Q. What are the different types of dc generation according to the ways in which fields are excited. Show the connection diagram of each type.
Ans.
Types of D.C. Machine : The magnetic flux in a d.c. machine is produced by field coils carrying current. The production of magnetic flux in the machine by circulating current in the field winding is called excitation.
There are two methods of excitation, namely separate excitation and self - excitation. In separate excitation the field are coils are energized by a separate d.c. source. In self - excitation the current flowing through the field winding is supplied by the machine itself.
Direct current machine are named according to the connection of the field winding with the armature. The principal types of d.c. machines are :
1. Separately excited d.c. machine
2. Shunt wound or shunt machine
3. Series wound or series machine
4. Compound wound or compound machine.
The four types of machines given above could be either generators or motors.
Separately Excited D.C. Machine : As the name implies, the field coils are energized by a separate d.c. source. The connection showing the separately excited d.c. machines.
Shunt Wound D.C. Machine : A machine in which the field coils are connected in parallel with the armature is called a shunt machine. Since the shunt field receives the full output voltage of generator or the supply voltage of generator or the supply of motor, it is generally made of large no. of turns of fine wire carrying a small field current.
Series Wound D.C. Machine : A d.c. machine in which the field coils are connected in series with the armature is called a series machine. The series field winding carries the armature current since the armature current is large, the series field winding consists of few turns, wire of large cross - sectional area.
Compound Wound D.C. Machine : A d.c. machine having both shunt and series fields is called a compound machine. Each field pole of the machine carries two winding. The shunt winding has many turns of fine wire and the series windings has few turns of large cross - sectional areas.
The compound machine may be connected in two ways. If the shunt field is connected in parallel with the armature alone the machine is called the short - shunt compound machine.
If the shunt field is in parallel with both armature and series field the machine is called long - shunt compound machine
If the magnetic flux produced by the series winding assists (aids) the flux produced by the shunt field winding, the machine is said to be cumulatively compounded. If the series flux opposes the shunt field flux, the machine is said to be differently compounded. Either type may be long - shunt or short - shaunt connected.