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Sunday, 5 January 2020

DC Generator: Introduction, Construction, Working, Types, Applications.

DC Generator


Introduction: 

  • Electric Motors and Generators, group of devices used to convert mechanical energy into electrical energy, or electrical energy into mechanical energy, by electromagnetic means. 
  • A machine that converts mechanical energy into electrical energy is called a generator, alternator, or dynamo. 
  • A machine that converts electrical energy into mechanical energy is called a motor. 
  • In this report we will take a brief look about DC Generators.

Generator principle:

  • An electrical generator is a machine which converts mechanical energy (or power) into electrical energy (or power). 
  • Induced e.m.f.is produced in it according to Faraday's law of electromagnetic induction. 
  • This e.m.f. cause a current to flow if the conductor circuit is closed. 
  • Hence, two basic essential parts of an electrical generator are: 
  • a) Magnetic field. 
  • b) Conductor or conductors which can move as to cut the flux. 
  • Generators are driven by a source of mechanical power, which is usually called the prime mover of the generator (steam turbine, diesel engine, or even an electric motor). 

Fleming’s Right hand rule: 

Used to determine the direction of emf induced in a conductor The middle finger, the fore finger and thumb of the Right hand are kept at right angles to one another. The fore finger represent the direction of magnetic field The thumb represent the direction of motion of the conductor The middle finger will indicate the direction of the inducted emf . 


Construction: 
Yoke, Poles, pole shoes, Field Coils, Armature, Commutator, Brushes, & bearings etc are the constructional parts of DC generator.
Dc Generator
Construction of DC Generator


Yoke 

  • Gives mechanical support for poles 
  • Protects whole machine as a protecting cover 
  • Provides path for the circulation of magnetic flux 
  • Small generators – cast iron 
  • Large machines – cast steel 


Poles, pole shoes 

  • Field magnets has two parts 
  • Pole cores 
  • Pole shoes 
  • Pole shoes: spread out the flux in the air gap & reduce the reluctance 
  • Support the exciting coils 
  • Pole cores: solid piece made of cast iron & cast steel.
  • Pole shoes are laminated to the pole face by screws. 


Field Coils 

  • Function: To carry current due to which pole core behaves as an ELECTRO-MAGNET, producing necessary flux. 
  • It helps in producing magnetic flux. 
  • Field winding is divided into various coils called field coils. These are connected in series with each other and wound in such a direction that an alternated ‘N’ and ‘S’ poles are created. 


Armature 

  • More loops of wire = higher rectified voltage. 
  • In practical, loops are generally placed in slots of an iron core. 
  • The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature. The windings of armatures are connected in different ways depending on the requirements of the machine. 


Commutator 

  • To facilitate collection of current from the armature conductors. & it rectified. 
  •  i.e. converts the alternating current induced in the armature conductors in unidirectional current in the external load circuit. 
  • It is of cylindrical and made up of copper. 


Brushes & bearings 

  • Brushes : To collect current from commutator. 
  • Made-up by carbon or graphite with in rectangular shape. 
  • Bearing is used for friction less smooth operation of DC Machine. 


Types of Generator DC Generator 

Permanent Magnet Generator 
  • When the flux in the magnetic circuit is established by the help of permanent magnets then it is known as Permanent magnet dc generator. 
  • This type of dc generators generates very low power. So, they are rarely found in industrial applications. They are normally used in small applications like dynamos in motor cycles. 

Separately Excited Generator 
  • These are the generators whose field magnets are energized by some external dc source such as battery. 
Self-Excited Generator 
  • These are the generators whose field magnets are energized by the electric current supplied by them-selves. In these type of machines field coils are internally connected with the armature. 
  • Due to residual magnetism some flux is always present in the poles. When the armature is rotated some emf is induced. Hence some induced electric current is produced. 
  • As the pole flux strengthened, it will produce more armature emf, which cause further increase of electric current through the field. This increased field electric current further raises armature emf and this cumulative phenomenon continues until the excitation reaches to the rated value. 
  • It has three types depend on the connection.

1. Series Wound Generator 
  • In these type of generators, the field windings are connected in series with armature conductors 
  • whole electric current flows through the field coils as well as the load. 
  • As series field winding carries full load electric current it is designed with relatively few turns of thick wire. The electrical resistance of series field winding is therefore very low (nearly 0.5Ω ) 
2. Shunt Wound Generator 
  • In these type of DC generators the field windings are connected in parallel with armature conductors 
  • In shunt wound generators the voltage in the field winding is same as the voltage across the terminal. 
3. Compound Wound Generator 

  • In a compound-wound generator, there are two sets of field windings on each Pole. One is in series and the other in parallel with the armature. This combination of windings is called compound wound DC generator. 
  • Compound wound generators have both series field winding and shunt field winding. One winding is placed in series with the armature and the other is placed in parallel with the armature. 
  • This type of DC generators may be of two types- short shunt compound wound generator and long shunt compound wound generator. 


A. Compound Wound Generator (Short Shunt) 
  • The generators in which only shunt field winding is in parallel with the armature winding as shown in figure. 
B. Compound Wound Generator (Long Shunt) 

  • The generators in which shunt field winding is in parallel with both series field and armature winding as shown in figure. 

Applications of DC generators 

Separately Excited DC Generators 
  • Separately excited DC Generators are used in laboratories for testing as they have a wide range of voltage output. 
  • Used as a supply source of DC motors. 

Shunt wound Generators 
  • DC shunt wound generators are used for lighting purposes. 
  • Used to charge the battery. 
  • Providing excitation to the alternators. 

Series Wound Generators 
  • DC series wound generators are used in DC locomotives for regenerative braking for providing field excitation current. 
  • Used as a booster in distribution networks. 
  • Over compounded cumulative generators are used in lighting and heavy power supply. 
  • Flat compounded generators are used in offices, hotels, homes, schools, etc.  
  • Differentially compounded generators are mainly used for arc welding purpose. 


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