SWT Technology Summary
Small Wind Turbines, LLC has commissioned the commercialization of a unique electrical generator technology for utilization in wind turbine applications. The technology was developed in Australia by GMI (Global Motors Inventions, LTD) over the course of several years and stemmed from their successful efforts to produce very efficient motors for “proof of concept” electric vehicles. As applied to electrical power generation from wind, the result is a family of three phase AC generators encompassing the power production range from 5 kW to 100 kW.
The primary design goal for the generator was to achieve dramatically higher efficiencies, lower weight and lower cost than previously available in the marketplace. Considerable efficiencies had been proven through GMI’s motor development efforts, which established their baseline for generator design and development.
With wind turbine system design considerations in mind, such as gearless drivelines and reliable performance over a wide range of wind speeds, GMI looked to optimize the direct drive, permanent magnet AC generator. In order to achieve this end, GMI established three primary design goals: 1) minimize the amount of iron required, 2) minimize the amount of magnetic material utilized, and 3) maximize the amount of active copper (windings). The achievement of these goals in the implementation of their design has resulted in a state of the art generator that surpassed anything available in the current market.
The salient feature of this design is the incorporation of dual stators with the rotor turning between the inner and outer stators in a radial flux configuration. This “generator within a generator” permits a much higher utilization of the magnetic remanence (field strength) of the permanent magnets incorporated in the rotor and contributes greatly to lower costs achieved by less magnetic material being required per unit output. Neodymium (NdFeB) magnets are incorporated in the rotor because of their superior remanence and good coercivty (resistance to demagnetization). Operating temperatures of 80 C are well below the Curie temperature (where magnetism is lost) of NdFeB magnets.
Mechanical design of the generator provides for a high level of flux density across the rotor/stators air gap thereby producing a high output voltage and corresponding high output efficiency. This design also dictates good flux consistency which results in low cogging torque and corresponding low torque ripple in the generator output waveform.
The design has been incorporated into three prototypes, one each 10 kW, 20 kW and 40 kW generators. All have successfully met initial test criteria in Australia and are undergoing further test verification locally.
By way of example, results for the 20 kW generators are examined below:
To measure against design parameters, an output test was undertaken at a fixed 185 rpm. The test resulted in an output of 21.39 kW (with an input torque of 1595 Nm) and with an efficiency of 96.6% (+0.6% on design specifications). Efficiency was determined as a function of output/input power ratio and remained above 94% from 7.75 kW to 40.75 kW output. The generator was then tested in a 232% overload condition at 46.76 kW (with an input torque of 2,631 Nm). Even at this heavy overload condition, the efficiency was 92.0%, thereby exceeding efficiency design parameters.
Voltage regulation and waveform tests indicate that the generator exhibits excellent RMS voltage regulation. The RMS voltage drop from no-load to full load (21.4 kW) was 4.4% (or +/- 2.2%). This fluctuation is well within the plus or minus 5% RMS requirement for line transmitted electrical power. This level of voltage regulation, however, was only maintained up to the rated loads of 21 kW. Once the generator was placed into overload conditions, the voltage regulating effects started to diminish as the load increased. This may explained by a second effect known as torque efficiency exhibited by this generator design. Torque efficiency only becomes evident when the generator is in overload mode (beginning at approximately 17 kW output). As the load current increases beyond 50 amps, the flux is dragged off center and the inductive current no longer helps to increase the voltage but starts to act as a driving force to assist the input (rotational) driving force. This is reflected by a drop in the torque required to produce a given unit output, and is reflected in a drop in the Nm/Amps ratio. The voltage regulation effect drops off as the torque efficiency increases.
The generator design has both very good voltage regulation and exceptional overload efficiency due to a switched reluctance effect evident at high loads. It also exhibits an exceptional torque to weigh ratio, in excess of 30 Nm/kg for high torque applications. This is 1.5 times better than any other designs currently being used.
These factors, along with large savings in weight and cost, result in a much higher power to weight ratio and higher cost efficiency than existing generators currently on the market. This design incorporates 40% fewer active components and weighs 40% less than competing designs. Its simplicity results in significantly reduced production costs.
WIND TURBINE SYSTEM IMPACT
The benefits of this generator design are reflected in resulting wind turbine applications. The gearless, direct drive, low rpm turbine provides for low energy loss from mechanical resistance. Turbines may now achieve lower startup speeds due to this low cogging torque and low resistive torque design. These factors result in higher turbine efficiencies over a broad range of wind regimes, allowing maximum energy extraction from a wide variety of wind speeds. Lower weight results in lower shipping costs and lighter supporting tower structures. Superior voltage regulation results in more cost effective inverters and control systems.
* Douglas Wogstad, Technology Manager of Small Wind Turbines, LLC, has a Bachelor of Science degree in mechanical engineering from NorthwesternUniversity and an MBA in finance and management information systems from the University of Minnesota. Doug has over 30 years experience in technologies businesses with the past ten years having a primary focus on the harnessing of wind energy. At Control Data for almost 20 years, he was responsible for the full product development cycle including design, manufacturing, quality assurance and the customer interface. Doug also gained experience in the marketing of technological products as a product manager and a marketing manager. firstname.lastname@example.org cell: 612-859-4713
Small Wind Turbine, LLC:
Wind Turbine Direct Drive Generators
Brief Product Specifications for the 10kw, 20kw and 40kw units
kW 10, 20, 40
weight (kg) 127, 236, 406
diameter (mm) 506, 777, 777
length (mm) 298, 289, 381
rpm Rated 250, 186, 133
Max 335, 285, 200
voltage V RMS 186, 210, 218
3 Phase 3 Phase 3 Phase
amps Rated 31., 54., 112.
Max 88.0, 117., 200.
efficiency at rated load 94.1% 96.4% 97%