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Friday, 6 December 2019

Classification of DC Generators

CLASSIFICATION OF DC GENERATORS


D.C Generators are classified depending on the type of connections made. If the field coils are connected in parallel to armature, it is called shunt generator. If the field coils are excited separately, it is said to be separately excited generator, and if the field coils are connected in series to armature it is known as series generator. There are two combinations possible viz shunt and series and if both are used, then it is said to be compound generator. In compound generator two combinations are possible

(a) Short shunt compound generator. and
(b) Long shunt compound generator.

The generator classification tree is given below.
Separately Excited DC Generator :

When the field winding is excited by a separated d.c. current, it is called separately excited DC generator. In this case the field current can be varied through a variable resistance connected in series to the battery circuit.  The Load is connected directly across the armature. The polarity of the generated e.m.f. depends on the direction of rotation of the shaft and the polarity of battery source. The connection diagram is shown in Figure.

D.C. Shunt Generator :

On the shunt generator the current for the magnetization of the field is tapped from the armature itse1f. The shunt field is connected in parallel to the armature. As such the shunt winding has to take full voltage of the armature and so it consists of thin wire of many turns. The sum of the load current and shunt field current is the armature current. The connection diagram is shown in Figure.



D.C. Series Generator :

In the case of a series generator, the field winding is connected in series to armature and the current in the armature, series field and the line are same. Since the voltage drop iii the series field should be as least as possible and also be capable of taking full load current, the field winding consists of less number of turns with thick wire. The total current is the same as that of series field or the armature. The connection diagram is shown in Figure.


D.C. Compound Generator :

D.C Compound generators are of two types

(a) Long shunt compound generator and
(b) Short shunt compound generator.

The difference between the two lies in the connection of the shunt winding.

Long Shunt Compound Generator :

Compound generator utilise both the shunt field and the series field. In the long shunt compound generator, the shunt field is connected parallel to the line as shown in Figure. The line current distribution is again a combination of shunt generator and series generator. In this case the armature current is equal to series field current but the line current is the sum of armature current and shunt field current.

Short Shunt Compound Generator :

When the shunt field is connected across the armature of a series generator, it is termed as short shunt compound generator. As in the shunt generator, the shunt field is impressed with armature voltage and series field takes the line current. The line current is the sum of armature current and the shunt field current. The connection diagram is shown in Figure.



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