Module 6. D.C. machines

Lesson 23

TYPES AND CHARACTERISTICS OF D.C. GENERATORS

23.1 Types of D.C. Generators

D.C. generators can be classified as:

1. Permanent magnet type

2. Electromagnet type

23.2 Permanent Magnet Type

In this type of generator the poles are made of permanent magnet. The poles do not require any windings. Such type of D.C. generators are of small size. These are employed mainly in dynamo in cycles and bikes. It is not used for industrial purpose because:

a. It would require large magnet which is economically not feasible.

b. Magnetic strength decreases with time so magnetic flux will not remain constant.

23.3 Electromagnet Type

The poles of D.C. generator is magnetized using windings. Electromagnetic type generators can further classifies as:

(i) Externally exited D.C. generator,

(ii) Self-exited D.C. generator.

23.3.1 External or separately excited D.C. generators

A D.C. generator whose field magnet winding is supplied from an independent external D.C. source (e.g., a battery etc.) is called a separately excited generator. Fig. (23.1) shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg =PΦ ZN/60 A). The greater the speed and field current, greater is the generated e.m.f. It may be noted that separately excited D.C. generators are rarely used in practice. The D.C. generators are normally of self-excited type

Fig. 23.1 External or separately excited D.C. generators

Armature current, I_a = I_L

Terminal Voltage, V = E_g – I_aR_a

Electric power developed = E_gI_a

Power delivered to load =

23.3.2 Self-excited D.C. generators

A D.C. generator whose field magnet winding is supplied with current from the output of the generator itself is called a self-excited generator. There are three types of self-excited generators depending upon the manner in which the field winding is connected to the armature, namely;

(i) Series generator

(ii) Shunt generator

(iii) Compound generator

23.3.2.1 Series generator

In a series wound generator, the field winding is connected in series with armature winding so that whole armature current flows through the field winding as well as the load. (Fig. 23.2) shows the connections of a series wound generator. Since the field winding carries the whole of load current, it has a few turns of thick wire having low resistance. Series generators are rarely used except for special purposes e.g., boosters.

Armature current, I_a = I_se = I_L = I(say)

Terminal voltage, V = Eg – I (R_a + R_se)

Power developed in armature = E_gI_a

Power delivered to load

Fig. 23.2 Series generator

23.3.2.2 Shunt generator

In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it. The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load. (Fig. 23.3) shows the connections of a shunt-wound generator.

Fig. 23.3 Shunt generator

Shunt field current, I_sh = V/R_sh

Armature current, I_a = I_L + I_sh

Terminal voltage, V = E_g – I_aR_a

Power developed in armature = E_gI_a

Power delivered to load = VI_L

23.3.2.3 Compound generator

In a compound-wound generator, there are two sets of field windings on each pole—one is in series and the other in parallel with the armature. A compound wound generator may be:

(a) Short Shunt in which only shunt field winding is in parallel with the armature winding (Fig. 23.4).