Which statement best describes how torque changes with speed in a typical permanent magnet DC motor?

• A. Torque increases with speed due to back-EMF
• B. Torque stays constant with speed
• C. Torque is proportional to armature current; speed has no effect
• D. Torque decreases with speed because back-EMF reduces current, so torque falls from stall to near no-load

Answer: (D)

Torque in a typical permanent-magnet DC motor is determined by the armature current: more current means more torque. The current, however, is set by the applied voltage minus the back-EMF generated as the rotor turns, all over the armature resistance. Back-EMF rises with speed, which slows the current because it subtracts from the supply voltage. Since torque follows current, it decreases as speed increases. At standstill there’s no back-EMF, so current is maximum and torque is at its peak. At high speed, back-EMF nearly cancels the supply, current is small, and torque is near zero. So the correct description is that torque decreases with speed because back-EMF reduces current, causing torque to fall from stall to near no-load.