In a DC motor, what is the role of back EMF?

• A. It opposes the applied voltage, reducing net voltage and current, limiting current and speed.
• B. It heats the windings to regulate speed.
• C. It provides torque directly.
• D. It adds to the supply voltage, increasing current.

Answer: (A)

Back EMF is the voltage generated inside a DC motor when the rotor spins in the magnetic field. It acts opposite the applied supply voltage, so it reduces the net voltage across the armature. Because the armature current is set by I = (V_supply − E_b)/R, this opposing voltage lowers the current as speed increases. Since motor torque is proportional to armature current, back EMF naturally limits the current and, in turn, the speed the motor can reach under a given load. At standstill, there is no back EMF, so the current is high and starting torque is available; as speed builds, back EMF grows, current falls, and the motor settles at a steady operating speed determined by the load. Back EMF can be modeled as E_b = K_e ω, showing its direct link to speed. It doesn’t heat the windings by itself or add to the supply voltage; its primary role is opposing the applied voltage to regulate current and speed.