Ozan Keysan

Design and Testing of a Linear Generator for Wave-Energy Applications

A linear-generator topology is proposed for wave-energy applications. The main significance of the generator topology is that the relative position of the magnets, copper, and steels has been chosen so that there are no magnetic attraction forces between a stator and a permanent-magnet (PM) translator. The lack of magnetic forces and the modular nature of the generator topology make the manufacture and assembly of the generator easier than a conventional iron-cored PM linear generator. Analytical modeling techniques are described with a genetic-algorithm optimization method. The proposed topology is implemented to an Archimedes-wave-swing wave-energy converter. A 50-kW prototype has been built to prove the concept, and the no-load- and load-test results are presented.

A Homopolar HTSG Topology for Large Direct-Drive Wind Turbines

For offshore wind energy, there is a trend toward larger wind turbines. The increased mass of a power-takeoff system increases the installation cost of the turbine. Direct-drive superconducting generators have the potential to reduce the installation cost of wind turbines. For a successful entry to the offshore-wind-energy market, a high-temperature superconducting generator should be as reliable as conventional generators. It is proposed that a stationary superconducting direct-current-field winding may increase the reliability of the generator. An axial-flux homopolar generator topology is proposed to be used in low-speed high-torque applications. The topology is modified by using two superconducting field windings to obtain a bipolar flux-density distribution for higher power density. Different core types and dimensions were examined to find the most suitable design, and a conceptual design of a 6-MW 12-r/min generator is presented.

A direct drive permanent magnet generator design for a tidal current turbine(SeaGen)

In this study, the feasibility of a direct-drive permanent magnet generator for a tidal turbine power take-off system, namely MCTs SeaGen - the worlds first full scale commercial tidal turbine- has been investigated. The investigated PM generator topology is called C-GEN which is an air-cored axial-flux generator developed in the University of Edinburgh. The C-GEN is prior to conventional PM generators by absence of magnetic attraction forces between rotor and stator, absence of cogging torque, ease of manufacturing, modularity and high fault-toleration. Firstly, the integrated analytical design tool that couples electromagnetic, structural and thermal aspects of the generator has been introduced. Then, an optimization tool based on genetic algorithm has been used to maximize the annual electricity generation and to minimize the initial cost of the generator. The optimized generator is validated using FEA tools and the specifications of the generator has been presented.

Higher order rotor slot harmonics for rotor speed & position estimation

Air-gap flux of all electrical machines contains harmonics. Some of these harmonics stem from rotor slotting of the machine. These harmonics can be utilized for control or diagnostic purposes. In the literature, rotor slot harmonics are mostly utilized for condition monitoring. There is also some research utilizing them for rotor speed estimation. However, it is observed that in the existing work spectral analysis techniques are employed which require long samples of data and in this from they are not suitable for vector control of induction motors. Accurate position information is vital for high performance vector control of induction motors. Furthermore using a sensor which requires some kind of modification on the motor is not desirable. A novel method has been proposed in this study that does not need any spectral estimation techniques. In this approach extracting the required information can be done within a typical control cycle of a vector controlled drive algorithm. Furthermore, any modification of the motor is not necessary. In this paper the proposed approach is presented. Experimental results illustrate that the position prediction accuracy of the approach is very high.

Linear generator for direct drive wave energy applications

A 50kW linear permanent magnet generator with a novel topology has been designed and built. The main significance of the generator topology is that there is no attractive force between the stator and permanent magnet translator. The magnetic force between the magnets is reacted within a self supporting structure. The lack of magnetic forces closing the airgap and the modular nature of the generator topology makes the manufacture and assembly of the generator easier than a conventional iron-cored permanent magnet linear generator. Photos of the manufacturing process are included in this paper. The modeling techniques used to design the generator are described and a summary of the design is presented.

A modular and cost-effective superconducting generator design for offshore wind turbines

Superconducting generators have the potential to reduce the tower head mass for large (~10 MW) offshore wind turbines. However, a high temperature superconductor generator should be as reliable as conventional generators for successful entry into the market. Most of the proposed designs use the superconducting synchronous generator concept, which has a higher cost than conventional generators and suffers from reliability issues. In this paper, a novel claw pole type of superconducting machine is presented. The design has a stationary superconducting field winding, which simplifies the design and increases the reliability. The machine can be operated in independent modules; thus even if one of the sections fails, the rest can operate until the next planned maintenance. Another advantage of the design is the very low superconducting wire requirement; a 10 MW, 10 rpm design is presented which uses 13 km of MgB2 wire at 30 K. The outer diameter of the machine is 6.63 m and it weighs 184 tonnes including the structural mass. The design is thought to be a good candidate for entering the renewable energy market, with its low cost and robust structure.

Speed & position estimation by demodulating rotor slot harmonics

It is well known that slotting harmonics exist in most of the electrical machines. These harmonics may supply valuable information for condition monitoring of bearings, windings or rotor bars. Utilization of these harmonics for rotor speed and position estimation is limited mainly due to long sampling periods required by spectral estimation techniques. In this study, a novel speed and position estimation method is proposed that does not need any kind of spectral estimation. In this approach extracting the required information can be done within a typical control cycle of a vector controlled drive algorithm. In the application of the method, higher order rotor slot harmonics have been utilized. The approach is explained in the paper and illustrated with experimental results.

Magnetic and structural analysis of a transverse flux claw pole linear machine

This paper details the design and testing of a novel transverse flux claw pole linear machine suitable for large superconducting generators. The machine utilises a modular claw pole transducer design with a stationary field winding which eliminates the need for cryogenic couplers and electrical brushes for a superconducting machine. The results from this prototype will enable a better understanding of the electromagnetic and mechanical structures before embarking on a more costly super-conducting design. The structure of the prototype supports a field winding and core; four pairs of claw poles with a stroke of 500 mm; three armature windings and core. Tests will be preformed to determine the deflection of the airgap, field core and armature core, the induced voltage in the armature coils, the forces acting on the structure and the flux density variation in the claw poles.

Application of high-temperature superconducting machines to direct drive renewable energy systems

Abstract: After the discovery of high-temperature superconductors, superconducting machines became more feasible and have been applied to many applications such as ship propulsion systems. Superconducting machines are lighter, smaller and more efficient than conventional generators. In this chapter, mass of different direct drive generators systems have been compared. Direct drive superconducting generators have the potential to reduce the nacelle mass and the installation cost of a large offshore wind turbine. Ongoing commercial projects and the requirements for a reliable direct drive superconducting generator are both listed.

Rotor loss prediction in air-cored permanent magnet machines

Despite the cost raise of rare earth materials, low and medium speed permanent magnet (PM) machines have gained big popularity among wind turbine manufacturers due to their high efficiency and reliability. Ironless stator machines have been proposed in the literature due to their reduced manufacturing cost. However, in the modelling process of the aircored machines, the rotor loss calculation is generally neglected. Through Finite Element Analysis (FEA) and experimental testing it has been found that rotor losses can be as high as 40% of the copper losses in an air-cored stator (ironless) low speed machine. This paper describes two methodologies to estimate the rotor losses due to eddy currents using static FEA, and the experimental data from the rotor-locked test. The equivalent resistance to predict the rotor losses can be obtained from the rotor-locked test; paying special attention to space harmonics induced due to the winding topology. Two prototypes have been analyzed: a radial and an axial flux machine. The rotor losses can be predicted with great accuracy applying the superposition principle, when the phase currents are nonsinusoidal (passive rectification).