In a DC motor, the armature consists of any number of windings, each one an electromagnet. The armature is immersed in a directional external magnetic field. This external field does not move, and can come from permanent magnets or electromagnets.
A direct current in a set of windings creates a polar magnetic field. A torque acts on the rotor due to its relation to the external magnetic field. Just as the magnetic field of the rotor becomes fully aligned with the external magnetic field, the direction of the current in the windings on the armature reverses, thereby reversing the polarity of the rotor's electromagnetic field. A torque is once again exerted on the rotor, and it continues spinning.
The change in direction of current is facilitated by the split ring commutator. The brushes remain stationary, but they are in contact with the armature at the commutator, which rotates with the armature such that at every 180° of rotation, the current in the armature is reversed.
A brushless DC motor has a permanent magnet or magnets for the armature. The external magnetic field comes from any number of electromagnets that are turned on and off at the correct times by a timing device.
The exact workings of different brushless DC motors depend on the type of timing device used. This example uses a Reed switch.
A reed switch is two thin wire contacts in a small glass tube filled with protective gas. When a magnet comes close, the two contacts become magnetized and attract each other. The circuit is closed as long as the magnet remains close. When the magnet is removed, the contacts separate, opening the circuit.
