What is the purpose of a constant-speed drive (CSD) in relation to aircraft generators?

• A. To vary generator frequency with engine speed to track RPM
• B. To maintain a constant generator output frequency as engine speed varies
• C. To regulate DC output voltage independently of engine speed
• D. To provide lubrication for the generator bearings

Answer: (B)

The main idea is to keep the electrical frequency produced by the generator constant even as the engine speed changes. Aircraft electrical systems that use AC power run at a fixed frequency (typically 400 Hz). The frequency of the generator is determined by its speed and the number of poles (f = P × N / 120). If the engine speeds up or slows down, the generator would otherwise run faster or slower, causing the output frequency to drift. A constant-speed drive uses a governor-controlled mechanism (often hydraulic) to keep the generator spinning at a fixed speed regardless of engine RPM. This decouples engine variations from the generator, so the output frequency stays stable. The generator still has its own voltage regulator to control voltage, but the CSD’s job is maintaining constant speed, not regulating DC voltage or providing lubrication.