TYPES OF DC MACHINE

TYPES OF DC MACHINE 

Please go through the DC MACHINE CONSTRUCTION before reading this topic. 

The mmf necessary to establish flux in the magnetic circuit of a dc generator can be obtained by means of 

• i. Permanent magnets
• ii. Field coils excited from some external source and
• iii. Field coils excited by the generator itself.

Generators are generally classified according to these methods of field excitation. On this basis, dc generators are divided into the following two classes:

1. Permanent magnet dc generators
2. Separately excited dc generators
3. Self excited dc generators

In permanent magnet dc machines, permanent magnet is used to establish flux in the magnetic circuit. These generators are not found in industrial applications because of the low power generated from it. Such generators are employed only in small sizes like dynamos in motor cycles. 

The behavior of a dc generator on load depends upon the method of field excitation adopted.

Separately Excited D.C. Generators

A dc 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. Figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/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.

Armature current, Ia = IL

Terminal voltage, V = Eg - IaRa

Electric power developed = EgIa

Power delivered to load = EgIa - I R = I E - I R = VIa 

Self-Excited D.C. Generators

A d.c. generator whose field magnet winding is supplied 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

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. Figure 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., as boosters.

Armature current, Ia = Ise = IL = I(say)

Terminal voltage, V = EG - I(Ra + Rse)

Power developed in armature = EgIa

Power delivered to load

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. (1.34) shows the connections of a shunt-wound generator.

Shunt field current, Ish = V/Rsh

Armature current, Ia = IL + Ish

Terminal voltage, V = Eg - IaRa

Power developed in armature = EgIa

Power delivered to load = VIL

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:

• Short Shunt in which only shunt field winding is in parallel with the armature winding.

• Long Shunt in which shunt field winding is in parallel with both series field and armature winding.

Long shunt

Series field current, Ise = Ia = IL + Ish

Shunt field current, Ish = V/Rsh

Terminal voltage, V = Eg - Ia(Ra + Rse)

Power developed in armature = EgIa

Power delivered to load = VIL

Short shunt

Series field current, Ise = IL

Shunt field current,

Terminal voltage, V = Eg - IaRa - IseRse

Power developed in armature = EgIa

Power delivered to load = VIL

In a compound generator the major portion of excitation is usually supplied by the shunt field. The shunt field is slightly weaker and the series field is considerably weaker than those of the corresponding machine in which the entire excitation is produced by a single shunt or a single series winding.

Compound wound generators are of two types, known as cumulative wound and differential wound generators. In cumulative wound generators the series field assists the shund field, whereas in differential wound generators, series field opposes the shunt field.