| || ||The motor is connected to a battery. When the switch is closed, the current starts to flow and the coil becomes an electromagnet. In this case the current is flowing anticlockwise in the top of the coil. This makes the top a north pole. This north pole is attracted to the south pole on the left. So the top of the coil turns towards the left. Notice that the bottom of the coil is a south pole and is attracted to the magnet on the right. |
Once the coil gets to the upright position, there is no turning force on it because the electromagnet of the coil is lined up with the permanent magnets. If the current in the coil were constant, the coil would stop in this position. However, to keep it spinning, the commutator breaks contact in this position. So the current stops for an instant. The momentum of the coil keeps it going and the contacts are reconnected. However, they are now the other way around. So, the side of the coil that used to be a south pole is now a north pole.
The commutator will keep swapping the contacts every half turn (when the coil is in the upright position). In this way, the motor keeps spinning.