DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE

DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE

DC GENERATORD.C generators may be classified as

(i) Separately excited generator,

(ii) Shunt generator,

(iii) Series generator and

(iv) Compound generator.

In a separately excited generator field winding is energized from a

separate voltage source in order to produce flux in the machine. So

long the machine operates in unsaturated condition the flux produced

will be proportional to the field current. In order to implement shunt

connection, the field winding is connected in parallel with the

armature. It will be shown that subject to fulfillment of certain

conditions, the machine may have sufficient field current developed

on its own by virtue of its shunt connection.

In series d.c machine, there is one field winding wound over the main

poles with fewer turns and large cross sectional area. Series winding

is meant to be connected in series with the armature and naturally to

be designed for rated armature current. Obviously there will be

practically no voltage or very small voltage due to residual field under

no load condition (Ia=0). However, field gets strengthened as load will

develop rated voltage across the armature with reverse polarity, is

connected and terminal voltage increases. Variation in load resistance

causes the terminal voltage to vary. Terminal voltage will start falling,

when saturation sets in and armature reaction effect becomes

pronounced at large load current. Hence, series generators are not

used for delivering power at constant voltage. Series generator found

application in boosting up voltage in d.c transmission system.

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A compound generator has two separate field coils wound over the

field poles. The coil having large number of turns and thinner cross

sectional area is called the shunt field coil and the other coil having

few number of turns and large cross sectional area is called the series

field coil. Series coil is generally connected in series with the

armature while the shunt field coil is connected in parallel with the

armature. If series coil is left alone without any connection, then it

becomes a shunt machine with the other coil connected in parallel.

BASIC PRINCIPLE OF OPERATION

The generator is an application of electromagnetic induction. It works

on the principle that when a wire is moved in a magnetic field, then

the current is induced in the coil. A rectangular coil is made to rotate

rapidly in the magnetic field between the poles of a horse shoe type

magnet. When the coil rotates, it cuts the lines of magnetic force, due

to which a current is produced in the generator coil. This current can

be used to run the various electrical appliances.

CONSTRUCTION

A simple D.C. generator consists of a rectangular coil ABCD which

can be rotated rapidly between the poles N and S of a strong horse-

shoe type magnet M. The generator coil is made of a large number of

turns of insulated copper wire. The two ends of the coil are connected

to the two copper half rings (or split rings) R1 and R2 of a

commutator. There are two carbon brushes B1 and B2 which press

lightly against the two half rings. When the coil is rotated, the two

half rings R1 and R2 touch the two carbon brushes B1 and B2 one by

one. So the current produced in the rotating coil can be tapped out

through the commutator half rings and into the carbon brushes. From




the carbon brushes B1 and B2 we can supply current into various

electrical appliances like radio, television, electric bulb etc.

WORKING PRINCIPLE

Let us suppose that the generator coil ABCD is initially in the

horizontal position. As the coil rotates in the anticlockwise direction

between the pole N and S of the magnet the side AB of the coil moves

down cutting the magnetic lines of force near the N-pole of the

magnet and side DC moves up, cutting the lines of force near the S-

pole of the magnet. Due to this, induced current is produced in the

sides AB and DC of the coil. On applying Fleming's right hand rule to

the sides AB and DC of the coil we find that the currents in them are

in the directions B to A and D to C respectively. Thus the induced

currents in the two sides of the coil are in the same direction and we

get an effective induced current in the direction BADC. Due to this the

brush B1 becomes the positive pole and brush B2 becomes the

negative pole of the generator.

After half revolution, the sides AB and DC of the coil will interchange

their positions. The side AB will come on the right hand side and

starts moving up whereas side DC will come on the left hand side and

start moving down. But when sides of the coil interchange their

positions, then the two commutator half rings R1 and R2

automatically change their contacts from one carbon brush to the

other. Due to this change, the current keeps flowing in the same

direction. Thus a DC generator supplies a current only in one

direction.



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DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE