Iron-cored generators dominate the wind industry. These machines functionally comprise a rotating part as well as a stationary stator part, and the rotor either turns inside (inner rotor) or outside the stator (outer rotor). The stator typically comprises copper winding and a magnetic material such as iron, which provides a strong magnetic circuit.
However, according to Gamble it also requires significant structural mass and complex assembly procedures to counteract the significant magnetic forces between the rotating and stationary parts of the generator. This complexity and mass penalty is exacerbated for larger multi-MW machines, he adds.Phenomenon
NGenTec’s concept is the outcome of research conducted at the University of Edinburgh. So far they built and tested three rotary generators using NGenTec’s novel axial flux air-cored generator technology. This consists of several standardised rotor and stator modules.
The benefits of the device include easy assembly and disassembly, while each module acts as an individual generator. This allows the turbine to keep producing power, albeit at a reduced output, if one of the modules fails.
Each stage of the rotor module consists of double-sided mild steel rotor plates, joined at one end to form a c-shape, with surface-mounted permanent magnets within it. These rotor modules can be assembled in a circular shape around a support structure without requiring specialist and costly tooling.
The air-cored stator modules consist of windings wound on a non-magnetic support structure. Gamble said that this technology feature eliminates the iron losses inherent in iron-cored machines, adding: "All magnetic forces are neutralised within the rotor module structure and thus cannot be transferred to the rotor or stator support structure. This allows for a flexible mechanical topology, simpler bearing arrangement, a lightweight structure and easier assembly."
Proving the technology
Gamble explained that NGenTec was completing the building of a full-size test generator. It will be duration tested at David Brown Gear Systems’ (DBGS) facilities starting Q1, 2012. Gamble said: "We will test and verify performance characteristics, vibration levels and other key performance variables. In addition main functions including thermal management, and validate the modelling of the generator as a whole and at component electromagnetic level."
Elaborating further on generator cooling, he added that initial results indicated favourable internal temperature performance enabling the use of less expensive, lower-grade magnet materials.
NGenTec closely cooperates with its global industrial partner DBGS, also headquartered in Scotland. Interestingly, Clyde Blowers, the owner of DBGS, recently acquired bankrupt Finnish gearbox supplier Moventas. The Finnish firm manufactures an integrated medium-speed geared system called FusionDrive.
Hybrids and direct drive
NGenTec initially focused on the direct-drive offshore market segment, aimed at offering stacked generators with power ratings of 6MW and beyond.
Gamble said: "We are working with wind-turbine manufacturers and developing concepts for hybrid solutions with either a single-stage low-speed or two-stage medium-speed gearbox. Based upon consultations with different potential clients we came to the conclusion that low speed and medium speed is the preferred wind industry ‘sweet spot’ for the next four to six years. However, we still consider direct drive a viable future technology option as ratings increase and as the industry moves further offshore."