There isn't any rational reason that an AC motor with a few resistors/potentiometer with a filter isn't the cheapest. DRM would involve DC power which means a transformer is required as well. Unless there was a good reason to have a DC motor, this wouldn't even be an issue for anyone who can combine a box fan with a filter.

I was building a DIY filter and took apart a purifier and was disappointed at it not having DC for me to connect my esp to for a sensor to check the purity.