PWM Frequency
It is the number of pulse repetitions per second in pulse width modulation and it sets the duration of one period: 50 µs for 20 kHz. In motor control, if above 20 kHz is chosen the switching noise moves outside the human hearing band; in practice 16–20 kHz solves most hum problems.
PWM frequency must not be confused with the duty ratio: the frequency says how often the pulses arrive, while the duty ratio says how much of each pulse passes as on. The motor's average voltage is set by the duty ratio; the frequency sets the noise, the current ripple and the driver's heat.
The motor winding behaves like a low-pass filter. The shorter the period compared with the electrical time constant L/R, the smoother the current. This is why small motors with low inductance demand a high frequency; motors with high inductance draw a smooth current at a lower frequency too.
The limiting side is heat. When you double the frequency, the switching loss roughly doubles as well. That is why the frequency is generally kept modest in high-current drivers. If you are complaining about hum in the field and your driver has no frequency setting, part of the noise may be mechanical in origin; also check the motor mounting, the coupling and the resonance of the mounting plate.
Context where this term is used: DC Motor Driver Selection Guide