What Are Constant Speed Drives (CSD) And Their Use In Aircraft?
In the realm of transmission technology, a constant speed drive (CSD) is an apparatus that utilizes the power supplied by a rotating input shaft with varying speed to create a constant output. For applications such as electrical generators that need a consistent speed for optimal operations, constant speed drives are a perfect solution. While constant speed drives may be used in a few settings, they are most prevalent in the accessory drives of aircraft gas turbine engines. In this blog, we will discuss constant speed drives in more detail, allowing you to better understand how they benefit modern aviation applications.
Within a jet engine belonging to a commercial airliner or military jet, a constant speed drive will be used to operate an alternating current (AC) generator. The electrical generator of such an aircraft will generally need to supply power at three-phase 115 VAC at 300 Hz, and this will require the CSD to drive the system at a speed of around 6,000 RPM. Since a typical AC electrical generator will vary in its speed from idle operations to peak performance, having a constant speed drive with a fixed output is extremely beneficial.
For the constant speed drive to manage the speed of the alternator, early designs regularly derived power from bleed air that was supplied by the engine compressor during flight. In other cases, constant speed drives were mechanically powered, utilizing an air turbine assembly for the means of conducting vernier speed adjustments, some being fully driven by the turbine assembly. For modern designs, constant speed drives now control alternator speeds through hydrokinetic means.
Depending on the aircraft in question and its design, a constant speed drive may either be an independent unit, or it may be mounted within the housing of the alternator itself. When the constant speed drive is a combined unit with the alternator, it will establish an assembly known as an integrated drive generator (IDG). The integrated drive generator can be found on a number of aircraft, examples including the Airbus A310, Airbus A320, Boeing 737 Next Generation, Boeing 747, and more. As compared to a standalone constant speed drive, integrated drive generators can make troubleshooting easier as the entire unit acts as the line-replaceable electrical generation unit for the engine.
While the constant speed drive is the most recognizable and widely used option, there are other alternatives that one may potentially use. One of these is a variable-speed constant-frequency (VSCF) generator, that of which can supply AC power with the help of an electronic tap converter. The variable frequency starter generator (VFSG) is another alternative that is commonly found on aircraft such as the Boeing 787, and it can be used both for the starting process of the engine and for basic electrical generation. Nevertheless, the constant speed generator is the most popular option, and at ASAP Aviation Supplies, we can help you secure all the parts and components you need for your aviation operations.
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