How does this simple DC Motor work?

Question

I built this DC Motor I found here

Its essentially a coil with a magnet connected to a battery. So we built it and it worked.

The main question is, why? I thought that the motor had to have a brush or something to change the direction of the magnet or the coil. Without it, the torque would change to the opposite direction once it flipped over, and I would expect a sort of wobbling motion. My friend thought it would be something to do with the change in flux and inducing magnetic fields, but according the Lenz's law, this should not produce a continous motion because the induced magnetic field will always try to oppose motion.

Why does it keep spinning?

(On a side note, it stopped working when we disconnected it from a small battery and changed to a bigger DC source. If anybody could maybe explain this too that would be nice.)

EDIT:

OK, so we didn't actually build it right. We more or less just looked at the pictures, pretended we knew what we were doing and improvised. We missed the crucial step of shaving only half of the plastic off of the wire. The strange part is that it was working even though we didn't do this step. Is there any explanation as to why this works even though we missed that step?

Answer 1

It's because on one of the wires leading into the coil, you only remove the enamel on one half segment of the conductor wire. This has a similar, but less efficient, effect to having a commutator. The rest is down to to angular momentum - basically you get a kick to the rotation when the conducting part of the wire enables current to flow, and angular momentum keeps the coil rotating until the next kick.

As for why using a "bigger" DC source doesn't work, I don't know, but maybe there is something in its electronics which goes wrong when the current is switched on and off rapidly.