You have a good point in Your reply.
I once PWM:ed a 48 volt fan. It could not be done the seller told. But what the hack I thought. Take the picture: Using loose cables in Your hand and You shortly make contact.. The fan makes a little rush, whatever type it is.
Done. I selected a very low frequency, maybe 25 Hz. Then I applied PWM in duty cycles of 0, 25, 50, 75 and 100%. It worked like a charm.