Why doesn't a commutator cause the rotation to reverse periodically or stop?
A normal DC motor has 3 poles instead of just 2. This solves a couple of problems:


the commutator doesn’t short out as it crosses from one pole to the other.
the energized poles are always phased with the field magnets such that they never get in a place where they’re ‘stuck’.


This Quora link has an animated illustration that shows the idea: https://www.quora.com/Why-do-most-brushed-DC-motors-have-3-armatures-and-not-2

And to save you the trouble of following the link, here are the ani-GIFs:




Wow, party like it's Internet 1999 again!