Yvette Cooper Didn't Dawn Butlers Post.
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No best answer has yet been selected by Bluenoser. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.Yes it will be cheaper. The mechanical input power for a fan blade is proportional to the cube of fan speed, but the motor power will not change in the same relationship because it will still draw some current when completely off load.
If for arguments sake you had a geared system with a constant speed motor (unusual, not seen one myself) and changed gear to a lower speed, the torque required from the motor would drop and therefore so would the motor input power, same as above.