Final answer:
In testing a tachometer generator, the variable DC drive motor in the test set controls the speed at which the device is driven, allowing for accurate simulation of various operating conditions.
Step-by-step explanation:
The tachometer generator to be tested is mounted on the tester, and the component that will control the speed at which it will be driven is a variable DC drive motor in the test set. Unlike a master tachometer generator which is used for comparison or a two-speed test pad which provides fixed speed settings, or the gearbox which just transfers the motion, a variable DC drive motor allows for precise control over a wide range of speeds. This enables the tester to simulate different operating conditions that the tachometer would experience in its actual working environment.