Why do aeroplanes work with 28V DC or 270 V DC and how do DC applications differ from 115/200 V AC at 400Hz?
By using multiple voltage levels, an optimal compromise between weight, efficiency, safety and performance can be achieved for different aviation requirements.
28V DC is the standard, especially for smaller aircraft and many on-board devices. 28V DC provides sufficient voltage to efficiently operate standard equipment such as lighting, avionics, hydraulic systems and low to medium power electric motors.
Compared to 12 V DC (e.g. in motor vehicles), less current flows at 28V DC for the same power, which reduces cable cross-sections and weight. On-board batteries and battery-backed emergency power supplies are also operated with 24 V DC, which makes operation and charging during flight more efficient.
28V DC was standardised early on in aviation, particularly in the military sector, and this standard has been transferred to civil aviation for smaller aircraft and helicopters. Low voltage is synonymous with safe operation: 28V DC is below the voltage limit classified as dangerous for humans, which increases safety during maintenance and operation.
Key facts 28V DC
- safe, lightweight, battery compatible
- ideal for low power requirements
270 V DC is used in larger or heavily electrified aircraft in which hydraulic and pneumatic systems have been replaced by electrical systems. Here, the current can be significantly reduced for the same power. This enables thinner and lighter cables, lower heat losses in the lines and more efficient energy transmission over greater distances in the aircraft compared to 28V DC.
Many high-performance devices such as actuators, electric drives for flight control or air conditioning systems work more efficiently with higher voltages.
Conversion losses are lower at the DC voltage level: devices that require high voltages can be operated directly at 270 V DC in parallel with the 28V DC system (standard for avionics systems) without a converter. This saves weight.
Key facts 270 V DC
- efficient for high-performance applications
- reduces weight and cable losses in heavily electrified aircraft
- 270 V DC harbours risks and is potentially life-threatening and therefore requires special handling rules
115/200 V AC (400Hz) alternating current is used in aviation for higher power requirements, especially for larger aircraft and systems that require high efficiency and reliability. It enables the supply of devices with higher power requirements compared to 270V DC and is the most widely used standard in aviation.
Key facts 115/200 V AC (400Hz)
- offers high performance and efficiency for large electrical systems
- enables the use of use of (standard) AC consumers, which are also widely used in aviation
Key facts DC-Systems (Direct Current)
- for the stable and reliable operation of avionics and emergency systems
- DC systems often require less complex devices such as transformers that would be required for AC systems
- DC systems enable lighter power power supply and distribution components
- DC systems are less susceptible to electromagnetic interference (EMC) than alternating current systems (important for the electronically sensitive environment of an aircraft)
- Aircraft use batteries as an emergency power source. Batteries inherently provide direct current, so direct use of DC systems reduces losses and complexity