To vary the speed of a DC motor generator set, which component is necessary?

To vary the speed of a DC motor generator set, a rheostat in the motor field is essential because it directly influences the field current, which in turn alters the motor's magnetic field strength. In a DC motor, the speed is inversely related to the field strength; reducing the field strength by increasing resistance in the rheostat allows the motor to speed up. Conversely, increasing the field strength decreases speed. This direct control mechanism makes the rheostat a critical component in the operation of a DC motor generator set, allowing for variable speeds based on the load and operational requirements.

The other components listed do not serve the same function in controlling the speed of a DC motor. Capacitors, for instance, are primarily used for filtering or to provide reactive power in AC systems. Transformers are utilized to change voltage levels but do not affect motor speed directly in a DC application. Rectifiers are designed to convert AC to DC, and while they are necessary in converting the output from AC generators or in certain configurations, they do not directly influence the speed adjustment capabilities of a DC motor generator set.