Yingzhen Liu

Feasibility Study of a Superconducting DC Direct-Drive Wind Generator

Superconducting (SC) direct-drive wind generators are proposed as a possible approach for offshore wind energy application. A lot of studies showed their benefits and superior performances. Small demonstrators up to several hundreds of kilowatts have been built and laboratory tested. More demonstrators will come in the future, and the success of these projects is very important for the large-scale prototypes to be realized. For most studies, the SC direct-drive generators are a synchronous generator concept. As the offshore wind farms are located far away from the land mainly due to the overall economic benefit, direct current transmission has been put forward. Hence, employing SC dc wind generators would be a good option. This enables a highly efficient and compact generator and, in addition, a new and also very efficient generator connection scheme at dc. For this reason, this paper investigates the feasibility of the large-scale SC dc direct-drive wind generator, in terms of torque, weight, and efficiency. In addition, the tape cost and commutation are discussed.

Experimental and Theoretical Research on Cogging Torque of PM Synchronous Motors Considering Manufacturing Tolerances

This paper investigates the cogging torque caused by manufacturing tolerances through analytical, finite element analysis (FEA), and experimental methods. Based on experiments, it is found that the stator and rotor tolerances of PM motor generate low-frequency components of cogging torque, such as the multiples of rotor pole and stator slot numbers. First, analytical calculation is used to find out the frequency spectrum components of the cogging torque. Then, FEA is used to confirm the analytical results and to determine which tolerances play dominant role on causing these components. Furthermore, the sensitivities of the key tolerances are illustrated. These can provide the designer a direction to design a motor with low sensitivity to tolerances. Because the tolerances of stator and rotor stacks are from stamping die, these tolerances are almost constant. A simple manufacturing method, i.e., rotation lamination, is proposed to manufacture the improved stator and rotor stacks. After using the proposed method, the effects of stator and rotor tolerances are individually validated through experiments.

A Large-Scale Superconducting DC Wind Generator Considering Concentrated/Distributed Armature Winding

The rapid development of renewable wind energy asks for new technology and the scheme of superconducting dc generators with superconducting dc transmission cables provides a possible and innovative solution to this trend. This paper designs a 10-MW superconducting dc generator with copper armature winding in the rotor. To find a suitable configuration of the armature winding, distributed winding, namely double-layer lap winding, and concentrated winding are analyzed and compared. The commutation principal of the two types of armature winding is briefly introduced by a simplified case. Electromagnetic performance, including torque, no-load voltage, and coil current, of the two types are calculated and compared based on the finite-element software. The reasons contributing to the performance difference are also elaborated.

Comparison of 2D simulation models to estimate the critical current of a coated superconducting coil

Superconductors have been being applied to a variety of large-scale power applications, including magnets, electric machines, and fault current limiters, because they can enable a compact, lightweight and high efficiency design. In applications such as those mentioned above, superconducting coils are always a key component. For example, in a superconducting electric machine, the superconducting coils are used to generate the main flux density in the air gap, which is significantly important for the energy conversion. It is the performance of the superconducting coils that plays an essential role in determining the performance of the device. However, the performance of a superconducting coil is limited by its critical current, which is determined by temperature and the magnitude and orientation of the magnetic field inside the superconductors. Hence, in-depth investigations to estimate the critical current of the superconducting coils are necessary before manufacturing. Available transient simulation models to estimate the critical current are through the H- and T-A formulations of Maxwells equations. Both methods consider the same current ramp-up process occurring in experiments. Besides these transient models, static simulations can also be used: a modified load-line method and the so-called P-model, which is based on the asymptotic limit of Faradays equation when time approaches infinity. To find the best way to calculate the critical current, the four methods are used to estimate the critical current of a double pancake superconducting coils and results are compared with experiments. As a conclusion, T-A formulation, P-model, and the modified load-line methods are recommended for estimating the critical current of the superconducting coils.

Iron loss measurement of nonoriented silicon and cobalt iron electrical steel sheets at liquid nitrogen temperature using ring specimen

The design of superconducting machines requires a knowledge of the magnetic characteristics of non-oriented soft magnetic materials at low temperatures. Therefore the performance of the electrical steels has to be examined under the given temperature constraints. This report shows that operating the steel at low temperature leads to increased iron core losses and an increased relative permeability. The losses are separated in different loss components and the loss components are quantified.

A novel linear machine with amorphous primary core

This paper proposes a novel DC-excited flux-modulated linear machine with new primary structure for utilizing amorphous core. Although amorphous core is regarded as a good candidate in high efficient electric machine, and being investigated for decades, it has not been widely used in electric machines yet. The key problem is the difficulty in slotting. For the new structure, the difficulty in slotting is avoided. Furthermore, the proposed machine is economical and has robust double salient structure without any permanent magnets (PMs), and both the field winding and the armature winding are located together in the primary core. In addition, both windings are single-toothed. By regulating the field excitation, the machine can operate at a wide speed range. Due to the low loss of amorphous core, this machine shows high efficiency at high speed.

Torque analysis of a DC-excited flux-modulated machine

Over the past decades, due to its simple and robust structure and low cost, switched reluctance machines (SRMs) have received some attention [1].

A Novel PM-Free High-Speed Linear Machine With Amorphous Primary Core

This paper proposes a novel dc-excited flux-modulated linear machine with a new primary structure for utilizing amorphous core. Although amorphous core is regarded as a good candidate in highly efficient electric machines, and is being investigated for decades, it has not been widely used in electric machines yet. The key challenge is the difficulty in slotting. For the new structure, the difficulty in slotting is avoided. Furthermore, the proposed machine is economical and has a robust double salient structure without any permanent magnets, and both single-toothed field and armature windings are located together in the primary core. By regulating the field excitation, the machine can operate at a wide speed range. Due to the low loss of amorphous core, this machine shows high efficiency at high speed.