A field coil has many designs
There are lots of different designs of field coils. In some units such as dynamos and alternators they supply the excitation for a generator. In a DC motor they provide the magnetic field which the armature reacts against in order for the armature to spin.
In a generator, the field coil can either be on the rotating part of the electrical machine (the rotor) or the stationary part (the stator). On an alternator the rotor coil provides the excitation. As the rotor spins it induces current to flow within the stator windings, and the output is taken from the stator.
In a dynamo, the field coils are fixed around the pole shoes. It is the armature that rotates and produces the output from the machine.
In some designs there can be as little as one field coil in the dynamo, such as the E3L as used on some vintage motorbikes. In this type, the dynamo is eccentric, that is to say the armature does not run through the centre of the diameter of the dynamo but is offset. This is because there is just the one field coil and pole shoe. This makes up one magnetic pole and the steel body of the dynamo opposite to the field coil, makes up the other pole. This type of design is also used on some early dynamos used on the Rolls Royce Ghost.
Most dynamo designs have two or four field coils. 2 pole or 4 pole respectively. In these designs the field coils are opposite one-another in the dynamo body, and are usually connected in series.
In some field coils they may have multiple windings within the same coil. These other windings are usually used to control the machine. They can be wound in opposition to the main field, so as current flows through them they produce a magnetic field with the opposite polarity to the main coil, and therefore weaken the strength of the overall field. This type of coil is called a compound winding.
In Dynastarts, the field coils have two windings. One is used in series with the armature which carries a heavy current and is used when the unit is working as a starter. The second winding is used when the unit is operating like a dynamo.
In some larger units they have small field coils called interpoles. They are smaller than the main field coils and are positioned half way between one field coil and the next, in the armatures neutral position, and are use to smooth the output.