Why is a low-pass filter often used on PWM outputs before driving a motor?

• A. To increase switching frequency.
• B. To measure velocity.
• C. To smooth high-frequency switching, reduce ripple and EMI, and deliver a steadier average voltage to the motor.
• D. To convert PWM to DC.

Answer: (C)

PWM outputs switch rapidly between high and low to approximate a desired voltage; the motor responds to the average effect of that switching rather than the instantaneous pulses. A low-pass filter attenuates the high-frequency switching components and passes the slower-changing average voltage set by the PWM duty cycle. This smooths the voltage and current seen by the motor, reduces ripple and torque ripple, and minimizes electromagnetic interference from fast transitions, giving a steadier, more controllable drive. The motor’s own inductance helps, but the filter strengthens this smoothing, improving overall performance. Keep in mind, adding filtering can introduce a bit of delay and phase shift, so it’s designed to balance fast response with adequate attenuation of the carrier frequency.