Ever wonder where a big jet gets all its electricity?
The system demands placed on jet aircraft electrical systems are higher than those found on typical light general aviation aircraft. Jets usually have more electrically powered components, including high-draw items such as electric hydraulic pumps or galley ovens. One result is that their electrical systems make greater use of 115-volt alternating current (VAC). Engine-driven generators are the main AC power-generating sources. That 115-volt current can also be supplied from either an auxiliary power unit or an external ground power unit.
Mechanical energy to turn the respective engine-driven generator comes from rotation of the engine itself. The engine's accessory gearbox is where this mechanical energy is applied to the generator (and to other components such as engine-driven hydraulic pumps). However, engine speed does not remain constant. Since most electronic components depend upon a relatively steady 400 cycles per second (Hz) current, a widely varying generator speed would be unable to maintain this value. The answer for many installations, especially those found on larger business jets and airliners, is the constant speed drive, commonly referred to as the CSD. The CSD is a hydromechanical transmission, designed to change variable engine rotation speeds to a constant generator speed.
Most of the time, a CSD works as advertised. However, just as a car's transmission can lose fluid, resulting in overheating and transmission damage, a CSD oil leak can result in overheating too. If left unchecked, the CSD can be seriously damaged. This can cause further harm to the accessory gearbox, or even the engine itself. If a pilot observes high CSD oil temperature, a first step (depending upon the model aircraft flown) may be to take the generator off-line to lighten the demand on the CSD. Should this be ineffective in lowering CSD oil temperatures, the CSD can be mechanically disconnected from the generator to prevent damage to the unit or engine. In this case, the generator is no longer usable in flight, and can only be reconnected to the CSD on the ground.
Some newer aircraft such as the Gulfstream V utilize an integrated drive generator, or IDG. These combine the electrical generator and CSD within a single integrated housing, rather than as separate components. But the essential function of an IDG remains the same: to allow the generation of steady AC power from variable engine operating speeds.
CSDs and IDGs are common on large jet aircraft. Some smaller jet and turboprop aircraft may utilize starter-generators instead. The Cessna Citation VII, for example, uses a 28-volt DC starter-generator driven by the accessory gearbox on each engine. Its name derives from the fact that it functions as a starter motor powered by the battery or external power source to turn the engine's high-pressure spool during engine start. When the engine attains idle speed, the unit becomes a 28-volt DC generator. Electronic generator control units regulate voltage levels.
Jets both large and small also rely upon 28-volt direct current for powering some instruments and components. Aircraft with 115 VAC generators produce needed DC voltage by routing a portion of the AC through transformer rectifier units, or TRUs. These change 115VAC input into 28-volt DC output.
The ship's battery can also supply 28-volt DC power, but this is primarily used for temporary load conditions such as initial engine or APU start. It is maintained in a charged state during normal system operation but remains available as a source of emergency power should all electric generating sources be lost. In flight, the battery must be capable of powering the aircraft's most critical DC and AC electrical buses by itself for at least 30 minutes if needed. In this situation, a portion of the battery's 28-volt DC output is routed through so-called static inverters. These solid-state units can be thought of as the opposite of TRUs in that their function is to change 28-volt DC input into 115 VAC output.
The alternator found on light general aviation aircraft is really an AC generator. It works by first generating alternating current, which is then rectified into 12- or 24-volt direct current by a diode assembly built into the alternator case.
Vincent Czaplyski holds ATP and CFI certificates. He flies as a Boeing 757 captain for a major U.S. airline.
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