DC Generators

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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 machineoperates in unsaturated condition the flux produced will be proportional to thefield current. In order to implement shunt connection, the field winding isconnected in parallel with the armature. It will be shown that subject tofulfillment of certain conditions, the machine may have sufficient field currentdeveloped on its own by virtue of its shunt connection.In series d.c machine, there is one field winding wound over the main poles withfewer turns and large cross sectional area. Series winding is meant to beconnected in series with the armature and naturally to be designed for ratedarmature current. Obviously there will be practically no voltage or very small voltage due to residual field under no load condition (Ia=0). However, field getsstrengthened as load will develop rated voltage across the armature with reversepolarity, is connected and terminal voltage increases. Variation in load resistancecauses the terminal voltage to vary. Terminal voltage will start falling, whensaturation sets in and armature reaction effect becomes pronounced at large loadcurrent. Hence, series generators are not used for delivering power at constant voltage. Series generator found application in boosting up voltage in d.ctransmission 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 calledthe shunt field coil and the other coil having few number of turns and large cross
 
DC GENERATOR, BASIC PRINCIPLE OF OPERATION, CONSTRUCTION & WORKING PRINCIPLE
sectional area is called the series field coil. Series coil is generally connected inseries with the armature while the shunt field coil is connected in parallel withthe armature. If series coil is left alone without any connection, then it becomes ashunt machine with the other coil connected in parallel.
BASIC PRINCIPLE OF OPERATION
The generator is an application of electromagnetic induction. It works on theprinciple that when a wire is moved in a magnetic field, then the current isinduced in the coil. A rectangular coil is made to rotate rapidly in the magneticfield between the poles of a horse shoe type magnet. When the coil rotates, it cutsthe lines of magnetic force, due to which a current is produced in the generatorcoil. 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 rotatedrapidly between the poles N and S of a strong horse-shoe type magnet M. Thegenerator coil is made of a large number of turns of insulated copper wire. Thetwo ends of the coil are connected to the two copper half rings (or split rings) R1and R2 of a commutator. There are two carbon brushes B1 and B2 which presslightly against the two half rings. When the coil is rotated, the two half rings R1and R2 touch the two carbon brushes B1 and B2 one by one. So the currentproduced 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 cansupply 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 themagnet the side AB of the coil moves down cutting the magnetic lines of force

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 forcenear the S-pole of the magnet. Due to this, induced current is produced in thesides AB and DC of the coil. On applying Fleming's right hand rule to the sides ABand 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 arein the same direction and we get an effective induced current in the directionBADC. Due to this the brush B1 becomes the positive pole and brush B2 becomesthe negative pole of the generator. After half revolution, the sides AB and DC of the coil will interchange theirpositions. 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 tothe 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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