UNIT1

ELECTRICAL MACHINERY & CONTROL

UNIT 1

DIRECT CURRENT (DC)

MACHINERY (PART I)

OBJECTIVES

General Objective

To understand the construction concept of direct current (DC) generator

Specific Objectives

By the end of this unit, you would be able to:

recognize a DC generator

identify main part of DC generator

explain method of excitation field by DC generator

identify the self-excitation circuit


INPUT

1.%2% INTRODUCTION TO DC GENERATOR

e will begin our study of electrical machinery & control with direct current

generator. Direct current generators are not as common as they used to be, because

direct current, when required, is mainly produced by electronic rectifiers. These

rectifiers are able to convert the current of an AC system into direct current without using

any moving parts.

Nevertheless, the understanding of DC generators is important because it represents a logical

introduction to the behavior of DC motors. Indeed, many DC motors in industry actually

operate as generators for brief periods.

DC generators are DC machines used as generators. As

previously pointed out, there is no difference between a

generator and a motor except for the direction of power flow.

In this unit we will discuss about mechanism of DC generators,

method of excitation and the techniques used in self-excitation

DC generator.

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…KEY WORDS… Mechanism method excitation techniques self-

excitation


1.1 MECHANISM OF DC GENERATOR

We have described the basic features and properties of direct-current generators. Now, we

will look at the mechanical construction of these machines, focusing our attention to the

yoke, the armature, the commutator, field pole and the brushes carbon.

Figure 1.1, shows us the cutaway view

of the DC generator. From this figure

we can see the main part of mechanical

construction of DC generators such as

the yoke, the armature, the

commutator, the field pole and the

brushes carbon.

Figure 1.1: Cutaway view of DC generator

(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)

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Figure 1.2: Cross section of a generator(Source: Electrical Machinery Fundamentals (3rd edition); Stephen J. Chapman)

We can also see the DC motor mechanism in different angle as example in Figure 1.2. It is show us the cross section angle.


1.2 THE MAIN PART OF DC GENERATOR

In order to get a better understanding about the mechanism construction of DC generator, let

us examine its main part. Figure 1.1 refers to the mechanism construction of DC generator

that we will discuss below.

1.2.1 Field Pole

The field produces the magnetic flux in the machine. It is basically a stationary

electromagnet composed of a set of salient poles bolted to the inside of circular frame. Field

coils, mounted on the poles, carry the DC exciting current. The frame is usually made of

solid cast steel, whereas the pole pieces are composed of stacked laminations. In some

generators the flux is generated.

In our discussion so far we have considered only two-pole generators. However, in practice a

DC generators or motor may have two, four, six, or as many as 24 poles. The number of

poles depends upon the physical size of the machine; the bigger it is, the more poles it will

have.

1.2.2 Armature

The armature is the rotating part of a DC generator. It

consists of a commutator, an iron core, and a set of coils

(see Figure 1.3). The armature is keyed to a shaft and

revolves between the field poles. The iron core is

composed of slotted, iron laminations that are stacked to

form a solid cylindrical core.The laminations are

individually coated with an insulating film so that they

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Figure 1.3: Armature of DC

generators showing the stacked

laminations, slots and shaft

(Source: Electrical Machines, Drives and Power

System 5th edition; Wildi Theodore)


do not come in electrical contact with each other.

The armature conductors carry the load current delivered by the generator. They are insulated

from the iron core by several layers of paper or mica and firmly held in place by fiber slot

sticks.

1.2.3 Commutator

The commutator is composed of an assembly of tappered copper segments insulated from

each other by mica sheets, and mounted on the shaft of the machine is shown Figure 1.4.

Great care is taken in building the commutator because any eccentricity will cause the

brushes to bounce, producing unacceptable sparking. The sparks burn the brushes and

overheat and carbonize the commutator.

Figure 1.4: Commutator of DC machine


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1.2.4 Brushes

A two-pole generator has two brushes fixed diametrically opposite to each other is shown at

Figure 1.5. They slide on the commutator and ensure good electrical contact between the

revolving armature and the stationary external load.

Figure 1.5: Brushes of a 2-pole generator


The brushes are made of carbon because it has good electrical conductivity and its softness

does not score the commutator. To improve the conductivity, a small amount of copper is

sometimes mixed with the carbon. The brush pressure is set by means of adjustable springs.

If the pressure is too great, the friction produces excessive heating of the commutator and

brushes, on the other hand, if it is too weak the imperfect contact may produce sparking. We

can see at Figure 1.6 ( a) and (b).

Figure 1.6 :

(a) Carbon brush and ultra-flexible copper lead

(b) Brush holder and spring to exert pressure


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(a)

(b)


ACTIVITY 1 A

1.1 Identify the main part in the DC generator mechanism in the box below. Circle and

list down all the answers.

Q F B R U S H E S QW D G Q S P R F D WE S H M D U M G S ER A J N T Y N H A RT P K A F T B J Y TC O M M U T A T O RY R L B G B V K K YA O Z V J V C L E UU I X C L Z X Z P IP O L E P I E C E O

1.2

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Test your UNDERSTANDING before you continue to the next input

WHO AM I?“I’m the rotating part of a DC generator. I consist of a commutator, an iron core and a set of coils.


FEEDBACK TO ACTIVITY 1 A

1.1

Q F B R U S H E S QW D G Q S P R F D WE S H M D U M G S ER A J N T Y N H A RT P K A F T B J Y TC O M M U T A T O RY R L B G B V K K YA O Z V J V C L E UU I X C L Z X Z P IP O L E P I E C E O

BRUSHES, ARMATURE, COMMUTATOR, YOKE, POLE PIECE

1.2 ARMATURE

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INPUT

1.3 TECHNIQUE OF FIELD EXCITATION BY DC GENERATOR

Now, we will differentiate the excitations in DC generator. Excitations in DC generator can

be divided into two major types; separately excited generator and self-excited generator.

Self-excited generator has three basic types such as series generator, shunt generator and

compound generator.

1.3.1 Separately Excited Generator

Instead of using permanent magnets to create the magnetic

field, we can use a pair of electromagnets, called field poles as

shown in Figure 1.7(a). When the DC field current in such a

generator is supplied by an independent source (such as a

battery storage, or another generator; called an exciter) the

generator is said to be separately excited. In conclusion, a

separately excited DC generator is a generator whose field

current is supplied by a separate external DC voltage source.

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Figure 1.7(a): Separately excited 2-pole generator(Source: Electrical Machines, Drives

and Power System 5th edition; Wildi Theodore)


The equivalent circuit is shown in Figure 1.7(b). The DC

source connected to terminals a and b causes an exciting

current IX to flow. The internal generated voltage is Eo

appears between brush terminals x and y.

Figure 1.7(b): Equivalent circuit separately excited generator

(Source: Electrical Machinery Fundamentals (3rd

edition); Stephen J. Chapman)

1.3.2 Self-excited Generator

There are three basic types of self-excited generator such as series generator, shunt generator

and compound generator. In this part, we will focus a shunt generator an example of the self-

excited generator.

A shunt excited generator is a machine which is called shunt field coil. The shunt field coil

is connected in parallel with the armature terminals, so that the generator can be self excited

as shown in Figure1.8(a). The advantage of this connection is that it eliminates the need for

an external source excitation. Similarly, a shunt DC generator is a DC generator that supplies

its own field current by having its field connected directly across the terminals of the

machine.

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a

b

IXRX

LX

EO

x

y

IA IL

RA


Figure 1.8(a): Self-excited shunt generator


The equivalent circuit is shown in Figure 1.8(b). In this circuit the armature current of the

machine supplied both the exciting current (Ix) and the load attached to the machine.

Figure 1.8(b): Equivalent circuit of self-excited shunt generator


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ACTIVITY 1 B

1.3 Match these equivalent circuit excitation generators to the right answer.

SET A SET B

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Test your UNDERSTANDING before you continue to the next input


separately excited generator

self-excited generator

1.4 Explain the difference between separately excited and self excited generators in:

i. connection generator

ii. excitation

FEEDBACK TO ACTIVITY 1 B

1.3

SET A SET B

separately excited generator

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a

b

IXRX

LX

EO

x

y

IA IL

RA


self-excited generator

1.4

(i)

separately excited generator self-excited generator

(ii)

separately excited generator : generator whose field current is supplied

by a separate external DC voltage source.

self-excited generator : DC generator that supplies it own field

current by having its field connected

directly across the terminals of the

machine.

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a

b

IXRX

LX

EO

x

y

IA IL

RA


INPUT

1.4 THE BASIC TYPE OF SELF-EXCITATION CIRCUIT

As we know there are three basic types of self-excited generator such as series generator,

shunt generator and compound generator. Table 1.1 will help us to identify these three self-

excitation circuit.

Table 1.1: Three Self-excitation Circuit

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Series Generator Shunt Generator Compound Generator

A series DC generator is a generator whose field is connected in series with its armature

A shunt generator is a machine whose shunt-field winding is connected in a parallel with the armature terminals.

A compound generator is a DC generator with both series and shunt fields.

ACTIVITY 1 C

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Test your UNDERSTANDING before you continue to the next unit.


1.5 Draw an equivalent circuit for series, shunt and compound self-excited generator?

1.6

FEEDBACK TO ACTIVITY 1 C

1.5


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WHO AM I?“I’m a machine whose parallel-field winding is connected in a parallel with the armature terminals.


1.6 Shunt Generator

You are approaching success. TRY all the questions ini this self-assessment section and

check your answers with those given in the feedback on Self-Assessment given on the next

page.

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SELF-ASSESMENT

If you face any problem, discuss it with your lecturer


Question 1-1

A. Complete the diagram below.

Question 1-2

A. Describe the construction of a comutator.

Question 1-3

A. Fill in the blanks by using the key words in the box

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(a)…………

(b)…………

(c)…………

(d)…………

permanent temporary field magnetic poles independent

dependent separately self external internal genarator


Thus instead of using ……(i)….magnets to create the magnetic …...(ii)….., we can use a

pair of electromagnets, called field ….(iii)…... When the DC field current in such a

generator is supplied by an …..(iv)…. Source, the generator is said to be …..(v)

…..excited. Summarize, a separately excited DC generator is a generator whose field

current is supplied by a separate …..(vi)……DC voltage source.

Question 1-4

A. Explain these three self-excitation generator. Include the equivalent circuit in your explanations

FEEDBACK TO SELF-ASSESMENT

Question 1-1

A.

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Question 1-2

A.The commutator is composed of an assembly of tappered copper segments insulated from

each other by mica sheets, and mounted on the shaft of the machine is shown Figure 1.3.

Great care is taken in building the commutator because any eccentricity will cause the

brushes to bounce, producing unacceptable sparking. The sparks burn the brushes and

overheat and carbonize the commutator.

Figure 1.4: Commutator of DC machine

Question 1-3

A.

Thus instead of using permanent magnets to create the magnetic field, we can use a pair

of electromagnets, called field poles. When the DC field current in such a generator is

supplied by an independent source, the generator is said to be separately excited.

Summarize, a separately excited DC generator is a generator whose field current is

supplied by a separate external DC voltage source.

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Question 1-4

A.


A series DC generator is a generator whose field is connected in series with its armature.

A shunt generator is a machine whose shunt-field winding is connected in a parallel with the armature terminals.

A compound generator is a DC generator with both series and shunt fields.

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UNIT1

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