How does PWM control motor speed in a DC motor?

• A. PWM modulates the instantaneous voltage level directly.
• B. PWM varies the frequency in order to change speed.
• C. PWM modulates the switching frequency rather than duty cycle to control speed.
• D. PWM modulates the average voltage by varying the duty cycle at a fixed switching frequency; higher duty cycle yields higher average voltage and speed.

Answer: (D)

PWM controls motor speed by changing the average voltage the motor sees. It does this by rapidly switching the supply on and off at a fixed high frequency. The fraction of time the switch is on in each cycle is the duty cycle. The motor’s average voltage is roughly the supply voltage multiplied by that duty cycle. So, increasing the duty cycle raises the average voltage and speeds up the motor, while decreasing it slows the motor. The high switching frequency makes these rapid on/off pulses feel like a steady DC voltage because the motor’s inductance filters the pulses. This approach relies on adjusting the duty cycle at a fixed switching frequency to set speed; changing the frequency won’t produce the same direct control over average voltage.