28-03-2012, 01:03 PM
DC GENERATOR
DC GENERATOR.docx (Size: 417.23 KB / Downloads: 47)
INTRODUCTION
GENERATOR is machine that converts mechanical energy to electrical energy.
DC generator or direct current generator generates a voltage when speed and flux are met. This machine is called a unidirectional (dynamo).It consists of the same basic elements as a simple AC generator like the multi-turn coil rotating uniformly in a magnetic field .The output of which is a series of emf pulses, all in the same positive direction resulting in an average EMF developed across the load. Increasing the number of coils thereby smoothen the output providing more pulses.
The generated voltage depends upon the number of poles and armature winding turns. It has armature with iron core and air gap which is uniform the vicinity of the center of the pole and which becomes larger as the pole tips are approached.This construction gives a uniform flux distribution under the main parts of the pole face with a reduction in the flux at pole tips. to produce an emf the conductor must cut the magnetic lines of force. The single coil is connected to two copper segments by carbon brushes. As before the output electromotive force is a result of the coil cutting across the lines of magnetic flux. With any inductive machine an e.m.f will only be generated while the lines of flux are being cut. Stop the rotation at any angle and the output falls to zero.
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) R1and 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 R1and 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
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
Components of a generator:
Yoke: Yoke is a outer frame. It serves two purposes.
(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and
(ii) It carries the magnetic flux produced by the poles.
In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually
Armature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.
Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).
Stator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.
Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.
Brushes: The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.
Commutator The Commutator is form of rotating switch placed between the armature and the external circuit and so arranged that it will reverse the connection to the external circuit at instant of each reversal of current in the armature coils.
The magnetic field in a d.c. generator is normally produced by electromagnets rather than permanent magnets. Generators are generally classified according to their methods of field excitation. On this basis, d.c. generators are divided into the following two classes:
(i) Separately excited d.c. generators
(ii) Self-excited d.c. generators
The behaviour of a d.c. generator on load depends upon the method of field excitation adopted.