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Filed coils in a DC motor provide the magnetic field for the armature, which is the part that rotates.
The armature has a commutator at one end comprising a number of copper segments.The armature is made up from a shaft, with a soft iron core wound with turns of copper wire making up a coil around the core.
The start and finish ends of the wire can be arranged so they are diametrically opposite each other, and are attached to the copper segments, such that when a current is passed throught the coil it too generates a magnetic field, causing the shaft assembly to rotate.
The purpose of the brushes is to provide an electrical contact for the armature........
The field coils and the brushes can be wired in various configurations depending on application, most common being "series wound" where the filed coils are wired in series with the brushes. A typical example of this would be your diy hammer drill.
In a totally different context, filed coils and brushes can be used to do different things when it comes to generators.
The field coils are the static part of the generator and are used to connect to the outside world..or your load......the rotor part now has "slip rings" in place of a segmented commutator. Here the brushes bear on 2 separate contact surfaces, an external voltage is applied to "excite" the rotor.........creating a magnetic rotating field. This in turn causes the stator to produce a voltage corresponding to the applied "excitation voltage". The frequency generated depends on the number of revs of the rotor.......
This is a basic rundown, Mechie probably knows more......Ahem!!
But have a search of the net, more info there...... 8)