The magnetic flux in the 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 the separate excitation the field coils are energised by a separate dc source. In self excitation the current flow through the field winding is supplied by machines it self.
Direct current machines are named according to the connection of field winding with armature.
The types of Dc Machines are:
- Separately excited d.c. machine
- Shunt wound or shunt machine
- Series wound or series machine
- Compound wound or compound machine.
As the implies, the field coils is energised by separate d.c. source. The connection showing the separately excited by dc machines are given in figure.
A Dc Machine in which field coils are connected parallel with armature is called a shunt machine.Shunt field is generally made of large number of turns of fine wire carrying small field current.since shunt field receives full output voltage of a generator or supply voltage of a motor. Figure shows the connection diagrams of d.c. shunt generator and d.c. shunt motor.
A Dc Machine in which field coils are connected series with armature is called a series machine. Series field winding consists of few turns of wire of large cross section area. Because series field winding carries the armature current. since armature current is large. Figure shows the connection diagrams of d.c. series generator and d.c. series motor.
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 windings. The shunt winding has many turns of fine wire and series winding has few turns of large cross section area.
The compound machine may be connected in two ways. If shunt field is connected in Parallel with the armature alone the machine is called the short shunt compound machine. Such machine shown in figure.
If the shunt field is in parallel with both armature and series field , the machine is called the Long-shunt compound machine.Such machine shown in figure.
If the magnetic flux produced by series field winding assists (aids) the flux produce by shunt field winding, the machine is said to be cumulatively compound. If the series field flux opposes the shunt field flux, the machine is said to be differentially compound. Either type may be long shunt or short shunt connected.
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