Analytical Modelling and Analysis of Magnetic Gears and Pseudo Direct Drives for Large Wind Turbines
The emergence of high energy rare earth permanent magnets (PMs) led to an increased interest in Magnetic Gears (MGs) in industry and academia. MGs facilitate a contactless torque transmission and may, therefore, offer improvements in efficiency and reliability for applications where mechanical gear boxes are traditionally employed. Furthermore, MGs can also be integrated with a PM machine resulting in electrical machines with high torque densities and high efficiencies. Recently these Pseudo Direct Drives (PDDs) have been considered for many applications, including wind turbines where they may offer an attractive alternative to other drive train solutions. The MG and the related PDD are fairly new topologies and so far they have mainly been designed, analysed and optimised employing time consuming Finite Element (FE) methods. Analytical models considering radially magnetised PMs have only been recently proposed for MGs. Therefore, in this thesis an analytical model for the prediction of the flux density distributions in the airgaps and PMs of MGs with arbitrary magnetisation distributions is presented, and is applied for radial and Halbach magnetisation distributions. The developed models are further refined, to take into account of the stator currents in a PDD. Furthermore, in order to predict the efficiency of a PDD, models are developed to predict the flux density distributions in the pole pieces (PPs) of the MG element and the average flux density distribution in the stator iron of a PDD on no load and on load conditions. Predictions from the analytical models are compared to those from 2 dimensional (2D) FE analysis. The developed models are employed for the analysis and optimisation of MGs and PDDs for a 10MW wind turbine. Design optimisation studies are undertaken in order to determine the effects of the leading design parameters on the key performance indicators (PIs) of the MG and the PDD. It is shown that for a 10MW wind turbine an MG with a PM mass of 13.5 tons, and a PDD with the same PM mass, an efficiency of 98.7% and a total active mass of 50tons, can be achieved. Furthermore, a 5kNm PDD has been commissioned and built by Magnomatics Ltd. to demonstrate the feasibility of a scaled version of a 10MW PDD, and predictions from the analytical models are also compared with measurements on the demonstrator PDD. Due to the variability of the PM price and the potential insecurity of supply, the reduction of the PM mass has emerged as an important driver in the design process of drive train solutions for wind turbines. Therefore, in this thesis a PDD, where the high speed (HS) rotor PMs are replaced by coils, is proposed. The developed models for a PM excited PDD are extended to accommodate for a coil excitation on the HS rotor, and are employed for the optimisation of the coil excited PDD. It it shown that for a 10MW PDD an efficiency of about 95% can be achieved with a total active mass of less than 85tons and a PM mass of only 4.5 tons. In order to investigate the suitability of the PDD for other wind turbine power ratings the effects of scaling on the masses and efficiencies of the PDD are investigated between 5 20MW.