Review of direct‐drive radial flux wind turbine generator mechanical design

Abstract

The direct-drive radial flux synchronous generator is considered as the modern wind turbine drive train. Both the electrically (e.g., Enercon) and permanent magnet (PM; e.g., Siemens) excited direct-drive generators are gaining popularity on the market today. Compared with the matured geared counterpart, the electrically excited direct-drive generator is heavier and more expensive but more reliable per unit capacity. The PM-excited generator is expensive, is simpler in electromechanical design, has a high power-to-weight ratio, and yields a higher energy conversion efficiency than its electrically excited equivalent. The PM generator technology has the potential to yield the highest energy-to-cost ratio. However, standardization of this direct-drive generator parts/subassemblies may overcome the existing cost barrier. Most current literature focuses on PM generator wind turbine technology, specifically on generator energy conversion optimization, and the scalability of technologies to capacities in access of 5 MW. Strangely, PM generator's mass and cost reductions through optimized structural design incorporating manufacturing, transportation, and installation constraints are less studied. This paper solely focuses on the mechanical and structural design aspects of large radial flux synchronous PM generators specific to direct-drive wind turbines. Generator topologies such as the common iron-cored and unconventional air-cored generator are discussed. However, design considerations specific to the iron-cored generator topology are studied. The design considerations investigated involve the geometries and the configurations of rotor/stator active and inactive structures, the interfaces, and the conductor/PM mounting methods.

Structural Analysis and Characterization of Radial Flux PM Generators for Direct-Drive Wind Turbines

“…Figure 1 displays the rotor structures that have been looked at in other papers [2], [4]. [4] This paper focusses on structures where discs connect the shaft to the rotor surface, as they are relatively simple to describe and model. Figure 2 shows a cross section of a direct-drive generator in a wind turbine.…”

Electrical generators for direct drive systems: a technology overview

“…Since magnetic bearings require active control, this concept would introduce additional control issues and increase the total cost of the direct drive machine due to the expensive magnetic bearings. 58 Despite the novelty of all the alternative concepts, a number of researchers who have compared the various RF designs conclude in favour of the conventional RF PMDD machine with permanent magnets mounted on the surface of the rotor because of its structural simplicity; the high energy yield torque density and reliability; and the reduced manufacture or maintenance costs. 6 , 10 , 16 , 19 , 44 , 58…”

“…The stator structure is eliminated in this case and the conducting material is embedded in a pocket made of non-magnetic material such as glass fi ber reinforced epoxy. 58 An ordinary rotor is employed with the permanent magnets mounted on its disc. The rotor and stator structures are relieved from maintaining the airgap clearance against strong attraction forces in such confi guration, leading to a more compact and lightweight design that is easy to manufacture and assembly.…”

Structural mass reduction by integrating active material in direct drive generator design

“…• replacing rotor shaft with rotor axle (Stander et al 2012), • minimising the amount of large diameter mechanical bearings (Versteegh 2004), • changing a double bearing configuration to a single bearing configuration with the addition of numerous smaller bearings at the air-gap (i.e NewGen concept) (Engström et al 2004), • increasing the wind turbine hub-generator structural integration (Versteegh 2004), • changing the generator topology from internal rotor to external rotor, • changing the generator magnetic flux orientation and active design (i.e. C-Gen concept) (Keysan et al 2010;Zavvos et al 2012), • introducing structural flexibility through the utilisation of active support systems like active magnetic bearings (Shrestha et al 2010), • the use of lighter materials such as aluminium and GFRE (Stander et al 2012).…”

“…C-Gen concept) (Keysan et al 2010;Zavvos et al 2012), • introducing structural flexibility through the utilisation of active support systems like active magnetic bearings (Shrestha et al 2010), • the use of lighter materials such as aluminium and GFRE (Stander et al 2012).…”