DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE
DC GENERATOR D.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. 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 1
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DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE
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
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DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE
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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