Constant speed drive


A constant speed drive is a type of transmission 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.
The term is most commonly applied to hydraulic transmissions found on the accessory drives of gas turbine engines, such as aircraft jet engines. On modern aircraft, the CSD is often combined with a generator into a single unit known as an integrated drive generator.

Mechanism

CSDs are mainly used on airliner and military aircraft jet engines to drive the alternating current electrical generator. In order to produce the proper voltage at a constant AC frequency, usually three-phase 115 VAC at 400 Hz, an alternator needs to spin at a constant specific speed. Since the jet engine gearbox speed varies from idle to full power, this creates the need for a constant speed drive. The CSD takes the variable speed output of the accessory drive gearbox and hydro-mechanically produces a constant output RPM.
Different systems have been used to control the alternator speed. Modern designs are mostly hydrokinetic, but early designs often took advantage of the bleed air available from the engines. Some of these were mostly mechanically powered, with an air turbine to provide a vernier speed adjustment. Others were purely turbine-driven.

Integrated drive generator

On aircraft such as the Airbus A310, Airbus A320 family, Airbus A320neo, Airbus A330, Airbus A330neo, Airbus A340, Boeing 737 Next Generation, 747, 757, 767 and 777, an integrated drive generator is used. This unit is simply a CSD and an oil cooled generator inside the same case. Troubleshooting is simplified as this unit is the line-replaceable electrical generation unit on the engine.

Manufacturers

is an American manufacturer of CSD and IDG units.

Alternatives

A variable-speed constant-frequency generator can be used to provide AC power using an electronic tap converter.