In a DC motor control circuit, what does PWM stand for and what is its purpose?

• A. Power-Width Modulation; it adjusts the voltage
• B. Phase-Width Modulation; it controls torque
• C. Pulse-Voltage Modulation; it modulates current
• D. Pulse-Width Modulation; it controls motor speed by varying the average voltage delivered

Answer: (D)

PWM is a method to control DC motor speed by turning the supply on and off rapidly and adjusting how long the signal stays on within each cycle. The fraction of time the power is on is the duty cycle. The motor’s average voltage equals the supply voltage times that duty cycle, so raising the duty cycle speeds the motor up and lowering it slows it down. Because the switching happens fast, the motor experiences a smooth average voltage rather than a pulsing supply, leading to steadier speed and torque.

This approach is implemented with a driver that switches the current and often a diode to handle the inductive kick from the motor. The idea isn’t about changing the voltage level per pulse or focusing on phase timing; it’s about modulating the width of the on-pulse to control the average voltage and thus the speed.