Why Do Aircraft Use 115V 400Hz Power Supply
When it comes to aircraft power supplies, 115V 400Hz is a commonly used specification. This article explores the reasons behind this choice, delving into the benefits and practicalities of using such a power supply in an aerial environment.
The Benefits of Using 400Hz Over 50/60Hz
The use of 400Hz in aircraft is not just an arbitrary choice. Higher frequencies offer several advantages, particularly in terms of weight reduction and efficiency. One of the primary reasons is that higher frequencies make components such as transformers and electric motors smaller and more efficient. This is due to the fact that higher frequencies result in more rapid variations in magnetic fields, reducing the size of the components while maintaining the same power transmission capabilities.
Weight Reduction and Space Efficiency
In the context of aircraft, weight is a critical factor. Every ounce counts given the limited space and resources available for equipment. By using 400Hz, aircraft components can be substantially lighter, which is crucial for overall weight management. The reduced size and weight of these components can lead to significant fuel savings and enhanced performance, making it an attractive choice for the aviation industry.
Why Not Higher Voltage and Why 115V
While higher voltage could technically reduce transmission losses and allow for smaller power cables, the decision to use 115V at 400Hz is not solely based on this factor. The choice of 115V 400Hz is a result of balancing multiple factors, including the complexity of switching, ease of adaptation, and the practical considerations of on-ground power supply.
Simplicity of Switching and Power Supplies
One of the most practical reasons for using 115V 400Hz is the simplicity of switching between ground power and the aircraft's system. A linear power supply designed for 115V 60Hz can be adapted for 115V 400Hz, although it may operate noisily and produce increased heat due to hysteresis losses. Ferroresonant power supplies, however, are less effective in this context, adding to the complexity of the system.
Historically, some UK aircraft, such as the V-bombers, used 115VAC 400Hz, which is interesting given that the UK mains supply was 240V 50Hz at the time. This suggests that there may be other practical reasons for the choice of 115V, beyond purely technical considerations.
Insulation Breakdown and Safety Concerns
Another important factor in choosing 115V 400Hz is the potential for insulation breakdown and safety. From a safety perspective, higher voltages can pose greater risks, especially in the event of ground faults. However, the breakdown voltage for air gaps increases as atmospheric pressure decreases, making 115V a safer choice in the reduced air pressure of the aircraft cabin. Additionally, it is generally understood that 220/240V is more dangerous than 115V under ground fault conditions, although the frequency dependency of this risk is not definitively established.
Variable Frequency Considerations
Modern aircraft often use synchronous motors and synchros, which are optimized for their design frequency. While variable frequency power supplies could potentially reduce the complexity of the power distribution system, the practical and historical considerations often favor a fixed frequency of 400Hz. For example, the 787 and A380 aircraft have a 115VAC 400Hz supply for most systems but use an inverter-driven by HVDC to generate a 115VAC 60Hz supply for minor loads, often generated through an inverter. This approach leverages modern electronics to achieve voltage conversion without the need for bulky transformers.
Conclusion
In summary, the choice of 115V 400Hz for aircraft power supplies is the result of a comprehensive analysis of technical, practical, and safety considerations. While higher voltage could reduce transmission losses, the benefits of 400Hz in terms of weight, size, and component efficiency make it a highly favored choice in the aviation industry. The combination of historical precedent and modern technological advancements ensures that this power supply standard remains effective and reliable.