As noted in the explanation for demonstration 68.50 -- Permanent magnet DC motor, demonstrations 68.30 -- Electromagnet; 68.34 -- Cathode ray tube, coil; 68.45 -- Field of current-carrying coil, solenoid, toroid; 68.46 -- Magnetic fields of coils, show that, by running current through a coil, you can produce a magnetic field similar to that of a bar magnet. Demonstration 68.48 -- Torque on current-carrying loop, shows that if you place a coil in a magnetic field, oriented so that a current in it produces a magnetic field that is not parallel to that of the external magnetic field, the coil experiences a torque, which, if the coil is mounted so as to allow it, rotates the coil in whichever direction aligns its magnetic field with the external field. With electric motors, we take advantage of this phenomenon to convert electrical energy to mechanical work. Whereas in a permanent magnet DC motor, the external magnetic field is provided by a magnet or pair of magnets, in a series-wound DC motor, a coil or pair of coils, the field coil or field coils, provides this magnetic field.

In the photograph above is a CENCO DC motor, connected so that the field coils are in series with the armature coils. One end of each of the field coils is connected to a brush (extending from the bracket hanging from the top center of the motor frame), which makes contact with one of the commutators on the motor shaft. Each commutator is connected to one end of one of the armature coils. (The two armature coils are connected to each other at the opposite end. They are connected so that current flows through both of them in the same direction, and their magnetic fields are thus in the same direction.)

When you apply power to this motor, current flows through all the coils, producing a magnetic field around each coil. The two field coils are wired so that their magnetic fields point in the same direction; the north pole of one faces in towards the motor shaft, and the south pole of the other faces in towards the motor shaft. The north and south poles of the armature coils are repelled by the (inward facing) north and south poles, respectively, of the field coils, and attracted, respectively, to the (inward facing) south and north poles of the field coils. This exerts a torque on the shaft, which rotates it until the armature coils line up with the field coils. When the armature reaches this position, the gaps between the commutators arrive at the brushes. Inertia carries the armature just past this point, which rotates the commutators so that they have now switched places. This switches the direction of the current in the armature coils, and they are now repelled by the field coils they have just reached (and attracted by the opposite ones), which causes them to continue rotating in the same direction.

Because all of the magnetic fields in this motor are produced by current flow in the motor, when you reverse the power supply connections, all the magnetic fields reverse, and the direction in which the motor rotates does not change. In order to change the direction in which this motor rotates, you must reverse the orientation of the brushes. That is, you must move the one that is in front, to the back, and the one that is in back, to the front. This reverses the orientation of the magnetic field of the armature coils relative to that of the field coils, which reverses the direction of rotation of the motor. (If it were possible to loosen the commutators and rotate them by 180 degrees relative to the armature coils, this would accomplish the same thing.)

You could also connect this motor so that the field coils are in parallel with the armature coils, though the motor would draw greater current. This would just require attaching one extra wire between the field coils. You could plug the power in where the armature coils are connected to the field coils, and short the top field coil terminals together (which flips the orientation of the field coils with respect to the armature coils and reverses the direction of rotation), or you could move both armature and power connections to the top terminals and short the bottom field coil terminals (which maintains the original orientation of the field coils with respect to the armature coils). Again, reversing the power supply connections has no effect on the direction in which the motor rotates.

When the motor is connected as shown in the photograph, the red field coil has its north pole facing outward (and south pole facing inward), the green field coil has its north pole facing inward (and its south pole facing outward), and the motor rotates in the clockwise direction as viewed from the top. A second CENCO motor available for this demonstration has all its coils (both field coils and armature coils) wound in the opposite direction to those in this motor. Wired identically to this one, the magnetic field of its red coil points north inward, and its green coil points south inward, and it, too, rotates clockwise as viewed from the top.