The 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 necessary needs 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, the alternator type generator is also used, which uses diodes to convert the generated alternating current into direct current.
When a conductor is moved in the amagnetic 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 the direction of lines of force, the direction of conductor motion and direction of current flow. If the 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 Figure, if the conductor moves towards the right, the direction of current flow induced in the conductor will be outwards, that is. towards the reader.
|DC Generator line diagram|
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 field is strengthened, that is, the number of lines of force increased, the current flow will be increased.
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 a single conductor will not be uniform, its shape will be as given in Figure. To make it uniform many such conductors are used in the generator. The magnets are not permanent magnet but they are electromagnets, the energy for which is taken from the current produced by the generator itself.
CONSTRUCTION OF D.C. GENERATOR
The figure shows a sectional view of a passenger car D.C. generator-commutator type. 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 makes the commutator. A steel shaft running 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 induced in the conductors flows through the load. Part of the current is induced and flows in the conductors 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 commutator and armature 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 change positions so that the current continues to be fed to one brush in the same direction.
Watch the animation video of DC Generator below
Applications Or Uses of DC Generator
- Testing purpose in Laboratory.
- Supply source of DC motor.
- Used for general lighting.
- Used to charge a battery because they can give a continuous voltage output.
- Excitation of the alternators.
- In the small power supply areas where a portable energy source is needed.
- Railway distribution system.
- DC Locomotives.
- Arc welding.
- Series arc lightning.
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