Why don’t devices like fans reverse their direction despite using alternating current (AC), which changes direction?
In: Physics
Once they’re running, momentum keeps single phase AC motors spinning in the same direction.
From a standstill, basic single phase AC induction motors won’t be able to start themselves. If you manually start it spinning it’ll go either way.
Single phase AC induction motors with starting circuits have an additional set of coils that will push/pull the rotor in a specific direction, then once the motor is spinning the starter coil gets switched off. Then momentum keeps the rotation direction constant. Reversable fans can swap the starter coil direction to push/pull the opposite direction..
Small so-called DC Brushless motors are AC motors with a controller that is aware of shaft speed and rotation, and can control stator coil energization to set direction and speed.
Large AC brushless synchronous motors start as induction motors then sync with the AC signal in the direction they are started. In a catastrophic mechanical failure with an exciter issue they can and will switch directions.
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