The fans as proposed/used by Accurascale, Bachmann and Cavalex are all separately powered. This can provide an extra level of realism - e.g on DCC fans can be coordinated with sounds and have various triggering features.
The roof fan on the Hornby 50 was, as mentioned above, driven by a belt off the main drive and was therefore on all the time when the loco was moving and not on at all when the loco was stopped. This was not prototypical. There was criticism as, if the belt was too tight, it would affect the performance of the loco. Again as stated above, this feature was dropped from more recent models. Hornby obviously passed on this manufacturing cost saving ;)
The second generation of motor driven fans are totally independent of the main drive and will not have this problem.