The speed of a pulse width modulated motor increases as the frequency of the pulses increases.

• A. False
• B. It depends
• C. True
• D. Cannot be determined

Answer: (C)

The essential idea is that a PWM-driven motor’s steady-state speed is set by the average voltage applied to the motor, not by how quickly you switch it on and off. In PWM, the average voltage equals the supply voltage times the duty cycle (the fraction of time the signal is high). If you crank up the pulse frequency but keep the duty cycle the same, the average voltage remains the same, so the speed does not increase. The frequency mainly affects how smoothly the current is shaped (reducing ripple and potentially improving torque response) and the amount of switching losses, not the final speed. To change speed, you adjust the duty cycle (the average voltage) rather than the switching frequency.