Power Electronics

Motor-Control Technologies Bolster Aircraft Efficiencies

Never before has the aircraft industry focused so much on lighter, more fuel-efficient airplanes. And for good reason: The cost of aircraft-grade fuel has doubled since the year 2000, according to a 2006 Subcommittee on Aviation hearing. Fuel costs constitute the second-highest operating expense for commercial aircraft carriers, with labor being number one. The importance of fuel consumption to the carriers manifests as aircraft purchasing criteria based on fuel efficiency. For example, lighter aircraft designs exploit composite materials such as carbon fiber instead of traditional metals. Reportedly, a 20% fuel-efficiency improvement derives from this use of composites.

Equal to the effort in material improvements is a focus on mechanical systems, including the replacement of many aircraft hydraulic lines and actuators with electric motors. Considering that current jumbo aircraft designs have wingspans approaching the length of a football field, not only are there dramatic weight reductions to be gained, but also higher reliability by eliminating the risk of hydraulic-line puncture or leakage.

Aircraft-engine technology is also changing. New engines eliminate the bleed-air systems older designs use. Bleed air steals engine intake and supplies this high-temperature, high-pressure air to the air-conditioning and anti-ice systems. Eliminating inefficient bleed-air systems increases the demand for motors to drive the same functions.

Additional weight reductions are attainable by replacing large, heavy and expensive induction motors with permanent-magnet (PM) motors. Induction-motor designs must accommodate operation from a variable-frequency power generator operating in the 400-Hz to 800-Hz range, which prevents motor optimization and results in oversized machines compared to their PM counterparts.

With aircraft motor use increasing throughout environmental, fuel, cooling and actuation systems, electric-motor drives have become critical to overall aircraft efficiency. Drive technology is as important as the motor construction. Motor drives must ensure reliable, efficient, lightweight control of the more than 100 motors possible in modern aircraft.

Electronic-motor drives within aircraft have evolved from fully discrete pc-board designs to more integrated implementations. Integration has mimicked the commercial industry with, for example, the integration of drivers and power devices and with the development of power modules. Integration by power-semiconductor and hybrid manufacturers has reduced the size and weight of these systems. Compared to commercial products, the only difference is that power modules for aviation must meet stringent vibration, temperature-performance and reliability standards.

As aviation-motor drives evolved, so grew the need for digital control — implemented on microcontrollers or DSPs — to ensure high motor efficiency. The use of software on an aircraft is a major reliability concern and there are certification standards for both the computation engine and the code itself, which add to the development time of any motor-drive system.

Aircraft motor- and system-level designers are now demanding more highly integrated motor drives where the digital control and power are all contained in one enclosure. This requires expertise in power semiconductor, digital signal processing semiconductor and hybrid packaging technologies. The module-design task is more demanding due to aircraft reliability, temperature and vibration requirements, which are more challenging than those for most other motor-drive applications.

Complete single-module drives simplify aircraft-motor design-ins and result in reductions in weight, size and manufacturing complexity. Integrated drive modules also practically eliminate the development time for motor drives. This short design-in time is even more important considering the different motor-drive designs an aircraft requires.

Improvements in motor-drive technology are enabling modern aircraft to make considerable gains in power and weight efficiency, both of which contribute to improved fuel efficiency. Many applications in commercial and industrial sectors already exploit integrated motor drives and enjoy the benefits of improved performance and faster design cycles. The aircraft industry has lagged the trend only due to its added reliability and environmental requirements.

Robert Gendron holds a BSEE degree from Clarkson University, an MSEE degree from Northeastern University, an MBA degree from the University of New Hampshire, a graduate certificate in management from Harvard University, and has 20 years experience in the electronics industry.

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