Low voltage DC power supply
Low-voltage DC power supplies were the earliest power supplies used in aircraft and matured during World War II. The regulation point voltage of the low-voltage DC power supply of modern aircraft is 28.5V. The main power supply consists of a generator directly driven by the aero-engine and a control protector. The rated capacity of the main generator is 3kW, 6kW, 9kW and 12kW, and the corresponding rated current is 100A, 200A, 300A and 400A. Auxiliary and emergency power sources are aviation batteries. The generator works in parallel with the battery or the generator and the generator.
In large aircraft, there are DC generators driven by auxiliary power units as auxiliary or backup power sources. The secondary power source is a rotating converter or a static converter, which converts low-voltage direct current into 400Hz three-phase or single-phase alternating current for use by instruments and other equipment on the aircraft.
The speed of the aero-engine changes with the change of its working state, and the ratio of the maximum working speed to the minimum working speed is 2. In order to keep the output voltage of the generator constant, a voltage regulator must be set up to adjust the excitation current of the generator so that the voltage at the adjustment point does not change due to changes in speed and load. When the generator and the battery work in parallel, in order to prevent the current of the battery from flowing into the generator when the generator is not working, there is a special reverse current protector, and the overvoltage protector is also used in the large-capacity aircraft DC power supply. At present, the generator controller with microprocessor has been used in the aircraft DC power system. It has various functions such as control, protection, self-checking, communication, fault memory and isolation alarm, which not only improves the quality of power supply, but also improves the maintainability and reliability.
The starter generator is a dual-function motor developed after the birth of the jet aircraft. Before the engine works, it works as a motor to drive the engine rotor to rotate. After reaching a certain speed, the engine is injected with fuel to ignite, so that the engine can work by itself. After that, the engine in turn drives the motor, making it a generator that supplies power to electrical equipment. One motor for two purposes, reducing the total weight, is an important development of DC power supply.
The main advantage of the low-voltage DC power supply is that it is simple and reliable, and it is very convenient to use the battery as a backup and emergency power supply. However, with the development of aircraft, the number of electrical equipment, especially AC electrical equipment, has increased, and the weakness of low-voltage DC power supply has been increasingly exposed. This is: ① The brushes and commutators of the DC generator limit the speed of the motor, thereby limiting the maximum capacity of the motor. The maximum capacity of the DC generator is 12kW; ② The weight of the aircraft DC power grid increases significantly after the power supply capacity is increased; ③ The efficiency of the secondary power supply is low and the weight is large.
The development of power electronic technology has provided new conditions for the development of low-voltage DC power supply. Brushless DC generators, static converters, etc. are its important symbols. Low-voltage DC power supplies are still widely used in small and medium-sized aircraft and helicopters.
High voltage DC power supply
A high-voltage DC power supply system is another alternative to a constant-speed and constant-frequency power supply system.
The 270V high-voltage DC power supply system has the advantages of simple structure, reliable operation, high efficiency, light weight, low cost, easy to achieve uninterrupted power supply and safe use, so it will be the development direction of aircraft power supply in the future. The F-14A fighter jets, S-3A and P-3C anti-submarine aircrafts of the United States partially use high-voltage DC power supply technology, while the F-22 fighter jets use a 270V high-voltage DC power supply system, and the F-35 fighter jets also use a 270V high-voltage DC starter power generation system. The aircraft electrical system adopts the DC power supply system, which is superior to the AC power supply system in the following aspects:
(1) There is no reactive voltage drop and reactive current in the power line.
(2) There is no skin effect when direct current passes through the wire.
(3) The power factor of the DC transmission and distribution system is 1.
(4) The corona initiation voltage of direct current is higher than that of alternating current.
(5) At the same voltage value, direct current is safer than alternating current in terms of personal safety.
(6) The parallel connection of DC generators is easy.
(7) The DC power system is easy to realize uninterrupted power supply.
With the development of high-voltage brushless DC generators, the problem of high-altitude commutation has been solved. In addition, the high-altitude arc extinguishing problem of switchgear is solved by using power electronic technology. The continuous development of solid-state power distribution technology makes the high-voltage DC power supply system have obvious advantages over the AC power supply system.
