There are two reasons, both of which are interrelated:
-
From the fan manufacturer’s perspective, it is best to give the motor full voltage at startup to get the motor up to speed as quickly as possible. This is due to rotational inertia at startup; the inertia must be overcome one way or another, and the most reliable way to overcome the inertia (especially with a cheap motor) is to start on the highest power setting.
-
As explained in #1, when you first apply power to an electric motor (with a load attached to it), the motor must overcome the rotational inertia necessary to get it up to speed. During this startup time the motor’s impedance is fairly low, and it draws quite a bit of current. And to make matters worse, there’s not much air flowing across the motor windings during this time. Once it gets up to speed the motor’s impedance increases, and thus draws less current. There’s also a lot more air flowing across the motor windings once it is up to speed. It makes sense, then, that you do not want the motor to spend much time in the startup phase, since the motor draws lots of current, the windings are getting hot, not much cooling air for the motor windings, etc. If you’re the designer, this phase will make you “nervous,” and you will want the fan to spend as little time in it as possible. Starting the fan on high minimizes the amount of time the fan spends in this phase.