Which statement about speed control in DC drives is true?

• A. Control of the armature or field voltage is used to control the speed.
• B. Speed is controlled only by mechanical gears and does not involve electrical changes.
• C. DC drives cannot alter motor speed once started.
• D. Speed changes must be achieved by changing the load torque only.

Answer: (A)

The main idea is that DC motor speed is controlled by changing the electrical conditions that set the back electromotive force. In a DC motor, speed is tied to back EMF (E = kΦω). If you vary the armature voltage while the field flux is held constant, you change the available voltage to drive the motor, which alters the back EMF and thus the speed. Similarly, changing the field current changes the field flux (Φ); with a given armature voltage, a stronger or weaker field alters the back EMF and the speed. So using either armature voltage control or field voltage control lets you set the desired speed.

That’s why the correct statement is that speed control is achieved by adjusting the armature or field voltage. The other ideas don’t fit: speed is not limited to mechanical gear changes and does involve electrical adjustments; a DC drive can change speed after starting; and changing load torque alone is not how you control speed, it’s a consequence of load changes rather than a deliberate control method.