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THE GEMINI ELECTRIC MOTOR TECHNOLOGY against permanent magnets means this area of switched reluctance research is still relatively unexplored. John Ettridge is an inventor who has invested the time required to pursue permanent-magnet- based switched reluctance designs. His research began out of frustration with conventional designs— which it seemed to him were grossly inefficient, difficult to repair and unimaginative. Figure 2 by John Patrick Ettridge with Tim Harwood, MA © 2004 lectric motor technology has Benes surprisingly static over the last 50 years. While there are a number of good reasons for conservatism in any design process, perhaps the most significant point is that it is only relatively recently that permanent magnets have become cheap enough, and rugged enough, to be appropriate for motor design. Up until the 1940s it was still customary to use electromagnets, as permanent magnets were not considered suitable for the demands of commercial applications. And it took even longer for magnets to be made available for low-cost private experimentation. The result was an industry that became surprisingly fixed in its ways: innovation could only come from the outside. While switched reluctance motors have found increasing popularity in recent years as the advantage of high efficiency becomes more widely appreciated, the traditional prejudice of motor designers co) 7 Design of the = Gemini Motor Figure 1 is taken directly from the Ettridge patent and illustrates the layout pursued in the early stages of research. Subsequently a variety of alternative geo- metrical layouts were developed, optimised for specific applications and load scenarios. When the six coils are aligned with the permanent magnets, an input pulse is provided to the apparatus that manifests a repulsion effect, pushing the stators away from the permanent magnets. The input voltage will determine the characteristics of the recession. The higher the input voltage, the greater the resultant mechanical force manifested. While the use of permanent magnets in such a manner is innovative, the Ettridge technology features further levels of design optimisation. Figure 2 illustrates how the conventional T-shaped stator can be replaced with an H-shaped stator. This enables both ends of an energised coil to be used to impart motion to the rotor section. Figure 3 places the two stator designs side by side to highlight the novelty in design. The resultant increase in efficiency and improvement in power-to-weight ratio means that the overall effectiveness of the technology is greatly enhanced without incurring a significant increase in manufacturing costs. Independent support for this view was sought from Paul Chandler, Research Engineer of the Northern Territory Centre for Energy Research, then part of the Northern Territory University [now Charles Darwin University]. He stated the following on 18 July 2000: "The general theory is that permanent magnet (PM) machines are capable of pro- ducing higher efficiencies over others such as induction motors and reluctance motors. They can be very robust and produce a large torque for their physical size. "The revolutionary feature of the Gemini motor is that it has the capability of producing more power for the same volume. Figure 1 NEXUS + 49 OCTOBER — NOVEMBER 2004 www.nexusmagazine.com