1. Constant speed and constant frequency AC power supply
In 1946, the United States invented the constant speed transmission device, which opened up the era of constant speed and constant frequency AC power supply. At present, the rated frequency of aircraft constant frequency AC is 400Hz and the voltage is 115/200V. The aircraft alternator is driven by the aircraft engine through a constant speed transmission. The constant speed transmission is referred to as CSD, which turns the changing speed of the aero-engine into a constant speed to drive the alternator, so the alternator can output 400Hz alternating current. The rated capacity of constant frequency alternator is 15kV·A, 20kV·A, 30kV·A, 40kV·A, 60kV·A, 90kV·A, 120kV·A and 150kV·A. The auxiliary power source for medium and large aircraft is an alternator driven by an auxiliary power unit (APU). Emergency power sources are ram air turbine generators or battery/static converters. The secondary power source is a transformer and a transformer rectifier. The application of constant frequency AC power supply eliminates the disadvantages of low voltage DC power supply.
For decades, the constant speed constant frequency power supply (CSCF power supply) has gone through four stages of development. The 1950s was the first stage, using differential hydraulic constant speed transmission, brushed alternator and electromagnetic mechanical regulating protector. The 1960s was the second stage, with gear differential hydraulic constant speed transmission, brushless alternator and electromagnetic control protector. Entering the third stage in the 1970s, the combined drive generator (IDG) was developed, which is characterized by: ① the integrated design of the constant speed transmission device and the generator simplifies the components; ②the generator is cooled by oil injection, and made of high-performance iron drill vanadium soft magnetic material, and the speed is increased to 12000r/min or 24000r/min. The power-to-weight ratio of this power supply is significantly increased, the overload capacity is enhanced, and the reliability is further improved. Entering the fourth stage in the 1980s, the design idea of AC power supply changed from focusing on reducing weight to focusing on improving maintainability and reducing full-cycle costs, so the controller of the microcomputer dominates, and standardization, modularization, and intelligence have become important considerations.
The advantages of CSCF power supply are: high working environment temperature and strong overload capacity. Its main disadvantages are: ①CSD manufacturing, use and maintenance are difficult; ②The power conversion efficiency is low, and the efficiency of the main power supply is about 70%; ③It is difficult to further improve the power quality; ④It is difficult to realize starting power generation.
2. Variable speed constant frequency AC power supply
The development of power electronic technology has laid the foundation for the variable speed constant frequency power supply (VSCF power supply). In 1972, the 20kV·AVSCF power supply developed by General Electric Company of the United States was installed for the first time (A-4 aircraft), and the VSCF power supply has developed rapidly for decades. The advantages of VSCF power supply are: ① high power quality; ② high power conversion efficiency; ③ less rotating parts, reliable operation; ④structural flexibility is large; ⑤It can realize brushless starting and power generation; ⑥It is convenient for production, use and maintenance. The disadvantages of this kind of power supply are: ① the allowable working environment temperature is lower; ② the ability to withstand overload and short circuit is poor.
The difference between the VSCF power supply and the CSCF power supply is only the main power supply. The main power supply of the CSCF power supply consists of a constant speed drive, an alternator and a generator controller, while the VSCF power supply consists of an alternator, a power converter and a controller. There are two types of power converters: AC-AC converters and AC-DC converters. The former consists of thyristors and the latter consists of power transistors. Since the allowable junction temperature of the power transistor is higher than that of the thyristor, and the transistor power converter uses fewer power devices, the reliability is high.