[Bradical]

What frequency? Direct current doesn't have a frequency. The motors would have to be AC for it to work- they may have them on modern cars, but they would have to be AC motors if that were the case.

For those reading this who don't know, PWM is converting AC current to a DC signal, and then modulating a second AC waveform by pulsing the output of the DC bus to the desired frequency which creates a fabricated AC signal. You would need an AC fan motor certainly otherwise you would just be turning the motor on and off rapidly, unless that's how you are saying it is done?