Question

in d.c motor, why must the current to the coil be reversed twice during each rotation

Answer 1

The current in the coil is reversed every 180 degrees. The current stops flowing in the coil from 90 degrees to 180 degrees as the split-rings are no longer in contact with the coil.  

If the current is not reversed, by Fleming's Left Hand Rule, the direction of the coil would be reversed. This would cause your coil to go clockwise - anticlockwise - clockwise - anticlockwise, now you wouldn't want that. You want the coil to go clockwise all the way.

hope it helps !!

Answer 2

hey mate here is your answer

On changing the direction of the current, repulsion takes place between the coil and the pole. It is relevant to reverse the coil of the motor because if not done the coil shall reach the other pole of the magnet and the motor will stop. If change in direction the method of repulsion and attraction takes place.

hope it helps you