How is the PWM frequency chosen in motor drivers?
Choosing the frequency is a balance between quietness and heat. Switching above 20 kHz moves outside the human hearing band and reduces the current ripple; in practice 16–20 kHz solves most hum problems, and 20–25 kHz is the common optimum on MOSFET based drivers. As the frequency rises, switching loss and heating increase.
Weigh three effects together when deciding:
- Acoustics: The switching frequency creates mechanical vibration in the motor winding. At low values such as 1–2 kHz this is an audible hum; above 20 kHz it is typically inaudible.
- Current ripple: As the period shortens, the motor inductance smooths the current better; the ripple falls, and the extra heating and torque ripple drop.
- Switching loss: Energy is spent in the MOSFET at every on-off event, and this loss grows in direct proportion to the frequency. If the cooling is inadequate the board overheats.
Practical approach: if audible noise is not a problem, staying at a medium frequency is thermally comfortable; in applications where people are close by, such as cabins, greenhouses and doors, going above the hearing band is the right choice.
Note: the switching frequency of the AXI DC drivers (ST42, KS250, PT500) is not stated in the product data and it is not a parameter the user can set. The settings open to the user on these models are speed (with a pot on the KS250 and PT500), ramp and, on the PT500, the A1/A2 current limits. The values above are general engineering references and must not be attributed to the products.
Full topic: DC Motor Driver Selection Guide