A forward/reverse magnetic motor starter is constructed from two regular contactors installed with a mechanical interlock that prevents both coils from pulling in simultaneously.
They are connected so that under normal conditions, one coil will connect L1 with T1, L2 with T2, and L3 with T3. When the alternate coil is energized, Lines 1 and 3 are interchanged with Terminals 1 and 3, allowing for a phase shift in the 3-phase circuit, driving the motor and causing it to rotate in the opposite direction.
Both coils power contacts feed through a single set of OLR to provide running protection in both directions.
To prevent unintentional energization of both the forward and reverse coil at the same time, the coils are equipped with normally closed contacts, which are then wired in series with the opposite direction’s coil to act as an electrical interlock. This means that if the coil that drives the motor in the forward direction is energized, a normally closed set of contacts wired in series with the reverse coils will prevent that coil from being energized as long as the forward coil remains energized.
Reference
Basic Motor Control by Aaron Lee and Chad Flinn is used under a CC BY 4.0 Licence.