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September 4, 2009

Unit 1

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



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.




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)



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





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

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


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

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.


Figure 1.8(a): Self-excited shunt generator
(Source: Electrical Machines, Drives and Power
System 5th edition; Wildi Theodore


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


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.


Series Generator

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

Shunt Generator

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

Compound Generator

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