Motors that count: air quality control at 36,000ft
21 November 2012
Cruising at an altitude of 11km above the surface of the earth, the modern commercial airliner must clearly provide a safe and pleasant ambient environment for passengers. Pressurised cabins with environmental control systems achieve this, but the new Boeing 787 Dreamliner goes one step further, providing a special air conditioning system that ensures optimum passenger comfort on long flights.
Aircraft environmental control systems involve three main elements: air exchange, pressure control and temperature control. Providing passengers with the required atmosphere in the cabin, at a suitable air pressure, with adequate oxygen levels and a satisfactory ambient temperature, means that aircraft air conditioning systems differ greatly from any ground-based system in terms of their design and energy requirements. Aircraft air conditioning systems not only need a higher capacity energy source, they also have to meet higher safety standards.
The circumference of an aircraft fuselage expands as a result of the need to maintain a specific air pressure within it, placing great stress on the airframe. Following take-off, air pressure in the cabin is gradually reduced as the aircraft gains altitude, reaching a level equivalent to that which would be experienced at approximately 2,400m. But regulating pressure alone is not the end of the story; climate control also depends on maintaining an adequate level of oxygen in the cabin atmosphere, that is also sufficient for the passenger numbers aboard.
But oxygen level and air pressure alone do not ensure a pleasant atmosphere. The temperature and humidity also play an important role, and modern computer-controlled systems are capable of regulating the cabin temperature to within one degree Celcius, taking into account the very low external temperatures, and the heat by the passengers themselves within the cabin (each person representing a ‘heat source’ of around 80 to 100W). The system must also compensate for differences in the source of the air supply. On the ground, the air conditioning unit is supplied with compressed air from an auxiliary power unit, while during flight most aircraft obtain the cabin air supply via the engines.
Going one better
Last year, Boeing launched a new long-distance aircraft, the Boeing 787 ‘Dreamliner’, which provides a step change in terms of improved passenger comfort. The Dreamliner fuselage includes carbon fibre composite materials in its construction and, according to Boeing, the resulting structure is considerably stronger than the more traditional all-aluminium shell we associate with most airliners.
As a result, this aircraft is able to provide passengers with a higher, more passenger-friendly ambient air pressure – equivalent to that at an altitude of 1,800m (as opposed to the usual 2,400m), making long-distance flights a more tolerable experience. Furthermore, the corrosion-resistant shell allows 15 percent air humidity in the interior, instead of the customary 4 percent. This has all meant a step change in the design of the climate control system.
On the Boeing 787, cabin air is not drawn from the jet engines under pressure, but is instead supplied as fresh air from the exterior of the aircraft to a cabin air conditioning system designed and manufactured by the US based company, Hamilton Sundstrand. Powered by the aircraft’s generators, this system provides enough heating/cooling effort sufficient for 25 private households.
No fewer than 48 motors are deployed in various parts of this air conditioning system, including specially modified drives for duties ranging from operating the air inlets supplying air from outside the aircraft, to controlling the cabin ventilation itself and providing cooling air for onboard electronic systems.
Differing considerably from standard units, these motors must be capable of operating over a much wider temperature range, have minimum sensitivity to vibration, offer long life and high reliability. The motors chosen by Hamilton Sundstrand for the Boeing 787 climate control system are all maxon units, variously modified to meet the specific requirements of the system. In particular, they are designed to operate over the temperature range –55oC to +85oC and withstand the shock and vibration of take-off and landing - over decades of the aircraft's service life.
The cabin ventilation system consists of 36 shut-off valves that are driven by maxon EC 45 flat dc brushless motors, lightweight units that can be installed within very tightly confined space envelopes. These high-torque motors will run at speeds of up to 20,000rpm without undue heat dissipation problems, though for the Hamilton Sundstrand system, they are only required to achieve maximum speeds of 4,000rpm.
The motor stator had to be adapted for this application and the printed circuit board modified with the inclusion of low-temperature Hall sensors. The motor case itself is treated with a special protective conformal coating. The modified stator magnetic path prevents movement when power is not applied to the motor, improving its overall efficiency.
Meanwhile, the linear drives for the air inlets use modified EC32 dc brushless motors which have also been equipped with low-temperature Hall sensors. A flame barrier is also included at the output shaft of the motor, along with special vibration-resistant screw fastenings and cogging detent brake modules.
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