Why is the civil aviation power supply 400Hz?

 Aircraft ground power supply units are divided into two main categories: one is DC power supply and the other is AC power supply.

According to the installation method and working principle of the power supply there are more subdivisions. General aviation aircraft use more 28V DC power supply, while civil aviation use more 115V/400Hz AC power supply.

 All we commonly see is 50/60Hz alternating current. AC is a sine wave and the frequency is expressed as the number of cycles of AC current per second. If the frequency of the current is low, then the losses in the transmission of electricity are reduced accordingly.

But if the frequency is too low, there is not enough power transmission and you will see light bulbs flickering. If the frequency becomes large, the transmission losses increase. Then, after increasing the transmission frequency, the losses can be reduced by reducing the size of the generator to get the equivalent power output.

For industrial and domestic electricity, the standard current frequency worldwide is 50/60 Hz (50/60 cycles of current per second), and in some countries, where power supply plants are generally located far away from cities, consideration of transmission losses is a key factor in determining the current frequency.

Some countries initially set it at 25-30Hz, and then gradually with globalization, 50/60Hz alternating current was adopted worldwide. There is only one frequency for an electrical system, and our country and most of the European countries in the world have 50Hz for their electrical systems and 60Hz for the Americas.

The JFADW-07K power supply vehicle is a trailer-type AC power supply vehicle. It uses a diesel engine as the prime mover to drive a generator to generate medium-frequency power. If DC is required, it can be generated through an external commutator. The entire generator set and electrical control system are loaded on the trailer chassis. The vehicle’s AC and DC power supplies can be output independently.

But airplanes are different. Weight is a key factor in airplanes, so designers make sure they are as light as possible when designing airplanes. The use of 400Hz alternating current reduces the size and weight of the electrical components on an airplane, especially the generator.

And because of the short distance between the generating part of the engine (the engine) and the operating end, the losses incurred during transmission are negligible. By using high frequency currents, the size and weight of the generating components can be reduced, and the weight saved can be added to the commercial load of the airplane.

Historically, airplanes were completely mechanical structures, so there was no need to have an onboard power system. In the earliest days, the application of onboard power was to start engines.

The wind-driven generators relied heavily on the wind induced by retractable landing gear. In order not to affect the aerodynamics of the aircraft, as aviation technology advanced, the wind-driven generators were phased out, and engine-driven generators began to be applied to the on-board power system.

The first engine-driven generator systems developed were 6V and 12V DC power systems. However, as more and more power-using components were added to the aircraft, such as lighting and air conditioning systems, the power systems were gradually increased to 28V DC power systems.

However, technology is advancing and the power requirements are getting higher. Since it was not so safe to load high-voltage DC power on an airplane, aircraft engineers began to consider AC power, and from the 1960s onwards, alternators were increasingly used on airplanes because of their light weight and larger power supply.

However, the choice of what power to use turned out to be a new and complex problem. The choice of frequency depended mainly on the maximum speed at which the generator would operate and the effect on the equipment involved.

At this point, suppliers around the world made various attempts to go from 60 Hz to 800 Hz. The first to standardize on 400Hz was the U.S. Air Force, who found that 400Hz current satisfied the equipment on the aircraft to achieve functionality, while maintainable generator speeds were stopped at 400Hz, and from there, in 1959, 400Hz became a military standard and has been used ever since (no. MIL-STD-704).