MM0704, Lesson 3
In a DC generator, the current flow in the rotating coil reverses in direction, however, the current flow in the external
load is constantly in one direction because of the action of the commutator. To reduce the amount of fluctuation in the
external current, the moving part of a DC generator is composed of a large number of coils arranged at different angles
about the axis. These coils are all connected to the commutator. The commutator has as many pairs of segments as
there are coils. The group of coils is called the armature.
Field Magnets. The magnetic field in which the coil rotates can be produced by permanent magnets or electromagnets
called field magnets. Permanent magnets are used only in small machines (magnetos). Since it is possible to produce
electromagnets which are much stronger than any permanent magnet, electromagnets are used in all large generators
and in most small ones.
The electric current necessary to excite a field magnet is occasionally supplied by some outside source such as a battery
or other generator, which is separately excited. More commonly current from a generator is used in the field coils of
such a generator. The current in a DC generator can be used to excite the field coils, but all AC generators must have
fields separately excited by an auxiliary DC generator.
Classification of Generators. Self-excited generators are classed according to the three general types of field
connections they employ: series, shunt, and compound. Compound generators are further classified as cumulative-
compound and differential-compound.
In the series-connected generator, all
of the current from the generator
through the field windings
because the external circuit and the field are in series (figure 3-2).
Shunt. Another way to excite the field magnets is to connect them in parallel with the load (See figure 3-3). This type
of generator is commonly called a shunt generator. Because the current in the field windings is wasted as far as the
useful load current delivered to the external circuit is concerned, the
Figure 3-2. Series Generator.