Tham khảo tài liệu 'wind turbines part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 630 Wind Turbines In terms of magnetic characteristics the rotor core can be either magnetic or non-magnetic. The use of magnetic irons can reduce the mmf required to establish the same field since core material forms part of magnetic circuits much better than air . Clearly the rotor mass would be increased accordingly and so is the rotor inertia. However the latter does not cause problems since in direct-drive wind turbines the actual rotation speed is quite low. In practice it is very difficult to twist the HTS coils to align with the field for the purposes of minimizing ac losses so that iron and the flux diverters should be used to guide the flux in the desired direction and away from the HTS. But the fact that iron saturates at approximately 2 T puts a limit on the maximum flux density. In theory the high current density in superconductors makes it possible to produce sufficient air-gap flux density without a rotor core. Therefore the rotor can be of air-cored type coreless rotor Ship Sykulski 2004 Lukasik et al. 2008 . This configuration provides a significant reduction in the weight of the rotor and the associated eddy current losses. Nevertheless it may increase the amount of superconductors used and the current level in the superconductor so as to produce the required flux density. Similarly because there is no iron core the support structure should be strong to transmit the high torque which is the case of direct-drive wind turbines. With regard to the rotor cooling arrangement the HTSWTG can use either warm or cold rotors as demonstrated in Fig. 6. In Fig. 6 a only HTS coils are cooled at cryogenic temperature so that the so-called cold mass is low. This results in short cool-down periods and reduced eddy current losses. But the supporting structure would be complicated to hold the HTS and also to prevent heat leakage. In contrast in Fig. 6 b the cold rotor structure is relatively simple and the whole rotor is cooled at cryogenic temperature requiring .
