Power P = 1x A
By Jim Sparks
A alternating Current (AC) is the power of choice for the manufacturers of large airline equipment. One primary reason is the ability to use smaller diameter wires resulting in a subsequent weight savings, plus AC generators can operate without brushes, which reduces maintenance.
The majority of aircraft will have some use for alternating current power. However, the way this power is produced can vary drastically. Most common are: mechanical means where the North and South poles of a regulated magnetic field interact with fixed windings, and a unit where solid state electronic components are used to produce a very close tolerance output.
Aircraft that are equipped with a Direct Current (DC) power may utilize inverters to develop AC for specific applications. In other cases, engine-mounted alternators are used as a supply source. In several new generation aircraft with primary DC power, individual components that require AC have the ability to convert the DC to alternating current. This is true for many motorized devices, such as fuel boost pumps and blower fans.
Early inverters used a motor supplied from the aircraft's main electrical system. This motor would then drive an AC generator at a predetermined constant speed to produce a specific frequency. Output voltage was regulated by varying the DC power to a main excitor winding included in the AC generator. This type of unit had several draw backs. First of all, if a DC-powered motor was used to drive the AC generator, brushes were required and therefore obliged periodic maintenance. These units were also susceptible to variations in motor speed which resulted in frequency drift.
Frequency of an AC signal is often viewed as a sign wave and is referred to as a cycle. This begins when a magnetic field begins to interact with a coil of wire. As the magnet begins to move relative to a stationary coil, voltage begins to build and will continue to increase until the magnet has moved 90 degrees. From this positive peak voltage will begin to decline until the magnet is 180 degrees from the starting point. At this point there is a zero potential in the coil and continued movement of the magnet will result in another increase in voltage, but in this case, it is moving to a negative peak that occurs at the magnets 270 degree point from the start position. Then the voltage will begin to decline until the magnet is back to its point of origin. The positive 90 degree potential is the same as the potential occurring at the 270 degree point, however the charges are opposite. Many AC generators and Inverters will include three of these sine wave generators spaced at 120 degree intervals. The result is three independent AC outputs. By utilizing different circuit wiring techniques these three phases can be utilized to produce a voltage almost twice as high as each phase independently. The most common voltage levels in aircraft are 115 VAC and 26 VAC. By combining phases, 208 VAC can be achieved while a 115VAC can be used simultaneously.
Most AC generators are brushless self cooled units with an integral Permanent Magnet Generator (PMG) and current transformers. The quill drive shaft is connected to a source of mechanical energy, such as an engine gear box. Changes in RPM of this mechanical drive will result in a frequency change. In applications where the generator is supplying power for heating elements, this variable frequency is not a problem, but in most other situations constant frequency is essential. Fixed RPM can be achieved by using a Constant Speed Drive (CSD) which employ hydraulics or a system of mechanical clutches to maintain a constant velocity output. Most CSDs will require periodic inspections and servicing. In many cases, these units are electronically monitored to ensure proper operation.
The drive shaft going into the generator will have a permanent magnet, excitation rotor, and main rotor all attached. Within the generator housing are the PMG stator, excitation coil, and the main stator.
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