An electrical machine which converts mechanical energy into direct current (dc) electrical energy is called generator. A generator which produces current that flows in only one direction is known as (DC) direct current generator.
Principle of operation:
All generators works on the principle of dynamically induced emf. Energy conversion in dc generator takes place in such a way that whenever a moving coil cuts the magnetic flux, emf (electromotive force) is induced in that coil or conductor according to the faraday law of electromagnetic induction.
Working of DC Generator:
As in any other machine, dc generator has two types of windings which are termed as Armature winding and Field winding.
Field winding applies to that winding which produces main magnetic field in a machine. In case of dc generator, field winding is applied on the stator (stationary part) of machine.
Armature winding applies to that winding where main voltage is induced in the machine. In dc generator, armature windings are placed on the rotor (rotating part).
DC voltage is applied to the stator field coils in order to produce constant electromagnetic field (Field winding). Armature windings are applied on the rotor which is installed on the rotating shaft of dc generator.
Armature coils of the rotor are made to rotate with the help of mechanical shaft through constant stator magnetic field. Once the armature conductors rotate in a steady magnetic field, an electromagnetically emf is induced within the armature conductors according to the faraday law of electromagnetic induction. The above discussion is explained by the following diagram.
The magnitude of induced emf depends on the number of magnetic lines of force linking with the moving conductor (i.e. rate of change of flux). Emf induced in the coil will cause the current to flow if the circuit is closed. Direction of induced current in dc generator is given by Fleming right hand rule.
Fleming right hand rule:
Fleming right hand rule is used to know the direction of induced current in generators. According to this rule, open the right hand in such a way that all the three fingers are perpendicular to each other. After that, align the right hand in such a way that thumb points in the direction of moving coil (force) and let the fore finger move in the direction of magnetic field. Once it’s done, middle finger indicates the direction of induced current.
If we analyze the above diagram, we can say that after every half rotation (when coil is perpendicular to magnetic field),direction of motion of that particular coil will be reversed. Hence direction of induce current also changes. In simple words, rotating coil(armature) produces alternating current at the output. The below diagram indicates the direction of induced current after every half rotation of coil.
Here the question arises:
How can we generate dc voltage at the output?
In order to have a dc voltage at the output, split rings are used in dc generator. These rings connect between the armature conductors and stationary brushes. As the name of split ring indicates, there is a vertical split between the rings (i.e. one portion of ring doesn’t make contact with the other portion of ring since there is a gap between them). Brushes are used for carrying current to the external load circuit.
When coil rotates and move from one magnetic field pole to the other (opposite) pole, direction of induce current changes. Similarly, split rings are also rotating change their position at the same time. Because of this, there is a reversal of making contact of split ring with the brush which can automatically change the contact of wire and brush. Actually both rings will make a contact with the other brush after every half cycle which makes the current to flow in only one direction .
In this way, rectification comes into takes place due to split rings. This whole phenomenon is called commutator. Commutator is used for obtaining a rectified waveform and thereby we get unidirectional induce voltage at the output. Moreover, the number of commutator segments are same as the number of armature conductors.
An alternating emf is generated across the armature conductors. Commutator converts alternating voltage produced in the armature winding into unidirectional (dc) voltage across the brushes. This simply means that the current through that is also unidirectional. Hence machine works as dc generator.