A constant speed drive (CSD) is a mechanical gearbox that takes an input shaft rotating at a wide range of speeds, delivering this power to an output shaft that rotates at a constant speed, despite the varying input. They are used to drive mechanisms, typically electrical generators, that require a constant input speed. Constant speed drives are usually found as part of the accessory drives of gas turbine engines, such as aircraft jet engines. Constant-speed drives are usually found supplying accessory equipment and are rarely needed for an engine’s main power output.
CSDs are mainly used on airliner and military aircraft jet engines to drive the alternating current (AC) electrical generator. In order to produce the proper voltage at a constant AC frequency, usually 3-phase 115 VAC at 400 Hz, a generator needs to spin at a constant specific RPM (typically 6,000 RPM for air-cooled generators).Since the jet engine gearbox speed varies from idle to full power, this creates the need for the Constant Speed Drive (CSD). The CSD takes the variable speed output of the accessory drive gearbox and hydro-mechanically produces a constant output RPM.The CSD holds the speed of the generator, and the frequency of the AC constant as the engine speed varies through its normal operating range. CSDs prevent power surges or breaks.
