I am wondering at what fan size/speed a direct drive motor is no longer sufficient to drive a fan? Phrased alternatively, what size/speed of fan requires motors with a transmission? For an axial fan.

  • $\begingroup$ What kind of fan? At what speed? Your question is too vague, I think. There's an edit link ... $\endgroup$
    – Transistor
    Mar 2, 2021 at 22:05
  • $\begingroup$ Pick a size and speed of fan, and for \$10 I'll give you a good argument about why it should be direct-drive. For another \$10 I'll give you an argument just as good for why it should be geared. $\endgroup$
    – TimWescott
    Mar 2, 2021 at 23:55
  • $\begingroup$ Smarty pants. How about I pay you $30 and you answer the question as it stands as today? $\endgroup$ Mar 3, 2021 at 0:00
  • $\begingroup$ @JordanMcBain So, have you paid up? $\endgroup$
    – Solar Mike
    Mar 3, 2021 at 4:50

1 Answer 1


Often because the “standard” off the shelf motor has a running speed different to the required fan speed or most efficient fan speed.

Then it comes down to a choice of transmission or motor speed control which is influenced by many other factors.



Fixed speed or variable speed

Ambient conditions

Can all be relevant.


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