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What is a DC Generator? Working and it's types

Introduction

It is obvious that dynamo is an electrical machine which converts either mechanical energy into electrical energy or electrical energy into mechanical energy. When it converts electrical energy into mechanical energy it is called a motor, and when it converts mechanical energy into electrical energy it is called a generator. In either case it is a converter of energy and not a producer of it.

The generator is usually driven by an electric motor, or a diesel engine or steam engine or steam turbine, which are called as prime movers. Thus, the generator converts mechanical energy stored in the rotation of these prime movers into electrical energy.


Definition

The electric machine which converts mechanical energy into electrical energy is called generator. The machine is driven mechanically by a prime-mover such as steam engine, diesel engine or water turbines etc.


Working Principle

A generator works on the principle of the production of dynamically induced emf. Whenever a conductor moving in a magnetic field cuts the lines of conductor moving in a magnetic field cuts the lines of force, dynamically induced emf is produced in it according to Faraday 's laws of electro- magnetic induction. This induced emf causes a current to flow if the conductor circuit is closed. The quicker the movement, the larger is the induced emf.

A generator consists of a large number of copper conductors and a strong magnetic field. The conductors are arranged in a system, called the armature, that they form a circuit. The armature rotates in the stationary magnetic field. When the conductor circuit is closed, a direct current flows in the circuit.


Simple DC Generator

Consider a rectangular conductor coil rotating about its axis in a magnetic field. The two ends of the coil are connected by a slipring which is split in two halves. It constitutes a slipring commutator, which rotates with the coil. Carbon brushes slip on each half of the ring through which the current is taken out and circuit is completed through a resistance. As in the case of alternator, when the coil rotates it cuts the magnetic lines of force and according to Faraday's laws of electro-magnetic induction an emf is induced in the coil. This emf causes a current to flow in the conductor when the circuit is closed. But this current is alternating. It flows first in one direction and then in the reverse direction. Also its magnitude is not constant while flowing in any one direction. But the commutator in d.c. generator makes the flow of current unidirectional in the external circuit.

In the first half revolution, the current flows along, brush No. 1 which is in contact with segment 'a' acts as positive end of the supply. and brush No. 2 which is in contact with segment 'b' acts as negative end. In the next half revolution, the direction of the current in the coil is reversed. But at the same time the position or segment (a) and (b) are also reversed. The current flows along, brush No. 1 still acts as positive end of the supply, and the current in the load resistance flows in the same direction from L and M.


A waveform of the current through the external circuit. This current is unidirectional, but not continuous (having the same magnitude) like pure direct current. Its magnitude rises from zero to maximum and again to zero for each 180° of rotation. If there are two coils mutually perpendicular to each other, and commutator is in four parts; then the resultant current would be very nearly constant. The periodic variation which still remain may be further reduced by increasing the number of coils.


Types of D.C. Generator

DC generators are of two types:

(1) Self excited DC generator

(2) Seperately excited DC generator


Self Excited DC Generators are of three types:

(a) Shunt Generator

(b) Series Generator

(c) Compound Generator

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