DC Generator: Construction and Principle

This Article explains the how the DC generator works, and constructional details of DC generator.  

          DC generator is a Machine which converts  mechanical energy into electrical energy. Rotating prime movers help to move the armature conductor in the stationary magnetic field, so that the conductor cuts the magnetic field, an e.m.f induced in the conductor (according to faradays law). 

    If induced e.m.f  collected by brush and slip ring the e.m.f is forming the sine wave (AC, i.e +ve and -ve values) due changing value of cutting the flux in different positions of the conductor.

simple ac generator, image source: Wikipedia

When it right angle to field, emf is zeo (at 0°,180°,360°), But when it align with magnetic field e.m.f values are maximum(at 90°,270°). in between these positions, the e.m.f values are in between zero and maximum, positive (between 0° to 90°, between 90°to180°) and negative (between 180° to 270°, between 270°to360°) respectively.

e.m.f for various positions of armature, Source:https://maritime.org/doc/radio/img/fig61.jpg

                                    

 But when it comes to DC generator the split ring or commutator makes the alternative e.m.f into unidirectional a.c not pure d.c but like d.c, by interchanging the position of  commutator segment in every half (180°). brushes are pressed against commutator to collect e.m.f and sending out.

Simple DC generator, image source: Pintrest

Construction: Main Parts of the DC generator:

A DC generator can be used as a DC motor without any constructional changes and vice versa is also possible. Thus a DC generator and DC motor can be broadly termed as a DC machine. It consists of two basic parts. i) Stator and ii) Rotor. 

Major constructional parts of a DC machines are: 

 

1. Yoke 

 2. Poles and pole shoes 

 3. Field winding 

 4. Armature core 

 5. Armature winding 

 6. Commutator and brushes. 

parts of DC generator,
Source:https://electricalacademia.com/wp-content/uploads/2018/08/Figure-7-1.jpg

Yoke: The outer frame of a DC machine is called yoke. It is made up of cast iron or steel. It provides mechanical strength to the whole assembly and it carries the magnetic field produced by the field windings. 

 Poles and Pole shoes: Poles are joined to the Yoke with the help of bolts and welding. They carry field winding and pole shoes are fastened to them. 

Pole shoes serves two purposes. They support field coils and spread out the flux in the air gap uniformly. 

 Field winding: They are usually made up of copper field coils and are former wound. They are 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.

Armature Core: Armature core is in the rotor of the DC machine. It is cylindrical in shape with slots to carry armature winding. As the armature core is also a conducting material, an emf is induced in the core. This emf produces a circulating currents(called as eddy currents) in the core and it leads to power loss called as eddy current loss. 

So the armature core is laminated(laminations reduces the effective area considerably thereby increasing the resistance which in turn decreases the eddy currents) to reduce eddy current losses. Air ducts are provided for the axial flow of air and is used for cooling purpose. Armature core is keyed to the shaft.

Armature winding: It is usually former wound copper coil which is placed in armature slots. The armature conductors are insulated from each other and also from the armature core. 

Armature winding can be wound by the following one or two methods. i) Lap winding ii) Wave winding Double layer lap and wave winding are generally used. Double layer winding means that each armature slot will carry two different coils. 

Commutator and brushes: Physical connection to the armature winding is made through a commutator and brush arrangement. A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes.

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