Types of Generators:

Generators are usually classified according to the way in which their fields are excited. Generators may be divided into (a) separately-excited generators and (b) self-excited generators.

(a) Separately-excited generators are those whose field magnets are energized from an independent external source of dc current.

(b) Self-excited generators are those whose field magnets are energized by the current produced by the generators themselves. There are three types of self-excited generators named according to the manner in which their field coils (or windings) are connected to the armature

(i) Shunt wound: The field windings are connected across or in parallel with the armature conductors and have the full voltage of the generator applied across them.

(ii) Series Wound: The field windings are joined in series with the armature conductors. As they carry full load current, they consist of relatively few turns of thick wire or strips. Such generators are rarely used except for special purposes i.e. as boosters etc.

(iii) Compound Wound: It is a combination of a few series and a few shunt windings and can be either short-shunt or long-shunt. In a compound generator, the shunt field is stronger than the series field. When series field aids the shunt field, generator is said to be commutatively-compounded. On the other hand if series field opposes the shunt field, the generator is said to be differentially compounded.

Brush Contact Drop: It is the voltage drop over the brush contact resistance when current passes from commutator segments to brushes and finally to the external load. Its value depends on the amount of current and the value of contact resistance. This drop is usually small and includes brushes of both polarities. However, in practice, the brush contact drop is assumed to have following constant values for all loads. 0.5 V for metalgraphite brushes. 2 V for carbon brushes.