DC generator is a machine which produces "Direct Current" electric power. It consists of two main pars namely, (1) Field Winding and (2) Armature Winding.
It is provided on poles which are necessary. These poles are fitted on stator and they project inwards towards the rotor. The total number of poles is always an even number. All these poles are identical in construction. Their cores are made of thin laminations of stalloy and their windings are made up of copper wires. All these poles are symmetrically fitted to the stator with alternate polarities i.e., a "North" pole is followed by a "South" pole which in turn is followed by another "North" pole and so on. The copper windings of all these poles are connected together to form a circuit which is known as "FIELD WINDING".
The field winding always require a D.C. Current. Relatively a small DC power is required by field winding to produce sufficiently strong magnetic field. The ends of the pole cores are given a special curvature which is known as "pole-shoe". With such curvature the flux produced by the poles is spread uniformly over almost the entire armature (i.e., the rotor) surface so that all the armature conductors are in the magnetic field of the poles.
The rotor of the machine is made up of thin circular laminations of stalloy. it is called armature core. Laminated structure is used to reduce magnetic losses and also to obtain convenience in the construction. Rectangular punches are cut at the periphery of the circular laminations. Due to these punches, slots are produced on the surface of the armature core. Copper conductors are placed in these slots. These are called "Armature Conductors". All these conductors are connected together to form a completely closed circuit which is known as "Armature Winding". It is winding in which electricity is produced.
It is part of the armature. It is fitted on the shaft. It consists of a number of copper segments. The ends of the armature conductors are connected to these copper segments. Thus the electricity generated in the armature winding is made available at the commutator segments from where it is picked up by two stationary carbon brushes.
The generator is driven by its engine in the correct direction at its rated speed (RPM). DC field current is then supplied to its field winding. This current enables the poles to produce a sufficiently strong magnetic flux which is spread uniformly over the armature surface. he armature conductors which are set in motion, cut the magnetic flux and hence electric power is produced in the armature winding. It is picked up by carbon brushes from the commutator as shown in the figure.
"Basic Marine Engineering" by J.K. Dhar