The operation of a DC motor is dependent on the workings of the poles of the stator with a part of the rotor, or armature. The stator contains an even number of poles of alternating magnetic polarity, each pole consisting of an electromagnet formed from a pole winding wrapped around a pole core. When a DC current flows through the winding, a magnetic field is formed. The armature A cross section of a simple direct-current electric motor. At its center is the rotor, a coil wound around an iron armature, which spins within the poles of the magnet that can be seen on the inside of the casing. Photograph by Bruce Iverson. Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. also contains a winding, in which the current flows in the direction illustrated. This armature current interacts with the magnetic field in accordance with Ampère's law, producing a torque which turns the armature.
If the armature windings were to rotate round to the next pole piece of opposite polarity, the torque would operate in the opposite direction, thus stopping the armature. In order to prevent this, the rotor contains a commutator which changes the direction of the armature current for each pole piece that the armature rotates past, thus ensuring that the windings passing, for example, a pole of north polarity will all have current flowing in the same direction, while the windings passing south poles will have oppositely flowing current to produce a torque in the same direction as that produced by the north poles. The commutator generally consists of a split contact ring against which the brushes applying the DC current ride.
The rotation of the armature windings through the stator field generates a voltage across the armature which is known as the counter EMF (electromotive force) since it opposes the applied voltage: this is the consequence of Faraday's law. The magnitude of the counter EMF is dependent on the magnetic field strength and the speed of the rotation of the armature. When the DC motor is initially turned on, there is no counter EMF and the armature starts to rotate. The counter EMF increases with the rotation. The effective voltage across the armature windings is the applied voltage minus the counter EMF.
