CUMULATIVELY COMPOUNDED DC

GENERATOR

Topic:

Group Members:

Asim Raza 13EL08 Imran Firdos 13EL09 Faizan-ul-Hassan Faiz 13EL34 Muhammad Tahir 13EL27

Course Instructor:

Engr. Abu Baker Siddique

Presentation FlowIntroductionCompounded DC Generator and its Types What is Cumulatively Compounded DC Generator?PrincipleCircuit DiagramDot ConventionFormulasTerminal characteristics GraphHow can we realize a voltage Characteristics in a single

generator?Voltage Control Applications

INTRODUCTION

What is Dc Generator? Generator is

the DC machine which is used to convert mechanical energy into electrical energy.

INTRODUCTION

Compounded Dc Generator and its Types

In the compounded dc generator, both Shunt and series field are present and their effects can be additive or subtractive.

Types of compounded Dc generator1)Cumulatively compounded2)Differentially compounded

What is Cumulatively compounded DC Generator?

Cumulatively compounded DC

generator is the generator in which both series and shunt fields are used, connected so that magnetomotive forces from two fields are additive.

Principle

The energy conversion in the generator is based on principle of production of dynamically induced emf.

Whenever the conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s laws of electromagnetic induction.

This emf causes the current to flow if the conductor circuit is closed.

Principle

Circuit Diagram

Dot Convention

Use of dot convention in cumulatively compounded dc generator has the same meaning as in transformer.

If the dot is present at that place where the current is entering into dotted coil ends of series and shunt coils.

Then the magnetomotive force will be additive.

Formulas

Terminal Characteristics

Suppose that the load on generator is increased, the load current IL increases.

Since armature current IA increases too. Two effects occur in generator.

Terminal Characteristics

(1)As IA increases , the IA(RA+RS) voltage drop increases as well. This tend to cause a decrease in the terminal voltage VT.

(2) As IA increases, series field magnetomotive force increases too. This increases the total magnetomotive force which increases flux in generator which cause to increase EA which tend to increase terminal voltage.

These two effects oppose each other with one to increase VT and other to decrease VT.

Terminal Characteristics

Which effect predominates in a given machine?

It depends how many series turns were placed on poles of machine.

It can be solved by taking several individual cases:

Terminal Characteristics

Case-1 Under-compounded For Fewer turns(NSE small)For that, resistive voltage drop becomes

predominant and voltage falls of just as shunt generator but not quite as steeply.

Generator in which Vt at full load is less than Vt at no load then it is called under-compounded.

Terminal Characteristics

Case-2 Flat-compounded (For more series turns)

For more series turns on poles, flux strengthening effect wins firstly and Vt rises with load.

As load continues to increase magnetic saturation sets in, and resistive drop becomes stronger than flux increase effect.

If Vt at no load is equal to Vt at full load, then generator is called flat compounded.

Terminal Characteristics

Case-3 over-compounded (Even

more series turns are added) If more series turns are added to

generator, then the flux-strengthening effect predominates for a long time before resistive drop takes over.

If Vt at full load exceeds Vt at no load, then generator is called over-compounded.

Graph

How all voltage characteristics can be realized in single generator?

It is possible to realize all these voltage characteristics in a single generator if diverter resister is used.

A diverter resistor is connected in series field. If it is adjusted to large value, most of armature current flows through the series field coil. If it is adjusted to small value, then most of current will flow through R diverter.

Circuit Diagram Using Diverter Resistor

Voltage control

There are two methods by which the speed of generator can be controlled.

(1) Changing speed of rotation ωm

(2) Changing field current IF

Applications

Among various types of Dc generator, cumulatively compounded Dc generators mostly used to get the desired terminal voltage by compensating the drop due to armature reaction and ohmic drop in the line.

Such generators has various applications.

Applications

(1)For lightening , because of constant voltage property

(2)Used for driving a motor

(3)For small distance operation as power for hotels, offices and homes in which generally flat compounded generators are used

(4)power supply purposes

(5)Over-compounded generator are used to compensate the voltage drop