A CSD unit drives a generator using which mechanism?

The correct mechanism by which a Controlled Start Device (CSD) unit drives a generator is a variable hydraulic pump and hydraulic motor. This approach utilizes the properties of hydraulic systems to effectively manage the startup and operation of the generator.

Using a variable hydraulic pump allows for variable flow and pressure, which gives the CSD the flexibility to control the speed and torque provided to the generator during the startup phase. The hydraulic motor then translates this hydraulic energy into rotational energy to drive the generator, making it an efficient and effective means of operation. This hydraulic system can also provide a smoother startup process, reducing wear and tear on the mechanical components of the generator.

The other options do not align with the mechanics of a CSD unit. A mechanical gearbox, for instance, may be used in different applications but is not typical in the context of a CSD, as it lacks the smooth control offered by hydraulic systems. An electrical circuit would not be a suitable driving mechanism for the CSD, as it serves more as a control and power distribution element rather than a direct driving force. Lastly, a pneumatic actuator relies on compressed air, which is not commonly used for tasks that require the precise control and high torque needed to start and run a generator efficiently.