Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
As load current increases, Ise increases. The flux is directly proportional to Ise. So flux also increases. The induced e.m.f. E is proportional to flux hence induced e.m.f. also increases. Thus the characteristics of E against i.e. internal characteristics is of increasing nature.
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
• Consider the d.c. shunt generator shown in the Fig. 3.25.1. The internal characteristics is E Vs IL while the external characteristics is Vt against IL
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
The characteristics of separately excited d.c. generator are divided into two types, 1) Magnetisation and 2) Load characteristics.
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
• The d.c. generators have following basic characteristics, 1) Magnetization characteristics and 2) Load characteristics
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
For every generator which is used as a self excited generator there must exist some residual magnetic flux. When armature rotates, conductors cut this small residual flux to produce the e.m.f. Er. This e.m.f. drives small current through field winding.
with Example Problems | DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Depending upon the connection of shunt and series fileld winding, compound generator is further classified as : i ) Long shunt compound generator.
with Example Problems | DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
• When the field winding is connected in series with the armature winding while supplying the load then the generator is called series generator. It is shown in the Fig. 3.20.1.
with Example Problems | DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
When the field winding is connected in parallel with the armature and the combination across the load then the generator is called shunt generator.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Based on how field winding is connected to the armature to derive its excitation, this type is further divided into following three types. i) Shunt generator ii) Series generator iii) Compound generator.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
• When the field winding is supplied from external, separate d.c. supply i.e. excitation of field winding is separate then the generator is called separately excited generator. Schematic representation of this type is shown in the Fig. 3.17.1.
DC Generators
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Key Point: The field winding is also called exciting winding and current carried by the field winding is called an exciting current.
Subject and UNIT: Electrical Machines: Unit II: D.C. Generators
Key Point: Every practical generator needs a prime mover to rotate its armature. Hence to avoid complexity of the diagram, prime mover need not be included in the symbolic representation of generator.