There are several types of Direct Current machines. In this chapter, the authors highlight the types and characteristics of DC generator. Load characteristics of DC generator are then discussed. They then discuss separately excited generators, self-excited shut generators, series generators, and compound generators. Finally, they discuss voltage build-up in self-excited generators and critical field resistance.
Top5.1 Types And Characteristics Of Dc Generator
D.C. machines are classified according to connection of field winding with the armature winding and accordingly these are called
- 1.
Shunt
- 2.
Series
- 3.
Compound
The generators are classified according to type of field excitation i.e.
- 1.
Self excited
- 2.
Separately exited
The fields winding in shunt machines are connected in parallel with the armature winding. The schematic diagram is shown in Figure 1 (a), excluding the interpole and compensating winding.
Figure 1. Self excited shunt generator, long shunt compound generator, and series generator
Field rheostat is an adjustable resistance connected in series with the shunt field to adjust the field current.
The shunt generator may be self excited/separately excited as shown in Figure 1 (a) and (b)
In series generator, as implies from the name, the field winding is connected in series with the armature winding see Figure 1 (c).
In compound generator, the field is splitted into two parts. A part is connected in series and the other is connected in parallel with the armature winding. The series part is called the series field and shunt part is called the shunt field. There two types of compound generator connections. The short shunt compound and long shunt compound as shown in Figure 1 (d) and (e).
Top5.2 Load Characteristics Of Dc Generators
The steady state performance of a generator is described by its load characteristics. The voltage at the armature terminals of a generator is given by
whereE= generated emfIa=armature currentra=resistance of the armature circuits between terminals of the armature
The generated emf is expressed by equation 2.3 which can be expressed as:
where
and
It is to be noted that the interpoles and compensating windings are considered as part of the armature circuit and the resistance of these windings and those of the brushes are included in ra. According to equation 5.1 at constant speed, E is proportional to the flux Ø and is therefore, a function of the field current and the armature current.
5.2.1 Separately Excited Generator
Let us consider a separately excited d.c. generator. If there were no armature reaction the E would be constant. But due to armature reaction the generated voltage because of the demagnetizing effect of armature field would fall as shown in Figure 2. This fall of voltage is not liner because the magnetizing vurve of the machine is not liner.
Figure 2. Demagnetizing affect due to armature
Since the field current of a separately excited machine is provided by an external source. The armature current is obviously equal load current. Separately excited generators are used on loads that require a wide variation in output voltage, such as motors that must operate through large speed ranges. The separately excited generators remain large speed ranges. The separately excited generators remain stable even at very low field excitation, which is not true for self excited shunt generators, as they become unstable after the terminal voltage is reduced below a critical value which is usually still a substantial fraction of rated value. A main disadvantage of separately excited generator is the need of separate source for excitation.