The speed of a DC motor is primarily controlled by adjusting which parameter?

• A. Armature voltage
• B. Armature current
• C. Load torque
• D. Mechanical linkage

Answer: (A)

Speed in a DC motor is set mainly by the armature voltage. The motor’s speed is tied to back-EMF, which rises with speed, and the armature voltage must overcome both this back-EMF and the resistance drop in the armature to drive current and produce torque. The fundamental relationship is V = I_a R_a + E_b, with E_b = k_e ω. For a given load, increasing the armature voltage increases the available driving force (torque) and, after the rotor speeds up and back-EMF grows, a new steady state is reached at a higher speed. Armature current responds to the voltage and speed, determining torque, but it isn’t the primary control input. Load torque and mechanical linkage affect the resisting force the motor must overcome, shaping the operating point rather than directly setting speed.