Chapter: Basic Electrical : Electrical Machines

DC Generator

Construction, Principle of Operation, Basic Equations and Applications of DC Generators

DC Generator



DC Generator:


To change the Simple Generator into a direct-current generator, two things must be done:(1) The current must be conducted from the rotating loop of wire(2) The current must be made to move in only one direction. A device called a commutator performs both tasks.


DC generator construction:


What is Generator?


An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.


The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.


The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. whenever a conductor cuts magneticic flux, dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction. This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.


Here is the construction diagram of dc generator:


Generator Construction:


Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets. In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes (of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional.

Components of a generator:


Yoke: Yoke is a outer frame. It serves two purposes.


(i) It provides mechanical support for the poles and acts as a protecting cover for the whole machine and


(ii) It carries the magnetic flux produced by the poles.


In small generators where cheapness rather than weight is the main consideration, yokes are made of cast iron. But for large machines usually cast steel or rolled steel is employed. The modern process of forming the yoke consists of rolling a steel slab round a cylindrical mandrel and then welding it at the bottom. The feet and the terminal box etc., are welded to the frame afterwards. Such yokes possess sufficient mechanical strength and have high permeability.


Rotor: In its simplest form, the rotor consists of a single loop of wire made to rotate within a magnetic field. In practice, the rotor usually consists of several coils of wire wound on an armature.


Armature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.

Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).


Stator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.


Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.


Brushes: The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.


Principle of operation:


DC generator converts mechanical energy into electrical energy. when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and emf produces in a generator and it is defined by faradays law of electromagneticinduction :emf causes current to flow if the conductor circuit is closed.


Applications of DC generator:


1.     Shunt generators are extensively used for general light and power supply, and for charging of batteries, since, in conjunction with a field regulator, a constant terminal voltage can be maintained at all loads.


2.     Series generators are mainly used as animation boosters in dc transmission system, in order to compensate for the drop of voltage due to the resistance of transmission conductors.


3.     Over-compounded generators find use in dc transmission, since it is possible to keep on a constant voltage at the load end, by generating a larger voltage so as to overcome the line drop.


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