Jeff Liebermann wrote:
On Thu, 11 Sep 2014 09:33:21 +1000, John G wrote:

Well I was told that so far back that I can't remember (1960s) I think,
Long before PCs, mainframes had lots of fans.
Although I never saw a failure I always thought it was because you
could grossly overspeed the fan if you were stupid enough to put too
much air in. B-)

Methinks you have hit upon the answer. Overspeed and reverse rotation
may have been a problem for the fans of the 1960's, but todays
brushless computah fans are largely immune to such abuse. Running a
commutator type motor backwards will probably do some damage to the
commutator because the brushes are inserted at a slight angle (to help
clear out debris between the brushes and commutator sections).
However, none of the computah fan motors I'm currently dealing with
have commutators or brushes.



Most ollder larger computers used ac fans. I've blown out a few fans, no
problems seen.

Greg