Question

why is the direction of electric current in an electro motor reversed after every half round?

Answer 1

Due to losing of contact of split rings by soft carbon brushes the split rings tend to change its direction

Answer 2

The two parts of a split ring in a d.c. motor act as commutator to reverse the direction of current in the coil. Thus, whenever the coil comes in vertical position, the direction of current through the coil reverses and the coil continues to rotate in the same direction