Little confusion about dc motors
Power to the armature coils is reversed because the brushes and commutator act as a reversing switch. Consider a simple DC motor with one winding and two commutator segments. When the armature rotates 180&deg;, each segment contacts the brushes on the opposite side than it did at 0&deg;, so current passes through the coil in the opposite direction. 

Current in DC motor



Practical motors need at least 3 segments (and coils) to ensure startup from any position. Larger motors may have many more commutator segments, but the commutator switching action is the same. 

In the example below, all the coils on the upper side are getting current in one direction (relative to start and end of each winding), while those on the lower side get it in the opposite direction. Each winding's polarity changes as the positive (or negative) brush goes from the segment connected to the start of the winding to the segment at the end of the winding.