Why don’t devices like fans reverse their direction despite using alternating current (AC), which changes direction?
In: Physics
Single-phase AC motors are made with the knowledge that current reverses. They coast while the current is going down and reversing, bringing an opposing pole into position for the rising reversal to continue to rotate the mechanism in the same direction. That’s why big ones sometimes require a capacitor to kick them started and build up the momentum needed.
Electric motors are electric generators running in reverse. The same mechanism that turns a rotating turbine into alternating current can be used to turn alternating current back into rotation in one direction.
If your legs just go up and down, why doesn’t your bicycle just move back and forth? It’s the same thing.
There are a few ways this can happen:
Some motors are designed so that the direction of current doesn’t matter, they still produce torque in a particular direction
Some motors have a rectifier, which is a device that basically causes the current to flow in the same direction regardless of the voltage polarity
To make an analogy with a physical object, think of a ratcheting wrench. When you move it in one direction, it turns the bolt. When you move it the other direction, the ratchet lets it go back without turning the bolt. That means you can move the wrench back and forth, but it only makes the bolt tighter.
AC is basically like that but with electricity.
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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