What is a constant speed drive control?

• A. Maintains engine RPM
• B. Controls voltage
• C. Keeps AC alternator at specific RPM and controls frequency
• D. Controls fuel mixture

Answer: (C)

A constant speed drive ensures the generator runs at a fixed rotational speed, so the AC output frequency stays constant regardless of engine speed changes. In aircraft electrical systems, keeping the generator at a specific RPM is crucial because the frequency (often 400 Hz) is tied directly to rotor speed. The CSD uses a hydraulic or mechanical drive with a governor to decouple engine RPM from the generator, maintaining that preset speed and thus a steady frequency. The generator’s voltage regulator then handles voltage, but keeping the speed constant is what keeps the frequency stable. The other functions listed—altering engine RPM, directly regulating voltage, or adjusting fuel mixture—aren’t the role of the CSD.