MIL-HDBK-1004/5
Section 2: GENERAL CONSIDERATIONS
2.1
Usage. Aerospace electrical equipment generally operates at an
input of 400 Hz. Electrical power is supplied by aircraft generators, which
normally receive their energy from the aircraft engines. Three-phase aircraft
generators deliver 3,000 to 4,000 RPM, depending upon engine speed, which is
synthesized into 400-Hz output voltage for distribution to aircraft equipment.
Large aircraft may have several hundred electric motors, and the use of 400 Hz
provides a considerable weight saving. Three-phase, 400 Hz, open-frame units
(1 to 15 horsepower in size, with speeds of 12,000 to 24,000 revolutions per
minute) developed for aircraft have weights averaging 2 pounds per horsepower
(0.9 kilograms per horsepower). An open, dripproof, 60 Hz, 1,800
revolutions-per-minute unit of one horsepower weighs about 40 pounds (18
kilograms). For an expanded description of aerospace electric subsystems, see
Fink and Beaty, Standard Handbook for Electrical Engineers (Section 23).
2.2
Types of Systems. Systems supplying 400 Hz for ground-power
operations use frequency conversion equipment to change 60-Hz input to 400-Hz
output. Rotary converters (motor generator sets) or solid state converters
are used for this purpose. Fixed service point units to which avionics
equipment and aircraft are connected are supplied from either nearby frequency
conversion assemblies over a low-voltage feeder system or from a more remotely
located 400-Hz central plant using medium-voltage feeders.
2.2.1
Rotary Converters. Rotary converters or motor generator (MG) sets
are used for both low and medium voltage systems. These units are usually
limited to installation in industrial locations due to the high level of noise
produced.
2.2.2
Solid State Converters. Solid state converters are used only for
low-voltage systems. The noise levels produced by these units as compared to
MG sets are substantially less. The industry trend is to replace rotary
machinery with solid state converters.
2.3
Distribution Systems. Fixed service point units to which avionics
equipment and aircraft are connected are supplied from either nearby frequency
conversion assemblies over a low-voltage feeder system or from a more remotely
located 400-Hz central plant using medium-voltage feeders.
Low-Voltage Systems. Generally low-voltage systems distribute
2.3.1
voltages less than 600 volts. Because the reactance of an electric system is
greater at 400 Hz than at 60 Hz, attention must be given to both circuit
length and conductor size to maintain acceptable voltage regulation.
Consequently, when loads and distribution distances increase, low-voltage
systems require use of excessive feeder sizes and installation of numerous
local frequency conversion assemblies. When numerous local frequency
conversion assemblies are used, the reliability of the system is increased. A
typical, 400 Hz low-voltage system is shown on Figures 1a and 1b. Detailed
requirements are provided in Appendix B.
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