Here is a very basic diagram of what a DC motor looks like:
There are six basic parts to the DC motor:
The Axle
The Field Magnet
The Commutator Bars
The Supply Battery
The Brushes
The Armature
The motor spins due to the opposing magnetic forces at the ends of the Armature to the Field Magnet. The Armature is just an electromagnet made by wrapping wire around a metal core (the green angled lines in the diagram represent the wires). Both
sides of the Armature are wrapped by independent coils of wire. One coil of wire is connected to one of the Commutator Bar, and the other coil is attached to the other Commutator Bar.
The Axle is part of the Armature, and the Commutator Bars are attached to the Axle. This entire setup spins around on the Axle.
The Brushes are stationary. The Supply Battery is connect to the brushes, each pole of the battery connecting to a single Brush.
The Brushes, in turn, make contact with the Commutator Bars which then polarize the coils on either side of the Armature.
The now polarized Armature has forces acting on it since initially the plus side of the Field Magnet is aligned with the coil that has the plus polarity and the minus side of the Field Magnet is aligned with the coil that has the minus polarity. The repulsive forces cause the Armature to rotate on the Axle (note that since the Commutator Bars are attached to the Axle, they rotate as
well).
Since the Commutator Bars also rotate, when the Armature rotates far enough the Brushes contact to Commutators switch. This in turn reverses the polarization on each coil so that as the Armature turns 180 degrees and comes into the other polarity of the Field Magnet the repulsion still exists, forcing the Armature to keep on rotating.
One neat thing about the motor we are using... it doesn't have a Field Magnet; instead, an electomagnet is used to provide the field for the motor. As more power is needed from the motor, the controller increases current to both the electromagnet and the
brushes. Thus, the field strength is increased when more power is needed, giving the motor an extra boost.