Electric Motor

Basic Components

Armature: Sometimes called a rotor. This is the part that spins. The armature can be either a permanent magnet or an electromagnet.

Stator: This is the part that doesn't move. The rotor spins in the magnetic field contained in the stator.

How a motor works
The force that that turns the armature comes from the magnetic field of the armature trying to line up with the external magnetic field of the stator. This force is called torque. This torque will cause the armature to turn until its magnetic field is aligned with the external field, but no further. How does the armature continue to spin? One of the magnetic fields must be changed so that the armature has to turn again. The armature will spin so long as there is always a torque acting on it. How this is accomplished is what sets each type of electric motor apart.

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.

Brushless Direct Current Motor

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.

Magnet 2 is near the Reed switch, so the electromagnet creates a field, repulsing magnet 1	There is no magnet near the Reed switch, so there is no field. The armature continues to turn due to inertia	Magnet 1 comes close to the Reed switch, closing the circuit. The electromagnet creates a field again, this time repulsing magnet 2. The cycle begins again.

If we apply alternating current to the simple direct current motor, the obstacle of how to reverse the current, and thereby the polarity of the electromagnet, is no longer an issue. Since the current alternates direction at a certain frequency, the magnetic field of an electromagnet can be made to change polarity at the same frequency.

If the same device is put to use in reverse, that is, if an outside mechanical force is used to turn the armature, an alternating current will be produced:

Most AC motors in use are induction motors, the workings of which are significantly more complicated than the simple AC motor discussed above. As implied by the name, induction motors rely on induction of a current in the armature by the external magnetic field.

The armature is sometimes called a 'squirrel cage', a somewhat archaic reference to it's hamster wheel-like appearance. Wires run the length of the armature, evenly spaced around the circumference. All are connected by plates or rings at both ends of the armature. No current is supplied to the armature; the only magnetic field is the external field of the stator.

Since the current alternates direction, the electromagnetic external field changes polarity at the same frequency. The field induces a current on the windings of the armature. This induced current, in turn, creates a magnetic field. The result is a torque on the armature. Due to the sinusoidal nature of alternating current, the external field more or less rotates continuously around the armature. The armature in effect follows the rotating poles of the external field.