Direction of rotation, brushless or brushed DC motor

[mjbourquin]

I still don't understand why these types of motors tend to spin in one direction or another. In theory the direction of spin should be random since when the north pole of the rotor magnet is lined up with the north pole of the electromagnet in the stator the rotor could be repelled clockwise or counterclockwise and so could spin in either direction. Then in another half a cycle the direction is known because of momentum but at the start how do you know?

 

[Hassan2]

This is because of the commutator on the rotor. It changes the poles just in time, by sending the current to another coil.

 

[rcgldr]

In theory the direction of spin should be random since when the north pole of the rotor magnet is lined up with the north pole of the electromagnet in the stator the rotor could be repelled clockwise or counterclockwise and so could spin in either direction. Then in another half a cycle the direction is known because of momentum but at the start how do you know?

You don't know which direction the motor will initially move. For a 3 (or more) phase brushless motor controller, the start up cycle just assumes the initial rotor will line up with some phase position after 1 or 2 steps using fixed delays between the first steps, then cycles "forwards" at an ever increasing pace until enough cycles and speed has occurred to switch to using back emf as input.

I'm not sure how a DC brushed motor with just two poles avoids getting stuck in a short circuit and/or zero torque state at startup. 3 or more poles solves the issue.