Measuring Techniques and Challenges of Moving to Variable Frequency Power
Larger aircraft typically have a power system that is able to generate electricity. This generation is generally performed by the aircraft’s engines and/or by an auxiliary power unit (APU). The most common power generation system on turbine-powered aircraft today is called an Integrated Drive Generator (IDG), which is a constant speed drive (CSD) and an alternator combined.
IDGs on aircraft typically produce power at 400 Hz (115V, 3 phase). In contrast, Variable Frequency Generators (VFG) don’t require mechanical speed conditioning systems. VFGs are linked directly to the engine shaft and thus there is a direct relationship between engine speed and output frequency.
A significant competitive advantage for an aircraft is how fuel-efficient it is – the less fuel required to fly a route, the more price-competitive the operating airline can be. One way to aid fuel efficiency is to move to a “more electric aircraft” (MEA). An MEA represents weight and fuel-saving for aircraft by replacing secondary hydraulic and mechanical systems with electric devices.
The advantages of VFG are their smaller size, lower weight and cost, and the fact they can generate power more efficiently than IDGs. This latter point is important as aircraft are increasingly using more electrical power, and thus more fuel is required to generate this power.
As aircraft design makes the shift from constant to VFP, monitoring the data during flight testing becomes a challenge because VFP requires new standards and monitoring equipment (to account for transient power events for example). An attempt to standardize was introduced in 2010 with ISO 12384 which intends to “explain, establish, and standardize specific methods for digital measurements of the aircraft electrical system” (ISO/TC 20/SC 1, 2010).
Most data acquisition units (DAU) currently on the market are incapable of fully meeting ISO 12384 standards in a practical application due their architecture throughput being too limited. The AXN/ADC/408 is the only viable solution on the market today, thanks to being hosted in Curtiss-Wright’s new Axon data acquisition unit (DAU). The Axon can handle a throughput in excess of 500 Mbps – this is enough capacity to host multiple modules into a single chassis.
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- Constant speed drives (CSD)
- Variable frequency generators (VFG)
- ISO 12384
- Data Acquisition Systems
- Rugged airborne test and monitoring equipment