- Basic types
The basic types of aerospace vehicle power supply systems are mainly divided according to the form and characteristics of their main power sources. Since the form of electric energy is divided into two basic types, DC and AC, the aerospace vehicle power supply system mainly includes three types: DC power supply system, AC power supply system and hybrid power supply system.
At present, according to the different voltage parameters of the power supply system, the DC power supply system is divided into a low-voltage DC power supply system with a rated voltage of 28V and a high-voltage DC power supply system with a rated voltage of 270V. AC power supply system is divided into 115/200V AC power supply system and 230/400V AC power supply system according to different voltage parameters; AC power supply system is also divided into constant frequency AC power supply system and variable frequency AC power supply system according to different frequency. Constant frequency AC power supply system is also divided into constant speed constant frequency AC power supply system and variable speed constant frequency AC power supply system.
On some airplanes, there are two main power sources of DC and AC at the same time, and the power supply system provides both AC and DC power at the same time, which is called a hybrid power supply system.
- Basic parameters of aircraft power supply system
The basic parameters of the aircraft power supply system refer to the electrical parameters, structure and connection methods of the system and other technical indicators and forms, which are closely related to the volume, weight, size and performance of the power supply system and electrical equipment. The selection of the basic parameters of the power supply system is related to factors such as technology, economy and historical inheritance.
The basic parameters of the DC power supply system are voltage, the rated capacity of the system and the corresponding rated current. The basic electrical parameters of AC power supply system are rated voltage, frequency, phase number and waveform.
In the early days, the DC power supply of airplanes used automobile power supply, and the voltage was 12V. However, with the increase of aircraft equipment, 27.5V DC was used in airplanes. During the Second World War, some large aircraft used 120V direct current, but due to the difficulty of switching arcs and motor commutation, it was not promoted further. After the birth of 115/200V400Hz constant frequency alternating current, it has been widely used. When the rated capacity, feeder length and current density are the same, the weight of the 115/200V AC distribution network is about 30% of the weight of the 28.5V low-voltage DC grid. Increasing the voltage can further reduce the cross-section of the grid wire, thereby reducing weight. However, the minimum cross-sectional area of the airplane wire is limited by the mechanical strength of the wire, usually the minimum is 0.2mm², and most of the airplanes are low-power electrical equipment, using small cross-section wires, so increasing the grid voltage too high may not significantly reduce the grid The weight is detrimental to the safety of personnel. In the 21st century, with the development of power electronics and computer technology, power systems with higher voltage levels have begun to develop and be applied. For example, the 270V high-voltage DC power supply system is used on the F22 and F35 aircraft in the United States, and the 230/400V three-phase AC power supply system is used on the B787 aircraft.
The size of the power source and the weight of the grid are the main factors to determine the grid voltage. When considering the rated voltage of the power grid, factors such as the safety of personnel, the reliability of high-altitude work, and the size of the short-circuit current are also very important. At the same time, historical inheritance and international versatility are involved.
The frequency is related to factors such as the weight, performance, material of the electromagnetic component and the technical level of the finished part.
The volume and weight of transformers and filters decrease with increasing frequency. There is a clear relationship between the generator’s speed, the number of pole pairs and the frequency of the alternating current generated. Its speed is mainly limited by the bearing life and the structural strength of the rotating part. It is currently 10,000~
20000r/min, in terms of motor structure, 2~3 pairs are extremely good, so a frequency of about 400Hz should be used. In some missiles, a gunpowder gas turbine generator is used, and the generator has a small capacity, a short working time, and a speed of tens of thousands of r/min, so the frequency can reach about 800 Hz. After the motor speed is limited, increasing the power frequency must increase the number of pole pairs of the motor, or increase the reduction ratio of the output reducer, which is unfavorable. If the motor speed is required to be around 40,000r/min, the 400Hz power supply is not applicable. If the frequency is too high, the voltage drop and loss of the feeder will increase. From the perspective of switching appliances and relays with contacts, when the power frequency is 400~600Hz, the circuit is disconnected, the arc burning time is the shortest, and the voltage growth rate between the contacts after the arc is extinguished is slower than the recovery rate of the dielectric strength of the air between the arcs. After the first voltage crosses zero, there will be no more arcs. It can be seen that it is reasonable to use 400Hz frequency on airplanes. As early as the 1940s, some aircraft alternators produced 115V/400Hz alternating current.
However, since the generation of 400Hz constant frequency AC power has strict requirements on the rotation speed of the generator or the electric energy converter, variable frequency AC power systems began to appear in the 1960s, and the frequency of the generator is determined by the rotation speed of the engine. However, the frequency range of the first variable frequency AC power supply is relatively narrow, and the power frequency range of the wide variable frequency AC power supply system developed in the 21st century is 360~800Hz.
(3) Phase number and waveform
400Hz AC power is commonly used in three-phase four-wire system. The reasons are: ①Three-phase generator and motor have high structure efficiency, and the power of three-phase motor is large when the volume and weight are the same; ②Three-phase motor is easy to start and has large starting torque; ③Three-phase The four-wire transmission and distribution system can obtain two voltages-line voltage and phase voltage. The metal body of the aircraft is used as the neutral line, and the power line is light in weight; ④The neutral line grounded three-phase motor can still rotate when one of the phase conductors is disconnected. But three-phase switches, relays and contactors are more complicated than single-phase ones.
Airplane AC adopts sinusoidal AC, so that the loss of electromagnetic components is small, and the level of electromagnetic interference is low.