrid rotor. In addition, the proposed loss calculation method, especially the core loss calculation model, is also applicable to BDFG with other rotor structure which is operated by the magnetic field modulation mechanism.

Brushless doubly fed machine (BDFM), a new type of adjustable-speed machine with special structure, shows promising features for the field of variable frequency speed regulation and wind power generation due to its inherent characteristics. However, the traditional loss calculation method is no longer applicable due to the special structure and complex magnetic field distribution of BDFM, especially for the proposed novel hybrid rotor structure which has better coupling ability than the commonly used rotor structures such as cage rotor and reluctance rotor. In this paper, in order to calculate the core loss accurately, an improved E&S model is applied to BDFM with hybrid rotor, and the analysis and calculation procedure are presented. In addition, based on the time-stepping finite-element model and harmonic analysis method, three different core loss models are established and compared with the proposed improved E&S model. Furthermore, the experimental result from a BDFM with hybrid rotor prototype is compared with the calculation results obtained by the above models, and the accuracy and feasibility of the improved E&S model are verified.

Brushless doubly fed generator (BDFG), a class of adjustable-speed machine with a special structure, shows great prospect in wind power generation. Since different rotor structures lead to great differences in the BDFG performance, the research of BDFG rotor structure has been a hotspot. However, only a few of the research studies have paid attention to the rotor and stator slot combination of the BDFG, especially for the BDFG with a novel hybrid rotor. Improper slot combination will lead to efficiency decrease, additional loss increase, and undesirable torque in an electrical machine. In order to study the principle of slot combination of the BDFG with the hybrid rotor, the combination approach of theoretical analysis and experiments is presented in this paper. Furthermore, a coupling factor which reflects the coupling ability of the BDFG with different slot combinations is proposed. Moreover, the best slot combination of the proposed BDFG is determined, and several parameters such as useful harmonic contents, total harmonic voltage distortion (THDu), core loss, and copper loss are considered as major criteria for judgment.