Abstract
The objective of this assignment is to get familiar with the electric generators and the DC generator in particularly to inform the reader more about electric generation and what is relation with the electrical generator and mechanical motor. The aim of this is to see how electric DC generator works and what is its components functions to understand its mechanism. In addition, what are the differences between the DC and the AC generators, and where is the dc generator is applied and how it is useful in our life.
Introduction
An Electric generator is a device that converts motive power into electrical power for use in an external circuit. Sources of mechanical energy include steam turbines, gas turbines, water turbines, internal combustion engines and even hand cranks. Electric generators fall into one of two broad categories, (dynamos and alternators). Dynamos generate pulsing DC using a commutator. Alternators generate AC.
DC Generator
A dc generator is an electrical machine which converts mechanical energy into direct current electricity. This energy conversion is based on the principle of production of dynamically induced voltage known as the EMF (electric motive force). The induced voltage can be used in electronic circuits, and electromechanical processors.
Constuction Of DC genarators
Mechanically a dc generator consists of a rotating part and a stationary part
Rotor is the rotating part of an electrical machine. Stator is the stationary part of an electrical machine, which surrounds the rotor. Figure2
Figure 2: The rotor and stator of a generator
One of these parts generates a magnetic field, the other has a wire winding in which the changing field induces an electric current.
The remaining parts of the DC generator are:
1. Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.
2. Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and
(ii) spread out the flux in air gap uniformly.
3. Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such a way that, when energized, they form alternate North and South poles.
4. Armature core: Armature core is the rotor of the machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.
5. Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils. The power-producing component of an electrical machine. In a generator, alternator, or dynamo the armature windings generate the electric current, which provides power to an external circuit. The armature can be on either the rotor or the stator, depending on the design, with the field coil or magnet on the other part
6. Commutator and brushes: Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.
Field winding or field magnet: The magnetic field producing component of an electrical machine. The magnetic field of the dynamo or alternator can be provided by either wire windings called field coils or permanent magnets.
Types Of A DC Generator:
DC generators can be classified in two main categories (Separately excited and Self-excited).
(1) Separately excited: In this type, field coils are energized from an independent external DC source.
(2) Self-excited: In this type, field coils are energized from the current produced by the generator itself. Initial emf generation is due to residual magnetism in field poles. The generated emf causes a part of current to flow in the field coils, thus strengthening the field flux and thereby increasing emf generation. Self-excited dc generators can further be divided into three types Series excited, Shunt excited, and Compound excited.
(A)Series Excited
field winding in series with armature winding
(b) Shunt excited
field winding in parallel with armature winding
(c) Compound excited
combination of series and shunt winding
Working Principle of A DC Generator:
According to Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field (OR a conductor is moved in a magnetic field), an emf (electromotive force) gets induced in the conductor. The magnitude of induced emf can be calculated from the emf equation of dc generator. If the conductor is provided with the closed path, the induced current will circulate within the path. In a DC generator, field coils produce an electromagnetic field and the armature conductors are rotated into the field. Thus, an electromagnetically induced emf is generated in the armature conductors. The direction of induced current is given by Fleming’s(right-hand-rule).
Need of a Split ring commutator:
According to Fleming’s right hand rule, the direction of induced current changes whenever the direction of motion of the conductor changes. Let’s consider an armature rotating clockwise and a conductor at the left is moving upward. When the armature completes a half rotation, the direction of motion of that particular conductor will be reversed to downward. Hence, the direction of current in every armature conductor will be alternating. If you look at the above figure, you will know how the direction of the induced current is alternating in an armature conductor. But with a split ring commutator, connections of the armature conductors also get reversed when the current reversal occurs. And therefore, we get unidirectional current at the terminals.