To reverse the direction of an AC door motor, which element must be switched?

To reverse the direction of an AC door motor, it is necessary to switch two of the phase windings. In three-phase motors, the direction of rotation is determined by the sequence of the phases connected to the windings. By interchanging the connections of two of the phases, the motor's magnetic field rotates in the opposite direction, which in turn causes the motor to reverse its rotational direction.

This principle is fundamental in understanding how AC motors operate since they rely on rotating magnetic fields produced by alternating current. The interaction between the magnetic fields of the stator and rotor determines the direction of rotation based on the phase sequence. Thus, reversing the connections of two of the phase windings effectively changes the phase sequence and alters the flow of the magnetic field, resulting in a reversal of the motor's output direction.

The other options do not achieve the desired outcome of changing the motor's rotation direction as effectively or correctly. Switching all windings would not specify which phases to reverse, as it could potentially cancel out the magnetic effects. Switching one of the phase windings would not suffice either, since the direction is dependent on the relationship between at least two phases. Finally, switching the motor itself would not ensure a directional shift so much as a physical relocation, which