A DC generator is an electrical apparatus that converts mechanical energy into direct current (DC), and it operates according to Faraday’s laws of electromagnetic induction. As such, DC generators are used in a wide range of applications, ranging from children’s toys to automobiles. To better understand how they work in numerous settings, this blog will outline the working principle of DC generators and their construction.
Generally, a DC generator can be utilized in the place of a DC motor without necessitating any major changes in terms of construction. In fact, a DC generator and DC motor will both fall under the umbrella term “DC machine” because they are composed of the same basic elements. That being said, we will outline the basic constructional parts of a DC machine.
Construction of a DC Machine
DC machines consist of six major components: a yoke, poles, pole shoes, field winding, an armature core, armature winding, a commutator, and brushes.
1. Yoke: The yoke serves as the outer frame, and it is made up of cast iron or steel. Apart from protecting the DC machine, it also carries the magnetic flux produced by the field winding.
2. Poles and Pole Shoes: Poles are bolted or welded onto the yoke, and they carry field winding. Meanwhile, pole shoes are fastened directly onto the poles. In general poles are incorporated to support field coils and spread out the flux in air gaps uniformly.
3. Field Winding: Usually constructed from copper, field winding is made up of a coil that is former-wound and placed on each pole in a series. They are wound so that, when energized, they form alternate North and South poles.
4. Armature Core: Also called the rotor, the armature core is cylindrically-shaped and has slots that carry armature winding. The armature is made up of thin, laminated, and circular steel disks that reduce eddy current losses. Some variations may feature air ducts that promote axial air flow for cooling purposes, and the armature may be keyed to the shaft.
5. Armature Winding: The armature winding is a former-wound copper coil that rests in armature slots. Keep in mind that the armature conductors are insulated from one another and from the armature core. The winding can be wound by one of two methods, those of which are lap winding or wave winding.
6. Commutator and Brushes: The physical connection to the armature winding is achieved through a commutator-brush arrangement. In a DC generator, commutators collect the current generated in the armature conductors, whereas in a DC motor, commutators help provide current to the armature conductors. Usually, a commutator consists of a set of copper segments that are insulated from each other, with the number of segments being equal to the number of armature coils. Brushes, on the other hand, are made of carbon or graphite, and they rest on commutator segments.
Working Principle of a DC Generator
As per Faraday’s laws of electromagnetic induction, whenever a conductor is placed in a varying magnetic field, an EMF is induced in the conductor. Please note that the magnitude of the induced EMF can be calculated from the EMF equation of the DC generator. If the conductor is provided with a closed path, the induced current will circulate within the path. In DC generators, field coils generate an electromagnetic field as the armature conductors rotate within it; thus, an electromagnetically induced EMF is produced in the armature conductors. For further context, Fleming’s right-hand rule dictates the direction of the induced current. According to this rule, the direction of the induced current changes as the direction of the conductor’s motion changes.
Types of DC Generators
DC generators can be classified into two main categories: separately excited and self-excited types. In separately excited generators, field coils are energized from an independent external DC source. In self-excited generators, field coils are energized from the current produced by the generator itself, and an initial EMF is obtained from the residual magnetism in the field poles. Furthermore, the generated EMF enables a portion of current to flow in the field coils, thereby strengthening the field flux and increasing EMF generation. To suit an array of applications, self-excited DC generators are subdivided into three types: series-wound, shunt-wound, and compound-wound variants.
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