Dc Generator:
Introduction:-Generator is a device to convert mechanical energy from the automobile engine into electrical energy. The generator replaces in the battery the current used in starting the engine and also supplies current for operation of electrical devices such as ignition system, lights, radio, etc. The automobile generator is usually a low voltage D.C. generator producing direct current, because the use of battery requires direct current.
The generator or the dynamo, is usually mounted on the side of the engine block. It is driven by the engine fan belt. As the engine speed is subjected to large variations, the generator speed will also vary. But the necessity demands that the voltage should remain nearly constant. When the generator is driven by the engine, it supplies electrical energy for all the circuits of the vehicle and keeps the battery fully charged. All the motor vehicles use generators to supply direct current. Many manufacturers use the commutator type shunt-wound, self energizing generator unit. In recent years, alternator type generator is also used, which uses diodes to convert the generated alternating current into direct current.
Generator Principle:- When a conductor is moved in a magnetic field, a current is produced in it. The direction of flow of the current is determined by Fleming's left hand rule. It gives the relationship between direction of lines of force, direction of conductor motion and direction of current flow. If left hand is placed around the conductor with fingers pointing in the direction of lines of force circling the conductor, the thumb will point in the direction of the current. The conductor must move across the magnetic field so that it cuts through lines of force. As shown in Fig. 23.1, if the conductor moves towards the right, the direction of current flow induced in conductor will be outwards, that is, towards the reader.
The conductor must move across the magnetic field so that it cuts the lines of force. If it moves parallel to the lines of force, it will not cut the lines of force and no current will be induced in the conductor. The amount of current induced in the conductor depends upon the rate at which the lines of force are cut. Thus, if the conductor moves more rapidly through the magnetic field, more lines of force will be cut per second and more current will flow through the conductor. If the magnetic
Instead of a straight conductor, a U-shaped conductor is rotated in the magnetic field, so that the direction of current flows in the two legs of U will be opposite, because of their opposite direction of motion. If the ends of the conductor are connected to the commutator through the carbon brushes, the current can be taken to the outer load. However, the current produced in single conductor will not be uniform, its shape will be as given in . To make it uniform, many such conductors are used in generator. The magnets are not permanent type but they are electromagnets, the energy for which is taken from the current produced by the generator itself.
Direction of electromotive force:- The direction of the electromotive force generated in a conductor within a magnetic field will vary with changes in the direction of the magnetic flux and the direction in which the conductor moves. If a conductor is moved (in the direction indicated by the large arrow in the illustration below) between the magnetic north and south poles, the electromotive force will flow from right to left (the direction of the magnetic flux is from the north to the south pole).
The direction of the electromotive force can be understood by use of Fleming's Right-hand rule.
Fleming's Right-hand Rule. With the thumb, forefinger and middle finger of one's right hand are outstretched at right angles to each other, the forefinger will indicate the direction of the magnetic flux (magnetic lines of force), the thumb the direction of conductor movement, and the middle finger the direction of the electromotive force.
CONSTRUCTION OF D.C. GENERATOR:-It consists of pole pieces fitted in a frame, armature, commutator and field winding. The pole pieces or shoes are the laminated iron cores for the field winding that furnish the magnetic field for the generator. Most generators contain one pair of poles (N-S), shaped to allow the armature to rotate between them with very little clearance. The armature is made up of conductors of insulated wire around on a laminated iron core. The conductor ends are soldered to copper wires, separated from each other by mica, which make the commutator. A steel shaft running the armature and commutator is supported by means of bearings so that the pulley and fan mounted on the front end can be turned by an engine-driven belt. Two carbon brushes are held in the brush holders, which make firm contact with the commutator segments in order to connect the coils of the rotating armature with the outside circuit.
When the armature rotates, the current is induced and flows in the conductors through the load. Part of the current induced in the conductors flows through the two field windings assembled around the two magnetic poles. This current strengthens the magnetic field between the poles, thus increasing the conductors as they move through the magnetic field.
The armature and commutator are designed to rotate together. They allow the generator to produce a flow of direct current that, is, the current continues to flow in the same direction. As the two ends of a conductor rotate and change position with respect to each other, the two segments of the commutator also changes positions so that the current continues to be fed to one brush in the same direction.
Voltage regulator. The voltage regulator maintains practically constant voltage in the electrical circuit. When the battery is low, it is supplied current at a high charging rate by the generator and when the battery is charged, the charge rate becomes slow.
No comments:
Post a Comment