DC Motors:

        Motor construction is physically similar to a generator, in fact,
a basic dc generator can also function as a dc motor. Both dc motors
and generators use external magnetic field excitation, armature coil
windings, and commutators.

        Motors operate because of the interaction between magnetic and
electromagnetic fields. A dc motor, like a dc generator, consists of  
end bells, frame, brush assembly, commutator assembly, armature assembly,
and pole pieces. The stationary field is not always a permanent magnetic
field. It may be supplied by an electromagnetic field, as with a dc
generator. The field windings are excited from a dc power supply, and
provide a steady electromagnetic field. The polarity of the field is 
determined by the direction of the current flow through the windings.
Thus, the polarity is adjusted such that one of the pole pieces becomes
an N pole and the other pole piece (in a two pole motor) becomes an S 
pole. This establishes the stationary field. However, a motor also
requires dc excitation to the armature. This could be supplied by a
separate dc power supply, but often the field windings (stator) and
the armature assembly (rotor) are supplied from the same source. By
adding more loops of wire to the armature, the motor principle is aided
by Repulsion and Attraction. Actually, the repulsion and attraction
effect becomes the dominant factor in dc motor operation when many loops
of wire are used. There are three types of dc motors: SERIES,SHUNT,
AND COMPOUND. This refers to the method in which the field coils
and the armature coils are connected. It is important to know the
characteristics of each type of motor in order to use them properly.
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